## 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.

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: the Microphone output level the Amplifier Gain (can be easily measured) the Speaker efficiency 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.

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

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

• 1st pass thru .... the return audio level will be 110% of the original audio
• 2nd pass thru .... the return audio level will be 115.5% of the original audio
• 3rd pass thru .... the return audio level will be 118.5% of the original audio
• 4th pass thru .... the return audio level will be 120.2% of the original audio
• 5th pass thru .... the return audio level will be 121.1% of the original audio
• 6th pass thru .... the return audio level will be 121.6% of the original audio

## 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
• Adjust the EQ of the system to the room (this can eliminate hot frequencies)
• reduce amplifier gain
• reduce the number of open microphones to only those currently needed
• use direction microphones
(obviously some of these methods won´t be useful in some situations)

2. Audio Delay
• A delay is put in the audio path. A tone would have to be sustained for the duration of the delay to add to itself (return audio) and cause feedback. This can increase the Gain−Before−Feedback. Often used in large arena situations.

3. Audio Frequency Shift
• The frequency of all of the audio is shifted slightly higher (or lower). The return audio would be the wrong frequency to add to the source and cause feedback. This can also increase the Gain−Before−Feedback. Useful only for voice announcements (definitely not music of any kind).

4. Feedback Suppression
• A popular method today. The audio is fed thru equipment which has numerous very sharp (ie narrow) notch filters. To set this equipment up, the system gain would be increased to the point of feedback. The frequency of the first notch would be adjusted to notch out the frequency. The system gain would then be increased to the new feedback point (should be a new frequency). The next notch filter would be adjusted similarly. Continuing with other filters would null out the worst offending frequencies.

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

6-Feb-07

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