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Author Topic: Ambient sound & Jecklin disc for quad?  (Read 3270 times)

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Offline kuba e

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Re: Ambient sound & Jecklin disc for quad?
« Reply #30 on: March 15, 2021, 06:12:50 AM »
It looks very nice. Thanks for the photos. I can't estimate if closing and filling in the pentagon is necessary.

I see you sealed the microphones in the board. That's good, I think that's important. You should probably pull them out a little. The microphone diaphragm should be at the same level as the board surface. It might also be enough to stick the windscreen just around the microphone. You probably don't need to wrap the whole pentagon.

The comparison of pentagon and star will be interesting. In addition to the coincident and spaced configuration, it will be interesting to compare how they affect high frequencies. At the asterisk, if you moved the microphones directly to the corners, you could get a higher gain for high frequencies. If I'm not mistaken, there should be theoretically a gain of up to 18db (3x6db - when microphone is in the corner of three solid surfaces). But the asterisk is made of damping material, it is difficult to estimate if the boundary layer will be formed and how it will behave. It may happen that the boundary layer does not form there and the baffles will only create the directionality of the microphone. The Pentagon has one solid surface, the gain for high frequencies will be 6db.
« Last Edit: March 15, 2021, 06:46:14 AM by kuba e »

Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #31 on: March 15, 2021, 12:59:38 PM »
Solid construction!  Looks like the recorder will fit inside, which is probably one of the more appropriate metrics for determining the correct size.

Getting more technical about it and speculating about the most optimal spacing between microphones based on playback imaging-
The polar pattern of a stereo pair of microphones in combination with the angle between them and spacing between them determines the width of the auditory image heard at playback.  This relationship is explained in Michael William's Stereo Zoom paper and in his subsequent work and the work of others.  If one knows the polar pattern and angle between a pair of microphones, and there is a specific imaging playback width one wishes to achieve, the appropriate spacing needed to achieve it can be determined.

Refer to the Improved Point At Stacks table linked in my signature.  It applies in this case not because this baffled microphone arrangement is intended to be used PAS style for recording an amplified public address system with adjacent pairs of microphones pointed directly at the PA stacks, but because the table solves for the specific Stereo Zoom condition were the resulting image capture angle is the same as the angle between microphones (they typically are not).  It would be important in this case if you wished the directional imaging of each adjacent microphone/speaker pair segment to link-up accurately with its adjacent pair along the edges without either too much overlap or a hole in the middle.  To be clear- This is not necessary for achieving an immersive sounding recording that will effectively transport the listener back to that acoustic space.  It is only important if you want to achieve relatively accurate directional image placement of specific identifiable sounds across in the full 360 degree playback image when played back over 5 speakers arranged in a similar pentagonal arrangement.  In other words, don't sweat this unless its important to you to be able to be able to point in the exact direction of a specific croaking frog or squawking bird or whatever.

I'm not sure what polar pattern will be most closely approximated above the frequency where the baffle becomes effective, but lets assume it's essentially cardioid-like in that range.  There is a 72-degree angle difference between each adjacent face of the pentagon.  Given a pair of cardioids angled 72 degrees apart, the spacing between them which would be needed to achieve an approximate 72 degree wide imaging angle on playback is around 13-15" (30-40cm).  At lower frequencies where the baffle becomes ineffective and the polar pattern reverts to omni-directional, the necessary spacing would be more like 30" (77cm) or so between each adjacent pair of microphones.  That's a big baffle, and likely too large and unwieldy to be used effectively.  Fortunately, given the stated intent of these recordings, it's not critical to get this directional imaging aspect fully optimized.

For reference, with four channels and a square baffle there is a 90 degree angle between microphones, and the dimensions get smaller due to this increased angle.  In that case you'd ideally want a spacing of about 8" (20cm) between adjacent cardioids or about 26" (67cm) between adjacent omnis.
« Last Edit: March 15, 2021, 02:44:14 PM by Gutbucket »
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Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #32 on: March 15, 2021, 02:14:21 PM »
You can find specific references to 4 and 5 channel recording/playback arrays on pages 6 and 7 of this Michael Williams AES paper, available on his Multi-Microphone Array Design (MMAD) website here - https://www.mmad.info/Collected%20Papers/Multichannel/24th%20ICP%20Banff%202003%20Paper%20(16%20pages).PDF 

That indicates that in order to achieve seamless imaging across segments, the optimal spacing for 4 cardioids in a regular square arrangement with a 90-degree angle between pairs intended for 4 channel (quad) playback is 25cm (9.8").  For 5 cardioids in a regular pentagonal arrangement with a 72-degree angle between each intended for playback over 5 speakers arrayed in a matching pentagonal arrangement, the spacing is 39cm (~15" or so).

In addition to providing good correlation with the answer as derived from the Improved PAS table in the previous post, William's MMAD methodology and website helps in figuring and choosing between various microphone arrays, representing a good resource for this kind of thing.

William's MMAD website- https://www.mmad.info/
His original AES paper on recording and playback using regular 360-degree arrays (square, pentagon, hexagon, octagon)- https://www.mmad.info/Collected%20Papers/Multichannel/3157%20New%20York%201991%20(13%20pages).pdf
^His later work including the first paper linked above extends things to irregular shaped playback arrays with a specific front direction, as in the defacto standard 5 and 7 channel layouts.



musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline m326

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Re: Ambient sound & Jecklin disc for quad?
« Reply #33 on: March 15, 2021, 02:25:23 PM »
Thanks for the continued interest and support  :)
 The microphones are now protruding about 5mm off the faces, I cut a smaller square of foam with the centre removed so nothing touches the mic elements and wrapped the whole lot with more foam.. which is quite effective at cutting down wind noise.
However, initially I was not so impressed with the sound. I look it into our garden, brick structures opposing each other, row of sheds behind and fence then road in front. It sounded so boomy and droney. Not good in an enclosed area. HP filter 60Hz.
I took it down our nearest woodland and got much better and more pleasing results this time with a 90Hz HP filter. It's a definite upgrade from my original which is comparison sounds sounds less defined and muddied. Recording (https://www.dropbox.com/s/vs5ylj4wuwd1ga2/F6_Tr_5edit.wav?dl=0) from today, recorded on F6 32bit float at 21db and gain adjusted up by 18db. The file is only the centre channel.
The string handle is more handy than I imagined, it allows hand holding of the device without introducing any handling noises.. but the whole thing is quite heavy, if the mics were 13 to 15" apart - it would be so much less portable for nature recording  :(
« Last Edit: March 15, 2021, 02:43:18 PM by m326 »

Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #34 on: March 15, 2021, 02:57:21 PM »
In exploring that in the posts above I mostly wanted to address what's going on acoustically with this type of arrangement so that you and others following along are aware of the underlying phenomena driving things.  As mentioned, I don't think you need worry about getting the spacing between microphones optimized to achieve optimal discrete directional imaging and suspect the arrangement you show above will work nicely for the intended purpose.

One thing the exploration above makes clear is how increased spacing is a good idea as channel count goes up and the angle between adjacent sections grows smaller.  That corresponds with why a 1st order ambisonic single-point microphone (4 recorded channels) works well for mono or stereo output yet becomes increasingly less ideal for surround output as the number of playback channels is increased, unless one shifts to an exotic higher-order ambisonic system requiring twice as many or more channels.  A spaced array configuration which introduces time-of-arrival between microphones that can be increased as playback channel count increases overcomes this limitation with the use of standard 1st order pattern microphones, but is certainly less compact and portable.

Try the microphones with their housings completely flush to the surface as well and see if that makes much difference in response. It may not make much difference, or you might find you can sort of tune any appreciable high frequency resonances imparted by the arrangement in that manor.
« Last Edit: March 15, 2021, 05:16:16 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

 

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