I think I may be having a microphone epiphany or at least what smug boffins might call "an intuitive understanding" but please feel free to correct any errors in my thinking. In using a six capsule (or 3 capsule) microphone setup with 3 figure of eight pairs this gives you localisation information based on the pressure gradient between each of these pairs (given by the antiphase ?) . This pressure gradient gives you the localisation of the sound on any of the given axis (X,Y,Z) and with this information you can get the localisation of the sound in a sphere ? Is this correct , or even vaguely correct ?
On 25 May 2012 18:05, Augustine Leudar <[email protected]> wrote: > Thankyou all for your responses - they've been a great help - especially > Eric Benjamin - that makes perfect sense thanks ! > > >> There are two ways to approach the construction of such a microphone. >> One of >> them is to use cardioid microphone capsules facing outwards at the six >> sides of >> your cube. The other is to use omnidirectional capsules and place them >> on the >> surface of a rigid object such as a sphere. I don't know which approach >> you're >> taking so I'll discuss both. >> >> Using a cubic array is intuitively simple for a microphone that is going >> to have >> outputs which are directed along the Cartesian axes, such as is the case >> for >> Ambisonic B-format. As you already know, in B-format the W signal is >> equivalent >> to an omnidirectional microphone located at the center of the array. X >> is a >> dipole or figure-of-eight microphone pointed along the X-axis. Since a >> dipole >> has two lobes, one positive and one negative, it must have a precise >> orientation >> to conform with X. X is a front-back signal, Y is a left-right signal, >> and Z is >> an up-down signal. >> >> Cardioid microphone capsules have a directivity that is heart-shaped, >> hence the >> name. They theoretically have maximum sensitivity in the front direction >> and >> zero sensitivity in the rear direction. They can be modeled as being the >> sum of >> a monopole (omni) and a dipole (figure-of-eight), and that is why it is >> easy to >> recover those components by adding or subtracting capsule signals. If >> you have >> two cardioid capsules, on a line and facing opposite directions, when you >> add >> the two microphone signals the two dipole components, being of opposite >> polarity >> to each other, cancel out. That is assuming that the microphones are >> identical, >> of course! If you subtract one of the microphone signals from the other, >> the >> two monopole components, being identical, cancel out, but the dipole >> signals now >> add to each other. So the sum gives a monopole and the difference gives a >> dipole. >> >> Note that it only takes two of the capsules to get a monopole. But you >> will >> have 6 capsule signals and you can sum all of them to get a monopole >> signal that >> is, at least in some ways a better omni than you get with just two. >> >> The process is the same, of course, with the other two axes and deriving >> Y and >> Z. >> >> Now, if your capsules are omnidirectional microphones the analysis is >> somewhat >> different. There really should be some sort of a baffle on which to >> mount the >> microphone capsules although it's certainly possible to have them just >> sitting >> in free space. But let's assume that they are mounted on (actually, in) >> the >> surface of a sphere. If we add all six omni capsules together we get >> another >> omni. Simple. But if two of the opposite-facing omni capsules are >> subtracted, >> we get a dipole. That may not be immediately apparent. It is the >> separation in >> space that makes it so. The simplest analysis is that, if the two omni >> capsules >> are identical, when you subtract the two signals you will get nothing. >> And >> that's partly true. But since the capsules are separated by some small >> distance >> there will be a small difference between the two signals. If a sound >> wave is >> approaching along the line separating the two capsules then it will reach >> one >> capsule first and the other one second. Assuming that the sound source is >> distant from the microphone array there will be no attenuation in level >> between >> the two microphone positions, although there may be some difference in >> level due >> to the spherical baffle. Mostly there is a difference in the phase of >> the two >> signals. When the two are subtracted, one from the other, a difference >> signal >> appears that is proportional to the phase difference. But that phase >> difference >> is proportional to the frequency of the sound! That is, at low >> frequencies, say >> 100 Hz, the wavelength of sound is about 3.4 meters. And high >> frequencies, say >> 10 kHz, the wavelength is 3.4 cm. If our capsules are separated by 1 cm >> (just >> to make the arithmetic easy), then that is 1/340 of a wavelength at 100 >> Hz and >> 1/3.4 of a wavelength at 10 kHz. So there will be a minute difference in >> phase >> at 100 Hz and a large difference in phase at 10 kHz. The result is that >> the >> frequency response of such an array is differentiated; it rises at 6 >> dB/octave >> from the lowest frequencies to the highest frequencies. That slope will >> need to >> be corrected by equalization. Furthermore, if the spacing were >> increased, say >> from 1 cm to 10 cm, the amount of signal will be increased by a factor of >> 10. >> But a sound with a wavelength of 3.4 cm, like our 10 kHz sound, will be >> ambiguous. >> >> It gets quite complicated. I have simplified things to make the >> explanation >> tractable. Typical spherical microphone arrays have diameters of about 5 >> to 10 >> cm, depending on what bandwidth is needed in the final design. >> >> One other comment; You could use a different orientation of the array >> than what >> I have assumed above where the capsules lie on the Cartesian axes. You >> could, >> for instance, rotate the array so that the axes run along the bisector >> between >> two of the capsules, or along the trisector between three of the >> capsules. That >> makes a difference in what you call 'front', of course, but also in the >> behavior >> of the array at the highest frequencies. >> >> I hope this helps. >> >> Eric Benjamin >> >> >> >> ----- Original Message ---- >> From: Augustine Leudar <[email protected]> >> To: [email protected] >> Sent: Thu, May 24, 2012 2:57:44 AM >> Subject: [Sursound] Why do you invert the phase of one channel of multi >> capsule >> microphones ? >> >> Hello all, >> I am building a six capsule ambisonic microphone. I have been told >> that with the opposite capsules (ie up/down, left/right, >> forward/backwards) I should invert the phase of one of the channels >> and then add them to get the X,Y,Z for the ambisonic b format. I've >> been struggling to find a good explanation - I was wondering if >> anyone could explain why this is in detail ? >> _______________________________________________ >> Sursound mailing list >> [email protected] >> https://mail.music.vt.edu/mailman/listinfo/sursound >> >> >> >> ------------------------------ >> >> Message: 7 >> Date: Fri, 25 May 2012 08:58:07 +0200 >> From: Bo-Erik Sandholm <[email protected]> >> Subject: [Sursound] Suitable students project - Creation of ambisonic >> player ? >> To: Surround Sound discussion group <[email protected]> >> Message-ID: >> < >> e023323b1ad21d44af70273b35e750151660a05...@esesscms0356.eemea.ericsson.se >> > >> >> Content-Type: text/plain; charset="us-ascii" >> >> >> I got the silly idea that if we have few a students with a lot of time at >> their hands... >> >> It could be possible to create a low cost ambisonic player using the >> following resources >> >> http://flac.sourceforge.net/format.html >> >> To encode .amb into, i guess att the moment there is yet no support for >> http://mchapman.com/amb/wavex in flac. >> Even if flack encoding is not neccessary but can save storage space. >> >> http://dream.cs.bath.ac.uk/researchdev/wave-ex/bformat.html >> >> Player >> http://www.alsaplayer.org/features.php supports FLAC ambisonic player >> http://offog.org/code/potamus.html or this simple one that i know works >> well via jack. >> >> Ambisonic decoder >> http://kokkinizita.linuxaudio.org/linuxaudio/index.html >> >> >> Operation system >> http://bkhome.org/blog/?viewCat=General >> - Barry Kauler has a A-10 Puppy Linux distribution and development >> environment, that seems nice - Puppy linux version >> Not shure jack is included, but jack is available for puppylinux >> >> Ubuntu is also available for A-10 >> >> Below 100 USD computers >> >> Computer with 4 Core processor and 8 channel sound card: >> >> http://www.aliexpress.com/product-fm/563764893-New-released-Android4-0-IPTV-google-tv-smart-android-box-allwinner-A10-Model-MK802-Notice-just-wholesalers.html >> >> Or >> http://rhombus-tech.net/allwinner_a10/hacking_the_mele_a1000/ >> >> Mele 2000 from dealextreme cost 6 usd more but still costs below 100USD >> and has 4 GB flash memory instead. >> >> With this platform it is is possible to have up to 8 speaker channels >> from the computer via hdmi, >> I have not at the moment a link to a 8 channel power amplifier to use for >> 2 rings of 4 speakers, >> But a horisontal 6 speaker array or other commeon speaker configurations >> can easily be powered via hometheater surround amplifiers. >> >> OR >> Maybe to offer a "novelity" integrate the ambdec decoder and a >> multichannel wave player and port it to the google TV android environmnet >> with a number of "suitable" decoding matrixes and publish it together with >> a pointer to a bit of ambisonic material. >> >> Have a nice summer >> Bo-Erik >> >> ------------------------------ >> >> _______________________________________________ >> Sursound mailing list >> [email protected] >> https://mail.music.vt.edu/mailman/listinfo/sursound >> >> >> End of Sursound Digest, Vol 46, Issue 7 >> *************************************** >> > > -------------- next part -------------- An HTML attachment was scrubbed... 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