20. How many speakers does Ambisonics use?
Description
This article is from the Ambisonic Surround Sound FAQ, by Martin Leese with numerous contributions by others.
20. How many speakers does Ambisonics use?
A major advantage of Ambisonic Surround Sound is that recording and
studio processing are disengaged from reproduction. The former
produce and operate on the W, X, Y and Z channels, but these can
be reproduced through any number of speakers. The more speakers
which are used the better, as this gives a larger listening area
and a more stable sound localisation. Using more speakers also
improves the illusion that the speakers have vanished; that is
to say, the listeners hear a single seamless sound field. For
horizontal surround sound a minimum of four speakers is
required.
Ambisonic technology places restrictions on the choice and placement
of speakers. Specifically:
* The speakers (power-amps) should have similar efficiencies
(gains) and phase responses. The easiest way to achieve this is
to use identical units.
* All speakers (and power-amps) should cover the full frequency
range. Unlike Dolby MP, there is no "surround channel" and it is
not band limited.
* Four speakers are placed in a rectangle, preferably with the
longer side running front to back, all facing a point in the
centre of your living room. A "layout" control compensates for
different aspect ratios. The four speakers can be driven from
either three or four power-amps.
* Five speakers are placed in a regular pentagon, all facing a
central point. They are driven from five power-amps.
* Six speakers are placed in a regular hexagon, all facing a
central point. They can be driven from between four and six
power-amps.
* A speaker (or speakers) can be moved closer to the central
point, but should then be fed through a delay line (or lines).
Some diagrams would be useful here.
The disadvantage of using only four speakers is that sounds with spiky
waveforms (audience applause, harpsichords, oboes) tend to be drawn
away from their correct location and towards a speaker. Using five or
six speakers gives considerably more robust side and rear imaging.
For periphonic (full-sphere) surround sound a minimum of six or eight
speakers is required, driven from at least four or five power-amps,
respectively. Readers interested in seeing the possible speaker
layouts should consult the Gerzon 1980 reference.
Auditorium decoders that can drive between 8 and 128 speakers are
available (from Cepiar Limited). For domestic use the limiting factors
are the cost of the necessary speakers and power-amps, and the
practical problem of squeezing them into your living room. (The use of
Ambisonics in auditoria is described in the Malham 1992 reference.)
The speaker feeds are each a simple weighted sum of the W, X, Y and Z
signals after they have passed through the shelf filters. Readers
interested in seeing the equation should consult the appendices of the
Gerzon 1985 reference.
With the "old-type" Ambisonic decoders described above, the apparent
position of the centre-front sound image varies as the listener moves
from one side of the living room to the other. This is not a problem
when reproducing music, but when used with TV the on-screen sounds can
become misaligned with the on-screen pictures. "New-type" Ambisonic
decoders, described in the Gerzon and Barton 1992 reference, produce a
stable centre-front sound image that solves this problem. These
decoders can be used with irregular speaker layouts, either five
speakers with one at the centre-front or six speakers with two at the
centre-front. Alternatively, it is possible to use an additional
channel to stabilise the centre-front sound image, as described in the
Gerzon 1992b reference. These newer types of Ambisonic decoders are
not yet commercially available.
Continue to:
My Books
