"Friends, Romans, countrymen, lend me your ears"
The outer ear extends as far as the ear-drum, a pressure-sensitive membrane. Beyond this point is the middle ear, in which three tiny bones transmit and amplify the vibrations of the eardrum. Beyond the middle ear is the inner ear, filled with liquid and contains the most intricate structures of all: the spiral-shaped cochlea, where sound is converted to nerve impulses, and the semicircular canals, the organs for our sense of balance.
The outer ear flap 'pinnae' surrounds each hole in the side of the skull and leads into the ear canal. This irregular cylinder averaging 6mm diameter and approx 25mm long narrows slightly, then widens towards its inner end, which is sealed off by the ear-drum. This shape and the combination of open and closed ends combine to make the canal look like an organ pipe shaped tube enclosing a resonating column of air. The ear canal supports sound vibrations and resonates at frequencies which human ears hear most sharply. This resonance amplifies the variations of air pressure that make up sound waves, placing a peak of pressure directly at the ear-drum. At frequencies between 2K - 5K Hz, the pressure at the ear-drum is approximately double the pressure at the open end of the canal.
Ear animation
The buzzing of a mosquito is less than one quadrillionth of a watt. Pressure movement less than the diameter of a hydrogen molecule causes the ear-drum to vibrate and can be heard. Sound 10 million million (1,000,000,000,000) times larger (short duration) will not damage the hearing mechanism. We can discriminate 400,000 approx sounds, (magnitudes greater than our eyes) and recognize a voice blurred by telephone. Hearing extends over a frequency spectrum of 3 decades (10 octaves) whereas sight is limited to less than 1/3 decade of the electro-magnetic spectrum.
Softly speaking Indigenous people living in remote areas isolated from traffic and amplified sound where the majority background sound, except for birds and periodic thunder, is about one-tenth of a refrigerator. They can hear a soft murmur across a clearing the size of a football field, and locate its source. They suffer little to no loss of acuity with age.
Hearing test
Of all our links to the outside world, hearing is the essential sense that makes us human. Without hearing we cannot master language; unless with heroic effort. Hearing with speech gives us capacity to communicate; to use and pass on knowledge and badly rule a planet. Hearing is the only sense that remains active every moment of our lives and cannot be negated when asleep or under anaesthetic. But in our consumerist image driven world, when asked to select the most precious of the five senses, few people would name hearing. Paraphrased from S.S.Stevens (1906-1973).
Brain hemispheres
Our mind is divided in 2 hemispheres which function in parallel. The hemispheres work together for almost every task. The specialties that each side possesses makes it easier for both sides to work together. For most right handed people, the right sensory system attends to detail whereas the left sensory system generalises context (at the same time). The hemispheres can be reversed for sensory perception at distances beyond arms length. This sometimes creates interesting illusions or confusion. Reading a book within arms length compared to reading a road sign at long distance.
This difference between the hemispheres is exaggerated when driving a vehicle with one eye closed then the other. One eye provides information to the brain hemisphere that discerns points of detail, whereas the other eye provides information to the brain hemisphere that processes general information on direction of road and traffic. Also many people tend to drive faster in fog white-out but slower in black-out. Model plane enthusiasts get confused when their plane hits the ground, and say "I couldn't see the ground coming up". Lunar pictures of crater/hill randomly reverse, similar to Escher drawings.
Trying to understand what someone is saying in a crowded noisy room by turning or closing one ear, then the other. Changing a telephone from one ear to the other; then comparing the difference to speaking with headphones with both ears. The experience of communication changes. Melody voice and higher may be referenced to brain hemisphere of the right, whereas rhythm backing and lower tones may be referenced to left. This can be used as an advantage when panning a voice toward the right to give an association of closeness or separateness and left to be associated with rhythm and backing
Synesthesia
Creating sound for Cinema requires understanding Synesthesia (one sensory system affecting another). Warm-colours associated with foreground as skin tones, red blood or pale yellow with deeper sounds of breathing, heartbeat and bass rumble etc. Cold-colours associated with background as white snow, blue sky and green hills with higher auditory tones of wind noise, chirping birds or screeching vehicle chases etc. Biasing sounds with spatial movement then reversing it for added effect.
The above information on perception is generalised and subjective. Colour does not exist in nature, but as a pigment of our imagination. Music and language cannot be described by its atomic structure. Evolving over millions of years, this beautiful subjective function of the cross integration of the sensory nervous system, reflects our perception of reality, resulting in music, language and consciousness of existence. The gift that makes us human. But the gift that makes us human also gives us arrogance, to create Gods in our own image, to justify greed betrayal and the delusional belief in magical cables, audiophile brand names, and over compressing recorded music.
Auditory perception
The above graph shows that at low level our hearing is most sensitive at frequencies between 1K - 4K Hz approx which is 1,000,000 (60dB) more sensitive than at bass frequencies eg 40Hz. Phons represent our hearing experience at different sound levels, described as equal loudness curves. At high sound levels we tend to experience all frequencies at a similar level. The sensitivity of our hearing reduces with increased level and so does the apparent frequency response. Muscular compression for the protection of the inner ear begins at approx 84dB SPL.
Sound direction The below pics show that at high frequencies the direction of sound is detected simply by amplitude (loudness) difference between our ears.
At low frequencies the wave lengths are so long that both ears will hear sound at similar level. Therefore below 1K Hz the direction of sound is by the phase difference in the wave between our ears. We are able to detect sound direction to approx 100Hz in a free field. But in a reverberant cave or room this is limited to approx 300Hz because the bass energy is reflected from all directions.
There is a small error area between 1K Hz and 4K Hz where it is difficult to detect the direction of sound. Locating a chirping cricket can be difficult if not impossible. Most animals can swivel their outer ears to locate the source of a sound; but the few humans who can do this use this skill to amuse children. Why is it not possible to detect sound direction at the most sensitive frequencies of hearing ?
One explanation is that frequencies between 1K - 4K Hz are required for speech intelligibility, for which both mental hemispheres are required. Nearly all single natural sound sources are broad spectrum, which allows each brain hemispheres to process comparative differences of frequencies either side of 1K - 4K Hz, to achieve sound location. This may have evolved to help us to detect hungry predators, and at the same time warn others. However after the first moment of direct sound, localisation processing is ignored for approx 35ms. This may have evolved to help us not to be confused with echoes from cave walls and ceiling within 10m 30ft.
Transients allows us to immediately localise the source of a sound. A hungry carnivore stepping on twigs creates small sharp transient clicks, which can be simulated with clicks in headphones. Two clicks heard at the same time and level in both ears, appears as a single fused centre click. As one click begins to lead the other from approx 0.3ms the fused click appears to shift toward the position of the leading click. From 0.65ms the fused click appears to come directly from the position of the leading click only. From 3ms (1m 344Hz) the illusion of fusion ceases and the 2 separate clicks are heard.
- 0 - 0.3ms (100mm 3K4Hz) single sound centred between clicks.
- 0.3 - 0.65ms (224mm 1K5Hz) single sound shift toward leading click.
- 0.65ms - 3ms (1000mm 344Hz) single sound heard from leading click only.
- 3ms sound is heard as 2 separate clicks.
The above pic shows a perception of left - right shift that occurs when the same sound as a click is played through both speakers. The listener at centre between the speakers, hears a single click as if from the centre. As the click from one speaker is advanced in time, the illusion is that the click which was heard from the centre, now moves to side of the speaker that is producing the advanced click. However as the click is further advanced the illusion that the click is coming from the advanced side stops, and we then hear the separate clicks from each speaker. This test is most noticeable with headphones but limited with speakers, which are environment and position dependant.
In a classroom this demonstration opens up discussion and possible use to obtain a left - right bias of an instrument in a stereo recording by delaying sound to one side. This appears to be most effective with instruments of high transients. Each student needs to practice this process with headphones and speakers to understand the limitations. Notice how this effect is reduced or lost when excessive compression/limiting is applied to the instrument. Once understood then deciding what to do for creative applications.
Localisation Most research on sound localisation is done with small passive speakers as in the above pic. This research is mostly used for applications of understanding left-center-right positioning from different mic techniques. The ultimate quest (pot of gold) is to create a 5.1 cinema surround illusion (with cleaver software) from only two small front speakers that give the illusion of sound coming from different directions other than speaker placement.
Time shifts and phase effects between speakers does create interesting and novel illusionary effects, but at the cost of reducing fidelity, and requiring the listener to be at a precise centred position. The contradiction to this research is that the majority of pop music and B grade cinema sound is recorded with excessive compression that removes transients and other detail that allows for direction of hearing.
Other sites with further information on hearing.
www.dasp.uni-wuppertal.de/ars_auditus
Diana Deutsch
www.philomel.com/description.html#tritone
Reciprocity of recording and playback chains
There are many web sites books and periodicals on the application and technicalities of microphones. This page is not intended to repeat information but provide a basic understanding for creative results to be achieved with microphones in the amplifying recording process. All who are passionate about music must be perplexed by the inconsistencies within amplified and recorded music. These inconsistencies are mostly the result of technical ignorance of the reciprocal nature of the recording and playback chains.
The application of a microphones can only understood in terms of how the result is heard by the listener through loudspeakers in a mostly chaotic unknown listening environment. This can be a distorted PA system at a live concert, in a large sports stadium concrete echo box, or a small low fidelity home cinema system in an excessively reverberant lounge room, or an audiophile stereo system in a correct acoustically dampened room. Last but not least is the musical discernment, or lack of, attached to a pair of ears.
The above vector analogy shows each 3 factors in the recording and playback chains that must be taken into consideration. The microphone is at the beginning of the recording chain, and the listener is at the end of the playback chain. The 2 triangles are therefore reciprocals of each other. The imaginary ideal is for both triangles to line up, which is impossible to achieve.
We must accept that the recording and playback mediums are entirely different. The objective is not to attempt the listener to experience being at the performance when original recorded, but for the listener to experience the performance as a reflection, as though it is alive. To achieve a recorded performance to appear alive requires understanding the content in both triangles.
"A l i v e"
First we must understand what is meant by alive as separate from real. A recording has the capacity contain greater detail with all instruments being heard at close range, which is impossible to hear at a concert when seated in the back rows. Listening to an opera in the back stalls may be real but it can sound completely dead.
Recording music has the capacity to sound more alive than actually being at the real concert. However there are some conditions that must be set. Instruments and voices can be animated with extended detail that is not available in the natural world. It is also not possible for any of us to listen to a world class opera singer within a meter or two. But we are able to listen to a microphone placed within a very close distance and hear the expression and detail that would not be heard at a concert sitting at a distance, with a lot of people coughing and rustling around us.
Reverberation-ion-ion-ion-ion-ion-ion
Because our ears are 180deg apart enabling us to hear sounds almost perfectly from every direction, any reverberation bouncing from walls and ceiling from different directions destroys localisation and 3D depth of field of the music. Therefore forward stereo 3D from speakers can only be heard in a non-reverberant environment. The visual equivalent of acoustic reverberation is a room full of distorted mirrors making it possible to judge direction or position of anything seen.
After the first moment of direct sound or transients from the speakers, localisation information is ignored for approx 35ms. With excessive reverberation that extends beyond 35ms it will not be possible to detect direction or depth of field within the music. Also Stereo imaging is rarely attempted in live productions because of the excessive reverberation of most venues.
Compression Over compression of recorded music is similar to reverberation remaining at a constant level. Over compressed music has little to zero spatial information. Repeat: Left right localisation and depth of field from recorded music is dependant on minimal compression and listening in a non-reverberant environment.
3 Dimension Stereo
The original meaning of the word stereo is 'solid'. The dual spaced (left-right) sensory system enables us to have a 3 Dimensional perspective of the world. 3D perception is dependant on angular perspective with comparative detail. The greater the amount of comparative detail; the richer the 3D perception becomes. This is easily noticed with our visual sense.
In pitch blackness it is difficult to see if a single spot of light is a firefly sitting in a tree or a galaxy a million light years from earth. Or how far a tennis/cricket or baseball is, when coming straight at you. Also with sound it is not possible to experience 3D sound with small low fidelity sound systems, or with music that has been excessively compressed, or in reverberant environments.
Techniques for stereo imaging have improved over time, and can be artificially created in post-production. Some classical music is skilfully recorded with stereo image, where there is consumer awareness.
Stereo photography with stereo viewers were popular pre 1970 and still have a small following today. The stereo 3D image requires that each eye separately view the left and right pictures. The eyes are very close and forward facing which is why the division is required. However it is possible to see a limited 3D image without division if focus can be shifted forward or back from the pictures. This requires skill and correct viewing position.
www.stereoscopy.com
Stereo recording was researched early in recording history. Our ears are at 180deg (6in 150mm) apart enabling a wide detailed 3D auditory experience. Two microphones spaced similarly to our ears, recorded onto separate tracks and played back through headphones. Similar to the visual 3D stereo experience, the recorded auditory 3D experience requires the ears to separately hear each sound source as with headphones. The result is a 3D quasi-spherical experience similar to reality. Many companies including Neumann have dummy head stereo mics available. There is also a low cost DIY dummy head project available from (ESP Elliott Sound Products) www.sound.westhost.com/project112.htm
Sweet spot A limited 3D stereo effect from speakers can be heard from a sweet spot of an equilateral triangle, in the forward plane. But this effect is limited to an off center position no greater than 3dB difference between the speakers. The larger the triangle the larger the sweet spot to include more people. This is acceptable because we naturally sit in front of a live musical performance. This effect is also noticed by people who choose to sit in the center middle of cinemas. Sound that appears from the other directions as echoes or people coughing is distracting.
Listening to forward facing stereo speakers (as in the above pic) enables both ears to hear both speakers, and therefore limits the ability of hearing localisation of separate sounds, except from the extreme positions of left and right. Another way to understand this, but must not be taken literally, is to imagine that the left right sparkers represent the perimeter of a single very large imaginary speaker.
Depth of field A symphony orchestra has a rich depth of field created by the immense comparative detail within the music. A single note of a flute has zero depth of field. Field depth is created by the richness of comparative and harmonic nuance detail within the music. Small low fidelity speakers can only reproduce a limited depth of field. Depth of field is also limited in reverberant environments and is not obtainable from excessively compressed recordings.
The enjoyment of listening to a live Jazz band (non-amplified) is not dependant on sitting in the middle to hear spatial left right orientation. Enjoyment is achieved by being close enough to hear a depth of field, which can be achieved from almost any angle. Depth of field is the primary influence for hearing realism, whereas localisation left right has little effect on realism. A single large full fidelity speaker system that gives a depth of field, will sound more enjoyable and realistic, than 5 small low fidelity home cinema speakers, that provide localisation only.
A simple experiment is to listen at low level to a single speaker within 100mm 4in. Notice that a depth of field can be heard within the music but disappears when the listening distance is increased. Then listen with 2 speakers and notice that depth of field can be heard from a greater distance as the speakers are moved apart. The depth of field also remains semi-stable over a listening angle of approx 60deg.
Reducing the level of one speaker, causes the depth of field to diminish, regardless of the listening angle. Therefore both speakers must be at the same level. Some teenagers will be seen lying down listening to a small stereo, at low level, very close to their ears. Others may listen at a higher level, at a distance, with speakers further apart.
Propagation is the ability for the sound system to provide full bandwidth of the music. Most small domestic 2 way speakers cannot achieve this. The speakers have to be very close together to enable lower voice and bass energy to combine to obtain lower frequency propagation. Therefore the majority of pop music is recorded as dual mono (pseudo-stereo) to increase the propagation of sound energy. True stereo to obtain realism depth of field is often ignored. Understandably each left and right speaker should independently be capable of providing full bandwidth propagation, for a true a stereo field to be obtained.
Mono comb filter Panning to center so both speakers are reproducing the same sound, to increase propagation, can only be heard accurately from exact center between speakers. But off center, the different path lengths from the speakers create a comb filter effect which causes cancellations, reduces hi frequency energy and decreases intelligibility. This effect can be used positively to decreasing the sharpness of a voice or instrument, particularly drums to obtain a softer splash. An instrument paned hard left or right will sound sharper and therefore closer.
3D Spatial realism
3D Spatial realism requires a minimum of 2 stereo fields. A single field from only 2 speakers enables one part of the 3D experience to be obtained. This point is rarely understood by recording engineers. As in the below pic three speakers can create three stereo fields from which fields 1 and 2 create spatial localisation. This means that musical instruments and be positioned and maintained into a left - center - right correlation over a 60deg listening angle. The 5.1 protocol for home cinema puts the rear speakers at too great an angle to be correlated into stereo fields. The rear speakers are for novelty effect only.
Panoramic 3D spatial realism with 5 speakers (as in the pic below) enables 10 stereo fields which can be correlated to replicate a full symphony orchestra, providing the speakers are full fidelity. Directly adjacent speakers 1, 2, 3, 4, provide the primary stereo fields giving accurate localisation of instruments which should contain high frequency detail with transients. Fields 5, 6, 7, can generalise sections of instruments or choirs. Fields 8, 9, 10, are wide apart, and most listening positions will be off center which will cause the closest speaker to be heard only.
The procedure for obtaining 3D spatial realism requires most if not all sound sources to be separately miked in stereo format. That is 2 microphones for each instrument or section of instruments and voices. This can also include separate stereo miking for the reverberant field.
Reverberant ambience The wider fields can be used for reverberant ambience where no localisation or hi frequency or transient information is required. The hi frequencies in the wider fields may require attenuation (-3db / octave) or -6db shelving above 250 Hz. These wider fields have an advantage to provide deep bass and bass effect localisation because of the greater distances favouring longer wavelengths.
Applying high frequency shelving to the reverberant field, (particularly the rear surround speakers), increases the feeling of envelopment, without the reverberant sound appearing to come directly from the speakers. However to avoid phase shift biasing the direction of the original sound, the reverberation should not come from the source, but only from the other speakers or the surround speakers. But there are other limitations. Longer path length reflections (50ms to 150ms) can overly create a sense of distance, at the cost of reduced intelligibility causing the whole sound to be cluttered and distant.
A simple technique to retain intelligibility is to put a delay between the original sound and when the reverberation begins. Early stereo recordings experimented with the original sound form one speaker and the reflected reverberation from the other. This often extended to a panned reversal to create a spatial moving effect. History proves that the greatest creativity is expressed when new technology is made available and then later substituted with un-imaginative conformity.
Cinema sound 5.1 only provides for one sub-woofer. Localisation below 250Hz is ignored. Commercial cinemas could provide the full bass register within each of the left - center - right speaker stack to enable bass localisation for thunder (movement of space ships?) and army tanks crossing the screen. This would require cinemas to be non-reverberant at low frequencies. The 1950s Cinerama experience clearly demonstrated this, but was considered not commercially viable for our modern low-cost high-profit driven world. There is a possibility that cinematic experiences of this magnitude will be re-created with digital audio management and digital projection in the future.
Compression of recorded music reduces left right localisation and depth of field. Over compressed music is similar to reverberation remaining at a constant level which has zero spatial information. Repeat: Left right localisation and depth of field from recorded music is dependant on minimal compression and listening in a non-reverberant environment. Also when music is over compressed it doesn't mater what mic or mic technique is used it will all end up sounding the same.
Microphone Applications
These descriptions are meant as a general guide as with each application the approaches will vary. It is essential to read as much information from other sites and books, and experiment with each approach to gain an understanding of the various applications. The same sound as mono from both speakers will only be heard as centred when sitting in the sweet spot in center of both speakers. With mono sound any position of center that results in a greater than a 4dB difference between the speakers will appear as the sound is coming from the nearest speaker only.
The traditional approach for a rock/pop band is to use robust dynamic cardioid microphones at very close distances. The aim is make the music as loud as possible through what is often a high powered low fidelity often distorted mono PA system. Some Pop musicians take little to no interest in how they are heard by the audience, this they believe should be managed by others. However many are obsessed by hearing their fold-back wedge monitors as loud as possible before feedback. Achieving loudness with minimum feedback is what governs what mics and techniques are used. Rarely do pop musicians become interested in specialised microphone techniques for creation of spatial sound scaping.
The above pic of the Woody Allan Jazz Band (of which I am a fan) is an excellent example of the basic correct principles of traditional live mic placement. The musician are well spaced for stage presence and best spatial microphone positions. Experience is required to know which microphone type is best suited to a particular production. In most application microphones are placed directly in front at a correct distance of each performer. This traditional approach is appropriate for applications where it is important to maintain a simple and constant outcome.
3:1 rule The often stated generalised rule is that the distance between mics should be at least 3 time greater that the distance between any one mic and the source. This simply means that the difference between mics hearing the same sound must be greater than 6dB. Following this simple rule will minimise the comb filter effect between mics.
Stereo applications to achieve sound scaping and increase depth of field require stereo application for almost every instrument or section of instruments. There are many stereo dual and triple mic techniques that place the microphones close together at varying angles which give slightly different results.
Each geometric arrangement is given a name.
- XY technique
- ORTF technique
- Decca Tree
- M&S Mid and Side or BDT Blumlein Difference Technique
- Coincident figure 8
It is essential to experiment with each arrangement to familiarise yourself with its behavioural pattern. There is no one arrangement that suits all applications. Each technique has an advantage for particular applications. Links to other web sites that give detailed explanations to the different names for each microphone arrangement are on the bottom of this page. If you are required to record as a team especially with trainees then each technique should be memorised with its correct name.
If you record on your own and have a good understanding of basic geometry and wavelength theory, you will be able to create the best outcome pattern without having to know the names for each arrangement. If you can find a geometric arrangement that has not already been named, you can claim it for yourself, similar to early explorers that have mountains named after them.
AKG tips
I n n o c e n t E a r (Essential reading)
A general rule is to achieve a difference in the sound level received by the mics of 6dB or greater. Sound level differences less than 6dB between mics will achieve a limited stereo effect but only if the listener is exactly in the middle between the speakers. Otherwise the sound will appear to come from the speaker closest to the listener as in the right pic below.
The above pic is a generalisation of showing the difference results between two basic stereo miking techniques. Placing two mics close together in any arrangement can enable the same sound to be heard by both as mono particularly the lower frequencies. The differences between some of the techniques can be subtle and it is essential to experiment with each approach to gain an understanding of the various applications. The intention of placing two mics close together is to achieve a similar outcome as with our ears. But when listening to the result through speakers from any position off center between the speakers will appear as the sound is coming from the nearest speaker only.
Stereo miking with short shotguns over a drum kit (percussive transients) gives the best demonstration of spatial detail that can be achieved when each mic is recorded separately and played back through separate speakers. The depth of field heard by the listener tends to remain constant over a wide listening angle. However if mixed to mono the listener would only hear the nearest speaker and the depth of field enabling a 3D image would disappear.
A common practice for pop music is to use a multitude of mics around the drum kit. As many as 20 mics can be used as in the above pic. The large number of mics interact creating an extreme comb filter effect which removes separation and transient detail. The mics are mixed to mono with added hyper-compression and short delays giving a thick fat percussive diffused wash. Two overheads may be added for stereo separation. There is simply no right or wrong in application of microphones for pop music, which is often driven by trends, momo-tonous conformity and a religious attachment to microphone brand names.
The above pic is an example of application choices by deciding the required outcome. Placing a single mic at a distance will insure the image is small. If mixed to mono the external comb-filter from the 2 speakers will diffuse transient and harmonic detail. Stereo close miking with each mic to each speaker will result in a larger than life image of the instrument, which will exaggerate both transient and harmonic detail. The size of the image is not dependant on level or comparative loudness with other instruments.
Sometimes it is required to have a large exaggerated image at a low level, or a distant diffused image at a high level. A vocalist can sing with a husky breathing voice into two microphones at close range which is then mixed at a higher sound level than the backing of the hard rock band. Also a film actor can be close micked whispering with exaggerated breathing, against loud diffused distance explosions and screeching vehicles. Close miking will exaggerate the sound stage of the instrument whereas distant miking will reduce the sound stage of the instrument.
The cello in the above pic can represent any type of instrument or section of instruments. The variation of mic choice applications needs to be pre-thought as to the type of image construction that best suited for musical production. Every variation of mic arrangement has a purpose that will be best suited of desired production result. The right cello pic shows a stereo line source of electret mics. This method increases horizontal directivity by 6dB, with a decreased vertical directivity. Line source directivity only works with frequency wavelengths less than the line length.
Understanding the spatial distance between mics to enable a depth of field can be viewed geometrically as in the above pic. The inverse square law states that sound energy decreases 6dB (1/4) each doubling of distance. The angular difference between the mics capture a depth of field within a 6dB difference between the front and rear of the sound source. Less than 6dB the depth will appear flat whereas greater than 6dB the front of the sound source can mask the rear. Its a juggling act similar to focusing a stereo camera lens.
Cinema screen. For cinema the objective is to make the train (any large sound source) sound as if it is actually present. This means that the sound field has to be heard in the same proportion as the image on the screen. In a commercial cinema the train can appear as its original size, whereas on a home cinema screen the train can only be a fraction of its original size. The recording of the train should be done separately for each application, which is rarely done.
Obtainable objectives
- The listening environment be non-reverberant. Reverberation only in the recording.
- Minimum numbers of mics with no boundaries or reflections.
- Sound system full fidelity 4 way active.
Un-obtainable objectives
- Each mic when recorded be played back separately with its own speaker.
- The playback speakers in the same position as mics.
- The listener be centred.
Web sites of Stereo Mic techniques
Chris Burmajster 'Innocent Ear' 'Essential reading'
www.paia.com 'How MS Stereo Works'
www.prosoundweb.com 'Stereo Microphone Techniques'
www.sounddevices.com/tech/ms_stereo 'ms_stereo'
www.tape.com/Bartlett_Articles/stereo_microphone_techniques 'microphone_techniques'
Magnetic Microphones
Stage dynamic mics are constructed in the same way as a small loudspeaker, often with a very light Mylar diaphragm. This design based on a loudspeaker construction is the most robust especially from spitting dropping and screaming, and therefore popular with stage musicians. Most are cardioid, although omni-directional are available.
A diaphragm is coupled to a voice-coil that is suspended in a strong magnetic field. As the diaphragm (and thence the coil) moves in sympathy with the sound waves, an electric current is generated. In a perfect microphone, the electrical current will be an exact replica of the acoustic signal, but in reality this is never the case.
The element (also known as a capsule) shown has a vented pole-piece (indicated with a *), and this is typically done to create the required directional characteristic. For an omni-directional dynamic microphone, the back would be sealed.
As you can see, this is similar to the construction of a small speaker, and a speaker will work as a microphone. Naturally, the speaker and mic are each optimised for their intended application. The majority of intercom systems use the speaker as a microphone and visa versa.
A typical dynamic mic is easily capable of 0.5V RMS (500mV). This may seem extreme, but the specification for the SM58 is 1.85mV at 1 Pascal (94dB SPL), so 185mV results at 114dB SPL. Therefore 500mV is achieved at just under 123dB SPL which many loud vocalists can achieve with the mic held at close range, at their lips or in their mouth.
Good mic technique includes 'pulling back' from the mic when singing loudly, and getting in close for soft passages. But unfortunately there are many pop/rock singers that don't have an understanding or feel for mic technique.
At these levels, one can completely forget using electret mics, as they will readily distort. Electret mic sensitivity is much higher than a typical dynamic mic. The electret mic may attempt to produce 3-5V RMS at the same SPL (123dB), which is not possible with standard electret capsules, especially if powered by an internal 1.5V battery.
Typical dynamic microphones have an impedance of around 150 - 300 ohms, although some are higher or lower than that. While it may seem tempting to match the impedance of the microphone and preamplifier, this is ill advised, as it will reduce the signal level by 6dB, and thus reduce the signal to noise ratio.
Ribbon microphones are also magnetic and have very thin (usually aluminium) ribbon (almost zero mass) suspended between the poles of an intense magnetic field, and generates a small electric current when it is moved by sound. They have special place in the hearts of many old recording engineers. They are said to be very fragile, which many are. However there are later designs that are robust. Because ribbon mics use a relatively large diaphragm (much larger than most other mics), they can be very sensitive to air movement - even at subsonic frequencies.
Ribbon mics have an inherent figure-8 pattern, (open front and rear) although this is often modified to produce more 'conventional' patterns. Because the impedance of the ribbon is extremely low (less than 1 Ohm), a transformer is used to raise the impedance and output voltage to a more usable level. The transformer is often in the same housing as the microphone. Because of relatively low output level (even after the transformer), a very quiet pre-amp is required. They have very low self noise, so pre-amp noise can easily exceed the microphone noise.
High SPL does not usually bother a ribbon mic in the least. Provided the ribbon remains in the gap, almost nothing will cause a ribbon mic to distort. However direct air movement (from breathing or blowing) can distort the ribbon, which then must be replaced.
www.lkmusic.co.nz/ribbonfix.htm
ribbon mics construction edu
www.vintage-microphones.de
'Planar ribbon' mics are a variation of the theme and are not a ribbon in the true sense of the term. These use a thin membrane with a planar (flat) metallised coil printed on a thin plastic carrier. These are very rugged according to the literature.
Capacitor Microphones
Historically, these mics have been known as 'condenser' mics. 'Condenser' is the old term for a capacitor. Capacitor mics are the backbone of the recording industry. 'Neumann U47' is the most recognised name. They have exceptional detail, and can usually tolerate very high sound levels. Distortion is very low, because the diaphragm movement is so small (comparable to that of the human ear drum).
Capacitor mics use a high DC voltage to polarise the 'plates' of the capacitor sensor, although some use the change in capacitance to modulate a radio frequency oscillator. The frequency modulated 'carrier' is then fed to a detector stage to be converted back to audio. Capacitor mics (of all types) require power - this may be supplied via the P48 (48V phantom feed) from a mixing desk, or may be an external power supply. Voltages up to 200V will be found in some examples.
A capacitor microphone is mechanically very simple, but the material quality is critical for good performance. Because the capacitance is so small (approx 80pF), the insulation resistance must be very high, as must the impedance of the following stage. It is not uncommon to find in excess of 100 Meg ohms input impedance for the impedance conversion stage. This places great constraints on the insulation, which is adversely affected by moisture.
The diaphragm of capacitor mics is conductive, and often use metallised plastic film (Mylar is popular). The metallisation film must be protected from moisture, and in most designs is on the inside of the capsule. The insert will have a tiny bleed hole to allow the air pressure inside the housing to match that of the outside atmosphere.
Many condenser mics use a dual diaphragm capsule, and switch one diaphragm to change the directional characteristic from omni-directional to cardioid. In some cases, the microphone capsule may have two diaphragms, each spaced as close as possible to the back-plate. This will create a microphone with a figure-8 directional pattern.
Excellent pics of condenser mic diaphragms repairs
www.pelusomicrophonelab.com/Repair&Restoration.html
Electret microphones are also called 'Electret Condenser' or 'Electret Capacitor'. These mics use the same general principles as a capacitor mic, except they use a permanently 'charged' plastic membrane so that a high voltage polarising signal is not needed (as is the case with true capacitor mics). Professional electret microphones are excellent for recording. But the majority are unsuited for high SPL.
Instead of needing an external DC polarising voltage, the back-plate is an electret material (this is a so-called 'back electret' hence the name). This material is a plastic that is subjected to an intense electrical field during processing. This causes the plastic material to retain a charge (more or less) permanently. The electret surface are metallised to make it conductive. Some electret mics use the diaphragm as the electret element (and use a conventional back-plate), and while this works very well, they do not have an indefinite life.
The FET (Field Effect Transistor) shown is almost always included in the capsule itself. This is the impedance converter, and in most cases there is no resistor from the gate to common (ground, mic housing). The FET gate circuit relies on surface leakage alone to bias the FET correctly. This is one reason that electret mics can react badly to sudden loud sound, and may lose sensitivity for a few seconds.
Electrets' can be used for stage work, they may (or will) distort at high sound pressure level (SPL). Loud vocalists can easily drive electret mics into distortion at close range. Also temperature and humidity (from the breath of vocalists) can adversely affect them.
The majority of electret mics are omni-directional. Cardioid inserts are available as well as multiple inserts to create different patterns.
Electret mic capsules only cost a few cents to make. Made in the countless millions they are the most common unseen microphone used in the world, mobile phones, telephones, answering machines, computer headsets, domestic stereos, walkmans, the list is endless.
PZMTM ...Pressure Zone Microphone (also known as a boundary mic) is a special application of the electret mic. A miniature electret sensor is mounted a small distance (less than 1mm) facing towards a flat plate. They are often used on floors, walls, tables for conferencing, and can be attached to any large flat surface.
www.crownaudio.com/mic_web/pzm.htm
The PZM has exceptional performance that closely resembles our hearing. The PZM is flat and is designed to be mounted to a wall or placed on the floor or a table top. The bigger the boundary underneath the microphone, the better it will perform. When applied the polar response is perfectly hemi-spherical.
There are many variations on the basic design, that allow for a single stereo mic unit. A cheap version supplied by Radio Shack (Tandy in Australia) called a boundary mic, but lacking the true characteristics of the original PZM.
History of PZM www.uneeda-audio.com/pzm/
"In 1978 sound engineers Ed Long and Ron Wickersham recognized the effects of a boundary layer in sound recording. In studying the behaviour of flush mounted microphones, they discovered that within a few millimetres of a large surface, sound levels from a pair of equal level signals add coherently because, in close proximity to the surface, the particles are still in phase as they accelerate after being brought to a stop by the boundary, thus creating what is called a pressure field, or pressure zone, in the boundary layer. The result is a 6 dB increase in acoustic pressure."
Written by Chris Steinwand www.svconline.com/mag/avinstall_boundary_microphones/index.html
There were AB demonstrations that proved many pop recording engineers could not hear the difference between some of the best know condenser mics and new PZM. For those of us that understood the PZM technology it proved how good many condenser mics were. The PZM is possibly the closest transparent recording device that does not appear to have its own sound.
It was thought at the time that the extraordinary performance of the PZM would make the majority of studio recording microphones obsolete (except for hand held mics) and revolutionise the recording process. But the pop recording industry is blinded by conformity, image, trends and fads, and very resistant to technological advancements and change.
www.shure.com/microphones/models/mxboundary.asp
www.beyerdynamic.com/cms/Studio_microphones
Transmitting mics are available in many variations. Transmitting mics for professional application can be very expensive. Mostly used for live dance musical productions and television. They require specialist knowledge and experience to use them correctly, which will be later covered on this page. Many transmitting mics have internal automatic compression/limiting that unfortunately restricts the dynamic expression of good singers, causing them to sound flat and lifeless.
Phantom Power
In the late 1960s, Neumann converted its valve (tube) capacitor microphones to solid-state. They decided upon a remote powering system called 'Phantom Power', which is a trade mark of Neumann. Although other manufacturers originally avoided the trade mark (using terms such as 'simplex' instead), with time the term 'Phantom Power' has become generic. DIN standard 45596 describes the powering of any device that uses the P48 phantom powering scheme.
The phantom feed supply voltage has been standardised at 48V, but there are many supplies that do not comply, with some operating at 30V or even less. The accepted voltage range for P48 is between 38V and 52V. A 'new' sub-standard has arisen, called P24 (20V - 26V), but this is a seriously retrograde step, creating potentially disastrous incompatibilities between competing standards.
Because phantom power is a common mode signal (it appears equally on both mic leads), plugging a balanced dynamic mic into a 'live' P48 powered mixer channel should not harm the microphone. The mic may make strange loud rude noises if the internal insulation is degraded (by age, saliva, beer, rum+coke, etc.). Always switch off the P48 supply unless it is not needed.
Phantom powering is not the only way that power is supplied to microphones. Another standard is called T12 - as well as transverse feed, A-B powering, parallel powering, and occasionally by its full name ... 12V Tonader (it originated in Germany). It is not commonly found outside the film industry, and is totally incompatible with P48 powering. Adaptors can be fabricated, but require a transformer.
The T12 system uses 180 ohm feed resistors and a 12V supply, but the DC is not sent as a common mode signal like phantom feed. Referring to an XLR mic connector, the positive DC is applied on pin 2, negative on pin 3, and earth (ground) on pin 1. However, there is also a reverse version, with positive on pin 3 and negative on pin2. T12 powering will probably damage dynamic mics that are inadvertently connected while the T12 power is on.
Capacitor microphones using valves (tubes) will almost always require a special outboard power supply, and multi-pin connectors are common. Because of the larger current needed by the valve heater, the 2 - 4mA available from P48 is completely unsuitable. These power supplies will be specific to the microphone and there is possibly no standard adopted by manufacturers, so each may be different.
Written by Rod Elliott (ESP Elliott Sound Products) www.sound.westhost.com/articles/microphones.htm#7.0
Directivity and Polar Response
Diaphragm size A large diaphragm gives improved directivity at higher frequencies and a larger output voltage and therefore a better the signal to noise ratio. But large diaphragms have a simple limitation similar to loudspeakers. The highest frequency is limited to the wavelength that co-insides with the diaphragm diameter. At high frequencies where the wavelengths are smaller than the diaphragm diameter, creates a comb filter effect in the response. However this comb filter effect above 10K Hz can be minimal depending on sound direction and often inaudible.
A small diaphragm diameter has a wider hi-frequency directivity and tends to give a flatter frequency response. The comb-filter effect is pushed well above the audio range. But small diaphragms result in less output level and therefore the signal to noise ratio is less.
Placement If a mic is placed very close to a surface (be it a wall, floor, drum skin or singer's face) it will no longer have the directional characteristics that are specified. Likewise, holding a mic with a hand that cups the back of the mic ball, will change directionality radically and unpredictably. Most microphone specifications only apply in non reverberant free field, at a distance from any reflective surface. The only exception is the boundary mic which is meant to be placed directly on a large flat surface.
www.mipro.com.tw
Live application the number of microphones should be kept to minimum. Unnecessary use of mics creates excessive comb filter distortion. This reduces intelligibility and increases feedback problems. Minimise using different mics. Exceptions are (directional overhead) for percussion and (dynamic high velocity) for bass drums. Placing any mic too close to an instrument or surface, affects its response.
Polar The terms Omni, Cardioid, Hyper-cardioid, Hypo-cardioid, represent polar, phase and frequency response, but these terms are loosely applied. The directivity of all microphones is frequency dependent, and become spherical as the frequency decreases. Directional microphones are also called 'pressure gradient' mics, because their directional characteristics are created by means of varying pressure to the front and rear of the diaphragm (the pressure gradient).
Only the omni has an inherent flat response. The other higher order polar responses must (by definition) alter the received sound to some extent. It is not possible to modify the directional characteristics without also altering the nature of the sound that is picked up. This is not necessarily 'bad', just different.
Increasing directivity is often achieved by sound being allowed to enter the rear of the diaphragm. This can have an added effect of reducing the frequency response at the lower end of the sound spectrum, as the distance increases (generalised in the above pic). There are other arrangements to create directional polar responses by using multiple inserts to achieve minimal effect on degrading the frequency response.
Likewise, many cartridge / capsule types (the actual transducer) have their own sound, whether real or imagined. This often influences the choice of microphone type for different tasks - for example, there are mics that are favoured for bass (kick) drums that may be deemed unsuited for anything else. This is not necessarily true, as experimentation can often demonstrate.
Omni Directional Omni refers to the frequency response being flat, and 'phase coherent' from all directions. Many stage Omnis' have good directivity This is not to be confused with the polar directivity of cardioid in which the phase and frequency response change off axis. Omni give less feedback problems, in live application compared to most cardioid.
Omni have minimal to zero proximity effect, are consistent with vocal movement, and therefore suited to singers who have good management technique, enabling the level to be easily controlled by distance. Omnis are also better suited to instruments. Omni mics are less used for live production compared to cardioid because of ignorance and limited understanding. However Omnis' with full spherical directivity are for recording and not suited to live productions.
Cardioid mics often have a proximity affect to colour and enhance the voice at close range. Different Cardioid mics suit male and female singers. Singers should own their mics and be skilled in the techniques of using them, as musicians owning their instruments.
Cardioid mics are often misused for instruments, typically used in very close proximity to drum skins (among other misuses). But if this gives a specific sound required, then it is no longer misuse.
Hyper Cardioid This is an exaggerated version of the cardioid mic, so it is more directional. A side-effect is that a small lobe is created at the rear of the microphone, so these mics must never be 'aimed' so that the rear lobe points towards a floor monitor (for example). Sometimes a distinction is made between 'super' and 'hyper' cardioid microphones, but other descriptions will consider them to be equivalent.
Fig 8. The figure-8 mic picks up sound equally well from the front and back, but rejects sound coming from the sides (as well as top, bottom, etc.). The pattern can be looked at as an extreme form of hyper-cardioid, where the front and rear lobes are equal in amplitude and frequency response. Many dual element microphones combine an omni and figure 8 capsule to allow switchable directivity.
Shotgun The length of a shotgun does not give it greater directionality but enables it to maintain directionality at lower frequencies. The slots along the microphone body enable sound from the sides to reach the rear of the microphone diaphragm and therefore cancel the sound from the front. The microphone must be suspended at a distance away from obstructing surfaces for it to function correctly. Many shotgun mics also use multiple inserts to improve directionality.
Proximity effect of Cardioid Mics
Proximity is the most controversial effect that plagues some cardioid microphones. When using some cardioid mics close to the lips the bass response increases in a way that appears to be out of control. The proximity effect can be used as an advantage giving a fuller sound for singers with a shallow voice. The proximity effect also gives male singers an artificial pre-emphasises of sounding like having larger balls. However at larger distances 3ft or 1m a cardioid mic may have the opposite effect, that is, the bass response is shallow.
Cardioid directionally is often achieved by enabling off axis sound to enter through a rear vent to the back of the diaphragm. However for the sound to cancel, it must arrive at both sides of the diaphragm, at the same level at the same time. When the mic is close to the lips, the distance to the front of the diaphragm is very short, and more than double the SPL level, than for the same sound to travel double the distance to the rear of the diaphragm (Inverse square law). This is the primary reason the bass response appears to increase dramatically.
As the mic distance is increased the level SPL difference between sound arriving at the front and rear of the microphone becomes less. The lower frequencies cancel more readily regardless of the direction of the sound source (Inverse square law). The rear vent is less effective at higher frequencies where the mic appears to maintain a consistent directional response. Some manufacturers go to extraordinary effort to minimise proximity effect and have various tuned vents to maintain a consistent cardioid polar response over the whole frequency range.
Whether a microphone or a speaker box has an extra hole (port) to improve its bass resonance, or control its directivity, the results for both are the same in creating other complexities that are often un-wanted. Because marketing focuses on the positives and not the negatives of a compromise we are sometimes quite happy to negate what we don't want to know. This is also seen on a larger scale in the marketing behaviour of drug companies.
The links below also give further explanations of the proximity effect of cardioid mics.
arts.ucsc.edu/EMS/Music/tech_background/TE-20/Proximity_Effect
www.rane.com/note1550.html Rane notes on proximity effect
Romantic un-technical language
The pop recording industry has a disproportional number of so called 'recording engineers' that do not have a background in electro-acoustic technology. Their language is often filled with romantic jargon that has no technical meaning. Regardless of whether microphone understanding is based on mumbo-jumbo or romantic symbolic language and religious attachment to brand names, it should not be dismissed. Many recording engineers may have inadvertently learnt by trial and error, or been informed from someone who passed on interesting techniques. The objective is to discern the technique separately from the non-technical romantic descriptive language. A sense of air, ambience, lightness, creamy, topy, fullness, thickness, fatter, body, open or closed.
A popular April fool joke played on many a non-suspecting brand name junkie pop recording engineer, is being easily fooled by a cheap microphone from radio shack with an imitation Neumann sticker created by someone from the art department, with a fabricated brochure describing it as the latest genetically modified homoerotic laser microphone technology.
Special note. We must remember that the majority of pop music is created for 12-16yr olds. Often over-compressed and downloaded to the lowest fidelity mp3 file. There is no test that can resolve which brand of mic, compressor, effects unit, etc was used to record the music. Also virtually no 12-16 yr old would possibly be interested.
Basic Specifications
Output level of Mics are rated in milli-volts per Pascal (mV /Pa)
1Pascal = 10micro-Bar = 94dB SPL
Average speech at 1 Meter = 0.1Pascal = 1micro-Bar = 74dB SPL
Links
Comparison mp3 demonstration of mics
www.coutant.org
www.coutant.org/contents.html
www.coutant.org/allmics
Early Creators of outstanding recordings.
www.asusd.edu 'History of mics'
www.asusd.edu 'History stereo research'
Orson Welles 'War of the Worlds'
Wikipedia 'Stan_Freberg'
www.george-martin.com
www.beatlesagain.com/bgeorgem.html
www.beatles.com
Seargent Peppers 1967 recording technology
Independent Microphone applications and Data
Wikipedia Microphones
Wikibooks Microphones
David Griesinger The Physics and Psychophysics of Surround Recording
schlemmer.gmxhome.de Introduction to Sound Recording
www.microphone-data.com
www.oktavausa.com Russian mics with excellent pics
Hansen Audio Good inside pics of mics
www.rane.com/library Rane library
Microphone manufacturers and suppliers The majority of sites have vast and useful information,
www.akg.com
AKG Recording basics 1
AKG Recording basics 2
The ABC's of AKG Microphone Basics & Fundamentals
AKG drum
www.appliedmicrophone.com
www.beyerdynamic.com
www.crownaudio.com PZM
www.dpamicrophones.com The DPA Microphone university page.
www.electrovoice.com
www.mannelectronics.com
www.neumann.com
www.neumann-publications_mic_book Neumann Microphone Book
www.popfilter.com Pop filters
www.royerlabs.com
www.royerlabs.com/faq
www.rycote.com Wind Socks
www.schoeps.de
www.shure.com
www.shure.com/cc/cc_reference.asp
www.shure.com/scripts/literature/literature.aspx
www.shure.com/booklets/default.asp
www.sennheiser.com
www.sennheiser.com/ mic application
www.sennheiser.com/sennheiser/icm_eng.nsf/root/products_microphones
Sennheiser mic technology
www.sennheiser.com/ Recording basics