REW Forum

**apbcom** · **03-16-07, 05:49 PM**

Hi Guys,
Got my new CM-140 today and laid it side by side with my RS 33-2055 Digital. Played a number of test tones (sine waves) from 30 Hz to 5000 hz and noticed that the SPL levels at different frequencies vary as much as 20 db! Could this be a bad meter? Which one? I'll try to chart them out this weekend and see what the difference is. Any thoughts or am I missing something.

**Ilkka** · **03-16-07, 07:12 PM**

Quote:

apbcom wrote:

Hi Guys,
Got my new CM-140 today and laid it side by side with my RS 33-2055 Digital. Played a number of test tones (sine waves) from 30 Hz to 5000 hz and noticed that the SPL levels at different frequencies vary as much as 20 db! Could this be a bad meter? Which one? I'll try to chart them out this weekend and see what the difference is. Any thoughts or am I missing something.

It is not wise to measure 5000 Hz frequencies with straight sine waves. Only a small difference in mic placement can have a huge effect at those frequencies (small wavelength). One shouldn't measure any higher than ~100 Hz with sine waves.

**apbcom** · **03-16-07, 08:05 PM**

What would be the proper way to compare the two? I'd like to see how close or not they are in measuring.

**Ilkka** · **03-16-07, 08:28 PM**

Quote:

apbcom wrote:

What would be the proper way to compare the two? I'd like to see how close or not they are in measuring.

Comparing frequencies higher than 100-200 Hz is very difficult unless you know what you are doing. But for low frequencies you can just plug them to your computer and run a few REW sweeps. Plain sine waves are also ok there, although you should still place both mics side by side. Use the same weighting for both (C-weighting).

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