Master A Common Audio Problem
Audio tracks are greatly influenced by acoustical and electrical phase. Ideally, when sound from multiple sources is mixed there is an expectation that the result will contribute to the overall quality of the final track. That's not always the case.
When a positive audio signal is added to a negative audio signal, the result is zero for signals of equal strength and frequency--or somewhere in between for unequal signals. That is the essence of phase. Phase errors are introduced by acoustics and/or wiring anomalies.
Complex waveforms which make up everyday sounds combine in both destructive and non-destructive ways. It is the artistry of the audio engineer that comes into play in the real world. The assets and liabilities of acoustical phase must be recognized and managed by the engineer.
Acoustical phase problems occur with all microphones. Off-axis sound being reflected from nearby surfaces will affect the equalization (timbre) of any microphone. Reflected sound also has a profound affect on directional characteristics.Acoustical phase problems occur with all microphones. Off-axis sound being reflected from nearby surfaces will affect the equalization (timbre) of any microphone. Reflected sound also has a profound affect on directional characteristics.
Cardioid microphones can become nearly omnidirectional in common use where there are reflective surfaces. A stand that elevates a microphone three or four inches above a table used in an interview is particularly susceptible to reflected sound coming off the table top for example.
Some shotgun microphones, in spite of their narrow field of response, are better than others at reducing this effect. Others, more sensitive to reflected sound become the superior choice for use in controlled acoustical environments such as on a sound stage. It's a matter of knowing when and why to use one or the other.
Observing the three to one rule for multiple microphone applications will reduce acoustical phase problems. If microphone A is 1 foot from a sound source, an actor for instance, then ideally microphone B needs to be at least 3 feet from microphone A. The situation becomes increasingly more difficult to manage as more microphones are added.
Tape two microphones together. If the microphones have small capsules, place something between them to separate them by an inch or so. Wire one with pin 2 high, the other with pin 3 high (phase reversing adapters are available).
Hold the microphones so that the lips just touch the capsule of one. Then speak as the microphones are moved back and forth. The resulting mixed sound will vary in timbre and volume as the source sound moves toward one microphone or the other. This is an easy demonstration of the 3:1 rule and the effects of phase.
Acoustical phase problems will be heard first at lower frequencies for multiple microphone setups. The point at which the low frequencies begin to roll off is a function of the distance between microphones. Thinner voice tracks will result with multiple microphones operating without regard for the 3:1 rule.
Electrical phase problems are conquered with diligence. In recent history the Audio Engineering Society (AES) established a worldwide standard making pin 2 on XLR connectors high for audio signals, pin 3 low (return), and pin 1 the shield connection. Positive pressure on the microphone diaphragm will create positive voltage on pin 2.
"I can remember an AES convention about five years ago where they handed out cards with a pointer that could be spun to indicate which pin was hot. It could land on either 2 or 3. That was their way of calling attention to the problem," recalls Ed Somers of Location Sound. "Nagra did not adopt the pin 2 standard until AES took a position. Modifying Nagra recorders also requires changing power polarity for A-B powered microphones," he advises.
AES14-1992 (r1998) is the published standard. It is available for downloading from the AES web site, aes.org. The document cites the historical background leading to the present day standard beginning with the introduction of the XLR connector as used by Ampex and connectors used by Magnecord in their tape recorders around 1950. It was not until 1975 that a standard was established for microphone wiring. The AES standard was published in 1992.
Older and used equipment needs to be checked first. Even today notable manufactures do not adhere to the standard in all cases.
Every item in the audio chain needs to be checked for proper phase. Microphones, cables, adapters and mixers. Universal cable testers are available from suppliers. LEDs indicate polarity status. Checking microphones, mixers and other electronics in the audio chain requires additional skill. Assume nothing. Check everything.
Phase reversing adapters should always be on hand. They are used to solve acoustic as well as electrical phase problems. In post it is helpful to have phase reversing patch cables. Grief relief can be as near as a phase reverser, especially when outside equipment is to be interfaced with a clean system.