I was sitting in a safety meeting today, and while “they” were telling us how to be safe, all I could think about was feedback.
I kept starring at the microphone and thinking “I’m sitting right in front of the person holding the microphone, and the sound coming from the speakers in the ceiling is louder than the presenter’s voice. Why the heck is the microphone not picking up that noise, and re-amplifying it over and over?” Where the heck is the feedback? I came up with a few possible solutions, but none of them really satisfied me. I don’t want to post them here, because I don’t want to pollute your fresh minds. Anyone know? Hell, I’m going nuts, so WAG’s are welcome too.
the simple explanation is that the microphone is highly directional. To the microphone, the voice of the person is louder than the sound from the speakers.
Caveat: I’m something of a rock n’ roll purist in that I don’t really know much about what the hell I’m doing when I play… I can never remember the names of the frets on my axe, for example… but my raw talent is blinding. Really.
There are several generic “types” of mics, eg, condensers, PZMs, monodirectionals, etc. Your speaker was probably using a run-of-the-mill monodirectional, which only picks up sounds in a very narrow field. Vocalists use them on stage for the very same reason you were wondering about: feedback. Guitar amps are probably much louder than whatever PA speakers your guy was using, plus, they’re usually placed directly behing the singer, so it helps to use a mic that’s gonna target the guys pipes and minimize picking up whatever other noise the rest of the band is making.
As I understand it, the “howling” noise most people associate with feedback is more a function of the magnets in the mic and the speaker protesting their close proximity than one of never-ending reamplification. That feedback that made everybody think Hendrix was a genuis worked on essentially the same concept, as guitar pickups are just magnets with a current running through them set up so that their magnetic field responds to the vibration of the string [where in a mic, it responds to vibrations in the air, ie, sound].
I’ll let someone else get into gain, power, impedence, etc to avoid saying something wrong… but you can bet that if you got on a ladder and held the mic right up to the cone of the speaker on the ceiling, you’d make your ears bleed.
Feedback is sampling a portion of an output signal and mixing it with the input signal.
There are 2 types feedback, negative which reduces the size of the output signal(plus lots of other things) and positive feedback which increses the output.
What you are talking about is when the output signal more than increases the gain. The system is actually oscillating (vibrating) around a frequency determined by a whole host of factors.You can get oscillation in amplifiers where the frequency is too high for human hearing.
With micropones the main determining factor in the feedback frequency is the speed at which the speaker cone and microphone coil can move at and since they are designed to operate at audio frequency it is not surprising that oscillation happens in this band.
The sound from the mike is sent to the amplifier and goes out to the speakers much increased, the speaker produces waves of sound which enter the mike and this is passed back down to the amp, hence the term loop.
Oscillation will only happen when the loop back signals timing works together with the speaker output otherwise it will tend to cancel out.
Microphones can be designed to pick up signals from all around them, omni-directional, such as might be used by someone recording an orchestra and these are the most vulnerable to feedback, or they can be designed to pick up from just in front of them, uni-directional, and also very close up, the type used by bands on stage.
Another way to eliminate or reduce feedback is to play with the equalization. For blaring painful feedback usually one has to either move the mic or turn it down however often one hears a “ring” or a “hum” coming from the speakers. This is because some frequencies of sound are picked up more easily than others. With very little experience a tech can usually identify the range of the noise and cut down the appropriate frequency on the EQ to eliminate the problem. This is most helpful when you really can’t bring down the volume any more or move the microphone.