On Sat, Nov 08, 2014 at 06:45:52PM +0000, Dave Hunt wrote: > There are obviously severe limitations in trying to make signals > appear close to the listener with ambisonics. As far as I know, all > first and higher order spherical harmonics go to zero at zero > distance, the central listening position, leaving W (omni) only.
You may be confusing a few things. When you describe or reconstruct the field at and around a point P in terms of spherical harmonic components they are clearly not zero at P. The contribution of the non-zero degree ones to the pressure at P will be zero - but that is not the same thing. In fact when P is close to the source S that the field corresponds to, the non-zero degree harmonics at P will be stronger and have a different phase (relative to pressure) than they would be in the far field. This is the well-known proximity effect. It is this that you'd need to reproduce in order to get the effect of a near source. But there are two problems when you try to do this: 1. It may require high level near antiphase signals from the secondary sources (the speakers), which almost cancel near the 'sweet spot', and 2. 'close' is measured in wavelenghts, and a soon as the distance to the source becomes a few wavelenghts, the differences w.r.t. the far field become very small, and the sound field contains virtually no information at all about the source distance. So reconstructing a near field can be done only for low frequencies, unless you go to absurdly high order systems (requiring the same number of speakers that a WFS system would need), and you need tricks like direct/reverb ratio etc. Ciao, -- FA _______________________________________________ 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.
