Monday, April 12, 2010

Partially Baffled Boundary Array with Improved Tone & Spatiality?

Impressed by the tonal and spatial imaging advantages demonstrated in Andrew Skeoch's and Paul Jacobson's comparison, I made an array combining small boundaries and short setback distances with the partial baffle principle employed in Andrew's array. I set-up this new array with Audio Technica AT3032 mics installed next to another stereo array I've been using for recording ambience and made some simultaneous recordings. The reference "Perp2Shphere" array uses two Rode NT2000 mics in omni mode mounted perpendicularly to a 7" diameter coated foam sphere. I've been happy with the tonality and spatial imaging of this array using two them daily for the past year but, I've kept hope alive that I might find something better.

Here's a 9mb QuickTime movie for browser viewing or .zip download  that alternates clips from both arrays starting with the Perp2Sphere. (The play cursor will line-up better with the change points in the graphic if downloaded and viewed with QuickTime Player. In the sample pairings, the PBB1 always comes second.)

The sound clips cover a wide variety of typical "far-field" recording subjects ranging from very faint, distant events at varied positions around the arrays to very delicate small mammal sounds to louder animal calls and human-produced effects.

The difference in the tonal response between the two arrays is immediately apparent to me. As hoped, the PBB1 exhibits a high frequency "lift" as Andrew's array does. From initial indications, the lift appears to begin around 500 Hz and extend to around 4K Hz.  I feel this lift contributes a sense of greater detail in distance sound sources and their reflections, In this stereo mic array comparison, its remarkable that the chorus of Peeper frogs a 1/2 mile away on the right channel "jumps out" in the PBB1 array's image but is very restrained in the Perp2Sphere's image.  The impact on the coyote and owl exchanges is not as profound but it helped me identify the number of callers and because the echoes are brighter, locate them with more accuracy.  There are several dramatic contrasts that occur with closer sounds. Consider the very delicate, high frequency rodent vocalizations at 2:12 where the PBB1 array provides much more relief. The audible differences in the passing deer at :30 and a small branch that drops on the right at 3:20 are also quite telling.  Both arrays were 7' from the ground, placed right next to each other and at very similar distances from the sounds included in the comparison.

 Spatially, the performance differences are equally striking. I detect additional sound reflections and more distinct echoes in the PBB1's samples. Take a listen to the additional slap back echo from a bluff on the left in the coyote bark at 1:30 and the additional "ring" and tree fluttering in the echoes from the PBB1's hand claps starting at 4:41. With the louder events like the passing trucks, cars and helicopter, I'm much aware with the PBB1 that these sources are coming from behind the mic arrays and that much of the sound is reflected from hillsides in front of the mic.  There's more upper harmonic content in the decay of all of the echoes once you are alert to it.

The width of the stereo image feels "tighter" on first impact than most spaced omni arrays I've studied, but with repeated listening, I think its because the the bass is very centered and in phase. The higher register sounds localize and spread very similarly to that of spaced omni Perp2Sphere's stereo image. I choose the Perp2Sphere over other popular arrays because I felt it improved "airyness" and depth, but when I listen to the difference in the distance imaging in the snipe calls from 1:32-1:47, it suggests to me realize the PBB1 could be in a different league.  I wanted to think that the PBB1 renders sounds more precisely across the stereo field after hearng the hand claps. but I think in many cases, the additional high frequency content aids localization more.  The shift of the owls from left to left center at :54 seems to be more the result of the tonal lift and increased definition of the sound reflections from the hillside than an improvement in sheer "stereo" abilities. (Its also helps to know that the direct sound of the owls is blocked on the left!) The additional high frequencies can also aid in localizing close sounds such as with the deer passing. 

As for the "narrowness" that the centered bass tends to produce, I found the array very responsive to using M-S processing in post. This allowed me to spread the image by lowering the volume of the center 1-2 dB and to use inserted EQ to lessen the "dirty" roar of the exaggerated traffic and human-made sounds that exist with the bass more towards in the center of the field without affect side content. I tried the same trick with the Perp2Sphere array, but it required a lot less EQ with the PBB1 and the result on the Perp2Sphere seemed "hollower."  There is less bass impact in the PBB1 array's "sound," but it seems smoother and less jumbled.

There is no question that the PBB1 array has a profound effect on tonality and spatial imaging. How much of it is the mics and not the arrays? The frequency response of the NT2000 in free air is actually brighter than that of the AT3032 in free air. The tonal difference in the lowest octaves in open air are very modest compared to that heard when the mics are mounted in the arrays.  I prefer the lower mid-range response (125-700 Hz) of the large diaphragm NT-2000's mics when mounted in free air as with most spaced omni arrays, but the PBB1 array gives a tonal advantage in this crucial range to the AT3032's. [I would install the NT2000's in a PPB1 array in a flash, but the capsules must be flu-mounted and the Rode's have "side address" capsule orientation. I may try to flush- mount some NT1-A capsules using the forward-facing mod, but I fear I'll get lots of RFI and noise without full shielding that will tend to obstruct the boundary. And they're cardioids.]

The heightened brightness that the PBB1 array seems to be especially useful in "far field" recording because it makes up for some of the loss of high frequencies absorbed by the air.  This pre-emphasis can mean less HF boosting in post to hear sounds clearer and less end-result audible hiss from mic self-noise.  I did notice, however, that robust, treble-rich sound sources like morning birds were a little brighter than I would have preferred when they took up perch within about 30 feet of the array. Others may feel this is an advantage too. I'm prompted to think of Walter Knapp's tall light stand that he has adapted to get his mic arrays further away from amphibians and to improve its "vista" of more distant subjects. 

It occurs to me that one could  place this mic array anywhere where one expects subtle, high frequency sounds and probably get amazing results. Insects, frogs and other sounds that have lots of mid and upper range frequency content will probably sound extraordinarily close with proximate micing.

Lots more to be learned about this array. hopefully, this test and my comments provide a useful start!

Addendum A
Some recordists have wondered if the AT3032 mics have an inherent treble lift compared to other omni mics. Paul Jacobson suggested that we could take another listen to the comparison recordings Walt Knapp made a few years ago featuring AT3032's and MKH-20's mounted in a modified SASS arrays. To facilitate this comparison, I've made a movie with short excerpts from outdoor presence Walt recorded at his rural home. The recordings were made back-to-back, not at the same time, so I selected the two parts which appeared most similar using sonograms. The clips are a bit boring, but I believe a potential difference in tonality and potential impact on spatial clarity will be easier to evaluate without close, discrepant bird songs.  

Blind Comparison
MKH-20 mics vs AT3032 in SASS (2mb) QuickTime Movie for browser viewing and for download.

I renamed the audio files with random strings and recorded the identities with screen captures before making the comparison movie so it would be a blind test for me as well. I'll going to wait until we have discussed our impressions before we find out which is which.
= = = = = = = =
A group of group of five field recordists proved to have pretty good ears. At least Sennheisier would like to think so:
Array                                        Selected as Preferred           Correctly Identified 
  A.   AT3203  mics /SASS                  1 time                                3/4 times       
  B.    MKH-20 mics /SASS                  3 times                              3/4 times 
The fifth recordist described the performance of the two arrays as a "tie" and didn't venture a guess as to which  was which.  

The recordists were not able to determine whether 3032's have an inherent "brightness" from this test, but if it does, it would probably be audibly small in comparison to the "lift" of the PBB1 array. Here's a QuickTime movie [2mb download] with a sonogram comparing an AT3032 in free air  to one in a PBB1 Array.

My thanks to Paul Jacobson, Curt Olson, Rich Peet and Allan Haighton for input and Walt Knapp for the original comparison files.

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