Jimi Hendrix harnessed it to great effect, but squealing feedback is not something you normally want at a gig. The trouble is, getting rid of it can affect the quality of the music. Now there鈥檚 a way to free sound engineers to focus on aesthetics, and allow bands who can鈥檛 afford sound engineers to give feedback-free performances.
During a performance, the sounds from instruments and microphones are routed to a mixer, where an engineer blends them, adjusting the relative volumes to achieve just the right balance. The signal is then sent to the amplifiers and speakers. Feedback occurs when a particular frequency 鈥� which one depends on the acoustic qualities of the room, the instrument and so on 鈥� reaches a critical volume that causes it to be re-captured by microphones and sent to the speakers. The microphones then pick up the frequency again, creating a vicious loop. Eventually the audio system gets saturated and a painful squeal rings out.
A soundcheck before the show will reveal the volumes at which particular frequencies lead to feedback. During the performance, the sound engineer keeps an eye on the mixer and lowers the volumes of those frequencies. However, it is a tedious task that distracts the engineer from concentrating on the quality of the mix. Automatic software filters help, but tend to remove some non-feedback sounds and allow feedback to slip through. 鈥淭hey are quite risky to use in live shows,鈥� says Joshua Reiss of the at Queen Mary, University of London.
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He and his team have created software that prevents feedback from occurring, rather than cancelling it out, and frees the sound engineer to focus on music quality. 鈥淥ur software doesn鈥檛 simply cancel the feedback, it prevents it in full,鈥� says team member Enrique Perez.
A soundcheck is still required to program the volumes at which the frequencies cause feedback into the software. During the performance, when a frequency rises above its critical volume, the software lowers it slightly. But it also lowers the volume of other frequencies, so that the balance of sounds is right.
The researchers tested the software by playing the song International Geophysical Year by in front of a microphone, which sent the song to a mixer, where the team played with the settings. The resulting signal was then sent to another speaker. When their software ran on the mixer, there was no feedback, but when the team switched the software off, they got the characteristic squeals.
鈥淚f the software can be used by non-experts and is going to be cheap or open source, it would be worth trying,鈥� says Simone Stopponi of Italian rock band . Reiss doesn鈥檛 know whether the software will be open source, but says it will be cheaper than hiring a sound engineer. Sound engineers are worried. 鈥淭his is the kind of stuff that will make us all unemployed,鈥� says Stuart Jarvis, a heavy metal sound engineer at the Hope & Anchor pub in Islington, London.
