Horn Sound when The mic is close to LoudSpeaker!

[Shahriar]

Hi

I have built a PreAmplifier and a Power-Amplifier and it works Very good

when I coonect my PC output to it.
When I connect My Electric Mic to it

and Increase the volume, a Sound Like a horn appears On the Loud

speaker.This also happen when mic is Close to LoudSpeaker.

Let me tell My Oponion about the reason and you tell me if I think right

or not.

The Mic take a harmonic and this Harmonic will be apmlified by PreAmp and

PA. When This harmonic comes out of the Loud Speaker the Mic takes it

again and the cycle Begins oNce more so an Oscillation occures and we

hear that like a horn Sound.

What should we do to solve this problem?

ThanX in Advance
Shahriar

[audioguru]

Hi Shahriar,
The sound is caused by acoustical feedback of any sound fundamental frequency (not a harmonic) that has a peak in the system's frequency response, when the microphone can hear the speaker's output and amplify it around and around.

It is stopped by:
1) Turn down the gain of the preamp or amp. Then speak closer to the microphone or speak louder so the output level is loud enough.
2) Position the speaker at a distance from the microphone and have them pointing in opposite directions.
3) Equalize the sound system so there isn't any peaks in its frequency response.
4) Add a frequency-shift circuit so that the frequency being fed-back is different each time the sound "goes around".
5) Record an announcement then play it back with the microphone turned off. 

[Shahriar]

Thank you very much.

Don't you know that if experts can overcome this problem by Digital Sound processing Tricks?
To examin this I conected my Mic to my Sound Card and then Increase the Speaker Volume. The same thing happened to it too. But I am Not sure if my Sound card is Based On Digital Sound Processing....  ::)

By the Way, Does Omni directional Microphones Solve this Problem?

Shahriar

[audioguru]

DSP can reduce feedback by analysing the sound then adding a frequency notch filter at a frequency that is oscillating. Your pc would need a program that can do that.
An omnidirectional mic picks-up sound from all directions so would probably make feedback worse. A unidirectional ot cardioid mic would help to reduce feedback.