Audio and Video Talk > Acoustics and Room treatment

Room EQ Measuring Equipment

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Timber_MG:
Here the results of a German company that calibrates these mics against a known reference. Scroll down for the graphs.

http://diylautsprecher.de/index.php?option=com_content&task=view&id=94&Itemid=35

Especially low frequency measurement is not always without its problems as many devices have filters of their own in the measurement chain. The better designed systems have a reference input (even voltage dividers of the amp signal feeding the DUT="device under test"). One also often requires a sufficient signal to noise ratio in the measurement or averaging (not the same as smoothing) to provide one with reliable results.

Timber_MG:
And here another link off that page outlining results from over 200 mic calibrations (including cheap Skype phone type capsules)

http://hifi-selbstbau.de/index.php?option=com_content&task=view&id=138&Itemid=1

I think I should start a seperate thread on calibration as there was some interrest from Gert and Gerhard to calibrate our ECM8000s. I had mine calibrated over 16 points (with very good results) but some recent measurements have cast some doubts on the long term stability of the results and we are looking at getting a suitable mic for calibration purposes.

Vaughan:

--- Quote from: Timber_GM ---Often times the frequency response of cheap electret mics below 50Hz is unpredictable (and some have significant production variations).
--- End quote ---

The frequency response inaccuracies down low can be corrected with a compensation file. There is a thread comparing a calibrated Behringer ECM 8000 versus a cheapie Radioshack analog model down to 20 Hz (with correction applied of course) and the results are very interesting (about a 1 dB variation at worst). I'll try to dig up the thread a bit later.

Regards,

Timber_MG:
The linked pages contain statistical analyses of accuracy of a couple of measurement mics (including the Panasonic capsules, ECM8000 and some cheap stand-mics) against a source calibrated against a LAB grade calibrated microphone.

An ECM8000 is not a reference, but it does very well for most crossover-design tasks faced by loudspeaker designers and when an individual calibration is performed it allows for some fairly accurate measurements.

The issue with a general correction curve is that it may well be representative for a given batch of mics (or even only a single mic), but one needs to be wary of the statistical variance in microphones to arrive at an uncertainty of measurement, a factor that can be reduced drastically by verifying (calibrating) the response against a known reference.

Vaughan:

--- Quote from: Timber_MG ---An ECM8000 is not a reference, but it does very well for most crossover-design tasks faces by loudspeaker designers and when an individual calibration is performed it allows for some accurate measurements.
--- End quote ---

I own one.  ;) But I understand. For those who want reliable results you don't need super expensive industry-grade equipment.

Even simple SPL meters that cost R500.00 (like the RS meter) with the correct filters applied can give very accurate results within the intended bandwidth of the design. High frequency measurements will be a problem however but then it is a cheap meter.

The Radio shack models do have slight variances from model to model (like 1-2 dB's) within the low frequency range which won't pose too much of an issue in my opinion.

Regards,

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