Understanding Acoustic Feedback

Acoustic feedback it that annoying loud continuous tone you get when the sound system is not set up or operated properly. Acoustic feedback can normally be reduced or eliminated if you understand and follow the basic principles and practices in this article.

What causes Acoustic Feedback?

Acoustic feedback is caused when all the right factors are lined up poorly. Acoustic feedback is basically the result of the sound level coming from the loudspeaker being picked up by the microphone, and being amplified. Then this amplified signal is again picked up by the microphone and gets amplified again. This acoustic feedback cycle continues to increase the signal level of the offending sound until it gets loud enough for everyone to stare at the sound person expecting it to be fixed.

Acoustic feedback is normally at a specific frequency. The frequency of the acoustic feedback will be frequency that is the loudest. That is, the frequency (or frequencies) that is amplified most by the microphone, amplifier, speakers and room acoustics.

Factors affecting Acoustic Feedback

The following are the main factors affecting acoustic feedback and what you can do to reduce it:

Microphone and speaker placement

Microphone proximity relative to the loudspeakers is very important. The closer a microphone is to the loudspeaker, the easier it is to have acoustic feedback. If the microphone is placed directly in front of the speaker, then the likelihood of acoustic feedback increases greatly. The greater the separation between the microphone and the speaker, the less the likelihood of acoustic feedback.

For this reason, most sound systems are set up with the speakers to the side and/or in front of the stage, and pointing away from the stage area. The microphones are placed on the stage, behind the speakers, and facing away from the speakers. The easiest and cheapest way to reduce the likelihood of acoustic feedback is to increase the separation between the speakers and the microphones.

Microphone gain

The greater the gain of the microphone channel, the greater the likelihood acoustic feedback will occur. Often the gain needs to be turned up because the person speaking is speaking too soft or is too far away from the microphone. Reducing the gain (volume or level of the microphone on the mixer) will reduce the likelihood of acoustic feedback.

This may mean placing the microphone closer to the person talking and or asking them to speak closer to the microphone. The person speaking may also need to speak louder into the microphone to avoid having to have so much microphone gain that acoustic feedback is induced.

Turning off all microphones which are not being used will decrease the overall system gain, therefore also reducing the likelihood of feedback. This may allow the microphone being used to have slightly increased gain before causing feedback.

Room Acoustics

The more reverberant a room is, the more likely sound will bounce around the room and be picked up by the microphone. If the room has sound absorbing materials on the wall and/or ceiling (or you are outside), then the likelihood of the sound bouncing off the walls back into the microphone causing acoustic feedback is reduced. This is not normally easily or cheaply achieved, so recognising and reducing the main frequencies bouncing back is required – this is a job for EQ.

Mixer and system EQ

A sound system should be set up to produce a near even sound level over the entire frequency spectrum. Such a system setup will reduce the sound level at frequencies that are amplified more by the speakers/room/system combination. It will also increase the sound level of those frequencies that are absorbed or attenuated (reduced) by the speakers/room/system combination. This system setup needs to be done every time speakers are moved or changed, or whenever a substantial change is made to the sound system.

This system set up could be called “balancing the frequencies” of the system, but is normally referred to as equalising the system. That is, it equalises the overall frequency response to make the loud frequencies less and the quiet frequencies more. Equalising is normally referred to as EQ for short. If a near even sound level is delivered over the entire frequency spectrum, then there will be less chance of any particular frequency being bounced around to cause acoustic feedback – the sound will also be more natural than before the system was equalised (EQ’d).

This topic of Equalising or EQ, is a large topic and is the subject of another article. Suffice to say if the system doesn’t have proper EQ, it is more likely to produce feedback that a system that is EQ’d correctly.

The tone controls on a mixer are also considered as EQ. Although designed more to EQ the sound of each channel, if incorrectly set, can cause acoustic feedback. This is likely to be the case if the system hasn’t been EQ’d, and the operator doesn’t know what the EQ controls (like bass, mid and treble knobs) do. If any of the channel tone controls are set to boost, then this will cause that frequency to be amplified more. This can increase the likelihood of acoustic feedback at that frequency. Consequentially, if there is some low level acoustic feedback at a particular frequency, it may be EQ’d out by reducing the gain of the appropriate tone control.

Summary

Acoustic feedback is simple, and it is complex. There are a number of factors (as listed above) to be considered when setting up a sound system to reduce the likelihood of acoustic feedback. All these factors need to be addressed correctly not to have feedback. Adhering to the basic principles outlined above will help you reduce the likelihood of acoustic feedback, and give you a better sound.

 

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