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How to properly measure subs?

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Frank Koenig:
The frequency resolution of the measurement (in Hz) is 1 / the window length (in seconds). For example, a 50 ms window results in 20 Hz frequency resolution. To achieve a pseudo-anechoic measurement the window must close before the first reflection arrives at the mic. So the distance from the speaker to the nearest reflective object plus the distance from the object to the mic minus the distance from the speaker to the mic must correspond to a time-of-flight no less than the window duration. As a result, the further the mic is from the speaker so too the reflective objects must be further from the speaker-mic axis.

Someone on DIYaudio did a rather thorough, and very interesting, comparison of the effect of different ground surfaces on ground-plane measurements. The short takeaway is that the harder and smoother the surface the higher the frequency to which the measurement is valid. You basically want the surface to have low absorption and be smooth at wavelength-scale so as to cause a specular reflection (just like a mirror in optics). A big, paved parking lot is ideal.

--Frank

Douglas Cyr:
This is great info, thanks everyone!

Ivan Beaver:
Just to add to the other comments, as the physical size of the sub get larger, the further you need to be away in order to minimize the close effects of the boundary of the cabinet.

There have been all sorts of "standards" presented by various people over the years.

We use 10m, as that is easy to back calculate (add 20dB), and minimizes any close boundary gains.

We drive the signal at 28.3V (a 20dB gain over 2.83V), so they distance loss and the voltage gain cancel each other out.

Frank Koenig:

--- Quote from: Frank Koenig on December 28, 2022, 01:46:56 PM ---Someone on DIYaudio did a rather thorough, and very interesting, comparison of the effect of different ground surfaces on ground-plane measurements.
--- End quote ---

Turns out I saved the link:

https://www.diyaudio.com/community/threads/measuring-response-how-close-is-close-enough-to-anechoic.353347/page-4

Look at the posts by "bikinpunk".

--Frank

John Schalk:

--- Quote from: Frank Koenig on January 03, 2023, 01:31:05 PM ---Turns out I saved the link:

https://www.diyaudio.com/community/threads/measuring-response-how-close-is-close-enough-to-anechoic.353347/page-4

Look at the posts by "bikinpunk".

--Frank

--- End quote ---
That is a great thread Frank, thanks for sharing the link.  Do you happen to know what software Bikinpunk was using that has gating (windowing?). Many of his screen shots were from REW, but I get the feeling that he was importing data into REW to make the comparison plots.  I've looked for that feature in REW, but didn't find it.

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