-- See bottom of this post regarding the influence of high density foam thickness on HF absorption --
In the process of studying the frequency absorption influences of five types of open cell, high-density foam, I may have confirmed my suspicion that some sound waves above 500 Hz can pass through the baffle of a PBB2 and interact with the short setback distances of the boundary mounted mic capsules. Surprisingly, the standard reference foam I measured transmitted almost all of the sound energy under 2000 Hz. That comparison is the last pair on this test:
So which foam would work best for the PBB2 array? I'm not sure. The dark gray acoustic foam (1.7 lbs per cubic foot with medium size pockets) accentuates frequencies between 500-1600Hz to take full advantage of the "partial baffling" effect:
But is this truly a beneficial "effect;" does it have a role in the increased "airyness" the PBB2 rig has exhibited? In theory, sounds above 1200 Hz rely more on timing differences between the ears, so passing more off-axis sound above 1200Hz through the baffle could obscure critical amplitude differences. If this is the case, then more left-right isolation above 1200Hz might work better and the "Luxury" foam might perform better. I have a much better idea of what the foam can do, but not what it should do. Simply matching the reference foam either by ear or by sonogram is not enough. Its also very possible that foam type is moot because a much larger percentage of the sound reaching the capsules does not pass through the baffle. Thanks to Paul Dickinson for the source of three of the higher density samples tested in this test. FoamForYou.com.
So, its looks like the next step is to compare the different types of foam in the PBB2 array within a stereo field to see if I can hear the indicated Hz absorption differences and impacts on stereo imaging. Please chime in if you ascertain different conclusions from mine! [For the above test, I used an AT 3032 mic pointed directly between the woofer and and tweeter of a two-way speaker a distance of 12 inches away. The foam was positioned between the mic and the speaker 1/16" from the front of the mic.]
= = = = = = = = = = The follow-up test below added 09-2010 = = = = = = = = = = = =
High Densiy Foam Thickness Affect on >1000 Hz Absorption
Click on image to see enlargement.
Michael Billingsley specifies that the SASS's baffle should attenuate frequencies between the sides above 1000 Hz by at least 9 dB. I tested the absorption rates of different thicknesses of high density foam to see how much foam is needed to satisfy this and found that a thickness of 4" -5" should be sufficient. I used 1.5 pound per cubic foot dark gray high-density acoustic foam for this test.
The test also confirms that foam even 14" thick does not appreciably attenuate frequencies below 1000Hz. It remains likely that the off-side passage of sound waves between 500-1000Hz through the baffle are playing a role in the spatial imaging of this array. Wavelengths under 500 Hz are shown to be passing around the foam unaffected as expected.