> building/using a soundfield type mic using omni's? If life gives you 4060s, then make lemonade. I mean, a B-format microphone!
The problem gets a lot easier if you resign yourself to making something that will have good utility as opposed to making something optimal. I would make a microphone array using a spherical baffle. One can find quite a variety of wood spheres. Here in the US there are spheres made of Birch with diameters of 1-1/2", 2", .... Wood is great because it's cheap, easy to drill, and if you make a mistake you just grab another one. If the user doesn't intend to make use of height, I'd want to make the array a horizontal-only one, primarily because the drilling is a lot easier! It's difficult enough to find the equator of a sphere without having to find the vertices of a tetrahedron inscribed in the sphere! The choice of diameter is tough, because as previous respondents pointed out there is a tradeoff between SNR and bandwidth. As you know, the 1st order patterns will be derived by subtracting the outputs of 2 or more of the capsules, and that means that the response will have to be equalized by apply an LF boost below a critical frequency determined by the diameter of the sphere. For an open array this is straightforward but for a spherical baffle you need to include the diffraction of the sphere. I can calculate this, but not on the back of an envelope. The spherical diffraction gives an effective gain of 6 dB and this is worth going before because the self noise of the 4060s is about 23 dBA as I recall, which is good enough to be useful but not so generous as to allow one to easily throw it away. So what we would like to do is to have that critical frequency be somewhere near the frequency at which the ear is most sensitive to mic hiss - about 2 to 7 kHz. And typical usable sphere sizes just happen to do that. This means that the array will only work well up to about 10 kHz, but then that is true of a traditional SF microphone too! The construction may be just a little bit difficult. It turns out to be difficult to find the center of a sphere once you have it in hand. You will really need to use a drill press to drill the holes. Routing the microphones into the sphere will also be difficult, depending on how the end of the microphone cables are connectorized. It may turn out that you will want to drill a large hole in the sphere at a direction not populated by microphone capsules, and use that hole for entry of the microphones and to route them each into their respective holes. Having done this before, I can give you a bit more specific info if you contact me off-list. Eric Benjamin ----- Original Message ---- From: Dave Malham <[email protected]> To: Surround Sound discussion group <[email protected]> Sent: Mon, June 20, 2011 3:54:38 AM Subject: [Sursound] B format mic using omnis? May seem a strange question, but anyone ever had any experience of building/using a soundfield type mic using omni's? I have been asked by one of the artists featured on The Morning Line if there's anything he could do with his collection of 4 DPA's (4060-bm's). Not something I'd ever really though about before, but as Angelo's B format hydrophone uses omni's ... (http://www.angelofarina.it/Public/UAM-2011/) Dave -- These are my own views and may or may not be shared by my employer /*********************************************************************/ /* Dave Malham http://music.york.ac.uk/staff/research/dave-malham/ */ /* Music Research Centre */ /* Department of Music "http://music.york.ac.uk/" */ /* The University of York Phone 01904 432448 */ /* Heslington Fax 01904 432450 */ /* York YO10 5DD */ /* UK 'Ambisonics - Component Imaging for Audio' */ /* "http://www.york.ac.uk/inst/mustech/3d_audio/"; */ /*********************************************************************/ _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
