# Inconsistent measurements



## X3MHC (Oct 23, 2011)

Gents,

This is my very first post on this forum and it is not exactly how I hoped it to be, but it will have to do.

I am fairly new to acoustic measurements, but I have used the “all in one” package XTZ Room Analyser for a while just to get acquainted with the subject of in-room measurements.

To take it to the next level, I decided to go for REW and register as a user on this forum. The equipment I am currently using with REW is as follows:

Microphone: Earthworks M30 w/tripod Product page
Soundcard: Sound Devices USBPre 2 Product page
Laptop: MacBook Pro (late 2006 edition) running REW under Windows XP (dual boot solution using Boot Camp) Product spec
Cabling, input: Balanced from mic to soundcard
Cabling, output: Balanced from soundcard to pre-amp/processor (unbalanced/single-ended is also an option)

The M30 mic is virtually flat from 10 Hz to 24 kHz (see the graph on its product page). Calibration data exists on a mic by mic basis from around 1 kHz and up, but since the deviation is very low and I am mostly interested in the lower frequencies, I am running REW without any mic calibration file.

The USBPre 2 is also flat from around 10 Hz to 20 kHz and the plot below shows a measurement of the soundcard in my set-up using REW. Because of this, I am running REW without any soundcard calibration file.









Enough said. Let me get on with my questions (before posting this, I did search the forum for similar reports and also browsed through the first 500 posts).

When performing repeated measurements to assess the robustness of a single measurement (i.e. without changing any factors involved), I get deviations from one measurement to the other in the order of 0-10 dB. Is this normal?! And if so, what can be done to counteract this, multiple sweeps for each measurement?

The plot below shows one series of measurements. I may later post other series where the deviations are even larger.

I am using a single sweep per measurement at the moment because I don’t like to perform averaging upfront unless someone here really recommends it. 

I should also add that I live out in the countryside of Norway away from civilisation and that the distance to the nearest neighbour is around 1000 feet. Perhaps the deviation in the measurements is due to the occasional Troll coming out from the nearby forest and stepping on my lawn. But then the decibel reading should be higher, right? 









My first measurements were made using the default sweep length setting of 256 KB. Due to the inconsistency in the measurements, I tried using a sweep length of 512 MB. Unfortunately, the inconsistencies remain and the deviation between measurements is of the same magnitude as for 256 KB.

I then investigated 128 KB, as well as 1 MB. The effect of changing the sweep length is, in some instances, huge (the blue curve below is for 128 KB, the red for 256 KB, the cyan for 512 KB and the green for 1 MB). From this I infer that my MacBook Pro is not fast enough to use the 512 KB or 1 MB option and that I should use either 128 or 256 KB. Is this assumption correct? 









FYI, here is a plot showing information about the first measurement I did today.









Lastly, I should mention that when performing measurements with XTZ RA, one would have the occasional odd measurement, but most repeated measurements would be close to identical - despite that the XTZ hardware and software is inferior to that of my current set-up.

Your help with getting my REW set-up to work properly is greatly appreciated. Thanks!

Alan/X3MHC


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## JohnM (Apr 11, 2006)

X3MHC said:


> When performing repeated measurements to assess the robustness of a single measurement (i.e. without changing any factors involved), I get deviations from one measurement to the other in the order of 0-10 dB. Is this normal?! And if so, what can be done to counteract this, multiple sweeps for each measurement?


No, it is not normal, and multiple sweeps is not a cure. It is probably caused by interruptions in either the generated or captured signal. If they are in the generated signal you would usually hear clicks or pops during the sweep. If they are in the captured signal you may be able to see them by examining the scope plot captured signal trace. You should also find the impulse response looks different on the bad measurements. Try changing the buffer size settings on the soundcard preferences to see if you can get consistent measurements with different values for replay (if getting clicks or pops) and/or record (if getting captured signal dropouts) buffer sizes.


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## X3MHC (Oct 23, 2011)

Thank you, John.

I will investigate this further. I can already report back that there are no clicks or pops during the sweeps. I also checked the scope plot for one of the measurements earlier today, but that could have been a measurement that was correctly executed. I did notice that the captured signal (red) continued for a longer time than the generated signal (purple) on this graph, which differs from the example in your user guide. The amplitude of the red was well within the amplitude of the purple, though. I will take a look at the impulse responses as well.

In light of your information, I should mention that the USBPre 2 card indicates that there is a signal synchronisation problem. As soon as I click the Measure button in REW, the Pre and PC meter LEDs on the soundcard start flashing simultaneously and this happens even before starting the measurement. I have contacted technical support at Sound Devices about this and they write: "When the PRe/PC light is flashing, it means it is sensing a clock signal that is different or conflicting with the computers sync signal."

I also have an M-Audio Fast Track Mk II card at hand that I could test and compare with the USBPre 2 card, but this has no loopback possibilities, which I really desire for a full utilisation of the features provided by REW.

This is possibly a stupid question, but perhaps I should use a sample rate of 44.1 KHz instead of 48 kHz since I am running Windows on a Mac? Perhaps running REW under Windows on a Mac is a flawed concept to start with. Unfortunately, that is the only laptop I have access to at the moment.

I guess I will find out soon enough...


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## X3MHC (Oct 23, 2011)

Finally, I had the house all to myself and could investigate this issue further. 

After trying different buffer sizes for the 'Record' option (as well as the 'Replay' option just to be sure) without success, I changed my set-up and started running REW under OS X instead of using Boot Camp. 

As the included picture shows, repeated measurements are now for all practical purposes identical, even for the different sweep lengths (including 1M option). Measurements 1-3 are with a sweep length of 256K, measurement 4 is with 512K and measurement 5 is with 1M.

=> Problem solved!

Now "all" that remains is to get a smoother bass response 

Alan


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