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What is Feedback?
Feedback is the screeching/squealing/hooting sound heard when there is an accidental loop in the PA system. That is when the sound from a microphone is amplified and fed to the speakers and then picked up again by the microphone, this sound then carries on round and round resulting in the nasty noise. The feedback frequency depends on a number of factors such as: the resonant frequency of the room, distance of microphone and speakers, frequency response of the microphone, characteristics of musical instruments.
To reduce or eliminate feedback this loop needs to be broken, here are a few ways of doing that.
Change Speaker and Microphone Position
If the speakers are behind the microphone ie:
Speaker --> Microphone --> Audience
Then moving the speakers forward, so that they are either in-line with the microphones or in front or them will mean that there is less sound entering the microphone from the speaker.
Microphone --> Speaker --> Audience
Also remember about your Foldback/Monitor speakers (see below).
Turn the Speakers down
Simply turning the speaker volume down will mean there is less sound entering the microphone.
Careful Use of Monitors /Foldback speakers
Monitors (aka Foldback speakers) are the speakers that the band use to hear themselves and the band leader. Monitors can be the cause of the feedback.
Position of Monitors
As with the main speakers, try not to point the monitors into the microphone! Place monitors behind the microphone:
Singer --> Microphone <-- Monitor
The singer will be using directional microphone which is best at picking up sound coming from directly in front of it, so in this arrangement it won't pick up the sound from the monitor.
Volume of Monitors
Keep the monitors as low as the band are happy to work with.
Equalise the Signal
A feedback squeal probably consists of one pure tone, ie, a single rogue frequency. By using the EQ controls on a sound desk, the frequency that is feeding back can be 'dialled out', leaving the rest of the audio material unaffected. A sound desk will typically have for each microphone channel:
- A High Frequency control
- One or Two sweeping mid-range controls, giving adjustment of frequency and gain
- A low frequency (bass) control
By finding the frequency that is feeding back and cutting it will suppress the feedback.
On a per-microphone basis, the basic steps to do this are:
- Ask the singer/preacher to sing/talk at their normal volume.
- Turn the gain up on the channel to just under clipping
- Turn up the master volume control until you hear the ringing
- Whilst it's ringing, use the EQ sweeping controls to sweep through the frequency range with the relevant gain control cutting (ie not boosting) until the ringing stops. With experience you will learn to guess roughly which frequency is squealing.
- Put the faders back to normal and check the singer/speaker still sounds good, and push the volume to check for feedback again.
Be careful not to wreck the sound by taking out too many frequencies or by cutting too much - be sure to check how it sounds!
Use of Microphones
Distance:
A major reason for feedback is the sound engineer having to turn the gain up high for a microphone as the speaker/singer's mouth is far away from the microphone. Vocal microphones are meant to be used close up to the mouth - ie within 1 inch. Having the microphone close to the source means means a lot of sound energy is entering the microphone, when this happens the Gain on the sound mixer can be lowered, giving much more head room before feeding back. Another positive benefit of having the singer as close as possible to the microphone is that it will pick up more bass and produce a warmer sound. When used for speech, the microphone needs to be 12 to 18 inches from the mouth so that, when the speaker makes natural head movements, the proportionate change of mouth-to-mike distance isn’t large and the PA operator doesn’t have to ride the faders. This also reduces ‘pop’ sounds on the spoken letters ‘p’and ‘b’. The extra distance (compared to sung vocals) makes the system more prone to feedback, but at least you can probably turn off monitors and all the other mikes. There are other ways to reduce the 'pop' sound, using compressors, but that is beyond the scope of this document.
Turning Off
Simply muting the microphone when not in use will mean that it won't be the cause of any feedback! If the PA operator decides on this policy, he/she then has to be constantly alert to fade that mike up again just before it is needed.
Use Microphone Stands, where possible
A mike on a stand, once sound-checked, behaves predictably for the rest of the event. A handheld mike, used by an animated singer, could be pointed anywhere and its distance to loudspeakers or monitors will vary. If a setup proves to be really feedback-prone, try to minimise the number of moving microphones, eg, persuade all thb backing vocalists to use mike stands.
Hand Held Microphone Tips
When using hand held microphones, either wired or wireless, apart from making sure the singer/speaker holds the mic close to their mouth, make sure they don't pace their hand over any part of the 'head', but use the handle. Even the ‘back’ half of the ball-shaped head shouldn’t be covered.If their hand is around the head part - the pick up part, then that will cause resonance within the microphone itself.
Wireless Microphones
Wireless microphones are useful as the can move around the stage/room easily. This as the potential for causing feedback if they are used near to the speakers or monitors. Try to find out the places on the stage where feedback may occur and then train the users on where they can and can't go with a radio microphone. If the users are confident is muting the microphone themselves (most have a mute button) then they can be in charge of muting/un-muting the microphone. This means if they wonder around the room with the microphone then they are free to go where they like. Often people will hold a handheld radio mic whilst walking around and at that low height it could end up close to monitors or pointing towards the main speakers.
Standardise on Microphones
 Every make/model of microphone is different in the way that it behaves. Different models will feedback at different frequencies If all your microphones are the same model, then this will help you a lot in diagnosing a feedback problem.
Cheaper microphones are also more prone to feedback. Where possible invest well in microphones. Microphones are the key item to quality in a sound system.
Rehearsals
Rehearsals are the ideal time to resolve feedback problems,especially where you may have a lot of microphones in different positions being used. It far better to spend time at this point eliminating feedback than in front of an audience!
More!
http://www.mediacollege.com/audio/ is a good webiste offering information/training on media, plus it's resources are free! |
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