On 11/07/2012 10:04 AM, Tommaso Perego wrote:
Dear all,
I was wondering how, knowing the diameter of a speaker octagon,
using 1st or 3rd Order ambisonics, to calculate precisely the dimensions of
the sweet spot area.
Any ideas?
Many thanks
the strict sweet spot is only a function of order and frequency, not of
the array diameter.
r < N/2 * c/f is an approximation i saw mentioned in one of franz
zotter's papers, where N is the order, c is the speed of sound and f is
the frequency. in words, the order N is the number of zero-crossings of
a given frequency which are correctly reproduced.
that sounds pretty dire until you realize that at frequencies above a
few hundred hertz, phase relationships are not that important anymore...
in practise, a larger diameter does help, because of two factors:
a) the sound pressure level varies much more slowly in the far field of
a loudspeaker. e.g. the level halves when you move from 2 to 4 meters
(i.e. over 2 meters), and again when you move from 4 to 8 meters (same
drop, but over 4 meters). that means that while the phase relationships
are quite wrong outside the sweet spot, the intensity ratios of the
speakers are more or less correct, which helps with rE localisation.
even more so if you can use highly directional speakers such as line arrays.
b) more distant loudspeakers excite more reverb and have a lower
direct-to-reverb ratio, which helps to mask some of the oddities of
ambisonic playback.
but these perceptional advantages do not change the basic fact that the
soundfield reconstruction is incorrect outside the sweet spot.
best,
jörn
--
Jörn Nettingsmeier
Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487
Meister für Veranstaltungstechnik (Bühne/Studio)
Tonmeister VDT
http://stackingdwarves.net
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