REW v. SMS Measurements

[Josuah]

I just got my SMS-1 and ran measurements using its sweeps, the REW sweeps, and the REW 1Hz tones and they all ended up producing completely different graphs. I can't explain this. I also had the SMS-1 displaying on screen while doing the REW measurements, and when playing the 1Hz tones, I saw a very broad hump of two or three octaves wide with the center point being the 1Hz measurements, only being maybe +6dB above the rest of the hump.

Anyway, I ended up doing manual calibration on the SMS-1's filters. In fact, the filters I placed into REW to try and reproduce the curve measured by the SMS-1, using the SMS-1 sweep, didn't end up flattening out the SMS-1 measured curve when I applied them to the SMS-1.

[MACCA350]

The graph produced by the SMS-1 will only work with its internal signal generator and mic. The SMS-1 will not display a correct graph if you use other software to generate the signal. Also, moving the mic by a few inches can sometimes dramatically effect the response curve due to room effects, this could account for some discrepancies.

Anyway, I ended up doing manual calibration on the SMS-1's filters. In fact, the filters I placed into REW to try and reproduce the curve measured by the SMS-1, using the SMS-1 sweep, didn't end up flattening out the SMS-1 measured curve when I applied them to the SMS-1.

Just the fact that you now have the SMS-1 connected in the line could effect the overall curve, you may need to adjust Phase settings, etc.

cheers

[Josuah]

Well, I was expecting the SMS-1 graph to display a peak if I play a single frequency sine wave. Shouldn't it do this, regardless of what is producing the signal? The mic and RS meter had their "openings" within .5" of each other and did not move while I was doing all of this. And likewise, the SMS-1 was in the signal path, with all filters at 0dB, while doing all three types of measurements.

[brucek]

they all ended up producing completely different graphs.

Very strange indeed.

I'm not that familiar with the SMS (although I've read the on-line manuals and understand what it does), but it's nothing more than a fancy parametric equalizer which should follow the same rules as a BFD and should provide filtering of the voltage signal sent to the subwoofer amplifier.

The end result should be the same as a BFD and as such REW (which is a tool for measuring frequency response) should provide the same results pre and post equalization.

I was expecting the SMS-1 graph to display a peak if I play a single frequency sine wave. Shouldn't it do this, regardless of what is producing the signal?

Absolutely. With the SMS in bypass, REW should display a raw measure of the room response for a sweep or for a single tone, providing the microphone used has a valid calibration file loaded and the soundcard used is compensated for with a calibration file.
The SMS system will have its own calibrated microphone and if placed in a similar position as the REW microphone, then the responses should be identical.

I have no explaination........ Is there someone else here who can perform this test. Our sample of one is hardly conclusive....

brucek

[Josuah]

I loaded the correct radio shack SPL meter calibration file off the forum, but I didn't load a sound card calibration file. I just clicked on the necessary red buttons to do the soundcard calibration?

[Chrisbee]

Josuah wrote:

I loaded the correct radio shack SPL meter calibration file off the forum, but I didn't load a sound card calibration file. I just clicked on the necessary red buttons to do the soundcard calibration?

Which microphone are you using with REW?

Sorry, I see above you are using an SPL meter.

Do you have the SPL meter in exactly the same position as the SMS-1 microphone?

An inch out is enough to produce considerable changes in response.

[Sonnie]

Chrisbee wrote:

An inch out is enough to produce considerable changes in response.

This was not the case with me... not sure if room size, layout, etc., etc., will make a difference, but when I measured my left ear and right ear, there was no significant difference... I posted the response graphs somewhere here.... (no comments about how much distance is between the two ears... )

[Chrisbee]

I saw a 6dB difference on the meter scale on REW's calibration pink noise with a 2 " change in the height of the SPL meter.

This variation between the SMS-1 and REW should really be cleared up.
They can't both be right. Though they can both be wrong.

[Josuah]

Pink noise will be full spectrum. At the frequencies we're talking about here, say 20Hz, an inch won't matter. The SMS-1 mic was about 1" above the SPL meter opening. Both were front facing. Not aiming upwards with either of them.

[brucek]

Can you post the REW and SMS graphs, so we can have a look at them?

I suppose the SMS one could be snapshot with a digital camera.

I'd like to see the scaling on both graphs....

brucek

[Sonnie]

Good idea and that should be interesting to see.

[oliverlim]

I experienced this myself when I got the SMS-1 and asked Bruce of Velodyne about it.

He says that the way SMS-1 measures is not by the freq that it hears at the time but the amplitute at the time the SMS-1 is outputting the test signal. So for example at 0.1ms when it outputs 15hz, it measure the volume it hears at the time and shows it on the graph and at 0.2ms when it outputs 16hz, it does the same and so on. It is suppose to have better then 1/60 octave accuracy and according to him is better in accuracy with the mic they are using which is basically the EM8000. that explains why you get a totally different result if you play a sweep from a DVD or from REW and try to see what SMS-1 measures instead.

Oliver

[Josuah]

Yeah, I figured that out later as to why I saw a broad hump on the SMS-1 graph when outputting a single-frequency sine wave from REW. But it doesn't explain why the SMS-1 measurements (using the SMS-1 procedure) and the REW measurements (using the REW procedure) ended up looking so totally different.

[brucek]

But it doesn't explain why the SMS-1 measurements (using the SMS-1 procedure) and the REW measurements (using the REW procedure) ended up looking so totally different.

But doesn't Oliverlim's explanation above tell us why?

brucek