It depends what you're after.
I have an older Behringer ECM8000, then bought a Beyerdynamic M1 because it came with calibration data. The Behringer data seemed a bit odd and I ended up buying an ISEMCON EMX7150, which is more expensive but had better calibration data. After using the ISEMCON for a few years, I found its sensitivity dropped by about a factor of 2 (!). The Beyerdynamic has become unusable (sensitivity changes from one measurement to the next). The Behringer sensitivity is still the same as it was a few years ago, but I have no independent calibration data for this. Also, the Behringers don't have a great reputation when it comes to long-term stability (although mine seems to do well). Last week I bit the bullet for an Earthworks M23. I payed for it, the dealer said he'd send it out asap, but nothing showed up so far. Dealer does not respond. Oh well...
Let us know what your requirements are so that you can get better help.
I have an older Behringer ECM8000, then bought a Beyerdynamic M1 because it came with calibration data. The Behringer data seemed a bit odd and I ended up buying an ISEMCON EMX7150, which is more expensive but had better calibration data. After using the ISEMCON for a few years, I found its sensitivity dropped by about a factor of 2 (!). The Beyerdynamic has become unusable (sensitivity changes from one measurement to the next). The Behringer sensitivity is still the same as it was a few years ago, but I have no independent calibration data for this. Also, the Behringers don't have a great reputation when it comes to long-term stability (although mine seems to do well). Last week I bit the bullet for an Earthworks M23. I payed for it, the dealer said he'd send it out asap, but nothing showed up so far. Dealer does not respond. Oh well...
Let us know what your requirements are so that you can get better help.
Just a data point, I have found Primo capsules to be very stable and have several that still measure their rated 10mV/Pa (within a dB or so) after 40yr. They also had a pinpoint omni that came with individual calibration for $40, times have changed.
The cheap two wire capsules have back to back diodes at their input which severely limits the SPL before clipping. For just frequency response plots at modest SPL they should be fine when calibrated. NASA has used large arrays of them in doing rocket engine noise studies. The one Kevin mentioned is probably fine.
Material properties of the suspension must be a major issue, elastic materials aren't often highly stable over decades, dirt build up on the membrane would change its mass, and for an electret change its charge? In older dynamic capsules magnet fade would be a possibility.
There are many mechanisms I can think of.
MEMS microphones are interesting, barring dirt they ought to be immune to aging effects I'd have thought.
The traditional metal diaphragm microphone (WE, B&K, GRAS etc.) can be really stable over time. Per B&K: "The predicted long-term stability is of the order of 1 dB over several hundred years at room temperature. Although difficult to verify this in practice, the sensitivities of several microphone cartridges measured periodically at Brüel&Kjær’s laboratory from 1974 to 1990 changed by only 0.05 dB, confirming the predicted long term stability." However you pay for what you get.
The electret based microphones use a plastic film that will over time and humidity cycles change some. Any microphone is a delicate transducer and even small stresses (temperature, humidity, loud noises) can stretch or loosen the diaphragm or degrade the electret charge. If absolute sensitivity is important (which it really isn't for loudspeaker work) you should have a calibrator. Frequency response testing for electret mikes requires a reference mike, a source and a controlled acoustic space for testing. Its actually more hassle and less accurate than the electrostatic method used for metal diaphragm mikes but its the only option. The prices above seem pretty reasonable for that type of calibration.
The electret based microphones use a plastic film that will over time and humidity cycles change some. Any microphone is a delicate transducer and even small stresses (temperature, humidity, loud noises) can stretch or loosen the diaphragm or degrade the electret charge. If absolute sensitivity is important (which it really isn't for loudspeaker work) you should have a calibrator. Frequency response testing for electret mikes requires a reference mike, a source and a controlled acoustic space for testing. Its actually more hassle and less accurate than the electrostatic method used for metal diaphragm mikes but its the only option. The prices above seem pretty reasonable for that type of calibration.
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