4 capsule ambisonic mic

[ambi_doug]

hi there,

i have been pointed here by one of the regulars (so it seems), in relation to ambisonics.

some time last year i started building my own ambisonic setup (b-format encoder, and UHJ encoders), but have been stalled for a while due to the (at the moment) prohibitive cost, precision and manufacturing methods needed for the capsule array.

having searched through these pages, i see that there has been some interest (coincidentally at the same time i started this project - last oct/nov), in constructing a DIY 4 capsule array - but nothing seems to have come of it.

i have got as far as building myself a b-format encoder, and have a solution for the 4-channel mic pres, and plan to do the decoding (probaby to 5.1) in software.

would any of you enlightened fellows like to discuss the practicalities of making an ambisonic array?

cheers,

doug.

(btw, my current progress on this project can be found here: http://ambisonic.hamsterfight.co.uk - as you can see, no updates in a while :sad: )

 

[Rob Flinn]

Doug

I take it you`ve seen this site:

http://homepage.ntlworld.com/henry01/cheap_soundfield/cheap_soundfield.htm

 

[ambi_doug]

hi rob,

i have indeed seen that site, and it was that site that got me started. my encoder circuits are henry's. (which is why my site has no schematics on it).

i guess my next step will be to at least try the system with some cheap back-electrets or something, but i would really like to expore the possibility of making a soundfield-esque mic.

 

[ToobieSnack]

nice ref rob
later
ts[/i]

 

[mikeyB]

Me 2 saw the site with the electret capsules - I,ve seen and heard some capsules like this from Maplins some years ago and was not impressed.
I was ordering bits from www.thomann.de and ordered a pair of "t-bone EM800" mics as a cheap drum overhead pair! I was impressed with the mics - especially at £50 for the pair - even better they do the capsules for £6 each! - yes I ordered them with the mics.
I will soon have a poke around in the mic and then do the 4 capsule array!

All going well - I would like input from other members regarding quality condeser capsules of say 10 to 20 mm dia - wouldn't a valve ambi mic be f***ing awesome!

 

[ambi_doug]

thanks for the link mikey.

how would a valve ambi mic fare?

would the valves mess up the phase accuracy needed for such recording? :?

 

[Scodiddly]

At least a couple of folks on the micbuilders group (at Yahoo Groups) have done ambisonic mics with electret capsules. Might wander over there and ask around...

 

[SSLtech]

Hey Doug,

Last night I recorded a British brass band in B-format, I wanted to experiment with "B+" but there really wasn't time to work out the logistics because two of the numbers that they were performing (the opening numbers of the first and second halves of the concert respectively) were 'choreographed' insofar that the pieces of music were arranged in such a way that the band walked on in sections, moving downstage for featured sections, then walked back to their traditional locations at specific cues... any additional mic stands would have been in the way and there wasn't time to fly the rest of the mics...

Anyhow, my specific thoughts were that someone connected to this forum might know somebody who'd be able to help make a series of angle-brackets which were specifically designed to work with perhaps the smaller Sound King capsules or similar...?

I can probably help to provide the information needed to fabricate the angles, the thing would probably have to be CNC'd...

By the way, the Brass Band concert was a lot of fun last night. (the band are the reigning US national champions and the current US open champions also) I can probably edit out a 3-minute piece if anyone can host it... I really have to generate a stereo version of the cobra recording for Clintrubber to host.... Hmmm, the "to-do" list is growing again!!!

Welcome, Doug!

Keith

Keith

 

[mikeyB]

Hi Ambi-doug
Am I missing something here and if so, is the idea of a valve ambi mic a non starter?
Tell me if theres something you know!
I know that the capsules are meant to be as small as possible - the bigger the capsule - the greater the phase error - or something like that.

 

[clintrubber]

I can probably edit out a 3-minute piece if anyone can host it... I really have to generate a stereo version of the cobra recording for Clintrubber to host.... Hmmm, the "to-do" list is growing again!!!

Keith, please take your time. I know how it feels, each day I'm adding pages to my list here as well... :thumb:

 

[rafafredd]

http://www.thomann.de/thoiw10_artikel-172815.html

http://www.thomann.de/thoiw10_artikel-172816.html

http://www.thomann.de/thoiw10_artikel-176112.html

Thanks for the heads up on those capsules. Not too much money for experimenting. Are those really nice? I´m considering those are electrets....

 

[ambi_doug]

i've actually just bought 5 different kinds of electret capsules from farnell (total order £20, for 20 capsules)... i figured i should at least see if i can make this thing work using the most basic suitable components before i even consider anything of a higher quality (ie. more expensive).


mikey: i've no idea - i put this question to those who know much more than i do about valves and mics (which is about nothing).

 

[Scodiddly]

[quote author="rafafredd"]http://www.thomann.de/thoiw10_artikel-172815.html

http://www.thomann.de/thoiw10_artikel-172816.html

http://www.thomann.de/thoiw10_artikel-176112.html

Thanks for the heads up on those capsules. Not too much money for experimenting. Are those really nice? I´m considering those are electrets....[/quote]

The 2nd & 3rd links show a capsule that looks a lot like the mid-sized Transound capsules, maybe the TSB-140A.

 

[Scodiddly]

Somewhat more interesting is the capsule for the T-bone SM-56 knockoff:

http://www.thomann.de/thoiw2_artikel-172817.html

Looks like I was a bit late with an orange microphone...

http://www.thomann.de/thoiw2_the_tbone_retro_jr_prodinfo.html

 

[SSLtech]

[quote author="ambi_doug"]mikey: i've no idea - i put this question to those who know much more than i do about valves and mics (which is about nothing).[/quote]
Doug,

Here's some info: http://www.gyraf.dk/gy_pd/g7/gic.htm

Lots of people here really like tubes (nothing wrong with that at all!) -I think that one big problem is that the levels need to be fairly closely matched. Different tubes have different gains and I wonder how age-stable they are. The matching in a true Ambisonic capsule can be quite critical and age-related drift can be a huge problem...

Apart fron that, the sheer size means that the mic body for a 4-capsule item (these would almost certainly need to be large-diaphragm capsules to compound the problem!) would possibly become a significant acoustic object..

Keith

 

[ambi_doug]

i agree, a tube or two in the right places can definitely be a good idea... just wondered if the concept of a tube ambisonic mic was a non-starter.

edit: also, forgive my n00b-ness, but i wonder if anyone could point me in the right direction for making a phantom powered balanced line to electret capsule circuit? i have found a few on the web, but as i don't really know an awful lot about electronics, wondered what you guys have to say on the subject.

cheers.

 

[SSLtech]

Nah, I have a U67 at home... I love it for some stuff. If you ever get the A-format end of your mic working properly, you can always try a 4-channel tube front end, the mechanics of the array and making a case will be the tough bit... by then you might only have to make a bigger case if you want to try an alternative tube stage. Once you get out of A-format I'd stay away from tubes, and even before that, calibration might be difficult without a home-made anechoic measurement arrangement...

I know that some folks have made a "speaker pit" outdoors in the floor firing up at the sky, then used a boom mic stand on a quiet night in order to calibrate gains, check side rejection etc... That might be a good idea, and you could even check for tube gain drift with age if you felt so inclined... :grin:

Keith

 

[SSLtech]

...Heym if you think a 4-capsule array is tough, try this:
http://www.ambisonic.net/pdf/CotterellThesis/

a 12-capsule array in the form of a regular dodecahedron... :shock:



Keef

 

[Scodiddly]

[quote author="ambi_doug"]edit: also, forgive my n00b-ness, but i wonder if anyone could point me in the right direction for making a phantom powered balanced line to electret capsule circuit? i have found a few on the web, but as i don't really know an awful lot about electronics, wondered what you guys have to say on the subject.[/quote]

Here's the one I use all the time:

 

[ambi_doug]

scodiddly, that's fab - just what i was after :grin:
although, is this for 48V phantom? and what voltage will the capsule see? (i presume 12V due to the zener?)

keith,
yeah, i've seen that paper before - maybe one day i'll work out if it's a feasible build - the decoding looks pretty hardcore :twisted:

cheers guys. your help is much appreciated.