# Sound Wave, Microphone and Electromagnetic Induction

1. Nov 14, 2013

### wlng81

Dear all, I have encountered an issue in understanding how microphone works and I hope you guys can assist.

There are two scenarios involved.
In the first scenario, there is tuning fork and a microphone. The microphone contains a small disc attached to a magnet and a fixed coil. (Please refer to here: http://www.flickr.com/photos/11353862@N02/10851144236/)
It is mentioned when the tuning fork vibrates, an alternating induced emf is induced in the coil.

In the second scenario, the tuning fork is replaced a source that produces a sound of greater loudness but the same pitch.

The question is: How will the amplitude of the induced emf change?

I had thought the magnitude induced emf should remain the same since the pitch (hence frequency -> rate of change of magnetic flux) was the same. But I also came across some audio websites saying the amplitude induced emf would increase.
As a result, I am utterly confused now...

2. Nov 14, 2013

### Bobbywhy

Remember that the acoustic waves that propagate from the source (tuning fork) to the detector (microphone) are pressure waves. When they strike that disc they push and pull the magnet, which induces the current in the coil.

When the tuning fork is replaced with a louder source, the acoustic pressure waves will be (louder) stronger and so will push and pull that mechanism in the microphone with a greater force than the tuning fork did. So, the alternating current in the coil will be the same frequency, and a greater amplitude.

Last edited: Nov 14, 2013
3. Nov 14, 2013

### wlng81

Hi Bobby, can I say that the frequency of the sound wave is reflected in the frequency of the induced current and the amplitude of the sound wave is reflected in the amplitude of the induced emf?

4. Nov 14, 2013

### Bobbywhy

Yes and yes.

5. Nov 15, 2013

### Staff: Mentor

At least ideally. That's why some microphones are better (and more expensive) than others.

6. Nov 15, 2013

### wlng81

Out of curiosity, what are the factors that separate a good mic from a lousy one?

7. Nov 15, 2013