please explain something to me (harvey?)

[Jonkan]

ok, here goes...
quote from the big harvey thread (by harvey):
"The system resonance is chosen for a mid band frequency. By itself, the capsule's response looks something like this:

......./\
....../..\
...../....\
..../......\
.../........\
../..........\
./............\ - just one big resonant peak, with the response falling off rapidly on each side of the peak. Now you can tame that peak by putting in a resonant chamber that's tuned to that peak, which will give you two smaller peaks on either side, like this:" etc etc...

Now, I always thought that resonance amplified the frequencys it was tuned for (like in a guitar), how come it here (in the dynamic microphone) lessens the diaphragms response in the frequency it was tuned? Does it absorb the acoustic energy somehow, so that the diaphragm doesn't recieve as much "acoustic attention" in these frequency areas and thereby smooths out the response or what? I hope you can understand my question, I'm looking forward to being taught about resonance and its ramifications on acoustics, and microphones in particular in the proper way.

Also, how does these resonance chambers actually look in a microphone? how do you calculate their size, materials, shape etc.. pictures anyone? Harvey, I know you could explain this to me in your usual informative and helpful way!
/Jonas

 

[Harvey Gerst]

Ports are very funny things; when they're active, the diaphragm moves less. So if the port is tuned to the center of the big honking resonance, the diaphragm motion is reduced at that frequency, and the overall response is smoother.

Damping the port absorbs the excess energy over a wide band, so it's not seen as output. In other words, it just sucks and swallows - it doesn't make the signal louder in that band, just the opposite.

As the peak goes down, you get the appearance of a flatter response, since the extreme ends of the frequency range can now be heard.

 

[Michael Jones]

Here's a picture of the acoustic chamber of an RCA 77D microphone: