How to Control Feedback in your Sound System
One minute everyone’s listening, dancing and happy, and the next minute it sounds like a screaming banshee has been let loose in the venue. We’re all familiar with the unpleasant rumbling, howling or screeching of acoustic feedback, but how do you stop it happening? Read on to find out.
First, what causes acoustic feedback in a sound system?
Acoustic feedback happens when the sound produced by powered PA speakers makes its way back into a microphone or guitar pickup, and is then amplified through the system again. The resulting sound loop creates its own frequency, and while some musicians use this to their advantage (hello, Jimi Hendrix) this sound is wholly undesirable more often than not, particularly through a microphone.
What are some methods of controlling audio feedback?
The overly simplistic answer is to check that the microphone isn’t too close to the powered PA speakers and that the microphone volume isn’t up too high. The further the mikes are from the speakers, the louder they can go without causing feedback issues. Beyond that, you can use your Alto professional mixer to “ring out” or EQ the sound. Ideally this is completed and perfected before the performance, but you can adjust at any time if required. Here’s how:
Start by bringing up levels within the system until you start to hear feedback issues. You’ll want to pay attention to the sound that’s produced (or learn to use a real time audio spectrum analyser), as this will give you an idea of the frequency band to adjust. If you’re learning to do this by ear it does take some time and practice to perfect.
If you’re getting howls or hoots, you probably need to adjust the 250-500 Hz range. If the feedback is singing, then look to the 1k Hz range, and if your ears are being assaulted by screeches and whistles then focus on bands above the 2 kHz level. Once you have identified the offending frequency, bring down that level by around 3 dB on your mixer. Bring up the overall levels slightly and rinse and repeat to reduce the next worst offending frequency, but be wary of over equalising by more than 6-9 dB which will impact on general sound quality.
Other tips for minimising acoustic feedback:
- Stop the performer from cupping the microphone with their hand, and instruct them to keep their mouth close to the mike while singing.
- Replace floor monitors with in-ear monitoring systems for performers.
- Turn off any unused microphones that could be adding to the feedback loop.
- Soften or remove any hard or reflective surfaces in the venue if possible.