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Discussion Starter #1
Greetings

I am having a problem with the signal generator and the sine wave sweep used for room measurements. The amplitude of the generator decreases as the frequency of the output increases. This occurs with REW running under both OS X and Windows 7. I have read and reread the instructions and can't find any info.

In an attempt to trouble shoot, I hooked up a FLUKE true RMS meter to the stereo mini jack of both computers - nothing else was connected. With the generator putting out 1 KHz, I get - for example - .5 V RMS. As I increase the frequency of the generator to 15 KHz, the amplitude drops to about .2 V RMS. I did this manually. Then - I looked at the amplitude on the "sweep" used for room measurements. I set it to loop and watched the meter - same behavior. Realizing that the FLUKE has some lag in it, the high frequency drop is probably greater than what I am seeing.

I could find no controls in the program for this. What am I missing? As noted, this happens on both OS X and Windows 7.

Thanks for any help
 

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Premium Member
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6,711 Posts
Hi,

There is nothing in REW that changes the output level with frequency, and most soundcards will still be within 0.5 dB or so of their nominal level at 15 kHz if they are operating at 44.1 k or above. You need to be a little careful when using a Fluke to measure voltages as the meter accuracy is typically only spec'd for inputs up to 1 kHz (which is a little odd as their frequency measurement range typically goes to 100 kHz). If the computers you are using have line inputs try connecting a loopback from output to input and measure the response to make sure there are no feedback/monitoring paths active, which would cause a series of dips and peaks in the response according to the level and delay of the feedback path (wouldn't cause a gradual drop with frequency though). With the loopback in place you can check what the input sees when using the signal generator by monitoring it with REW's SPL meter or RTA. If using the SPL meter make sure to select Z weighting, otherwise the C weighting curve will reduce the displayed levels at higher frequencies (C weighting is -3 dB at 8 kHz).
 

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Plain ole user
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11,121 Posts
Try measuring across a load rather than open ended. With no load, all the output device sees is the capacitance of the meter.
 
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