Before Audio Feedback by Al Whale

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Everyone knows what audio feedback is,
but what exactly causes it.

Take a simplified diagram of a microphone feeding an amplifier, which in turn feeds a speaker.

Typical PA Setup Text

In operation, the audio signal (Red Sinewave ), enters the microphone and is then amplified.
The output of the amplifier is then fed to the speaker. The gain of the system is
calculated by the amplitude of the final signal divided by the amplitude of the original signal.

Calculation of the System Gain must take into account:
  1. the Microphone output level
  2. the Amplifier Gain (can be easily measured)
  3. the Speaker efficiency
                             Sound Meter
A "Sound Meter" will help you calculate the gain.

The following discussion all relate to the audio volumes at the microphone position

If the speaker is in the same room as the Microphone, some of the amplified
audio from the speaker will combine with the original audio at the microphone.

Typical PA SetupText

(For this situation, assume that both audio sources are in phase at the microphone.)

Amplified audio level from the speaker at this location:

Example .... if the total equipment gain is 55%

(each 'Pass' is equal to the delay time thru the system)

Equipment Gain Chart

The Real World

The previous situation was with a single tone and with the two sources in perfect phase. In an actual situation, the source would be various audio frequencies and various phases. System response (and EQ) and room acoustics would alter the level of different frequencies returning from the amplifier. One frequency would be closest to being in phase and have the greatest level and thus the feedback would start at that frequency. Moving a short distance with the microphone may completely alter this resonant frequency (many other variables can also change this response).

Feebback is Caused by the Same Audio Signal being reamplified many times

Methods used to correct Feedback

  1. Natural
  2. Audio Delay
  3. Audio Frequency Shift
  4. Feedback Suppression

Notch Filters

Often this equipment would automatically set the notches for you when you follow their setup procedure. Some notch filters are usually left for dynamic operation. These would automatically be set by the equipment when feedback occurs in changing situations (ie stage movement), and can be reused for other frequencies when the situation changes.

OK now you can´t hear feedback

If you are marginally below the feedback point,
you still have a problem. ....... " Phase distortion "

See my article      Effects of Audio Phasing

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