Tuesday, August 24, 2010

Vicki Powys Foam SASS & Wedge Array Tests- Part 2


Download QuickTime Movie [.zip ]
For this second sets of test with the Foam Wedge mic set at 70 degrees, I selected segments with bird motion nearer to the arrays with more distant calling to better study stereo imaging performance. I assumed that the Wedge70 would have poorer imaging but I'm not so sure. What do you think?

[Test conditions: SD702 record level indication @ "60" dB; Olympus LS-10 at Low Setting "10", MKH-20's Diffuse Field EQ ON which produces noticeable high frequency "graininess"]

The next test features similar material comparing my Foam SASS and my SASS Model B with MKH-20's.

Download QuickTime Movie [.zip ]

There is certainly more noise with the mini PIP mics, but to my ears the sound is more open and natural. I would like to measure the Wedge70 directly against the Foam SASS but I only have one LS-10 recorder. Does anyone know how I could use the minPIP mics with my SD702?  [Rob D note: For using PIPmics on your SD702, Mike Wall used a battery box very similar to Richard's schematic. Mike Rooke suggests these resistor values: 8K resistor for normal sensitivity; 4.7K for higher sensitivity and 10K for lower sensitivity. Pin 3 on the XLR input should be left unused. There's more about the circuit on Richard's site.  Mike Rooke might know of other phantom powering options he can build for you.]




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Rob D. Comments:
First the question Vicki posed, Which of the DIY arrays has better stereo imaging?

(A) The SASS-B mod with the MKH-20's produces more "body" or emphasis in the lower mid range from approx 200-600Hz. This gives the SASS-B array a fuller tone and more for the brain to deduce distance from. It also has a noise advantage even when boosted to match the response of the PIPmics.

As for the runner-up:

(B) The DIY Foam Wedge array seems to be bunching the sounds together more in the center of the field compared to the DIY Foam SASS.  Vicki used deeper setbacks (5') and changed the WEDGE angle in this test.

(C)  The DIY Foam SASS seems to be producing slightly less dynamic range than the SASS B. The louder bird calls, though brighter, seem to have less separation from the background sounds which results in a little less audible clarity. I'm not sure where this could stem from, but I have a hunch that a coat of acrylic paint on this foam might help improve the boundary effect.

(D) I thought the DIY arrays were imaging pretty similarly to the SASS=B until I compared some presence clips between the Wedge and the DIY SASS arrays [ Download QT movie  3mb .zip ]. There are no simultaneous moments to compare, but its pretty audible to me that the SASS possesses some lower mid-range "magic" that contributes dynamic "airyness" that the Wedge lacks.  Another reason to make sure that the foam boundaries are  truly not detracting from the SASS DIY array's performance? Wood is not that much heavier and its easier to assure a seal around the capsule where it protrudes from the boundary.


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Another looming question posed by these comparison tests for me is why is the tonal balance for the small 10mm diameter PIPmics so much brighter than the larger MKH-20's capsules?  This is true for both foam boundaries.  I used parametric EQ on the SASS B-MKH20 samples to approximate the frequency response of the PIPmics in the DIY Foam boundaries: 

Download QuickTime SASS Arrays Matched with EQ movie [4mb .zip

Download QuickTime SASS vs Wedge Arrays Matched with EQ movie  [5mb .zip

The PIPmics have significantly more mid-range to high frequency respons when mounted in these foam boundaries. The most pronounced addition being a +9dB peak centered around 5KHz that extends all the way down to 1K Hz with some additional lift around 400 Hz-1K Hz. There might be a little too much HF emphasis in the PIPmics on the small boundaries but it does adds clarity to sounds in the distance as Vicki observes.

As to what can be causing this "lift" in the mid-range and high frequencies, Paul Jacobson, has suggested that the softer nature of the foam boundaries might result in less of the HF roll-off associated with boundaries and flush-mounted mics. Paul cited formulas in the Long & Wickersham  patent behind PZM technology that one can look at .pdf  Paul's is the best explanation I have though I'm wondering how reducing amount of HF roll=off would lead to a lift this dramatic that extends under 1 KHz.  Thanks Paul for this interesting lead to pursue!

If the softer boundaries have this effect on Hz response,one might be able to apply this variable beneficially-- such as making the lift a little less pronounced and more,.. This I propose a test with the below test samples to provide some evidence. I've included the larger diaphragm MKH-20 mics to eliminate the possibility of mic capsule size playing an unsuspected role.

(1) Pink Noise on boom box at 10' outdoors at 3:00 o'clock -> MKH20's in SASS-B  -> SD702 (max gain)

(2) Pink Noise on boom box at 10' outdoors at 1:00 o'clock -> MKH20's in SASS-B  -> SD702 (max gain)

(3) Pink Noise on boom box at 10' outdoors at 3:00 o'clock -> MKH20's in free air 13" apart facing front (removed from array)  -> SD702 (max gain)

(4) Pink Noise on boom box at 10' outdoors at 1:00 o'clock -> MKH20's in free air 13" apart facing front (removed from array)  -> SD702 (max gain)

(5) Pink Noise on boom box at 10' outdoors at 3:00 o'clock -> PIPmics in Foam SASS -> Battery Box Powering ->  SD702  (max gain)

(6) Pink Noise on boom box at 10' outdoors at 1:00 o'clock -> PIPmics in Foam SASS -> Battery Box Powering ->  SD702  (max gain)

(7) Pink Noise on boom box at 10' outdoors at 3:00 o'clock -> PIPmics in free air 13" apart facing front -> Battery Box Powering->  SD702  (max gain)

(8) Pink Noise on boom box at 10' outdoors at 1:00 o'clock -> PIPmics in free air 13" apart facing front -> Battery Box Powering ->  SD702  (max gain)

(9) Pink Noise on boom box at 10' outdoors at 3:00 o'clock -> PIPmics  flush-mounted in 3-5/8" X 4-7/8" hard finished wood boundary with 1" setback -> Battery Box Powering ->  SD702  (max gain)

(10)  Pink Noise on boom box at 10' outdoors at 1:00 o'clock -> PIPmics  flush-mounted in 3-5/8" X 4-7/8" hard finished wood boundary 1" setback -> Battery Box Powering ->  SD702  (max gain)

Avoid use of  wind screening materials for this test. 





2 comments:

pj said...

Hi Rob,

The effect of the foam is most likely a reduced boundary effect due to absorption of some sound waves. This reduction in gain will be frequency dependent, and the size of the reduction will be related to the properties of the material used.

Boundary gain would reduced to zero if the foam barrier was completely sound absorbent, which might be the case at some frequencies.

The foam won't prevent HF roll-off as such, rather it will reduce the amount of gain from the boundary effect in the 1-4.5khz range, so the level difference between mid-range and hf is smaller than if the barrier was completely reflective.

Another consideration is that the smaller capsule size of the MiniPIP mic is significantly less directional above 6khz than a MKH20 or AT3032 when mounted flush with a boundary. It's likely the minipip is picking up more hf due to it's more omni-directional polar pattern.

cheers
Paul

Rob D. said...

I just played with several types of closed-cell foam and there is considerable variety in the Hz impacts by ear. A lift of +9dB peak centered around 5KH seems like a lot for the foam to contribute, though. I thought I might be detecting a sound difference in the metal vs hard finished wood boundaries in my last test. More variables to test.