Friday, October 15, 2010

PBB2N vs Jecklin Disk Localization & Depth

Download QuickTime Comparison Movie [Full Res 34 mb .zip ; Compressed 256 kbs 9mb .zip ]
Localization test at 35 ft with 12 to 6 o'clock pan at the whole & half hour positions.

 This is a continuation of an exploration documented in this blog to improve the treble response in the center of the field for the PBB2 array without sacrificing the spatial nuance this array can provide. This far field. outdoor, side by side, test included a Spaced Boundary array (far right) with small boundaries, separated 14-3/4" and angled in 30 degrees to provide additional center of the stereo field coverage which demonstrates poor high frequency response probably because of the close proximity of the foam to the mic capsules in conjunction with a very small lateral boundary width.  The test provides a good opportunity to evaluate a Jecklin Disk, on location, beside the PBB2N.

The rigs were placed as physically close as I could get them without interfering with each other 6' above the ground in a wooded area. The mics are positioned 2 feet below the sound- treated cover I use to protect mics from moisture. Al arrays had single layer natural burlap wind screens. The Jecklin and Spaced Boundary arrays were recorded with with a 744T recorder near full gain and the PBB2N was recorded via a Symetrix SX202 Mic Pre/RME audio interface.

I have had quite a few opportunities to compare recordings made in the same location with a Jecklin Disk and PBB2 as I have been using the same pairs on my portable surround rig since last Spring. Following are my impressions about the strength and weaknesses of the PBB2N and Jecklin arrays when I evaluated them in the sweet spot formed by two Mackiie HR824 MKII speakers separated 60 degrees in a treated studio.

In whole, both the Jecklin Disk and PBB2N arrays create a stereo images with excellent lateral width in the stereo image and good depth imaging. Here are some distinctions I noticed when considering diffuse field applications:

Advantages of the Jecklin Disk
+ More Bass response under 125 Hz
+ More High Frequency response over 7K Hz adding "edge" and making objects feel closer
+ Better localization positioning at 12:30
+ Lowest octaves tend to separate producing less bass in center (can be adjusted in post)
+ Lowest octaves can be equalized to sound dynamic and spacious

Advantages of the SASS-P dimensioned PBB2N
+ Greater clarity of distant sounds-- especially those with 500-2200 Hz frequency content
+ Greater sound reflection detail leading to more subtlety in depth imaging
+ Better front to rear imaging distinction especially 4:30-6 o'clock (tone shifts, & more echo)
+ Bass centered and easier to equalize & balance.
+ More overall detail generated?  The PBB2N creates a .flac file that about 15% larger than the Jecklin.

Spaced Boundary Array
- The HF loss is too significant with the narrow boundary widths directed outside and proximate foam to merit use as-is. There does seem to be more bass-possibly from the narrow boundaries and/or wider separation. I can sense that both mics are covering a good part of the center of the field, perhaps through slight phase differences?  The resulting stereo imagery feels different, maybe a bit more confused, but when I just regard localization performance, there is better placement of 12:30  than with the other arrays. I suspect that 15-20 degrees of  inward angling might provide better overall balance between middle and sides.

Localization Tone & Depth Performance

click image to enlarge
Download Localization Test QuickTime Movie [ songram display 6 mb  .zip ;  clock position 6 mb .zip ]

I played the samples from one speaker of a portable CD player on the right side of the arrays at the positions marked with the crows in the graphic. I was not able to analyze the polar patterns of the arrays using pink noise as I hoped because of an obstruction in one clock position and a reflector in another.  The high frequencies in the pink noise sample were also not picked-up very well by the arrays at 35 feet even though the HF was boosted. (The speaker may not be reproducing the highest octave of HF; I'll have to look into that for future testing).

The crow sample does have enough frequency content above 6 K Hz to hear some of the differences I've ascribed to the arrays. Here's another localization movie in which the Jecklin and PBB2N samples from the same clock positions can be heard back to back:

Download QuickTime Comparison Movie [3mb .zip ]

As was heard in the Race Cars comparison an on this blog, flush to boundary mounting of large diaphragm mics results in a loss of high frequency response  that falls in the center of a SASS-P/PBB2N's' stereo field. This is very audible in the crow sample at 12 o'clock where the greater HF of Jecklin array adds so much more "edge" that the crow actually seems closer. Paul Jacobson has been able to use Vicki Powys' test to measure and display this.  I have added some obnoxious graphics  to Paul's pristine polar chart make the effect clearer:

Click on image to enlarge

The mics in the Jeckjlin array are mounted in free air so the comparisons are analogous. 

At the 1 o'clock and 2 o'clock positions, the difference in Hz response is a little less audible. By the 3 o'clock position, the difference in depth imaging seems to have the largest impact and this effect continues through to 6 o'clock. The PBB2N's image feels as present as the Jecklin's at the 1 and 2 o'clock positions. This is the range on Paul's plot where the boundary lifts response between 1K and 4K Hz. 

I do not know how much narrowing the nose of the foam baffle improved HF response but I'm pretty sure it didn't hurt it. There remains some question as to whether close capsule proximity to the baffle foam effectively lowers HF response.  It certainly sounds like it does when I place a piece of foam parallel to a HF sound source within an inch or so of the sound path. To test this I'm including more capsule-baffle separation (setback) in my next rig.

With this Center- HF loss liability of the PBB2N in mind, I found that it did help to add sparkle to the middle of stereo field with a 4-6 dB lift starting at 6 K Hz.  The equalized clips in the main video comparison are the result of trying to compensate for the weaknesses of both arrays.

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