Odd tweeter measurements with repetition larger than 1

soundster111

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Jun 9, 2020
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hi ,

been using REW for some time. also with the latest beta release (currently on the most recent one out).

I went back to fine tune my tweeter crossover , and fired up REW with my UMIK1 as usual. I've run few measurements for the tweeter unit alone. some strange behaviour is seen on the graphs when I'm selecting repetition of x2 and above.
like something is oscillating. when repetition is set to 1 it's almost smooth. can any one suspect what the issue is? I can't recall the other times I did that in the past...

thanks!
 

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John Mulcahy

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When you say "repetition" do you mean multiple sweeps? Multiple sweeps cannot be used when the input and output devices are different, as they are when using a USB mic, because the OS cannot maintain synchronisation between different devices. REW disables the multiple sweeps option when a USB mic is selected as the input device or when the acoustic timing reference is used.
 

soundster111

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yes that was exactly what I was referring. it's interesting , because after you choose to "level test" it does let you choose number of sweeps repetition. so it's the synchronisation that's causing this wield waves.

does number of sweeps per test make it more accurate? currently I'm not planning on switching for a line level MIC instead of the USB option.

thanks
 

John Mulcahy

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after you choose to "level test" it does let you choose number of sweeps repetition
I'll fix that.

does number of sweeps per test make it more accurate?
From the help:

REW allows multiple sweeps to be averaged, although best results are generally obtained by using single, longer sweeps rather than multiple, shorter sweeps. Multiple sweeps are offered when not using a Timing reference or when using a loopback as a timing reference. Do not use multiple sweeps if the input and output are on different devices (for example, if the input is a USB mic). If Repetitions is more than 1 REW uses synchronous pre-averaging, capturing the selected number of sweeps per measurement and averaging the results to reduce the effects of noise and interference. The pre-averaging can improve S/N by almost 3 dB for each doubling of the number of sweeps. Averaging can be useful if the measurements are contaminated by interference tones, whether electrical or acoustic, as they typically will not add coherently in the averaging and hence will be suppressed by the process.
 
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