Topic: How to make an Omnidirectional microphone Directional? (Perhaps sound insulation

[Leo]

Hello.  I'm new here, so Hello there!  I have bought some Panasonic WM-61A microphones to use with a digital SLR camera (Canon 550D).  They are omnidirectional but I would like to make them more directional, and configure them in a stereo XY configuration.  I don't want to space the capsules far apart and rely on a phase delay for stereo, which would also pick up sounds behind the camera (including me).  It's for amateur use but I want the best quality I can achieve easily and cheaply.  I aware this can be done using a parabolic reflector, but this is too cumbersome for my intended use, and I have read there is a way to make a directional microphone by using many omini's in a grid on a wall and somehow processing it into one directional signal, but this sounds rather complicated and cumbersome.

(And of course I realise I could have bought directional electrets but I was unable to find ones of similar quality at less than 20 euros each including UK delivery.  And the WM-61A is only about €1.50 each, tiny (6x4mm) and even gives good low frequency performance.  And my understanding is that "directional" microphones generally pick up sound directly behind them as well as in front of them.)

I'm thinking that if I phyiscally sound-insulate the capsule in all directions apart from one it should become much more directional.

My current idea is to roll the electret fairly tightly in a small piece of thin cotton fabric (eg from an old bed sheet) to produce a short tube (eg 3cm long with a 4mm wall thickness) and plug the back end, so there is only a clear hole at the front, hopefully making the electret much more unidirectional.  Other ideas included foam, neoprene or silicone/soft glue.  And probably with a few smears of glue to keep it together.

I would then position the two tubes in an XY configuration for stereo, which seems (from my reading) to be optimal.

So, my question is:  Is this likely to work well, and do you have any advice or suggestions?  Perhaps another material is better than rolled cotton?

Many thanks in advance!

Leon

[Gutbucket]

Welcome Leo.

I recommend forgetting about X/Y, especially with bastardized omnis.  Space between microphones, even just a little bit, is going to be your friend for getting good natural sound, regardless of what you end up doing.  Also forget about parabolic reflectors, they sound bad and are cumbersome.

Three choices:
1) Space them apart by a few feet on a rod of some sort 
2) Mount them closer together with a sound blocking/absorbing barrier between them
3) Mount them directly onto hard surfaces as flush to the surface as possible

Option one uses spacing alone.  Try this first even if you are convinced it is not what you want to do.  Compare all other options to what you hear with this setup.  It will pickup sound from all directions. (Search TS threads for the terms: spaced omnis, AB, A-B, etc.)

Option two lets you place the mics closer together and creates Left/Right directionality due to the barrier.  Suitable barriers block sound from the opposite side and absorb sound reflecting off the surface from the same side.  Possible options are your head, someone else's head, an LP record with foam and fur glued on it, whatever.  Small baffles only do their thing in the higher frequency range, larger baffles are more directional to lower frequencies.   This setup will also pick up sound from all directions as a stereo pair, but increases the directionality of each individual mic to each side as the frequency rises. When you use someone’s head it’s called, HTRF, when you use a baffle it’s called a Jecklin disk, or baffled omnis. (Search TS threads for the terms: Jecklin disk, J-disk, baffled omnis, HTRF, etc. for details)

Option three makes the mics hemispherically directional from the surface.  Like option two, the directionality depends size of the surface. Small surfaces only do their thing in the higher frequency range, larger baffles provide directionality to lower frequencies.  Also, in the range in which it is effective, you effectively increase the sensitivity of the mic by 6dB without raising the mic’s self-noise.  6dB free gain.  You can point the surfaces as you like.  If you simply space the two mics apart on the same surface you have A-B spaced hemispherical mics.  If you hinge the surface down the middle, or use two separate surfaces you can angle them apart more like cardioid microphones.  You might try making a wedge and placing the mics on each face, angled away from each other and spaced apart a bit. (Search TS for the terms: boundary mount, PZM, boundary layer, etc.)


If you absolutely need mix-to-mono compatibility more than good stereo sound quality, then you can boundary mount the mics as close as possible on opposite sides of a single, thin, rigid barrier.  You’ll then have coincident, boundary-baffled omnis with similar directionality to option two, but without any time information.  But try the wedge.  That gives you both some forward directionality and a bit of time info too.  One minimalist audiophile recording label here in the states uses a plexiglass (perspex in UK speak?) wedge with 4' square surfaces.  Smaller works too.
 
The most basic thing you can do is simply stick the mics on opposite sides of the camcorder facing away from each other. Trying to baffle all directions except one like you suggest is not likely to work with reasonable sized baffles, but might with  a construction of big gobo sized barriers.

[bryonsos]

Mod an omni to make it directional?! Blasphemy I say!! 

[Gutbucket]

Mod an omni to make it directional?! Blasphemy I say!! 

Not blasphemous, but canon!.. an attempt to mod a directional to make it omnidirectional is what would raise an inquisition. 

[Church-Audio]

You can use a baffle with sound absorbing material. Or you can look at the DPA 4060 Series. They actually made the mic more directional by using the body of the mic it self to force the mic into more of an omni pattern. The ca-10 mics I have are pretty directional mics. One way to increase directionality with out affecting frequency response too much is to mount the capsule on a flat baffle and experiment with the size. Jon from Niant does this with one of his mics. It works quite well. I would do it with a sound absorbing material so that reflections from the baffle sound more natural. And that will help smooth out the frequency response.


Chris