# Subwoofer testing



## tkc (Sep 11, 2007)

I am always impressed with tests like Ilkka's at this site which use a standardized testing methodology to obtain measurements, which can be compared to each other with a reasonable degree of error. 

There is one method of subwoofer testing that I've only seen in one place, though, and that is comparing a waterfall plot taken directly from source material to a waterfall plot of the signal as played back through a loudspeaker. Keith Yates did this in his remarkable "Way Down Deep" tests from 2004. If you've not seen this, one example is at:

http://www.ultimateavmag.com/images/archivesart/604way.waterfalls.jpg

I realize there are difficulties with this approach, but I am surprised this is not done more often, as it seems like a very direct way of seeing if a subwoofer is able to reproduce the signal that it is "supposed to", without adding a disagreeable amount of distortion, within a desired frequency range.

Does anyone have any comments about this? This seems like a knowledgeable group!


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## Wayne A. Pflughaupt (Apr 13, 2006)

Welcome to the Forum, tkc!



tkc said:


> here is one method of subwoofer testing that I've only seen in one place, though, and that is comparing a waterfall plot taken directly from source material to a waterfall plot of the signal as played back through a loudspeaker. Keith Yates did this in his remarkable "Way Down Deep" tests from 2004. If you've not seen this, one example is at:
> 
> http://www.ultimateavmag.com/images/archivesart/604way.waterfalls.jpg


I'm sure someone will correct me if I'm wrong, but I don’t think those are waterfall plots...

Regards,
Wayne


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## eugovector (Sep 4, 2006)

According to Wiki: The term waterfall plot is sometimes used interchangeably with spectrogram.

However, here's what I understand a waterfall plot to look like: http://www.hometheatershack.com/roomeq/images/screenshotsmall.jpg


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## Sonnie (Apr 11, 2006)

I'm like you Marshall... I think of REW's type waterfalls when I see the word "waterfall"... BUT, as you say, I think a spectrograph is also considered a waterfall.

Spectrum Labs calls them waterfalls as well...

http://www.qsl.net/dl4yhf/spectra1.html


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## phaseshift (May 29, 2007)

Spectral images like the ones shown at the top and traditional waterfall plots are not really the same. Here is a link to a more traditional waterfall plot and the color version which you can see resembles the color spectrogram that is shown in the original post. http://www.cliowin.com/7wat.htm 

In general, you would want to take a waterfall measurement in an anechoic or semi-anechoic environment. Spectral decay measurement is time critical and any reflections in the room really mess up your measurement. The measurements at the top are probably looking at the signal + the room. 

I have never done any sort of waterfall analysis on a woofer; normally only on tweeters and mids, but I suppose you could if you can run the time out far enough. Since your woofer is generally operating in piston mode, I really would not worry too much about CSD measurements; not nearly as much as I owuld look at other factors more directly related to the output of the DUT.


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## tkc (Sep 11, 2007)

Thanks for the replies!

In regards to the terminology, Yates himself uses the term "waterfall diagrams" for the spectrograms, and it seems to have become common to use that term in various Internet fora, not that that in and of itself legitimizes the usage.

Whatever the term used, wouldn't comparing the diagrams taken from the source material to those taken from the loudspeaker itself in an anechoic environment give you a quick picture of the speaker's ability to accurately reproduce the recorded material at a given reference level? Wouldn't it show deviations due to compression, distortion and other factors affecting the speaker's frequency response? I suppose it would depend on the resolution of the plots, but I would think you could get a relatively good idea if a speaker were even close to "accurate" when set to play back the material at different sound pressure levels.


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## cjwhitehouse (Jan 31, 2007)

If you have ever tried to plot comparative spectrograms for different subwoofers, one of the biggest difficulties is how to normalise the various plots in a fair way. No subwoofer has a completely flat frequency response. Whether you choose to normalise at a single frequency or as an average over a frequency band, it is hard to come up with something that is fair and objective. If the plots are not normalised then comparing and interpreting the different colour ranges presented becomes very difficult. Having tried various methods of wavelet analysis on subwoofer responses myself, I have found this normalisation issue the hardest to overcome. :nerd:


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