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SoundCheck simulated anechoic response

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Helge A Bentsen:
The latest VoiceCoil magazine has a feature about how to make a simulated anechoic response measurement in a ordinary room.
From the manufacturers website:

https://www.listeninc.com/products/test-sequences/free/anechoic-response-of-a-loudspeaker-without-an-anechoic-chamber-splice-sequence/#tab-id-1

Anybody tried it?

Chris Grimshaw:
I have, and found problems.

The main problem went like this:
- 15" subwoofer. Ported box, port fires out of the front of the cabinet, next to the driver.
- When testing via impedance and visible excursion, the port tuning frequency was about 39Hz.
- With the microphone placed close to the cone, you'd expect a dip in output where the cone stops moving (ie, at port tuning). This wasn't the case. I got a notch in the frequency response curve anywhere from 25Hz to the high-30s. Centre of the cone was mid-30s IIRC.

Following that, close-micing a woofer like that cannot be an accurate representation of what's happening at LF when there's effectively more than one source (ported box), and some phase difference between the two.

See attachments for the graphs.


I'd also point out that the near-field LF response of even a sealed box (the simplest LF system) isn't an accurate representation of what'll happen: effects pertaining to the baffle dimensions (search: baffle step compensation) shouldn't be ignored.

The only believable (to me) way of doing this stuff* would be using the Klippel Nearfield Scanner, which is a really serious bit of kit.

* By "this stuff", I mean taking an effectively-anechoic measurement in a typical room.

The guy that runs Audio Science Review actually has one of those Klippel setups, and the data produced is extraordinary.

Chris

Helge A Bentsen:
Chris:
I've done a few experiments with close-micing drivers and while it seemingly produces a nice looking curve, it doesn't necessarily represent what I'm actually hearing. Tested eqing a box in the near field and as an average of the far field with several mics, thought the far-field method sounded better.

That's why I find this setup interesting. Maybe I'm missing something in my methods. Your points seem to correlate with my own results.



Frank Koenig:
So far as I know this is the classic paper on the subject:

https://www.aes.org/e-lib/browse.cfm?elib=1717&if=1

A good place to start.

--Frank

Mark Wilkinson:

--- Quote from: Frank Koenig on February 28, 2021, 12:18:23 pm ---So far as I know this is the classic paper on the subject:

https://www.aes.org/e-lib/browse.cfm?elib=1717&if=1

A good place to start.

--Frank

--- End quote ---

Yep, that seems to be the paper that started it all.
Here's a link to it from Keele's website...free...http://www.xlrtechs.com/dbkeele.com/PDF/Keele%20(1974-04%20AES%20Published)%20-%20Nearfield%20Paper.pdf

I share Chris's and Helge's reservations about the technique.  Makes pretty curves, but doesn't seem to tie to hearing.
Plus, even in the paper, it says vent measurements made to stitch together with primary response are only vailid to a max of 1.6x vent tuning freq.

Me....I'll take a parking lot and a 4 meter minimum.

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