Newbie in need - Help with graphic interpretation


New Member
Thread Starter
Oct 11, 2019
Hello everyone,

I am new to using REW and I need help to correctly interpret the frequential SPL graphics that I obtain when I measure the same sounds.

First of all, I am unsure about what reference is used to calculate the dB values shown on graphics obtained by using the "Measure" feature. The previous software I worked with was using the threshold of audibility (0.00002 Pa) as the dB reference and I am wondering if it is the same for REW.

Second of all, for now, I have taken a few measurements with REW, using the "Measure" feature where I have muted the sweep (by muting my speakers) and I have generated a tone of 500 Hz. I have run the measurements four times, and I obtain significant differences between the graphs. In fact, three of them are very similar, just as I expected them all to be, but there is one of them that is quite different. I understand that the SPL may be slightly different from a measurement to another, but what I would like to understand is why there would be such a sudden change in my measurements (see attachment).

Indeed, when I look at the first graph, the one of the ambient noise alone, I obtain a relatively linear and consistent spectrum. However, when I generate a tone, I obtain results where the low frequencies are under the 0 dB line, whereas the high frequencies are over it, and vice-versa. In short, I don't understand why those results do not even overlay with the ambient noise measurement I have made.

Can someone plus help me understand this? I would really appreciate the help.



Senior Member
May 22, 2017
Arizona, USA
My AV System  
Preamp, Processor or Receiver
Marantz AV7705 Pre/Pro
Main Amp
Rotel RMB-1066 6 chnl P-amp x 2
Additional Amp
Behringer DCX2496 x 2
Universal / Blu-ray / CD Player
OPPO BDP-103 Universal Player
Front Speakers
DIY SEAS H1456/H1212 Spkr x 5
DIY JBL 2235H 15" SW x 2
Video Display Device
Da-Lite Da-Snap 39105V - 92"
The measurements are consistent. The problem is that the nature of the test signal is not consistent an provides no normal impulse peak for REW. REW locates the window reference time at the impulse peak. Below is 1 of the measurements showing the window location on the captured data. The window overlays some of the noise prior to the test signal is introduced. This distorts the result as you noted.

Below The window was moved to place it properly within the range we are interested in. This was done for all measurements.

Below is the overlay of all measurement to show where 0 ms is now located on this overlay window. Note the IR window reference, left and right locations was to reset to defaults for all measurements as shown.

The SPL chart is then consistent for noise floor and tone level (about 86.4 dB).


May 22, 2017
To the OP.

I think that all you really need to do is to open REW's builtin RTA//Spectral Analysis window ( and then fire-up REW's tone generator set to 500hz ) to accomplish what you apparently want to accomplish ( seemingly > measuring harmonic distortion of single frequencies ).

Understanding why your creative HACK of REW's impulse capture window doesn't work well ( while interesting and educational in its own right ) it doesn't really address your real issue ( which I believe is using the wrong measurement tool for the job at hand ).

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