noise in public address systems

[samcheetah]

ive seen that public address systems are more vulnerable to noise than any type of amplifier system. maybe im wrong but thats my observation.

if someone is speaking on the microphone then for no reason a loud "teeeeeeeeeeeeeeeeeeee" voice starts to sound. what is that? i read something about ground loops in an article and it said that ground loops produce a 50Hz hum in audio-video systems. is that sound the 50Hz hum produced by ground loops? (i wish i could attatch an oscilloscope to my ear, then i wouldnt have been asking this question :lol: )

i know that this sound continues because a feedback loop begins in which the noise from the speakers enter the microphone and this further amplifies the noise and so on. but why is it so common in public address systems.

ive also seen this sound starting when a person unplugs the microphone from the jack without first switching off the amplifier. is this because the jack catches interference? does this have some connection to the ground loops?

 

[Nigel Goodwin]

ive seen that public address systems are more vulnerable to noise than any type of amplifier system. maybe im wrong but thats my observation.

I would say you're wrong, but you can't really compare totally different systems.

if someone is speaking on the microphone then for no reason a loud "teeeeeeeeeeeeeeeeeeee" voice starts to sound. what is that? i read something about ground loops in an article and it said that ground loops produce a 50Hz hum in audio-video systems. is that sound the 50Hz hum produced by ground loops? (i wish i could attatch an oscilloscope to my ear, then i wouldnt have been asking this question :lol: )

This is just feedback, it's obviously going to happen with any system where a microphone feeds a loudspeaker in the same area. The microphone simply picks up the sound from the speaker, the amplifier amplifys the sounds and puts it out the speaker - the microphone picks it up again, and an oscillator is formed. It's just like any oscillator, it requires positive feedback (which is from speaker to microphone) and a gain greater than it's losses - the gain is in the amplifier, and the loss is in the audio path from speaker to microphone.

There are various methods used to minimise it, firstly point the speakers away from the microphone - which is why the speakers are usually in front of the singer in a band. Secondly, keep the volume down, lower gain means less chance of feedback. Thirdly, you can buy (or build) electronic devices which allow you to use higher gains, they basically frequency shift the audio a few hertz, so instead of a constant feedback tone (which reinforces itself) the feedback rapidly goes higher (or lower) in frequency which prevents the oscillation until the gain if a fair bit higher.

i know that this sound continues because a feedback loop begins in which the noise from the speakers enter the microphone and this further amplifies the noise and so on. but why is it so common in public address systems.

Simply because you've got a microphone and a speaker in close proximity, and generally running at high volumes. Nothing else really has a microphone feeding a loudspeaker, so it doesn't apply elsewhere.

ive also seen this sound starting when a person unplugs the microphone from the jack without first switching off the amplifier. is this because the jack catches interference? does this have some connection to the ground loops?

No, it's because the jack plug shorts the socket out as you pull it out, don't forget you are talking extremely small signals - 1mV wouldn't be uncommon, so you have huge amounts of gain in the system. A good size amplifier might output 100V, from a 1mV input - that's a voltage gain of 100,000 - and you get massive current gain as well.