Saturday, August 21, 2010

Vicki Powys Light-Weight Closed-Cell Foam Mic Arrays

Vicki Powys, of New South Wales, Australia, has begun testing some experimental boundary mic arrays she constructs from foam. One is a SASS-shaped construction made of "Yoga" EVA foam blocks (ethylene-vinyl acetate). This material is described as having "barrier properties." Thee other rig is less dense, slightly dimpled, closed-cell foam that is used for making life preservers that she paints with acrylic paint.

For several years, she's has been using an expensive and fairly heavy SASS Model B unit modified by Walt Knapp with flush-mounted Sennheiser MKH-20 mics in a Crown SASS MKII-P body. Her goal is to make a lighter-weight, comparable quality array that is easier to hike with.  In this test, she has incorporated small (10mm diameter) microphones made by PIPmics with a self-noise of 14 dB(A)--about 4-6 dB higher than the Sennheiser 20's.

From the test materials there are three QuickTime comparison movies and links to photos of the arrays. She also sent some photos of her DIY Lycra wind screens for her Wedge and SASS arrays which could be of general interest to recordists.

A.  Blind comparison QuickTime movie (4mb download .zip)  to assess the performance of the two DIY arrays in relation to her Crown SASS Model B:

    Powys Adjustable Wedge Closed-Cell Foam Array (constructionfurry windscreens)

    Powys SASS-shaped EVA Foam Array (construction)

    SASS Model B - Knapp Mod with MKH-20 omni mics.  DIfusse EQ and HP filter OFF.

The self-noise of the PIPmic "Mini" mics -> LS10 combination in the DIY arrays proved to be fairly audible in comparison to the MKH-20->SD702 combination [download 4mb comparison QuickTime movie .zip].  The Olympus LS-10 recorder [measured EIN -121 dBu (A)] has noise that extends lower into the spectrum and could be adding a touch of input noise. In any case, I feel that its likely the PIPmics will create a quality hit when recording in lower sound level settings. Its unfortunate, but not surprising.

To aid in the evaluation of the stereo imaging performance of the arrays, I used parametric "notch" EQ to lessen some the of the "hiss" from the PIPmics between 4-7Khz in the test movie samples.

The samples in the movie graphic are unlabeled. The dark samples are the SASS Modle B reference.  Of the green and red samples representing the two DIY arrays, which sounds like a Wedge array and which sounds like a SASS-like array?  Do these DIY arrays have potential? 

B.  The outcome of the first comparison surprised me as the imaging of the DIY Wedge array (bottom, red regions) with its sizeable, 3-1/4" setback compared very favorably with the SASS B mod.  I would have preferred  more spaciousness in the imagery as a whole, but Vicki commented that a running stream nearby or distant coal trains might be masking some lower mid-range acoustical cues.  We checked to make sure her MKH-20 capsules were mounted flush to the boundary and they were. The tonal character of the DIY rigs was noticeably brighter to good effect and we noted that the "Diffuse EQ" option for high Hz emphasis on the MKH-20 mics had been turned OFF.  [She has done some preliminary tests with "Diffuse EQ" turned on after this test and finds the increased HF response beneficial.]

I compared her SASS-B's recording with some I made with my PBB2 and like her DIY arrays, it had more response above 3K Hz. Wondering if Vicki's SASS Mod B array was performing up to snuff, I made movie comparison of her recordings to a recording made by Andrew Skeoch with another SASS Model B Knapp Mod and Vicki's DIY Wedge. Note that the "Diffuse EQ" was OFF on Andrew's array as well.

QuickTime Movie (12mb download .zip)

Obviously, Andrew's recording was made in a different location but it seems to have more HF response than Vicki's SASS Model B array given these samples. If the two rigs truly differ, I'm not sure why. Vicki reports that she'll continue to use the "Diffuse EQ" setting on the reference SASS array for her tests in any case.

The DIY Adjustable Closed-Cell Foam Wedge array holds-up quite well stereo image-wise. The array seems to benefit from more amplitude in the center of the stereo field compared to her DIY EVA Foam SASS array. When recording very distance sounds in open spaces such as these, more amplitude in the center of the stereo field can increase the sense of depth. This effect can also be realized to some extent in post using M-S processing so it might be more instructive to conduct careful localization performance testing on a few DIY array options before assuming the Wedge is superior. I know she has conducted tests with the wedge at set at a 70 degree angle. Maybe I can convince her to post the results of this test on the blog.

It doesn't seem like the multiple bumps that can be seen the painted foam boundaries of the wedge (inherited from the life preservers) would help the boundary effect but how much negative impact they have is unknown. One is tempted to assume, not much. 

The effectiveness of the EVA material on the boundary effect remains unknown, in part because of the question about the HF response of the reference mic and in part because the performance of this array was less impressive in this test. It could be that the paint coating is important. 

To determine if these materials work as well or better than traditional, hard smooth surfaces, one would have to make up three arrays with the exact same dimensions out of (1) EVA "Yoga" block foam, un-coated, (2) Closed-Cell foam, painted (3) Wood, fine sanded and sealed. It might require a careful localization test done outside at 100'  to produce definitive differences. In the past, I found that high density foam (probably EVA) produced a different lift compared to wood, but this test hasn't been repeated.  The "hardness" requirement may not mean that the boundary surface be perfectly smooth because as Vicki points out, the boundaries on the Crown SASS array have a dimpled surface that is not too dissimilar to the texture of her painted, close-cell foam. Its tempting to forge ahead and assume that there is no significant change in at least using the painted foam, but one could be wrong.

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