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Author Topic: Ambient sound & Jecklin disc for quad?  (Read 3237 times)

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Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #15 on: February 16, 2021, 04:29:10 PM »
A directional microphone oriented more or less tangentially along the surface of the floor, in your case angled upward toward the lecturer but still close to the floor, may not benefit from significant sensitivity increase, but the arrangement should reduce the effect of the floor bounce interfering with the direct path arrival- addressing the "hollow sound" issue mentioned in the lower red-boxed portion of the Shure literature in DSatz's post above.  Placing the arrangement on a yoga mat probably won't hurt.

You may recall my previous posts about on-stage taping using directional mics positioned very close to the stage floor.  I think the reduction of floor bounce lead to increased clarity in those situations.  Those mics were angled upward at angles of 45 degrees or less toward relatively close drum kit and acoustic bass sources.  Noted and certainly related, although I'm not sure of all the details of the underlying phenomena, was the unusual and exceptional clear projection and pickup across larger distance of the guitar amplifier cabinet placed directly on the hardwood stage floor and projecting tangentially across it toward to the microphones.  Perhaps that was doubling the effect by it's application at both the source and receiving end of things.

On the frequency-selective sensitivity increase of a stereo sphere baffle like the Schoeps KFM6 using flush-mounted omnis in the +/- 90 degree positions, the maximum sensitivity increase should occur at +90 degrees in one channel and -90 degrees in the other.  That is in combination with the sensitivity decreasing shadowing effect toward the opposite direction across the same frequency range.  Because of that I imagine the averaged response across all directions is likely to be flat for each channel or close to it.  And I imagine the response along the median plane (source at 0 or 180 degrees), which is on-axis with respect to the stereo array but 90 degree off axis to either microphone, to be flat as well.  DSatz, please correct me if I'm incorrect about this.  Any additional information you have access to regarding the response of KFM6 would seem quite applicable to these questions.

The first stereo recording array I purpose-built after finding Taperssection 15 years ago was modeled on the KFM6, although modified as a boundary-mounted 7" diameter hemisphere mated to a larger diameter flat disk, intended to be placed on the floor, on a stool or a table rather than a full sphere placed in free space.  It was designed for recording instructor lectures, demonstrations and performances in a classroom setting at an acoustic guitar camp, as well as something I could place in the center of a circle of players, optionally housing the preamp and recorder within the hemisphere to form an easily portable all-in-one device that would fit into a guitar gig bag front pocket.   It worked well, yet was best when the sources were relatively low to it's 'horizon' such as the players seated on the floor itself, or with it placed on a table or high stool.  It lost directional differentiation as the angle to the sources grew higher, similar to how our own directional hearing becomes less differentiated for sources with significant elevation above the horizontal plane.

The original discussion is here - https://taperssection.com/index.php?topic=70134.msg939807#msg939807  I dubbed it Boundrisphere. Unfortunately it looks like the photos were stripped from that thread and I cannot find one of it on this computer, but I still have it at home if you'd like me to dig it up and take a photo. [edit- Hmm, I now see them again and am inserting a few from that thread below]





« Last Edit: February 17, 2021, 10:19:47 AM by Gutbucket »
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Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #16 on: February 17, 2021, 10:04:26 AM »
Apologies for getting OT w/r/t the OP, but to follow up on my reply to kuba e-

You may recall my previous posts about on-stage taping using directional mics positioned very close to the stage floor.  I think the reduction of floor bounce lead to increased clarity in those situations.  Those mics were angled upward at angles of 45 degrees or less toward relatively close to the drum kit and acoustic bass.  Noted and certainly related, although I'm not sure of all the details of the underlying phenomena, was the unusual and exceptional clear projection and pickup across a larger distance of the guitar amplifier cabinet placed directly on the hardwood stage floor and projecting tangentially across it toward to the microphones.  Perhaps that was doubling the effect by it's application at both the source and receiving end of things.
^
Below are photos of this from three different evenings recording a jazz trio.  The mics aren't close enough to the floor to achieve boundary effect through the highest audible frequency range, but no high frequency comb-filtering was apparent.  The guitar cabinet mentioned can be seen to the right and was farther away than seems apparent in the photos, partly because it was placed further away from the front of the stage than the drums and bass, and partly because the microphone array was intentionally not centered directly in front of the drum kit but positioned to the right of it (stage left) for increased proximity to the acoustic bass (amplifier-reinforced) and to avoid having the mics directly on-axis with the kick drum.  The 3 large diameter diaphragm ADK TL microphones in the small-format triangular decca tree-like arrangement on the stage were set to a supercardioid pickup pattern.  The room/audience/ambience/surround microphones clamped just below the stage-lip facing out into the room are Microtech Gefell supercardioids.







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 kuba e

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Re: Ambient sound & Jecklin disc for quad?
« Reply #17 on: February 17, 2021, 12:02:55 PM »
Congratulations, you overcame Schoeps. I like the ears you made to the ball. The musicians certainly liked the ball too.

Yes, that's a good note. Boundary microphone's  advantage is avoiding reflections from the floor. I will experiment with it more. I will try the microphones on the stand and on the ground. I will see how the reflections from the ground affect the recording.

Thank you for nice photos and explanation of onstage recording. It is beautiful setup. Photos are very good for creating an idea. Maybe the recording went also well because the cabinets were placed on the floor, so the microphones were at the same level.

Unfortunately I didn't find much information about sound boundary layer. I only found a mention that the width of the boundary layer depends on the frequency and that the gain of 6db only applies to sounds incident perpendicular to the surface. This is really just a guess, but if I recalculate it by simple relations for ideal flow (without friction), the gain in the boundary layer for sounds at an angle of 30 degrees would be only 1.5db. Anyway, this is not important. It is important to avoid reflections from the floor. I'm looking forward to trying it and being able to compare boundary and classic settings.
« Last Edit: February 17, 2021, 12:14:14 PM by kuba e »

Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #18 on: February 17, 2021, 01:08:34 PM »
Don't know about overcame.  A modified emulation inspired by Schoeps, and based upon the same underlying principles, I'd say

Most of the times I've used "standard" boundary mounting involved attaching the miniature omnis directly to the back wall of the room facing the stage, in a spaced A-B arrangement.  That produced a more advantageous direct/reverberant ratio in the resulting recording with better clarity and less low frequency "muddiness" than using directional microphones placed a similar distance away from the stage (perhaps a meter or two in front of the back wall - either way, more distant from the stage than desirable).  As the only option was to record from back there or not at all, the boundary mounted omnis placed on the back wall produced the best result in that situation.  If I could have moved closer and used directional mics in a typical arrangement I would have chosen that.  I tried this same back wall boundary-mounting arrangement in the onstage recording scenarios pictured above.  Ironically it picked up too much direct sound from the stage for the room/audience/ambience/surround channels role.  Moving them to the front wall underneath the stage facing out into the room did work well as an alternate configuration to using the MG supercards.

Also, along with other directional microphone arrangements for the trio above I tried boundary mounted omnis on the stage surface facing upwards, but preferred elevating the omnis a few feet above the stage and ultimately preferred the L/C/R directional microphone arrangement pictured above which I stuck with for the remainder of the series.
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 kuba e

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Re: Ambient sound & Jecklin disc for quad?
« Reply #19 on: February 18, 2021, 08:00:50 AM »
Don't know about overcame.  A modified emulation inspired by Schoeps, and based upon the same underlying principles, I'd say

I meant it's a very good job and it has style. I understand that Schoeps had to make black ball on the stick, because it's dark at concerts and they don't want it to be seen. But for lectures or workshops, your ball is better, it's happier. It is easier for musicians to accept it.

Also, along with other directional microphone arrangements for the trio above I tried boundary mounted omnis on the stage surface facing upwards, but preferred elevating the omnis a few feet above the stage and ultimately preferred the L/C/R directional microphone arrangement pictured above which I stuck with for the remainder of the series.

This is good information that is worth experimenting with the height of the stand when recording onstage. I like onstage recording, but I haven't tried this yet.

Offline m326

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Re: Ambient sound & Jecklin disc for quad?
« Reply #20 on: March 11, 2021, 04:02:10 PM »
Thanks for you input guys, some of the terminology is rather foreign to the newbie me :-) but I'm sure will be helpful to me.

Bit of an update on my project.. I spent the whole of January in hospital and they kindly gave me Covid while there but when able I've been busy.
My idea of a quad went up by one to include a centre channel. I cut 2 x 12" discs from foam board and glued them together for the base. I then cut 5 radial dividers again from foam board and spaced them according to the mic's position, in hindsight I should have arranged the angles differently as the L, C and R channels occupy exactly 180 degrees and the surround channels the same. I would have preferred the L & R to gain an extra 20 degrees or so. Holes were made near centrally in the base for the mics and a quick test revealed the surfaces were too reflective so the interior surfaces were covered with 2.5mm fun foam (it's what I had left over from another job).
To offer some support for the wind protection fabric I 3D printed a set of different sized circles (rings) which were siliconed into position. The prototype was finished off by; reading up on making parachute gores and making a dome shaped wind protection cover out of 1" squirrel fur fabric. I didn't think I could create so much mess cutting that stuff!

The result is a really cool looking dome hiding a rather heath robinson contraption inside. I've only done a couple of outdoor test recordings - but not in ideal test conditions. I'm finding that the microphones are quite sensitive and pick up noise from quite far away  :shrug: and that includes noise form the bypass. On the plus side, wind protection seems to worth all the mess and effort. My next task is to get to grips with surround sound in Reaper, tutorials either seem too basic, incomplete or do way more than what I want to do with it  :help:

A couple of pictures:

Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #21 on: March 11, 2021, 06:38:48 PM »
Nice work!  Very cool to see this actualized. This is very similar to what I was thinking about 10 years ago.  Hoping you didn't suffer that unfortunate illness too badly and are free of any lingering effects moving forward! A few comments-

Quote
Holes were made near centrally in the base for the mics and a quick test revealed the surfaces were too reflective so the interior surfaces were covered with 2.5mm fun foam (it's what I had left over from another job).

I'll start with this, as it makes for a logical continuation of the discussion of boundary-layer-effect in the preceding posts above.  The walls of your arrangement serve as both baffles that modify the directional pickup pattern of each microphone, and as acoustically reflective boundary surfaces which increase sensitivity and the ratio of direct to reverberant pickup.  Both of these effects will come into play above a particular frequency which is determined by the dimensions of the baffles and the area of the boundaries which they form.  Below that frequency, both the baffling and boundary effects are minimal.  Ok, so what are the implications of this?

A basic one is that boundary-mounting benefits from mounting the microphone as close as possible to a hard, acoustically reflective surface.  A baffle does not require the microphone to be close to it as long as the surface of the baffle is soft and acoustically absorbent within the frequency range in question.

Taken to the logical extremes, this means the microphones should either be mounted as deep into the corners as possible, or the baffles should be as acoustically absorbent as possible.  There are arguments for each approach.

To optimize the boundary effect, the surface of the boundary should be acoustically reflective (hard), and the microphone diaphragm should be mounted as close as possible to the boundary surface - flush embedded into it if possible.   It is the flush-to-the-surface mounting which avoids the problem of reflections off the boundary interfering with the sound reaching the microphone directly.  In your case there is not just a single boundary surface but three acting together to affect each microphone - two vertical wall boundaries and a floor boundary.  Ideally, the microphone should be mounted with its diaphragm as deep into the vertex as possible (buried as far as possible in the corner facing outward). 

It looks like you can easily get closer to that kind of arrangement without much trouble.  In your photo above it appears each microphone cable passes through a hole in the "floor panel" which is about the same diameter as the microphone itself and is positioned well back into the corner.  Try installing each microphone so that it barely protrudes through that hole, with its diaphragm facing directly upward, leaving most of the microphone body extending down below the "floor panel".  This will get the microphone element much closer to the vertex and reduce combfiltering from reflections off the boundaries. You may also notice increased high-frequency response as the microphone is moved closer to the vertex, which is the boundary effect working to a higher frequency given the closer-to-flush placement in the corner.  It might be that high-frequency boost nicely offsets the attenuation imparted by the squirrel fur.  If so, great.  If not you can correct for it with EQ, or play around with using foam and/or fur on the boundary surfaces.  If the microphone is placed flush enough to avoid comb-filtering reflections, making the boundary surfaces less acoustically reflective and more acoustically absorbent will reduce the sensitivity boost effect in the range in which the boundary effect is occuring.

Alternately, if the microphones are not flush mounted, you can make each baffle surface as acoustically absorbent as possible and rely on just baffle effect, like a multichannel Jecklin disk baffle. It will take a bit more than thin foam to effectively achieve this.  When I was playing around with Jecklin disk baffles, I made a bunch of tests to determine how much absorption I needed to reduce angle-of-incidence reflection across the frequency range in which the baffles were effective.  I initially thought I'd just use a layer of foam and fur, but ended up with a baffle core made from corrugated cardboard supporting a layer of cut-pile carpet, atop which I placed a layer of felt (further covered by fur) on each side.  In other words, it took a lot more than I first imagined it would to effectively damp reflections off a surface in close proximity to the microphone.
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 Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #22 on: March 11, 2021, 07:33:51 PM »
Second-

Keeping the above in mind as you play around with the construction, consider whether you want some degree if time-of-arrival/phase difference between channels or not. In terms of stereo recording, this is the difference between a coincident microphone technique and a near-spaced one, and the same applies to a multichannel array such as this. 

With the microphones tucked as deep as possible into the corners, you effectively achieve a coincident array.  With them spaced away from the center, you have a near-spaced array. This is similar to how far the microphones are spaced from the face of a Jecklin disk.  Folks here at TS have made Jecklin-baffled recordings with the microphones tight up against the baffle as well as spaced farther away from it.  I like some spacing myself, but the more spacing the larger the baffle needs to be to effectively shadow the microphone on the opposite side to the same extent.

If you prefer near-spacing but want the microphones boundary mounted and avoid the need for acoustically absorbent baffles, you could make something that looks more like a star rather than an asterisk when viewed from above.  That pushes the inside corner between each point of the stair outward away from the center to provide near-spacing.  It would also allow you to place the microphone bodies inside the center of the star facing outward, rather than underneath with the diaphragms facing upward, while keeping the diaphragms tight in the corners.

If the overall diameter of the baffle remains the same, the larger you make the diameter of the center portion of the stair, the shorter each point of the star becomes.  Push it out far enough and you get a square (4 channels) or pentagon (5 channels) with outside corners instead of points and no inside corners.  Push it further still and you get a smooth cylindrical or spherical surface with the microphones flush mounted around it.  In that way you can trade off the degree of baffle-isolation between adjacent channels verses the spacing between adjacent channels.
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 Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #23 on: March 11, 2021, 08:08:16 PM »
Third-

Quote
I then cut 5 radial dividers again from foam board and spaced them according to the mic's position, in hindsight I should have arranged the angles differently as the L, C and R channels occupy exactly 180 degrees and the surround channels the same. I would have preferred the L & R to gain an extra 20 degrees or so.

This very much depends on what you want to achieve, what you are recording, and how you are playing it back.

If the primary goal is the sensation of immersion in the recorded acoustic natural outdoor environment, you'll do well to array the channels evenly around 360 degrees.  Reproduction will work best with your playback system arranged similarly with an even angular spacing between speakers all the way around.

In recording and reproducing live music events in surround, there is far more energy coming from the front, and a need to retain forward focus along with increased imaging clarity across the front. Much of the challenge in this case is keeping the energy from the front from bleeding into the surround channels, while at the same time limiting ambient pickup as much as possible from center channel so that the proportion of sound arriving directly from the sources of interest on stage dominate in that channel.  The Left and Right channels serve as intermediaries between the dry, forward focused center and the wet ambient surrounds, with a more-balanced distribution of both aspects.  So for surround recording of a musical performance with an audience, an angular distribution like you show above with the surround channels taking up a full 180 degrees or even more of the circle is likely to be advantageous. Likewise it might be helpful to narrow the acceptance angle of the center channel to less than that of the Left and Right channels.
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 Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #24 on: March 11, 2021, 08:15:41 PM »
If you are open to experimentation, I'd suggest comparing your 5-sided asterisk baffle arrangement with a flat-sided pentagonal box that has the same outside circumference and the microphones flush mounted in each face

I'd love to hear that comparison.
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 Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #25 on: March 12, 2021, 11:18:35 AM »
You might do a web search for articles and photos from Mike Skeet, who was based in the UK and died in 2015.  He was a champion of home-built baffled and boundary-mounted microphone arrays designed for nature recording, experimented with and documenting numerous unusual configurations.  His contraptions and those built by others who were inspired by him are frequently referenced on nature-recording forums (or at least used to be, I've not checked in the last few years).

Here are a couple examples of commercial surround microphones using these techniques:

DPA 5100
A "bicycle seat" shaped microphone containing 5 baffled omnis-


Holophone
An egg-shaped hard-surface boundary with flush-mounted omnis arranged around the periphery-


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 #26 on: March 12, 2021, 05:07:35 PM »
Many thanks for all the info, lots of reading to do..
Worst thing about Covid was a lingering cough for weeks  :(
Regarding the pentagon, do you mean something like this (below)? Points are based within a 12" circle making the sides 7 x 7" with a mic mounted centrally in each face. Should the pentagon be enclosed top and bottom?
Next time I have the table saw out I can knock this up  :)

Offline Gutbucket

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Re: Ambient sound & Jecklin disc for quad?
« Reply #27 on: March 12, 2021, 09:02:57 PM »
Yes, that looks promising.  I can't think of any reason you'd need enclose the top or bottom.

I don't know if that will really make any audible difference or not, but it might be beneficial to move the microphones up or down slightly to shift them away from the center of each face, and/or make each face a rectangle instead of a square.  Reason being that the distance from the microphone to the side edges will differ from the distance to the top and bottom edge, and that may help smooth the response around the corner frequency where the boundary effect imparts its high-shelf boost.  This is a phenomena akin to optimizing placement of a tweeter on the front of a speaker baffle.  Similarly, baffle-step-response in loudspeaker design is related to boundary-effect-response in this kind of microphone array design.

It's a minor detail though, and may not matter. Build it however is easiest and most practical.
« Last Edit: March 12, 2021, 09:24:29 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)

Offline kuba e

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Re: Ambient sound & Jecklin disc for quad?
« Reply #28 on: March 13, 2021, 06:30:12 AM »
M326, great that Covid didn't cause you any trouble.

The pictures and comments are great. I'm curious to compare the original coincident asterisk to the pentagon. I don't have surround playback, but I wonder what you say about the difference.

I also can't think of why the pentagon can't be opened. Maybe it is good to enclosed it to avoid a sound in chimney (melusine) if you are recording in a strong wind. It also occurs to me that you will have to solve the windscreens. Maybe stick them on the board?

I was looking for details about the boundary layer. But I didn't find anything. So I wrote to Dpa if they could give me an example of a polar pattern for their boundary microphone. I also asked if they would have a document explaining the origin and behavior of the boundary layer. They sent me a message that this must be answered by a specialist and that it will take longer. If I get any information, I'll write here.
« Last Edit: March 13, 2021, 03:35:54 PM by kuba e »

Offline m326

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Re: Ambient sound & Jecklin disc for quad?
« Reply #29 on: March 14, 2021, 02:43:40 PM »
Made more progress on the pentaboundrythingy, twice I got the table saw out of the shed yesterday and twice it rained! That's UK weather for you.
While at it I made a top and a bottom, the bottom is fixed but the top is removable & has a carry handle. I'm thinking of perhaps adding something inside to deaden internal reflections.
Kuba, I have some 15mm acoustic foam which I'll wrap around it and cover with fur - it was a big squirrel  ;D
One of the pictures makes it look squarish, the last one is of another diy project on the go at the moment.

 

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