Sunday, September 12, 2010

Comparison of SASS-P and PBB2 Localization & Depth - Race Cars

Download folder with QuickTime Comparison Movie & Select Pairs [ 38mb .zip ]

I wanted to study some outdoor recordings made with Rich's stock Crown SASS-P with ones from my PBB2 array (a DIY version of the SASS Model B but using flush-mounted 21 mm AT4022 omni mics).  A trip to a race track to see my brother race provided me an opportunity to do some simultaneous, side by side recording. The dynamic and robust outdoor subject does not handicap the SASS-P for its lower self-noise performance.

The differences I can detect are striking and potentially instructive. The SASS-P produces higher levels of brighter toned sounds in the center of the stereo field. Take a listen to the separate pairs in the folder.  The proportionally higher volume in the center of the field  enhances the impression of depth though it comes with some loss in width and lateral localization performance. In Pair_1, the car moves deeper, more vividly into the center of the field in the first SASS-P sample, yet the car moves off further off to the right at the end of the of PBB2's rendition. In  Pairs 2 & 6, the middle of the SASS-P's stereo field feels substantially more populated and dynamic. In Pair 7 with the cars off to the right, the depth imaging of PBB2 seems wider and the  reflections picked up on left side feel more vibrant and airy. 

I personally feel that the tonal balance of the SASS-P borders on being too bright, but its depth imagery in the center of the stereo field is considerably more engaging. The PBB2 sounds too dark in comparison. Paul Jacob's interest in getting brighter response from flush-mounted AT4022's could be on target. See more discussion of this below.

In order to learn more about specific, contributing qualities, I tried to equalize the PBB2's recording to sound more like that made by the SASS-P.  One octave wide +4dB boosts centered at 4K Hz and 8K Hz helped, but the slurred, superimposed mid-range pitches of the cars created numerous, different peaks between 500-2K Hz in both arrays. Boosting the level of the center of the PBB2's Stereo field using M-S EQ helped the most but the number and varied distribution of peaks over 3K Hz made it clear that the two array have many complex differences. In the end, I opted to equalize the two recordings separately to hear how the EQ'd versions would sound next to each other. It seems that the under-expressed center of the PBB2 was easier to correct than the narrowness of the SASS-P.  Both arrays produce spatial "airy-ness" in the lower registers, but the PBB2, post-equalization, has a bit more of this "magic" to my ears. Taste plays a big role at this point.  Neither array produced results with this loud subject that were that were impressive as-is.  I also noticed that the SASS-P's lower mid range had more "body" to work with and smoother bass response. This was surprising to discover given the overall warmer sound of the PBB2.

As Paul Jacboson pointed out in relation to some confirming measurements he made on his SASS B-like rig, "Billingsley and Bartlett...noted that the SASS was designed to work with flat pressure response mics, rather than flat free-field response."  The SASS-P's mics are are flat pressure response type and the AT4022's are made to have flat free-field response in free air, not flat response when mounted in a boundary.  If this phenomenon is responsible, why is the SASS's better HF response mostly evident in the center of the stereo field? (I thought that flat pressure response mics had reduced HF response at 0 degrees). I need to dive into the Lipshitz and Vanderkooy AES paper again with these new insights.

The race car test is consistent with Vicki's recent localization tests and those I did with PBMB1, that the HF lift for the flush mounted AT 4022 rigs is from the sides around 3 - 4:30-- including the Knapp/Elliot MKH-20 SASS-P body mod.  This correlates directly with the boundary "grazing effect" which could be more influential with the flush-mounted AT 4022 rigs than we have grasped up to now.

Assuming the goal is to get more HF response in the center of the stereo field using low noise mics, here are some possible variables to explore: 

(1) Make AT40022 mics into flat pressure response type mics and mount them in a PBB2-like body. Paul says we can buy the AT4022 capsules separately. 

(2)  Take the above flat pressure response modded 4022's and mount them PZM-style closer to the boundary than I was able to do with the stock 4022's in the Barn test. 

(3) Vicki's recent localization comparisons remind us that "Wedge" arrays have always exhibited stronger center of the field imaging. Her foam wedges have 5" setbacks and the increased grazing effect towards the front could be a bit too pronounced [see still image from the test showing waveforms with 12 o'clock impulses matched .gif  and movie  .zipIts seems logical to make an array that retains head-spacing and extends the leading edges to create effective 1.5" to 2" setbacks. The drawback with the wedge arrays has been the disappearance of the lower mid range "airyness."  Maybe we can discover how the PBB2 body is creating this airy-ness while improving center gain and center HF response.  Using MS-EQ to boost the center of the field is always a post option, but getting more from the start would be great. Vicki has proposed making her Foam SASS Lite's baffle smaller but having a less boundary "nose" with baffle modification might move this intuition forward. I might try head-spaced capsules with a 1.5" to 2" setback in boundaries that are 4- 7/8" wide and  2.5" high to get more HF lift.

(4) There's also a chance that the painted close-cell foam boundaries that Vicki has been experimenting with have impact on the greater HF response she's getting (with 10mm capsules). Paul J has suggested it might have more to do with the capsule size, but its probably time to do an exhaustive test of boundary materials on flush mounted AT4022 mics.  We might choose to add thin, metal plates after the results are in. The test I did of this variable before was with Perp2boundary with large diaphragm mics.

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