6speed
Active Member
I have noticed IMD testing of speakers mentioned a few times, but no results...or even suggestions. As a starting point, can we just use an equally spaced multi-tone signal and run an RTA?
What is happening if you use the same size FFT for higher rates, is each FFT bin is twice or four times as wide. So it has 3db or 6db more noise on the same signal. And this wider FFT bin size means lesser resolution at low frequencies. So your results are what is expected. Does this make sense or should I go into more detail?I tried running REW RTA on the 31 tone test in yellow in my previous post and here are the results. The mic is just 2 feet from the speaker being driven and the non-level peaks show the room modes. I was surprised to see lower sampling rates with less noise and better low frequency resolution. I tried 192kHz and it was incrementally worse than 96kHz. The noise floor is really high though and I was unable to find a way to lower it. What does rise above the noise floor does not always seem well correlated between the two sampling rates, making it all rather suspect.
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I did not have time to run an IM sweep, but will try to soon.
I don't think that is what was done. I think the same FFT size was used for first 48 khz, and then 96 khz. In the latter case, each FFT bin is twice as wide, has 3 db more noise, and less frequency resolution due to the wider FFT bin size. If you double the FFT size with doubling the sample rate/bandwidth, resolution and noise would have stayed the same.Larger FFT size usually means higher resolution.
Yes that will do it.So every time I double the sampling rate, I need to double the FFT size to keep the bin width the same?
I have noticed IMD testing of speakers mentioned a few times, but no results...or even suggestions. As a starting point, can we just use an equally spaced multi-tone signal and run an RTA?
@6speedThanks for the guidance so far @Blumlein 88, but it looks like REW's sweep generator (the tab meant for measuring) uses logarithmic spacing because sweeps from 2Hz - 19kHz and 1,002Hz - 20kHz are not the same length of time. I also don't think they are quite the same thing as a normal sweep, which means I can't use a third party tool to merge the wave files.
You can play the multitone wav using a bit perfect player, and REW RTA for record. You won't get the distortion panel though.@Blumlein 88 Thanks. Your AP multitone looks a lot cleaner than the one I generated using REW.
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Unfortunately, REW will not use your sweep for measurements. When I import it, REW says I need to use "a measurements sweep saved from this version of REW." With a single tone sweep, REW knows to record the fundamental and 9 harmonics, but does not have an IMD measurement facility, so I do not know how it would record an IMD sweep where we want to be able to see n-number of sum and difference products. Furthermore, I don't know how one would want to display an IMD measurement other than via fixed tones.
You can play the multitone wav using a bit perfect player, and REW RTA for record.
I'm sorry, I should have been more specific. Just play and record the linear sweeps I posted. Then analyze them with Audacity or another program which will do FFT analysis. You won't get the convenience of REW spitting out the numbers for you. You'll have to look at them yourself and see what the levels are, but you can determine how much IMD distortion is occurring.I plan on doing that for the multi-tone, but how do you record a 2 tone sweep? You could use the RTA again...and then what the average forever setting (or peak trace) will ensure that the swept tones and IMD products remain?