Monday, August 23, 2010

Affects of PZM, Flush-Mounted & Boundary-Facing Orientation on Tonalty & Spatial Imaging

Download QuickTime Comparison Movie 19mb .zip

I would like to thank Paul Jacobson, MIchael Billingsley and Bruce Bartlett for their invaluable input on this test and probably several more to follow. Thanks also to Rich Peet for the kind loan of his SASS-P microphone.

The test is an initial study of tonality, localization and depth imaging properties that accompany three ways of mounting mic capsules within the pressure zones of the small boundaries of a SASS-like enclosure:

A. Flush-mounting of medium size omnidirectional microphones (21mm AT3032's) similar to the SASS Model mods made by Walt Knapp and my PBB2.

B. "PZM" mounting with small diameter omni capsules facing the boundary (as in the SASS MKII -P)

C. Boundary-facing positioning similar to PZM-mounting except with medium size omnidirectional microphones (21mm AT3032's) (I refer to the orientation as "downward-facing" in the current movie graphic).

The above mounting variables are configured in arrays that are very similar in size and construction. All arrays have 3-5/8" wide X 4-7/8" high boundaries. The capsules in arrays A & C have a 1" setbacks from center of mic capsules to the leading edges.  The SASS-B has a .8" setbacks to the center of its capsules. The capsule positioning of A & C are offset vertically close to 1/3 proportioning. Both of the SASS-P's capsules are  2" down from the top boundary edges, The PBB2 array has hard finished wood boundaries. C has hard smooth metal boundaries and the SASS-P has slightly dimpled, hard plastic boundaries. The void behind the foam baffle in A & C is filled with deadening material (rubber-coated carpet underlayment). The void behind the SASS-P is in the original state manufactured by Crown. The foam baffles for A & C are made of high density dark gray acoustic foam (1.8 lb./sq.Ft). 

Rather than wait a month or more for this Summer's robust insect crop to wane and conduct the test in my usual outdoor site, I decided to try this test in my rectangular pole barn. In addition to pink noise, I used two audio clips, "Low Hz" to address response <1000 Hz and time of arrival imaging and "Mid Hz"  for >1000Hz to correspond with amplitude difference imaging. The pink noise results are presented in another movie (4 mb .zip download)

Download QuickTime Comparison Movie of Pink Noise Samples ( 4 mb .zip )

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Rob D. Follow-up & Observations August 30, 2010.

The pink noise test sonogram differences are interesting to look at for pronounced differences such as the HF drop-off for on-axis sounds with the boundary-facing configuration (also referred to as "down-firing"). Also, the overall brighter response of the SASS-P in conjunction with less bass. I think one asset of the SASS arrays that could distinguish them from many other (or all?) arrays is their depth perspective and the "room" character that one can sense in the nature of the reflections.  Here are some of my initial observations along these lines and then a follow-up test.

(A) The SASS-P is too bright but its sense of depth and "room" character is quite striking compared to the other arrays. (I suspected this would become evident and its why I included the boundary facing larger mic with low self noise to see if there was a similarity). Small diaphragm mics supposedly work much better in PZM-like applications. This creates a bind as they best of the small capsule mics have ~6 dB(A) more self noise than the mics I'm used to for recording natural ambience.  A good application of the small mics in PZM mounting might be my PBB3 cap plan but I might want to re-think the boundary sizes as the extra brightness might become excessive .(I wonder if the small capsule size itself is somehow contributing to the HF boost Vicki is getting with her small capsule flush-mounted boundary experiments.)

(B) Bruce Bartlett predicted some excessive resonance with a larger diaphragm mic facing the boundary in PZM like fashion, and this test seems to confirm this phenomenon Though the impact of the depth imaging has some pleasant, unusual qualities, there's a dramatic peaking in the mid-range that creates dissonance.  The 4022's have stock "free-air" grilles in place and there's a chance this could change a bit if the mics had none, but the lower frequency location suggest to me that the resonance problems are associated with the larger capsule size.

(C) The Free Air mics have very different tonal character and they seem to exaggerate the resonances in the large barn space a lot more than the other capsule mounting configurations. Some people like this quality but from me they mask other complexities, nuance and overall clarity of the space and contained sounds.  The forward facing or  "A/B" style array with  13" separation does a fairly good job of localization but the depth is much flatter. For example, the 1 o'clock position feels closer,, more in the same plane as the 2:30 sample compared to the other arrays.

(D) The PBB2 seems to be a compromise, and a pretty good one. It has some of the faster depth drop-off of the SASS-P, more room character than the Free-Air and is tonal balance is much warmer and more pleasant than the SASS-P.  I just wish is had more "room" character of the SASS-P.

Next, I equalized the low (mostly <1KHz) and mid (mostly >iKhz) clock position samples (2:30, 1:30 & 1:00) with parametric EQ like I might If was trying to modestly bring-out spatial clarity. 

Download QuickTime Comparison movie 14mb [ .zip ]

I'll give folks a chance to listen to this test while I work on my observations.

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