inconsistent waterfall plots ?

D. Tester

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I'm finding consecutive runs of REW - with no changes to the room, levels or mic position - do not produce the same waterfall plot results (though the SPL and phase plots are always the same). REW is running on Mac OS 10.13.6 with a 4 core i7 chip at 3.2 GHZ base speed using all SSD storage directly off the PCIe buss.

REW is using its own dedicated Metric Halo (MH) audio interface via audio over ethernet - which is quite fast. The digital output from the MH interface goes into a second (apogee) interface running on the same machine connected thru a PCIe card - also a fast audio connection. Some latency is introduced by a DAW program processing the REW output on the Apogee interface. The DAW program implements a crossover for the monitors via filter plugins that rolloff to the sub. The filters are setup to be minimum phase with the impulse response customized to be low latency, but the audio buffer for the DAW needs to be 512 samples to run these filters. Then the analog out from the second interface goes to a 2.1 monitor system. I assume the round trip latency REW sees would be = latency of interface one + 512 samples (DAW program) + latency of interface 2.

REW 5.18 seems to mostly work with this setup, e.g., checking the main and sub levels via the preferences menu. Similarly, in the measurement window, REW 5.18 can check the levels which always come in at the same +/- 0.1 dB. You can see there is some latency between when REW generated the test signal and it is picked up by the mic. But it makes not difference if I enable acoustic timing reference or not - waterfall plots are not the same 2X in a row.

With acoustic timing reference enabled the delay REW calculates is not always the same. At 48K doing 20 second sweeps, for 2 consecutive measurements the delay reported came in at: (1) -0.066 +/- 0.01 msec; (2) -0.085+/- 0.01 msec. At 96K the sweeps are 10 seconds and for 3 consecutive runs the delays came in at: (1) 0.101 +/- 0.005 msec; (2) -0.043 +/- 0.005 msec; (3) -0.044 +/- 0.005 msec. But it does not matter if I use 48K or 96K sampling rate, e.g., if on the first run if I see some ringing on the waterfall plot at ~200 Hz, on the next run that can be gone and I see ringing somewhere else were there was none.

This machine has no disks to go to sleep - and those settings are turned off anyway in MacOS. But after REW has been running for a bit, the check for the levels starts to fail. The latency visibly increases between the time when REW generates the tone and when it gets sent out thru the system and picked up by the mic. This happens even if I only use 1 interface w/o any crossover filtering. But unlike REW 5.18, REW 5.19 release and the current 5.20 beta can fail the first time I try to check the levels because of too much delay between the input and output. But if I quit REW and re-start it, then at least on the first try checking the level with 5.18 works. So I've ended up using 5.18 and not 5.19 release or 5.20 beta.

It seems like I'm dealing with some latency in REW that accumiulates after sweeps and the level checks have been run a few times; but does not happen when the program has first been launched. It's not clear to me if this is contributing to some kind of time skew that causes waterfall plots to never be the same 2 times in a row. In REW I tried changing the time to end the impulse response from 1.1 to 4.4 to keep all - but it made no difference. The windowing for the IR left, right, and decay is Tukey 0.25. Happy to hear any pointers about how to get more consistent waterfall results.
 

John Mulcahy

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Please attach an mdat file with some measurements showing the behaviour you are seeing.
 

John Mulcahy

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Those waterall plots look quite consistent to me, and correspond with their respective frequency responses. The later parts of the waterfall will show the effects of any ambient noise, but they are not really the relevant part anyway.

The impulse responses show a lot of noise and ringing ahead of the initial peak. That is often seen if the OS is resampling the audio, which could also be contributing to latency any synchronisation problems especially with longer signals. Make sure that the rate selected for the card in Audio Midi setup is the same as the rate selected in REW, use 256k sweeps (also less potential for background noise to intrude with a shorter sweep) and set the IR truncation back to 1.7s.
 

D. Tester

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OK - sorry if I am making a fuss over low signal levels that are not really of consequence. I just want to make sure that I am viewing the scale of the waterfall plots correctly. I've attached a screenshot of the waterfall plots from the 2 measurements in the first file as meas_1.png and meas_2.png respectively. In both I have set the top of the visible y-axis to be at 90 dB and marked the highest point in the waterfall trace to be 81.82 dB. Then if you look ~40 dB down from that peak, I do see differences, e.g., in meas_2.png there are some trails for frequencies above 200 Hz that decay out to 600 msec on the time (z-axis) that do not show up in meas_1.png. And meas_1.png has a decaying component at about 65 Hz that does not show up in meas_2.png. I will check and re-adjust what you mentioned above and also measure and look at reducing the noise floor.
 

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Matthew J Poes

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You may want to look at the rooms actual noise level. Those trails that you are attributing to ringing don’t look like ringing and are very low in level. They look like room noise.

Try measuring again after raising the volume level 3dB. If it’s ringing then the ridges will rise in level. If it’s just noise they would just show up randomly.
 

John Mulcahy

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Noise is a substantial contributor down there, in particular anything that has a constant level is a background tone picked up during the measurement - things like car engines idling outside the building or air conditioning systems are common sources. The relevant info in the waterfall is in the ridges that descend from the top.
 

Matthew J Poes

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Noise is a substantial contributor down there, in particular anything that has a constant level is a background tone picked up during the measurement - things like car engines idling outside the building or air conditioning systems are common sources. The relevant info in the waterfall is in the ridges that descend from the top.

John, someone else mentioned doing a test of the Impulse to Noise ratio test, it got me to thinking that this might be a good calculation. I assume REW doesn't innately provide this, but the data is there to do it. I wonder if that might be a good feature to add? Maybe it could even provide some additional guidance when the INR isn't high enough? Maybe this wouldn't provide any useful information above the noise floor plot you already provide.
 

John Mulcahy

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If you look at the impulse response plot in dB mode the level of the noise floor is pretty apparent, see below as an example.

noisefloor.jpg
 
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