The other thing to add to this point is the 3dB (not 9dB) improvement in signal to noise ratio ONLY APPLIES TO THE W CHANNEL, which ignores the noise on the other eight channels of your B-Format.
You get a 3dB increase of noise per order of ambisonics, which is why eight-capsule hybrid second order microphones can actually sound noisier than first order microphones. Hence the various settings in A-Format to B-Format encoders that essentially bring down the level of the second order components to reduce the hiss, at the expense of a certain amount of spatial resolution. Cheers Jack Sent from my iPhone > On 9 Nov 2023, at 08:51, Jack Reynolds <jackreynolds...@gmail.com> wrote: > > Hi Len, > > As we’ve discussed before… > > This simply isn’t true. > > If you add two omnidirectional microphones together you get a 6dB increase in > peak audio signal (voltage) but only a 3dB increase in noise because noise is > an average power. P=I^2*R and V=IR. The square makes the difference. > > If you double the number of capsules twice, i.e. adding eight omnis, you get > a 9dB increase in noise (3*3) and a 24dB increase in peak signal (3*6). > > A 15dB increase in signal to noise ratio (SNR, the difference between the > peak signal and the noise power, expressed in decibels). > > Adding eight cardioids together to create a single omni W channel for > ambisonic use is not the same thing at all. > > Cardioids are made up of an omnidirectional, pressure component and a > velocity, figure-of-eight component. > > If you add two cardioids, pointing in opposite directions, the figure of > eight components cancel out, leaving just the omnidirectional components. > > The omni component is only half of the energy of the cardioid microphone, so > you only get half the peak signal benefit when adding cardioids arranged in > A-Format to produce a B-Format W channel. > > Adding eight cardioids in A-Format arrangement will give you 9dB more noise, > but only a 12dB (24/2) increase in peak level. An increase in signal to noise > ratio of 3dB. Not 9dB. > > So if you are expecting a 9dB increase in SNR and your published 15dBA SNR, > which is equivalent to 79dB signal to noise ratio, is based on that > assumption, those numbers can’t possibly be accurate. > > If your mic has eight capsules and you claim you are getting a 9dB SNR > benefit from adding the capsules that would imply 70dB SNR per capsule > (79-9). > > I suspect that number should actually be more like 21dBA or 73dB SNR for the > W omni channel and 70dB SNR per single capsule, since you only get a 3dB SNR > benefit when adding eight. > > We will confirm these numbers by measuring a single Tetramic capsule on axis > with a known SPL white noise signal and subsequently comparing the single > capsule numbers to the W channel SNR. > > I’m sorry to have to point these things out. I just want transparency in our > industry and any misleading claims or doubts cast on our mic comparison study > must be addressed. > > Cheers > > Jack > > Sent from my iPhone > >> On 9 Nov 2023, at 01:37, lenmoskow...@optonline.net wrote: >> Do Many Small Capsules Mean More Self-Noise Or Less? Less! >> >> >> As we've discussed before, the more capsules an array microphone has, the >> lower its self-noise. That's because as you double the number of capsules, >> the combined output voltage goes up 6 dB, but the self-noise only goes up 3 >> dB. >> >> The noise increases only 3 dB because it's uncorrelated across capsules. So >> each time you double the number of capsules, self-noise drops 3 dB. >> OctoMic's combined eight capsules have 9 dB lower self-noise than a single >> capsule. >> >> That results in a low self-noise specification. We specify it >> conservatively as 15 dBA. That's the same as DPA's wonderful 4003, or only 1 >> dB more than Schoeps' Mk 4. >> >> >> >> Len Moskowitz (mosko...@core-sound.com) >> Core Sound LLC >> www.core-sound.com >> Home of OctoMic and TetraMic >> -------------- next part -------------- >> An HTML attachment was scrubbed... >> URL: >> <https://mail.music.vt.edu/mailman/private/sursound/attachments/20231108/3a4c2ad2/attachment.htm> >> _______________________________________________ >> Sursound mailing list >> Sursound@music.vt.edu >> https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit >> account or options, view archives and so on. _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.
