The rotating head figure (above) was unashamedly
borrowed from Jérôme Daniel's Ambisonics and 3D Research
website. His website, doctoral thesis, and highly informative
PowerPoint presentation (in French) on high-order Ambisonics can be accessed here:
gyronymo.free.fr/index.html
"Ambisonics goes further than
stereo in that what it does... is to sample the acoustic
field in such a way that the combination of the signals from
all the loudspeakers in the array produces… a reconstruction
of the original acoustic wave field (both traveling and
standing wave components).
If a listener puts their head in the sound field, because
the wavefronts are similar to the original, the perception
of directionality and space should correspond to the
original too. It is a ‘wavefront reconstruction’ scheme in
the small. The ear signals (crosstalk and all) will be
correct if the reconstructed wavefronts are correct. This is
just like natural hearing.
Increasing the number of loudspeakers in Ambisonics (each
fed its correctly-decoded signal) increases the accuracy of
the reconstruction and the region over which it holds up.
The interaural arrival-time differences also correspond to
natural hearing in Ambisonics.”
[Reference:
ambisonic.net quoting Professor Stanley Lipshitz]
I will be
adding a comprehensive bibliography on Ambisonics, to
include direct web links to articles when possible. A number
of sites are quite good, but I avoid providing
direct links to sites that have pop-up ads (the ads may be
benign, but I don't want to alienate anyone because of
dubious ads). For
readers who are new to Ambisonics, a good place to start is
ambisonic.net (noting that it's ambisonic, not
ambisonics). My goal is to demonstrate how Ambisonics is an
appropriate technology for researching the effects of noise
using normal- and hearing-impaired listeners. Controlled,
real-world listening tests will enhance the external
validity of research and claims regarding hearing aids,
cochlear implants, and assistive communication devices.
Another highly
recommended source of information on Ambisonics is the Audio
Engineering Society (aes.org),
particularly their anthologies on Spatial Sound Techniques
(Parts I and II). I am currently reviewing a number of web sites
regarding Ambisonics in addition to reading a long list of
journal articles on 3D recording and playback techniques.
I have
observed that a number
of authors attempt to explain Ambisonics terms. Their explanations
generally make it clear that the Ambisonic W, X, Y, and Z
signals are NOT speaker feeds (i.e., they're not the same as
the signals ultimately routed to the loudspeakers). What most authors fail to explain,
however, is
that the W, X, Y, and Z signals aren't the signals
originating at the microphone capsules either. If one
attempts to make Ambisonics recordings using separate
microphones (versus, for example, the Soundfield mic), the
unprocessed microphone signals, as with the speaker feeds,
are not the same as the ubiquitous Ambisonic W, X, Y, and Z
signals.
Ambisonic
B-format signals are not speaker feeds. Neither are these
power lines (egad!).
Brief note regarding Ambisonic microphone techniques:
Michael Gerzon, Peter Craven et al expanded upon the stereo
concepts pioneered by Alan Blumlein to develop the concept
of a microphone system that could reproduce a full
three-dimensional soundfield. Both Blumlein and Gerzon
realized that only when a sound wave is captured at a single
point in space can it be reproduced faithfully and without
the phase distortion anomalies inherent in spaced microphone
techniques. Microphones designed for first-order Ambisonic
recordings use four carefully-matched (sub)cardiod
microphone elements situated on a tetrahedral mount. The
Soundfield DSF-1 is one such microphone, as is the
Core
Sound TetraMic. Higher-order recordings can be made using mh
acoustics' 32-element
em32 Eigenmike®
microphone array.
Keith Howard's
audiosignal.co.uk website
is a nice resource for Michael Gerzon's early writings on
audio-related topics. This resource includes articles that appeared in Hi-Fi
News, Studio Sound, and Wireless World (please note that a
lot of these articles are not related to Ambisonics). Keith
Howard's "Gerzon archive" can
be accessed directly here:
Gerzon archive.
