On Thu, Apr 25, 2013 at 02:50:24PM -0600, Bearcat M. Şándor wrote: > As i'm still learning about Ambisonics (mostly trying to wrap my fuzzy head > around the math), there's something i don't understand. > > If the 4 channels of a b-format mic give you all you need for the > mathematical computations for 3-D space, why do we have Ambisonic mics with > more than 4 channels and orders with 8, 16 and more channels? What does > having a 4 channel (w, x, y and z) mic restrict you?
There is no simple answer to your question. As Sampo as already pointed out, first order AMB signals provide all the info there is about the sound field in a single point. The physics and maths being what they are, they also define the sound field in an area around that point. The size of that area depends on the frequency. For first order the 'extrapolation' works well up to a distance of around 1/4 to 1/3 of a wavelength. So the area it covers will be quite large at low frequencies (LF), but gets very small for medium (MF) and high (HF) frequencies. Above a few hundred Hz it is so small that it is impossible even for a single listener to have both ears inside the area in which the sound field is accurately reconstructed. So for MF and HF Ambisonic decoders use an ad-hoc approximation which is based on psycho-acoustics and which works well in practice. That is the reason why you need a frequency-dependent decoder, either dual-band or using shelf filters. So far for first order. Higher order Ambisonics provides two things. First, the area in which the sound field is reconstructed 'exactly' becomes larger, more or less proportional to order. But in practice little is gained from this and we still need the 'approximation' for the mid and high frequency range. But HOA also allows this approximation to be much more accurate, to the point that for third order or above, in practice it can be 'almost perfect'. This allows HOA to work in a much larger listening area than first order, i.e. to serve a large audience, and also to be usable in situations were for practical reasons (related to speaker placement) first order would not work well. To really fully understand the why and how you'll need the maths. What I wrote above is an attempt to explain things in intuitive terms, which means to simplify things, but hopefully not to the point where the essence is lost. Ciao, -- FA A world of exhaustive, reliable metadata would be an utopia. It's also a pipe-dream, founded on self-delusion, nerd hubris and hysterically inflated market opportunities. (Cory Doctorow) _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
