Need help with strange measurement results in REW

[ptaskov]

Hi everyone,

This is my first post here. I've been reading forum topics for a long time and I really like this place, for which I thank you!

I'm helping someone to find the problem with incorrect acoustic measurements and soundcard calibration in REW and I'm wondering if any of you have encountered similar issues.
I don't have access to his measurement setup because I'm helping him remotely.
He's trying to make gated measurements of a 6.5" mid-woofer and 1" tweeter mounted on speaker cabinet, measuring with Dayton imm-6 mic and PC built-in soundcard. Here is what I've observed so far:

1. Soundcard calibration has a linearly rising +6dB/oct response. I expect overall flat response with some roll-off at lower and higher frequencies.
2. On the IR graph the first pulse looks strange - IMO, not what is expected from a raw woofer. If I remove the soundcard and mic calibration files and gate with a window over the first pulse I'm getting also a linearly rising +6dB/oct response similar to the soundcard calibration file.
3. The second pulse on IR graph looks correct and if I place the gating window over there and also remove the calibrations, I get similar to the expected woofer response.
4. Tweeter measurements have similar issues.

I expect to see some reflections on the IR graph after the main pulse but I'm puzzled with the first pulse and the soundcard calibration results leading to a rising +6dB/oct response. Personally I'm measuring with EMU0404 USB interface and ECM8000 mic from years, and have never observed something like that. Has anybody seen similar results and what could be causing them? I'm attaching some screenshots from the incorrect measurements.

Appreciate your help!

This i

 

[sam_adams]

Soundcard calibration has a linearly rising +6dB/oct response.

That is where the issue is. That soundcard 'calibration' file is completely wrong. How was that generated?

 

[ptaskov]

That is where the issue is. That soundcard 'calibration' file is completely wrong. How was that generated?

He said calibration was perdormed via a loopback cable but he got inconsistent results every time with the calibration.
His laptop has a 4-pole connector and he used a splitter with an aux-aux cable to connect both ends. Calibration was performed in REW Preferences menu -> Soundcard -> Calibrate soundcard. Maybe his splitter connection is causing some issues.

However, I think that soundcard calibration is not the only issue. If I remove it from the measurements I still can see the first pulse on the IR graph with a rising +6dB/oct slope, and the second pulse looks fine.

The first pulse looks like a doublet which confirms the +6dB rising slope of the frequency response when I place a gating window over there. Ringing period of the impulse corresponds to the Nyquist frequency - about 22kHz.

 

[Devnull]

Has he turned off all the different windows "effects" for the sound card and mic? Maybe look for software associated with the sound card and see if it's doing anything.

 

[ptaskov]

Has he turned off all the different windows "effects" for the sound card and mic? Maybe look for software associated with the sound card and see if it's doing anything.

Thanks for the tips! Will ask him to check the soundcard effects and system sounds and re-measure.

 

[Keith_W]

You should tell your friend to get rid of that piece of junk and get himself a proper measurement microphone. The Dayton IMM-6 is a condenser mic. Condenser mics are effectively capacitors - sound causes the plates to vibrate with respect to each other and thus changes the capacitance. Because of this, condenser mics need to be charged with a bias voltage. Your ECM8000 needs 48V Phantom Power, and different condenser mics have different bias voltage requirements. USB mics have the bias voltage built-in since they are powered.

The problem with the IMM-6 is that it has a 3.5mm jack, so its bias voltage depends on what device the mic is plugged into. It is likely to give strange results. A quick Google search found this thread on ASR and this thread on DIYAudio. Read from this post onwards.

The other problem with his measurement setup is that he is using a PC sound card. Make sure that he is using WASAPI Exclusive to take measurements - in REW it is labelled "EXCL" in Preferences.

 

[Sokel]

I have used more than 10 piece of audio interfaces and PC soundcards, all of them was ruler flat when measured for calibration, to the point they didn't needed it, even the crappy ones.
Something is amiss here, big time.