It depends on the type of capsule.
But one effect that applies to all types is that the cavity in front of the diaphragm(s) is increased and the resonant peak this gives will move down and be higher. You get this in all condensor mikes and the real mike designers use it to flatten & extend the HF response.
If it is a single diaphragm cardioid or fig-8, it also increases the path length between front & back. I'm too tired to remember what happens for a double diaphragm.
Have you got a copy of the Neumann PDF book on mike design on their website? It will explain the significance of greater path length.
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ln76d, I think it is high time you looked seriously at how to do quasi-anechoic measurements of microphones at home.
Chapter 10.4.3 ACOUSTIC FREQUENCY RESPONSE
of the Clio 8 manual is the best explanation of this. http://www.audiomatica.com/wp/?page_id=34
I highly recommend Clio as an inexpensive R&D tool. The calibration of their inexpensive measurement mikes are one of the few I believe. ClioQC is THE system for production testing.
Eric Benjamin’s Extending Quasi-Anechoic Measurements to Low Frequencies
http://www.aes.org/e-lib/browse.cfm?elib=12875
explains some advanced methods to extend useful measurements to lower frequencies.
These methods measure an impulse response and use FFTs to get the (complex) frequency response. The comparison is done by (complex) dividing the response of the DUT by the response of the reference in the Frequency Domain.
There’s only a few free programs that allow this to be done conveniently.
- Prof. Angelo Farina developed the Log Sweep method that is now the favoured method from Clio to the latest Audio Precision to yours truly. If you have Audition, try his Aurora plugins. There is a version for a special version of Audacity.
- Our own Les Watts uses & recommends the full version of ARTA. There is a free version.
Read the Clio manual regardless of what you use. It is the ONLY clear & accurate explanation of quasi-anechoic that I know of. If you find a better one ... or even another .. please let me know.
I haven’t used any of these methods but have my own version of Angelo’s method in DOS.
If you do use them, please consider writing a little report on how they can present graphs with 25dB/decade aspect ratio.