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Old 20th June 2011, 10:21 PM   #11
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Quote:
Originally Posted by alexf View Post
After reading a few web pages about using WM-61A as a test microphone I decided to try it.
I have Sony MDR-V6 headphones that are supposed to have a "flat" response from 5 Hz to 30K.
So I plugged them into the output of my sound card (Audiotrak Prodigy HD2) and put the mic close to the center of the speaker. The response is anything but flat: severe loss at both low and high ends. Any ideas?
Hi Alex, what are you calling high and low ends? Headphones flat down to 5Hz sounds extremely optomistic to me could be marketers taking liberties!

on the high freq side, you have effectively done a nearfield measurement which tends to be inaccurate at high frequencies... WM61A's typically have a response peak at the high end, usually approaching 10Khz.

Tony.
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Old 20th June 2011, 10:28 PM   #12
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I am working for years now with the original Woofer Tester, made by Smith and Larson. They have models from simple to professional, and I can only say that I see clear advantages of using a dedicated box in stead of using a sound card.
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Old 20th June 2011, 11:30 PM   #13
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I still use a signal generator and small power amp, a resistor and a meter or scope. I don't like the added mass method and usually knock together a test box or an adapter for some existing box. The numbers I get are generally very close to manufacturers numbers (at least the good ones). The signal generator can be any cheap thing, from antique to modern and the scope can be equally as decrepit. Most meters do OK at low frequencies and it doesn't have to be true RMS. The stuff just has to work and be reasonably accurate. I have a sound card and all the rest but unless I were doing a large number of woofers it's as easy to use the dinosaur methods just like Weems and Speaker Builder described them.
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Old 21st June 2011, 12:04 AM   #14
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Hi Alex, what are you calling high and low ends? Tony.
Sure I was not expecting anything crazy as 5 Hz. In my test the response was very erratic below about 500 Hz, there was a plateau from 500 to say 4000 and a steep decline after that. As in 20 dB down at 18 KHz.
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Old 21st June 2011, 03:32 AM   #15
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ok that definitely sounds a bit sus then.... I'd try pointing it at a known good quality tweeter at perhaps a 1/2 meter distance and see what you get (for the high freqencies) and a nearfield measurement of a decent woofer should show you down to well below 500Hz.... , but it sounds like you have a problem with the mic or perhaps the preamp. did you do a linkwitz mod on it? if so you may have damaged it.

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Old 21st June 2011, 05:36 AM   #16
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I've borrowed the WT3 and it was quite easy for me to install the software and be up and getting all the T/S params calculated for me automatically in less than an hour.

I also have an old Mitey mike with MM1 preamp/power module which I'm going to have to try out again now that I realized a cable I was using in the past wasn't working.
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Old 22nd June 2011, 08:05 PM   #17
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Tried a no name mic from a phone headset and a calibration mic that came with a high end Denon - very similar results. Apparently even great headphones are not good for this. Same steep decline after a few KHz and no low frequency response at all unless the microphone almost touches the inside of an earpiece.
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Old 22nd June 2011, 08:15 PM   #18
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headset mics are not meant to capture all noise in a room, instead they would be optimal if centred around vocal frequencies and blocks souds that are not close or very loud.

I'm waiting for a secondhand function generator I got on fleecebay, then I also want to get started with measurements, allready got the scope
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Old 9th July 2012, 03:09 PM   #19
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What coupler have you used? Headphone response may be coupler dependent around below 50Hz and WM-61A microphone dependent above 10kHz. Microphone is likely to have a resonance in high frequency band, before response rolled-off. And what is IN-SERIES resistance seen by headphone? Headphone specs you have might be derived from setup with a coupler. There are a few coupler types, depending on headphone / eat bud type, examples are here Ear simulators Have you tried T/S measurements for your headphones? I have Dayton Audio WT3P WT3 Bundled with Precision Scale it is extremely nice to work with, easy Impedance Test, but often it does not allow T/S test after Impedance Test completed, I need to figure out if I am using it properly. Sometimes USB cable straightened or bent gives slightly different results... need to keep it straight all the time, Peter
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Old 10th July 2012, 04:21 AM   #20
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Originally Posted by alexf View Post
Tried a no name mic from a phone headset and a calibration mic that came with a high end Denon - very similar results. Apparently even great headphones are not good for this. Same steep decline after a few KHz and no low frequency response at all unless the microphone almost touches the inside of an earpiece.
Don't waste your time trying to calibrate your mic using ANY headphone. There are too many variables here AND the phones generally never have absolutely flat response. Good LF also depends on very close coupling with the head and you probably cannot achieve that properly with a mic set up. If you do the enclosed volume could create all sorts of other response errors that the 'ear' can take without a hassle but not a measurement mic !

Best option would be to measure a known tweeter and a known woofer and keep that as reference. Compare everything you make with that and you know what's hapening. What you will not know is what it's like in absolute terms. In any case the WM-61A can safely be used probably from about 200Hz to 5Khz without bothering about calibration. It's pretty flat there.
Do a search and you will find plots of several WM-61A capsules compared. Most vary beyond 5Khz .
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