On 2012-10-11, Fabio Kaiser wrote:
So that means, for sound source directions in the near of the horizon, speakers below the horizon also contribute and if I leave them out I am missing something and get a false localization!?
Exactly so. Controlled opposites (or in-phase) decoding can make some of that go away, but then it will also reduce the maximal localisation performance of your array at the same time. The best ambisonic experience requires a roughly symmetric array, especially at low orders, and psychoacoustic optimization for your precise array geometry.
There is pretty much no proper optimization theory for hemispherical arrays.
So, Ambisonics in the end is also just creating phantom sources, just using a little fancier approach!?
Absolutely not. It is about always creating a full sound field, even if you have a low order system and a slightly irregular array with which you reproduce it. Placing synthetical point sources within the field is just a particularly simple application of the technology.
The trouble is, ambisonic *really* tries to be a fully surrounding system even at low orders, that is, with very few transmission channels. It really does do full 3D at four transmission channels, and as few as six speakers. There it pays a certain price: all of the speakers will necessarily have to participate in recreating even a single synthetic source, they will have to do so from roughly all directions at the same time, and there will have to be some psychoacoustic trickery.
If you only have a hemispherical array, you can't do all that at the same time. You will have to reoptimize the decoder for the new array, it can't work as well as the full periphonic one, and there is no possible way the system would be fully compatible with the basic B-format. Not at low order.
Still, practically I cannot place source below the horizon, I even have to shift the horizontal ring in elevation by somewhat 15°.
If I wanted to place something beneath the floor level using only speakers above the horizon, I'd utilize reflections from a lacquered, hard floor at the lowest frequencies, then somewhat phase dispersed, HRTF weighted sound from all round at the mid frequencies, and fully specular sound over the upper hemisphere at HF, smoothly interpolated to fully dispersed and somewhat attenuated from the lower hemisphere. Something like that can be done even at first order, though it will sould weird. With DirAC and other active, "infinite order decoders", it can be done much better.
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