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brucek,

Which is more accurate down to 10Hz?

1. CM140
2. RS meter
3. ECM8000 set-up

I will guess the ECM.
 

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The ECM8000 is the best setup but you still need a SPL meter to get proper levels set, The CM140 is quite a bit better then the RS meter.
 

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Yeah, I would say the ECM and the CM-140 are both pretty good down to 10Hz.

If you look at the correction files of the ECM and the CM-140, they're down ~-9dB and -10dB respectively. Below that they drop off pretty quick and you would be get into noise. The RS meters should only be used to about 15Hz.

brucek
 

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Discussion Starter · #7 ·
This graph looks pretty consistent which leads me to believe it is accurate way down low.
I guess it really does not matter but am I flat to 5Hz or am I pushing it to say that?:scratchhead:
The was measured with a C-140.
 

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I guess it really does not matter but am I flat to 5Hz or am I pushing it to say that?
Yeah, it may be OK to 5Hz. You would have to look at the impulse response to be sure and check where the signal is dropping into the noise.

You have to consider the amount of boost that the cal file offers at the lower frequencies. When we created the cal file for the Galaxy, we found them to be quite a good mic with very good consistency between units. The 5Hz entry required 24dBSPL boost. That's quite a lot. If the response of the meter has dropped off 24dB, you are starting to get into the noise floor.

Personally, I wouldn't trust any meter below 10Hz...

brucek
 

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What are the appropriate limits of the graph
That depends on the noise level. In the vertical you want to see 0dBFS and then whatever scale is required to properly display the noise. The horizontal also depends on the bandwidth or your measurement.

Post the impulse response so we can see. I just want to see how far down your noise is. Obviously if it's -30dBFS, we can assume the response it shows at 5Hz is noise.
Again, either way, there's no usable information below about 10Hz, so I wouldn't be too concerned with anything below that.

brucek
 
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