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Primer: Acoustic Characteristics Of Live Sound Reinforcement

In particular, the logarithmic relationship means that to make a 6 dB change in the value of PAG the corresponding distance must be doubled or halved.

For example, if a microphone is 1 ft. from an instrument, moving it to 2 ft. away will decrease the gain-before-feedback by 6 dB while moving it to 4 ft. away will decrease it by 12 dB.

On the other hand, moving it to 6 in. away increases gain-before-feedback by 6 dB while moving it to only 3 in. away will increase it by 12 dB. This is why the single most significant factor in maximizing gain-before-feedback is to place the microphone as close as practical to the sound source.

The NOM term in the PAG equation reflects the fact that gain-before-feedback decreases by 3 dB every time the number of open (active) microphones doubles.

For example, if a system has a PAG of 20 dB with a single microphone, adding a second microphone will decrease PAG to 17 dB and adding a third and fourth mic will decrease PAG to 14 dB. This is why the number of microphones should be kept to a minimum and why unused microphones should be turned off or attenuated.

Essentially, the gain-before-feedback of a sound system can be evaluated strictly on the relative location of sources, microphones, loudspeakers, and audience, as well as the number of microphones, but without regard to the actual type of component. Though quite simple, the results are very useful as a best case estimate.

Understanding principles of basic acoustics can help to create an awareness of potential influences on reinforced sound and to provide some insight into controlling them. When effects of this sort are encountered and are undesirable, it may be possible to adjust the sound source, use a microphone with a different directional characteristic, reposition the microphone or use fewer microphones, or possibly use acoustic treatment to improve the situation. Keep in mind that in most cases, acoustic problems can best be solved acoustically, not strictly by electronic devices.

General Rules
Microphone technique is largely a matter of personal taste—whatever method sounds right for the particular instrument, musician, and song is right. There is no one ideal microphone to use on any particular instrument. There is also no one ideal way to place a microphone. Choose and place the microphone to get the sound you want. We recommend experimenting with a variety of microphones and positions until you create your desired sound.

However, the desired sound can often be achieved more quickly and consistently by understanding basic microphone characteristics, sound-radiation properties of musical instruments, and acoustic fundamentals as presented above.

Here are some suggestions to follow when miking musical instruments for sound reinforcement.

1) Try to get the sound source (instrument, voice, or amplifier) to sound good acoustically (“live”) before miking it.

2) Use a microphone with a frequency response that is limited to the frequency range of the instrument, if possible, or filter out frequencies below the lowest fundamental frequency of the instrument.

3) To determine a good starting microphone position, try closing one ear with your finger. Listen to the sound source with the other ear and move around until you find a spot that sounds good. Put the microphone there. However, this may not be practical (or healthy) for extremely close placement near loud sources.

4) The closer a microphone is to a sound source, the louder the sound source is compared to reverberation and ambient noise. Also, the Potential Acoustic Gain is increased—that is, the system can produce more level before feedback occurs. Each time the distance between the microphone and sound source is halved, the sound pressure level at the microphone (and hence the system) will increase by 6 dB. (Inverse Square Law)

5) Place the microphone only as close as necessary. Too close a placement can color the sound source’s tone quality (timbre), by picking up only one part of the instrument. Be aware of Proximity Effect with unidirectional microphones and use bass rolloff if necessary.

6) Use as few microphones as are necessary to get a good sound. To do that, you can often pick up two or more sound sources with one microphone. Remember: every time the number of microphones doubles, the Potential Acoustic Gain of the sound system decreases by 3 dB. This means that the volume level of the system must be turned down for every extra mic added in order to prevent feedback. In addition, the amount of noise picked up increases as does the likelihood of interference effects such as comb-filtering.

7) When multiple microphones are used, the distance between microphones should be at least three times the distance from each microphone to its intended sound source. This will help eliminate phase cancellation. For example, if two microphones are each placed one foot from their sound sources, the distance between the microphones should be at least three feet. (3-to-1 Rule)

8) To reduce feedback and pickup of unwanted sounds:

a) place microphone as close as practical to desired sound source

b) place microphone as far as practical from unwanted sound sources such as loudspeakers and other instruments

c) aim unidirectional microphone toward desired sound source (on-axis)

d) aim unidirectional microphone away from undesired sound source (180 degrees off-axis for cardioid, 126 degrees off-axis for supercardioid)

e) use minimum number of microphones

9) To reduce handling noise and stand thumps:

a) use an accessory shock mount (such as the Shure A55M)

b) use an omnidirectional microphone

c) use a unidirectional microphone with a specially designed internal shock mount

10) To reduce “pop” (explosive breath sounds occurring with the letters “p,” “b,” and “t”):

a) mic either closer or farther than 3 inches from the mouth (because the 3-inch distance is worst)

b) place the microphone out of the path of pop travel (to the side, above, or below the mouth)

c) use an omnidirectional microphone

d) use a microphone with a pop filter. This pop filter can be a ball-type grille or an external foam windscreen

11) If the sound from your loudspeakers is distorted the microphone signal may be overloading your mixer’s input. To correct this situation, use an in-line attenuator, or use the input attenuator on your mixer to reduce the signal level from the microphone.

Supplied by Shure Incorporated.

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