Common Feedback Frequencies
Every single sound guy deals with feedback. It’s actually part of the job. It doesn’t matter if its a band, a seminar, a tradeshow, a festival, a concert or you by yourself practicing in the basement you will deal with it.
What are the common feedback frequencies?
If it sounds like a howl its likely in the 250 to 500 Hz range.
If it seems less pronounced, like singing your looking at 1kHz.
High pitched whistles and screeches Most likely above 2 kHz.
The above is a general guide but lets really dive into what is happening and how to fix it.
What is feedback?
At its basic core feedback is an audio loop. The audio input and audio output are at the right levels to cause a loop that builds on itself and that’s when you get feedback.
The first frequency that feeds back is the one requiring the least amount of energy to excite resonance. You can look up resonance on your own time…because its a technical way of saying things get louder. That is because the frequencies amplify and stack on top.
So if the frequency 1k is the resonant frequency in the space it will take the least amount of energy to excite and that’s why you hear feedback at 1k.
What are the common causes of feedback?
Open microphones are too close to a monitor or speaker
Gain structure is too high
The compressor is set wrong
The PA or monitor hasn’t been properly tuned to the room
Singers are in front of the PA
How to ring out a room?
To ring out a room is actually pretty simple in its application. In it’s simplest form you need a microphone and a live PA.
Turn the PA on and make sure that at least one speaker is live. For the purposes of most live situations, you want to do each speaker individually because they are located at different places in the room.
Open a microphone. Typically you want to put the microphone exactly where it is going to be used and then test each speaker box relative to that. Most times when you are in a hurry it is ok to just test the vocal mics in the monitor that it is closest too and the main vocal in the FOH.
Turn the gain at least 10-20% more than you would have it during the show. It’s sometimes difficult to guess but I would do a quick vocal “check one-two” into the mic and see where the meter is on the channel. If it is around unity then thats where it should be for the show. Turn the gain 3-6db higher than that.
Make sure nothing else is on and all other channels are closed.
Make sure that all of the EQ, Compression and gates are off on the channel you are testing
Push the fader of the mic channel up slowly. You should start to hear a ringing sound as you get close to exciting the frequencies.
In a live bar/gig setting, you may need to continue “check one-two” into the mic as you ride the fader. I have found that this gets youto feedback a little sooner and it’s not quite as loud. Makes it more manageable.
If your mixer or gear has an RTA then as the mic is feeding back in the system there will be a frequency that will show hot in the RTA. Grab that frequency and bring it down till it stops ringing.
If the ringing stops then push your fader till you get more ringing. It will be at a different frequency now. Bring it down.
Repeat until you are at least 6 db above where you think you will need the fader for the show. Again this is kind of a guess but as you get better at this you will know how much volume you will need to push in each monitor to make your musicians happy.