DIY Stereo Boundary Mics

Olson "Wing" Array Comparison with Danielson PBMB2

2011-10-08T20:54:00.000-07:00

Download Blind Comparison Movie [13 mb .ZIP] -Use QuickTime 7 Player with Progress Bar
[Movie has 256Kbs ACC* soundtrack. Uncompressed soundtrack is here: .AIFF 105 mb]

The black rig at the bottom of the photo is Curt Olson's "Wing" array constructed from a 8.5" length of 2X6 standard lumber (1.5" X ~5.5") with 1.5" wide wings. AT3032 mics are setback 1-7/8" from the front edges. The array was titled downward about 20 degrees. At the top of the photo is Rob Danielson's PBMB2 array larger SASS-like array; specs are available here. The PBMB2 is wrapped with fiberglass furnace filter media.

Olson's array was recorded with 59.8 dB of gain on a SD 744T and Danielson's used a 60 B of gain on Symmetrix SX-202 Mic Preamp into a Focusrite Saffire Pro 24 audio interface. Both arrays used AT3032 mics. The flush-to-boundary mounting of the mics in the PBMB2 array seemed to contribute more gain than the double perpendicular-to-boundaries arrangement of the "wing" array, but the exact amount could not be determined.

The systems recorded simultaneously through one night at a rural location in Southwest Wisconsin and includes Barred owls, an Eastern Screech Owl, a Coyote and a Dawn Cardinal on October 6-7, 2011. Other surround recordings from this location may be found here.

*Note: The ACC compression in the movie soundtrack affected the tonal balance of the recordings-- particularly the impact of the insects. Download the original .aiff for a more accurate evaluation. Equalization and stereo image adjustments were based on speaker monitoring in a setting with minimal background interference.

Comments and observations are welcome below or on the Boundary Mic List (subscribe)

Danielson PBMB2 Medium Boundary Array - Lift Centered Circa 1400 Hz

2011-02-07T08:52:00.000-08:00

This PBMB2 above is where my search for an array with increased "reach" for recording distant sounds in expansive quiet settings has settled for a while. In Fall 2010, I made 6 arrays with different size and shaped boundaries. After I experimented with different boundary angles, baffle sizes and set-back distances and other variables, this array exhibited the best "reach" performance in diffuse field (distant micing) applications. I'm inclined to think the boundary effect "lift" centered around 1400 Hz and other traits we've learned to associate with SASS-like constructions are responsible. In examining recordings that I and others have made with a variety of array type using omni mics, I began to notice that a lift or boost in this in the range was often beneficial largely by making the upper harmonnics of local sound reflections more audible.

The stereo imaging is very good, as-is, and it responds very well to tonal adjustments using M-S processing.

I have not gotten around to testing for a subtle "double-boundary" effect with these SASS-like arrays to incorporate the mild boundary effect that Paul Jacobson and I detected using high density foam and the corner placed capsules. The roles of setback distance and other baffle-related variables could very well lead to further refinements.

Here is a more detail plan for the array:

Click on image to enlarge it. Right-click to download

John Hartog's "Wooden Bowl" Array with Curved Boundaries

2010-12-05T22:58:00.000-08:00

With the possibility of making front to rear transitioning smoother, John started with a large, wooden bowl cut into pieces [download construction video 1 mb .zip]. Using high density close-cell foam for substructure, he assembled the pieces into an array with proportions that echo a Crown SASS-P body. There are several photos of the finished array on his website.

Click on graphic to enlarge

John's idea was that curved boundaries might "wrap" the pressure effect evenly around the array creating a more even gradient in the polar pattern of each boundary-mounted mic. Mic orientation after construction was more forward-facing at 45 degrees compared to 70 degrees for the Crown SASS. He design even includes s partially-covered baffle of high-density open-cell foam quite similar to the SASS P's except its nose is narrower.

He took the array to a large open field in the Mill Creek Wilderness near Prineville, Oregon and conducted two localization tests, compared frequency response with the same mics in free air and produced an example recording at the site for us to evaluate.

First, I looked at the free air comparison to evaluate the tonal nature of the lift created by the curved boundaries:

Click on movie still to enlarge

Download QuickTime Comparison Movie [ 13mb .zip ]

I used excerpts from John's 30 foot localization test of air "presence" with embedded footsteps a the same spots. The compared sounds are very similar but not identical. Pink noise would be more conclusive, but we have been able to detect a frequency response shift from boundary lifts using airy location presence in previous tests.

To the best of my ability, I cannot hear a boundary effect "lift" in the wooden bowl sample. The audible change between the samples appears to be increased left-right separation from the addition of the baffle between the mics. Pressure layers should be formed with the curved, hard boundaries and the flush-mounted mics should be affected; I can only guess that the build-up is quite small compared to flat boundaries. The reason would be interesting to explore, but Its sufficient for our purposes to observe that there is negligible frequency change produced under 2000 Hz by the curved boundary arrangement in the wooden bowl array. The sonogram made from calibrated samples seems to support this conclusion as well.

Next, I looked at John's localization test made for his new array at distance of 30 feet:

Download QuickTime Movie [ 25 mb .zip ]

Monitoring with speakers in my studio, clicks at 11, 12 and 1 imaged in "phantom" positions between my left and right speakers and clicks 2, 3 and 4 and 8, 9 and 10 seem to be stationary at the speaker positions. There is a distinguishable drop in level from 3 to 4 and an increase in level from 8 to 9. Positions 5, 6 & 7 image between the speakers and with decreasing tonal brightness making it easier to sense that these positions are to the rear. Taken as a whole, front to rear imaging is distinct but the goal to image many evenly spaced positions from front to rear was less successful.

The recording John provided made of a distant Coyote chorus and a Great Horned Owl is excellent for considering diffuse field performance [recording in entirety .mp3 ].

Download QuickTime Comparison Movie [ 13mb .zip ]

I went through my usual experiments with equalization and stereo field balance. There's more noise in the recording than I'm accustomed to with AT 3032 mics. After quite a bit of very narrow notch filtering, the upper harmonics in the coyote calls were coming through with good clarity. I suppose we could come up with an explanation for the additional noise if we went through all of the factors. Perhaps I'm getting used to the noise performance advantage of arrays with larger flat boundaries and the 3032/4022 AT mics.

I've found that increasing the playback level of the mid range often improves clarity and depth imaging in diffuse field recordings. With John's recording, I created a smooth, +6 dB curve with Firium centered about 1200 Hz and tapering gently to about 4.5K Hz. Using the left channel of parametric EQ inserted into an M-S chain, I attenuated some of the lower mid range rumble in the center of the stereo field (see yellow dips from 50-250 Hz in far right EQ graph). There was a little more bass rumble on the left which was addressed with the yellow cuts centered at 90 and 220 Hz in the middle graph. I added a tad more mid-range lift to the sides and center of the field and boosted the overall level of the center 2.5 dB.

I believe that some of the traits noted in the localization test can be heard in the stereo imaging of the distant sound sources. With the center boosted and rumble reduced, the echoes have longer decay and provide greater sense of depth. Note, however, that the distant echoes seem to be concentrated in the center of the stereo field with a pronounced gap between the center and the right speaker. Very likely there were reflections originating between these two angles that are not well-represented.

The recording responded to adjustments in post pretty well. I would have to look into the noise question before I could tell what direction to take the design. Its hard to tell from these tests how the array shapes crucial mid range frequencies coming from the rear. Even if it proves to perform well in this regard, one would have to weigh this advantage against the gain loss of the curved boundaries. I would never have guessed that the spherical shaped boundaries would behave so differently. Maybe we can find add a sphere shape to a sound wave animation applet to see what happens in the pressure zone where the mics are flush-mounted. Rob D.

Powys SASS "Lite" vs. Crown SASS-P/MKH-20's

2010-11-15T22:37:00.000-08:00

Vicki Powys, of New South Wales, Australia, made some refinements in her SASS "Lite" array and to illustrate her progress with the design made simultaneous dawn recordings in the Bush of New South Wales, Australia. To me, the comparison suggests that one can very closely approximate the sound impact of a pricey array with a fraction of the cash and DIY labor. Here are the equipment particulars and a QuickTime Movie comparison:

Crown SASS-P body

modified with 2- Sennheiser MKH20 mics ( Diffuse Field EQ turned ON)

Sound Devices 702 Recorder, Record level "60"
2 x MKH20 mics with Diffuse Field ON
No Low Cut
Lycra windshield

DIY SASS Lite Narrow #2

with PIPMics "Twin" capsules (two on each channel; PIP)

Olympus LS10 Recorder; low sensitivity; record level 10
No Low Cut
Lycra windshield + Beanie

click on image to enlarge

Download QuickTime comparison Movie [.zip 12 mb]

The same arrays were also included in her outdoor pink noise test and sound and silent versions of the localization test can be evaluated here:

click on image to enlarged animation

Download QuickTime comparison Movie with sound samples [.zip 12 mb]

Comments Rob D. November 16, 2010:

The only significant difference I can detect by-ear between the two arrays in the recordings is slightly more "body"/lower frequency representation in the SASS-P body mod with the MKH-20 mics. This difference might be lessened if harder boundary surfaces were employed in the DIY mic. See Paul Jacobson's polar plots below where the lift <1000 Hz is greater for unfinished wood.

The harmonic shift between the pink noise samples is not detectable in the spectrum of the sounds presented in the field recordings. I think this shift is the result of the unique tonal balance presented by the capsules and not problematic coloration. If we think of each capsule/array as a filter or producing an unique tonal balance, these "curves" cause some bands of pink noise to get boosted and others to become attenuated. The net effect is some notes get louder and others get softer-- thus a shift in key.

Pink noise confuses my ability to evaluate overall tonal "balance" so I used the visual sonogram displays in analyzing the test using pink noise. The greater low-end response of the SASS-P/MKH-20 mod is evident in the field recording sonogram but not in the pink noise generated sonogram by the portable CD player. This is not at all surprising considering its 4" speaker.

It appears that SASS-P/MKH-20 mod has more response from 3K to 4.5K Hz from the 12 to 2 o'clock positions. The SASS Lite's response is better in the 5500 Hz to 9500 Hz range at most angles. Probably due to the Diffuse EQ option on the MKH20's, there's more response above 12K Hz at most angles and especially from 10:30 to 2:30. It looks like much of the extra brightness with Vicki's current SASS Lite configuration is associated with sounds arriving from 2:30 to 3:30. There may be some evidence of this in the field recording that Vicki can pick-out.

The most audible change in the pink noise comparison is the SASS Lite's greater response between 1K Hz and 3KHz. The effect starts at 10:30 and increases to 1:30. From 2:00-3:30, it disappears. Its not standing out to me in Vicki's field sample but settings with more distant subjects and "airyness" might make this difference more apparent. The difference is not just between hard and soft boundaries because the capsule sizes differ. Paul Jacobson's comparison of the 10mm "Jack Mics" placed in SASS-P dimensioned unfinished wood and foam boundaries might be better for isolating the hard soft boundary differences:

click on image to enlarge animation

At some point we might want to compare the self-noise and phase performance of single 10mm capsules to the "twin" sets Michael Rooke made for Vicki. I assume the twin capsules use series or parallel wiring to take advantage of the higher PIP voltage of the LS-10 and increase output.

I am not struck by a huge noise performance difference in the recordings. The < 500 Hz input noise we've detected with the mic pre of the LS-10 is not surfacing noticeably. There is proportionally more soft background "grunge" in the Lite/LS-10 sonograms between 500-1000Hz. Perhaps the input noise is effectively masked by the background sounds she happened to record? (Note LS-10 owners that Vicki uses special recorder settings to get lower input noise performance). Those with better high frequency hearing than mine can probably hear more self-noise difference above 10K Hz. Under that, the considerable high-frequency content in the setting seems to be masking the potential 4 dB(A) difference in self-noise ratings [MKH-20's at 10dB(A) and Pip Mics at 14 dB(A)]. The high gain associated with the quiet conditions would normally reveal a difference this large, so the twin capsules may be improving self-noise performance.

PBB2N vs Jecklin Disk Localization & Depth

2010-10-15T08:05:00.000-07:00

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.

Powys Localization Pink Noise Test -Capsule Size, Boundary & Baffle Effects

2010-10-06T01:03:00.000-07:00

Click on image to enlarge it.

Vicki played a pink noise sample from a small CD player at 15 positions at 4.5 meters from the test arrays in a open, flat area. The test results can be used to study a number of boundary micing variables because she made efforts to control boundary size, boundary materials, capsule positioning, baffling and capsule types:

Paul Jacobson Oct. 20, 2010 Polar Pattern Plots

Unfinished Wood Boundary; 25mm capsule; SASS-Dimensions; No baffle;

Frequency Response relative to Free Air Response at 1K Hz, 0 degrees.