# Need help diagnosing/solving problem with CM-140



## adeeb (Sep 20, 2008)

I have been using a CM-140 in my home theater for the past 6 months to set speaker levels. I have probably used it in 4 sessions - 30 minutes each.

The problem is that the meter previously measured ambient noise at around 49dB (i.e. all sources off, AC off, etc -- basically as quiet as the room gets). However, when I took it out last weekend the ambient level read at close to 76dB. I always use the C weighting and set the meter to Slow. I tried turning the meter off and back on several times, but the performance did not change.

Also, ambient noise has not changed in any way (I even verified it with another software based SPL), so the meter must be acting up.

I take very good care of the meter and it sits protected in its original packaging secure in a drawer in an air conditioned room when not in use. There is no chance that there has been any physical damage to the unit.

Any ideas what could be wrong and how to resolve it?

Thanks!


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## brucek (Apr 11, 2006)

It does indeed sound like you have a fault on it. I suppose you could try removing the battery and then putting it back in, just in case there was a weak connection there, but other than that there isn't much that can be done..

brucek


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## adeeb (Sep 20, 2008)

brucek said:


> It does indeed sound like you have a fault on it. I suppose you could try removing the battery and then putting it back in, just in case there was a weak connection there, but other than that there isn't much that can be done..
> 
> brucek


Thanks. I had already tried that but there was no change. I'll try a new battery and see if it fares any better.

Just curious, does a reading of around 49dB for baseline ambient noise seem about right? (or has the meter been defective all along)


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## brucek (Apr 11, 2006)

> Just curious, does a reading of around 49dB for baseline ambient noise seem about right? (or has the meter been defective all along)


That's maybe a bit high for a typical room. Normal talking would be between 40-60dBSPL. A quiet room without the air or furnace on would be around 30dB.

Maybe someone else can take a quiet room measure for us with a Galaxy meter?

brucek


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## Wayne A. Pflughaupt (Apr 13, 2006)

adeeb said:


> (i.e. all sources off, AC off, etc -- basically as quiet as the room gets)


Hopefully that included refrigerators and computers, and your HT system as well?

It may be that you were picking up extraneous low frequency noise from outside the house. Keep in mind the difference between A and C weighting. A-weighting rolls out everything below about 500 Hz, while C-weighting includes low frequencies down to ~40 Hz. So what you might do is try switching between A and C. If the reading increases when you switch to C, then there is some low freq noise present.

Regards,
Wayne


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## adeeb (Sep 20, 2008)

Wayne A. Pflughaupt said:


> Hopefully that included refrigerators and computers, and your HT system as well?
> 
> It may be that you were picking up extraneous low frequency noise from outside the house. Keep in mind the difference between A and C weighting. A-weighting rolls out everything below about 500 Hz, while C-weighting includes low frequencies down to ~40 Hz. So what you might do is try switching between A and C. If the reading increases when you switch to C, then there is some low freq noise present.
> 
> ...


Yes, I had absolutely everything off that was in the room (air filter, all A/V electronics, and central AC). This is the same procedure I did before and previously had consistent readings.

I always read C-weighted and never really compared to A-weighted. However, the A is about 50dB now as compared to the C in the 76dB range.


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## Wayne A. Pflughaupt (Apr 13, 2006)

adeeb said:


> Yes, I had absolutely everything off that was in the room (air filter, all A/V electronics, and central AC).


Only in the room? Low frequencies pass right through walls, especially interior walls...

Regards,
Wayne


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