Testing the M50

In May 2012, I decided to reward myself for something or other, and bought the M50 I have long planned to buy.

The Earthworks M50 is the king of measurement microphones nowadays, with calibrated (and quite flat) response to 50kHz.  It costs far less in real terms than the standards of old, such as B&K's with their custom amplifiers and such.

I think John Atkinson of Stereophile uses an M50, either that or an M25.

Until now, I've had no proof that my system even has frequency response above 20kHz or so, something I've invested thousands of dollars and endless fine tuning to achieve.  I have no idea what the proper calibration for my previous custom 25kHz calibrated Dayton Audio above 25kHz.  Given the different calibration curves for different angles, it's also hard to know which of several calibrations to use.  I didn't really trust it to 20kHz let alone 25kHz (but in fact I was wrong...it matches the M50 amazingly closely!  Perhaps I needn't have bothered.  Nah.  Anyway I plan to use my old mike for general purposes and the M50 for calibration and "final" measurements.)

For the measurements here, I recall I put the microphones straight forward.

Here is my system response with the 25kHz Dayton Audio microphone:

Here is the response with the M50, which I finally figured out how to calibrate correctly (the calibration they supply cuts off below 700 Hz..for ARTA I had to add in fake points at that exact same level).

The M50 shows a slightly cleaner HF peak at 22kHz, and about 3dB more output at 40khz, and that's about it.  If I had an accurate calibration above 22kHz for my Dayton I could probably make them match even better.

I think the speaker system response, shown in much higher resolution that you usually see above 100 Hz (and less below, thanks to FFT techniques) is pretty good.  It slopes nicely downward from 1kHz (though perhaps a bit too early and there is more loss than desirable above 8kHz or so.  The inaudible 22kHz peak merely restores the highest treble to the midrange level (and less than the deep bass), and also makes possible the response extension to 40kHz where there may still be more output than at 14kHz.  Many highly esteemed tweeters have much larger ultrasonic peaks from fancy construction.  My 21mm Dynaudio D21AF's have factory response curve showing flat from 1200Hz to 39kHz measured with close microphone but they don't account for a lot of variables here, including the polar response dilution of a cloth dome tweeter (which is good enough to even make 40kHz response possible--which can't be done with many materials including of course my electrostatic speakers).

Anyway, there clearly IS ultrasonic response at useable level at 40kHz, which has been my goal, and now proven with a top calibrated microphone.

Instead of huge peaks and valleys in the bass in the unequalized bass response, or where the wall bounce isn't properly corrected (I dodge that with my 4 foot wall spacing and crossing subs at 125hz with 8th order linear phase crossover) it's pretty flat 80-800 Hz (amazingly flat in my building and measuring experience), with just a soft rise below 80 Hz (which I may have reduced with subsequent adjustments as I'm writing this months after these measurements were recorded.)  It sounds natural.  1/6 octave omni measurements on my phone show less difference below and above 80Hz than these FFT measurements.

Some measurements last year were made before I figured out how to run the supertweeter miniDSP at 96kHz.  So it was running only at 48kHz like my other DSP's (the 48kHz plug in is more flexible than the 96kHz...but that flexibility is not an issue for my supertweeter...so I run only the supertweeter crossover at 96kHz where the highest sampling rate matters).  So measurements through the middle of last year showed a steep dive at 24kHz, the Nyquist Frequency of 48kHz sampling.  It took me awhile even to figure out that sampling rate was a problem, and until I did I greatly feared that I was basically getting zero response above 24kHz.  I remembered that fear into the present.  By that time I was thinking the microphone calibration explained the rolloff above 22kHz...but in fact the peak there, and the general response, shown by the Dayton...extending the highest calibration to all higher frequencies...is very close to that shown by the M50, with hard-to-see difference at 40kHz where useable response is ending with a precipitous roll off.  Also the Dayton curve shows a little notch where the calibration ends, whereas the M50 shows a smooth peak at 22kHz.