Cross-Spectrum Microphone Calibration Service - USA

[Anechoic]

And again, since this is for home use for most of us, a few dB isn't going to cause any problems either way, since most of us are using a generic calibration file...

Yep. When it comes to acoustical measurements the reality is that there are always limitations that we have to live with. For example, if you really want to know what's going on a 20 kHz (say, for a speaker measurement) you really need to be using a 1/4-inch mic, since a 1/2-inch mic is so big relative to the wavelength so that you get diffraction effects and the diaphragm doesn't necessarily act like a piston (there's a reason why IEC and ANSI standards for Type 1 mics go to -infinity above 16 kHz). If you're careful, you can get a "good enough" number up that high but it's not going to be anywhere near as accurate as what you're getting in the 100 Hz - 3 kHz range.

How much would a an ECM calibrated down to 5hz be with an additional 45 and 90 degree measurement? I'm most likely going to get on the waiting list for one.

I won't charge extra for that, but you'll have to ask for it when you order it. And yes, there is a waiting list - it's been really crazy lately, everytime I get some in stock, they immediately go out again.

[redliner]

What's the turnaround time once you're on the waiting list?

[Anechoic]

redliner wrote:

What's the turnaround time once you're on the waiting list?

It really depends on where you are on the queue and my schedule. Right now I have no back orders and zero stock (which should get refreshed early next week) so the first few orders I get now should go out no later than March 2.

[Dennis H]

Hi Herb,

Good to see you posting here. Do you have any guess about the high and low end roll-offs of the Behringer? Say X dB/oct at Y Hz? With a 96K sound card and a 128K FFT, you could be generating measurement files covering .7-48K Hz. It would be nice to have some non-zero numbers in the cal file at those extremes, even if they were just a wild guess, so you didn't have your CAD software assuming there was a sudden big jump in the response. Seems like that could lead to phase errors in your crossover design.

[Anechoic]

The ECM8000s have so much unit to unit variation (see here) that I'm not comfortable giving generalized numbers - well other than +/- 15 from 20Hz to 20kHz .

As for the extremes, yes it is possible with a long FFT and wideband sound card to give a wide frequency response, but acoustically it's very difficult to generate reliable test data at those extremes. I could give numbers at those frequencies, but they would be meaningless.

[lcaillo]

Amen to that!

[Dennis H]

Anechoic wrote:

The ECM8000s have so much unit to unit variation that I'm not comfortable giving generalized numbers - well other than +/- 15 from 20Hz to 20kHz .

As for the extremes, yes it is possible with a long FFT and wideband sound card to give a wide frequency response, but acoustically it's very difficult to generate reliable test data at those extremes. I could give numbers at those frequencies, but they would be meaningless.

Okay, forget the Behringer. Generally speaking, what is the slope of the roll-off of generic condenser mics on the high and low end? 2nd order? Steeper? I'm just trying to learn how these mics work.

[Anechoic]

Dennis H wrote:

I'm just trying to learn how these mics work.

So am I.

The short answer is "it depends on how the microphone and microphone pre-amp are designed." I have to head out for the day, but I'll give a more detailed explanation later this evening.

[Dennis H]

Thanks, Herb. To keep things simple, could we start with the response of a plain old electret capsule with no extra circuits? Most software takes 2-channel measurements these days so the response of the preamp gets subtracted out.

[Anechoic]

Dennis H wrote:

Thanks, Herb. To keep things simple, could we start with the response of a plain old electret capsule with no extra circuits? Most software takes 2-channel measurements these days so the response of the preamp gets subtracted out.

Just to make sure we're all on the same page: when I talk about the capsule, I'm refering to the "tip" of the microphone that has the microphone diaphragm, blackplate, isolator and electrical terminals(and grill if applicable). When I talk about the pre-amp, I'm not referring to the phantom power supply or the sound-card interface, I'm talking about the electronics that convert the varying charge into an output voltage and steps up the voltage. In the parlance of my reference equipment, this is the mic capsule, and this/this is the pre-amp. In mics like the dbx RTA-M or ECM8000, the unit contains both the mic and the preamp (which gets plugged into another pre-amp to provide phantom power and bump up the output more).

A condenser mic can go down to DC, in which case you could use it measure atmospheric pressure. In practice, the rear of the diaphragm is usually vented to prevent the diaphragm from breaking during sudden pressure changes or exposure to very high SPLs. This vent creates a low-frequency roll-off which limits the low-frequency performance of the microphone. The vent hole and the volume of air behind the diaphragm is a small resonant chamber; the size of these parameters determine the roll-off, so you need to know those parameters to predict the roll-off of any particular microphone.

The theoretical roll-off on the high frequencies for condenser mics (normal incidence) is 12 dB per octave if you only consider an undamped system. However, at mid-audio frequencies (8-13kHz depending on a variety of factors including diaphragm diameter) there is a resonance that manufacturers try to damp out though adjusting the vent sizes, grill design, diaphragm tension and diaphragm mass. This damping can change the slope of the high-frequency roll-off depending on the proximity of the resonance to the high-frequency limit of the mic. So if you want to use a number for the high-frequency rolloff, use 12dB/octave but it's going to be a wild guess at best.

We also have to account for the performance of the pre-amp response. Manufacturers may try to use the electronics to tweak the frequency response of the capsule, which may also have an effect on the rolloff.

HTH.