Thursday, July 22, 2010

Partially Baffled Arrays - Small vs. Medium Size Boundaries

The tree that was holding-up my outdoor S1 array collapsed on it last month and I decided to replace the ad hoc "Perp2Sphere" array with a Partially Baffled Boundary array. There are two qualities in my PBB2 array I'd like to improve upon if I can: (1) More deep bass < 125Hz and (2) A little less "sharpness" in the frequency response between 3-4KHz. A logical attribute to experiment with is the size of the boundaries. 

The boundary area of my new rig, PBMB1 (Partially Baffled Medium Boundary ver 01), is about 9 times as large as the PBB2's (10.25" high X 12" wide vs. 4.75" x 3.6"). The middle section where the baffle is located is identical-- its just twice the height. The PBB2 has AT3032 mics and the medium boundary array has AT4022 mics. The AT4022 mics have more bass response on their own accord so some of the increased bass response will be from the mics themselves and some from the medium sized boundary. To get some sense of the former, see Paul D's 3.4 mb QuickTime movie comparison you can download and assess with QuickTime player.

Note: Paul Jacobson astutely detected that I had a 80 Hz -18dB "roll-off" filter enabled on my 744T recorder thus affecting the small boundary array recording. Even with this error, I felt the test was still useful so I added matching roll-off filtering to the PMMB1's recording. As a result, comparing the arrays in terms of bass performance will have to be resumed in another test.

Method: I set-up and oriented both arrays as close as I could get them in space about 6' off the ground. Both arrays received 60dB of gain which is typical for recording ambience in quiet locations. The PBB2 was connected directly to a SD744T and the PMBM1 is connected to a Symetrix SX 202 external pre and Hammerfall RME interface A-D.

Here's the comparison of the mic arrays as a 30 mb QuickTime Movie to download and play with QuickTime. You can also open the movie in any Quicktime compatible sound editing app which will place the 48K/16bit soundtrack in waveform view. The PBMB1 sample always comes before the PBB2 sample. 
= = = = = = = = = = =
Assessment: There is less "sharpness" in the recording made with the medium size boundary (compare the close sparrow calls at 27 secs and 35 secs)-- so that is good news. Another mentionable plus is animals with lower pitched voices like the distant trees frogs (2:21) and distant coyotes (0:42) receive improved presence or separation from background sounds when recorded to the side with the medium boundary array. Unfortunately, these improvements come with an overall degradation of stereo imaging. The smaller boundary array has better depth imaging, better phantom-imaging between the speakers and better reproduction of sound reflections.  If I can understand how these traits are produced in the small boundary array, I might be able to adapt them to the special circumstances of capturing distant subjects in quiet settings.

The bass content in the medium boundary samples tends to emanate directly from the speakers more so than from the middle of the stereo field as it does with the small boundary array. This creates more horizontal "spread" in the medium boundary's stereo image. One can hear this quite readily in the first section with the distant motorcycle where the location of the motorcycle jumps from hard right to just left of the right speaker in the stereo field.  All small and medium sized boundary arrays suffer poorer very low frequency response compared to omni mics in free air.  According to Crown's boundary mics application guide (pdf p13), this effect is less pronounced for sound waves that are parallel to or "rake" the plane of the boundaries.  Curiously, looking at a map of the setting,  the sound waves from the motorcycle do seem to be arriving on the ~4 o'clock "raking" angle.  An increase in volume is similarly confirmed by louder impulses coming from the 3-4 o'clock range in the localization tests for the medium size boundary.  The Crown literature explains that the bass "lift" from a 12" boundary becomes effective at about 188 Hz (188/D where D= 1 Foot). The lift from the smaller, 3.6" boundary becomes effective considerably higher at 564 Hz.  These different lifts can be seen in a sonogram of the first pair of motorcycle samples and also confirmed when equalization is used on the PBMB1 recording to make it sound, tonally, more like the PBB2.

Its my opinion that small boundary array exhibits less muddiness in the 200-500 Hz range than the medium boundary, and that its higher pitched lift accentuates ~800 Hz sound reflections from a bluff at 2 o'clock thus making this additional spatial detail audible.  A hill blocking direct sound passage along the raking angle probably contributes to the muddiness of the medium boundary array, but even if these sounds waves were uninterrupted, the stereo image would still have exaggerated width. This explanation seems consistent throughout the samples where there is more volume and reverberation nuance in the center of the field with the small boundary and more bass emphasis on the sides with the medium boundary.

I was tempted before I made this new array to only extend the width-- not the height of the boundaries.  That may have been good intuition. A boundary width of around 9-10" should create emphasis around 1300 -1500 Hz compared to the 3.8 K Hz fundamental added by the 3.5" dimension in the small boundary array. This compromise might reduce sharpness and increase warmth with less muddiness. An array with lateral boundaries that start at 4-1/4" with hinged  extensions to about 10 inches would allow me to test the variable more easily. If 10" is too wide, I can trim down the extensions a little at a time.


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