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Old 16th December 2008, 08:46 PM   #921
Pan is offline Pan  Sweden
diyAudio Member
Join Date: Sep 2002
Originally posted by HiFiNutNut

3. In the Phoenix subwoofer documents Linkwitz mentioned that the XLS 12" has better distortion figures when working in OB than sealed and he published some measurements / graphs to support this. I guess the same applies to the SS 10".


what you mention is true for a certain excursion but not for a given SPL.

On the OB the driver will have to move more air to make up for the cancellation around the baffle in order to achieve the same SPL as when the driver is mounted in a box.

Sooner or later as the frequency is lowered the OB will for sure produce much more distortion than the box.

Possiby there is a small region where the OB need to move more air for a given SPL but till resuls in lower distortion. Just speculation and it would be easy to find out.

I measured 3x Seas W22 on a small open baffle, 100Hz @ 90dB (one or two meters, don't remember) I had 0.2%THD. That's better than one would have guessed. Measurement done outdoors without any reflections and some ground support.

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Old 16th December 2008, 09:19 PM   #922
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Join Date: May 2004
Location: Sydney
I intentionally did not apply dipole roll-off EQ (6dB/oct) so that excursions, hence distortions, are kept low. This makes the 115dB-120dB SPL capability of the speakers throughout most of the audio bandwidth within driver linear excursions possible. This was one of the objectives set at the beginning of the DIY project. The Fequal was designed to be at 65Hz and the subs would take over below 60Hz. I was pleased to find even in OB U-frame, the woofer cones rarely moved more than 2-3mm even if I played heavy organ music or Japanese drums. No low pass filter was used. This came out as a (good) surprise.

I found with OB U-frame it was weaker below 50Hz. This is as expected.

However, do remember that above Fequal and below the first dipole peak, and in my case for most of the passband of the woofers from 65Hz to 180Hz, as how it was designed, the U-frame gives up to 6dB more SPL with the same excursion as in sealed boxes. I am going to EQ it flat but still need no bass boost.

Perhaps for this reason, OB U-frame bass would have far less distortions because the driver excursions are nearly halved for a given SPL, provided that you don't apply the 6dB/oct boost and use the driver mainly from Fequal to the first dipole peak.

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