# ECM8000 uncalibrated - inaccurate



## rjpcardoso (Oct 23, 2013)

Hi,
I have been using REW with an Alesis io2 soundcard and with an ECM 8000 mic.
I am about to receive my active crossover and started doing some measurements for treble and bass in my speakers to measure the crossover point when found out that my ECM might be giving me inaccurate measurements (by a big margin). Always thought they were at least accurate in the bass department...

Can you please help here trying to find if its only a mic problem or any other?
I have attached the tweeter and full range plots. The tweeter is showing bass response...

Are ECM8000 so inaccurate?

Thanks for the help


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## AudiocRaver (Jun 6, 2012)

For accurate results you will need a measurement mic that has been individually calibrated. Then you will have a calibration file for it that is used in REW to compensate for the microphone's inaccuracies.

You can have your microphone calibrated by a calibration service or purchase one that has been properly calibrated for about the same cost. Most HTS members deal with Cross Spectrum Labs, very reasonable and reliable.


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## gazoink (Apr 17, 2013)

rjpcardoso said:


> Hi,
> I have been using REW with an Alesis io2 soundcard and with an ECM 8000 mic.
> I am about to receive my active crossover and started doing some measurements for treble and bass in my speakers to measure the crossover point when found out that my ECM might be giving me inaccurate measurements (by a big margin). Always thought they were at least accurate in the bass department...
> 
> ...


From the image file names, the top graph is the tweeter, the bottom full range. Try to shoot the tweeter again, trying several measurements and adjusting the power amp level a few dB each time, see if the bottom end changes along with the top, or if the junk at the bottom is just noise. The basic response looks about right for many tweeters.

Mic inaccuracies would never be 10s of dB off, they're a few dB error or fraction of dB, unless it's not a real measurement mic at all, which yours is. And even if the measurement mic's response was wrong, it wouldn't create artificial bass response. It's likely just noise, there's always noise, and the measurement system is not immune. You may not hear it, but it's there. Low level LF noise is inaudible, but easily measurable. 

It would help to know what exactly the tweeter is, what the speaker is, etc. 

I tried to verify a few things about how the Alesis works, but the Alesis sound thing has the worst instruction manual. I was looking for a possible direct path from PC out to PC in. I assume it's there in the headphone monitor because of the knob, but shouldn't ever permit recording of the PCs own output...at least I hope not.


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## rjpcardoso (Oct 23, 2013)

Hi,
Yes tweeter is the upper image and full range bottom. I've tried several readings and this noise was always there in the same place. Also tried different volumes but the noise in the bass is constant.
Even tested in mute and the noise is still there. Also appearing in the rta graph... 
The speakers are b&w 805s.

Is this normal from an uncalibrated mic??

Where in the uk can I send the mic for calibration? 
Thanks for the help


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## gazoink (Apr 17, 2013)

rjpcardoso said:


> Hi,
> Yes tweeter is the upper image and full range bottom. I've tried several readings and this noise was always there in the same place. Also tried different volumes but the noise in the bass is constant.
> Even tested in mute and the noise is still there. Also appearing in the rta graph...
> The speakers are b&w 805s.
> ...


Yes this is normal, but not just for an uncalibrated mic, it's normal for any mic. You can ignore it.

Even uncalibrated the ECM8000 is essentially flat. Yes, it's not perfectly flat, and if you were to do any precise measurements you'd want to load a cal file for it, but the errors are nothing like what you see. The differences are at the extremes. It can be a few dB down at 20Hz, and there's a slight rise above 10KHz. Cal files should be here somewhere. Regardless, you shouldn't be chasing single dB errors.


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## gazoink (Apr 17, 2013)

ECM8000 cal file info is *here* (post #3).

The same thread has info about outside calibration services.


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## rjpcardoso (Oct 23, 2013)

rta screen attached captured now...
Maybe Im in a noisy area?? will try again at late night to see if it changes


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## rjpcardoso (Oct 23, 2013)

Thanks for the info. I believe those calibration services are from USA. Does anyone knows any in the UK ?


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## gazoink (Apr 17, 2013)

rjpcardoso said:


> rta screen attached captured now...
> Maybe Im in a noisy area?? will try again at late night to see if it changes


EVERYONE is in a noisy area! You can't get away from it except in a few very special situations. It's just a question of degree. Every mic will show this, calibrated or not. _The noise you see will not look any different after calibration_. Noise is not a reason to calibrate. 

If you want to improve your noise immunity you have to measure at a higher volume level to put more "distance" between the desired signal and residual noise. There are also adjustments that can be made in the measurement parameters, but at the cost of speed, etc. You can't make any changes other than signal level that affect the RTA.

Again, noise seen in a measurement, is usual and typical, and IS NOT a reason to have your mic calibrated! The calibration files found online on this forum will calibrate your mic well enough for any measurements you'll be doing.


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