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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