Tips for Avoiding Feedback in Meeting Room Audio Systems

Feedback is an annoying and potentially damaging squealing or humming sound that can be extremely distracting during a meeting. If the feedback in a boardroom is too intense it may force the meetings to put on hold until the issue is resolved. We explain why feedback occurs and how it can be eliminated.
 

Feedback occurs when a signal that is emanating from a loudspeakers arrives at a microphone both in phase and at a suitable level that it's picked up and looped through the system. That signal is then amplified again and played back through the loudspeakers. At this point, one or more frequencies may begin to ring or howl and produce a terrible and off-putting noise. Every audio system has a maximum amount of gain or volume that can be applied before feedback is introduced. In order to avoid problems, first we need to understand how and why feedback is caused.
 

Acoustic Gain vs. Potential Gain

When designing an audio system for a conference room a balance should be established between the needed acoustic gain and the potential acoustic gain that the system can provide. The further you move away from a sound source there is a decrease in the amount of direct sound that you will hear, as you will being to hear more of the reverberant field.

The potential acoustic gain that can be achieved by a sound system can be influenced by the acoustics of the room itself, the directionality of the microphones, the positioning of those microphones and the amplification level within the room.

When designing a conference room audio systems it important to ensure that those closest to the sound source are able to hear at the same level as those furthest away, as this helps maximise speech intelligibility by ensuring that everyone is hearing more of the direct sound versus the reverberant sound. The amount of gain we require in order to achieve even coverage is known as needed acoustic gain; however, each sound system has a maximum potential acoustic gain that can be achieved with regards to eliminating feedback.
 

Best Practices to Optimise Meeting Room Sound

When designing a new meeting room sound system the mathematics behind needed acoustic gain and potential acoustic game can be extremely valuable. There are also crucial best practices that should be followed to optimise the performance of an existing sound system:

Move the microphone closer to the sound source - Poor microphone technique increases the amount of gain required in order to capture sound. The more gain, the closer you are to introducing feedback.

Move the loudspeakers closer to the audience - Distributed ceiling loudspeakers can ensure even coverage. Use delayed surface mount loudspeakers to provide a suitable level to those seated further away. (Read this article to find out more about proper ceiling speaker placement).

Reduce the number of open microphones – Every time the number of open microphones is doubled, the amount of potential gain available is reduced by three dB. Unwanted noise is introduced into the system which creates additional paths for potential feedback loops to occur.

Use directional microphones where possible - Omnidirectional microphones are designed to capture sound from all directions, whereas the directionality offered by cardioid or super cardioid capsules helps minimise any unwanted noise from entering your system and maximises the amount of gain available.

Consider signal processing – Some dedicated signal processors offer built-in feedback reduction. This process introduces feedback into the system and then applies notches in the equalization to reduce those troublesome frequencies.  This can also be done manually with a parametric EQ

Consider acoustic treatment - Each room will have certain frequencies that are more resonant than others. Hard surfaces such as glass or hardwood tables contribute to that reverberant field. Use absorption to reduce the amount of reverb and reverberant field in that room to ensure that more direct sound is reaching the audience, therefore requiring less gain to ensure good intelligibility.

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