Greetings to All,
The patent 'finds' are interesting and, as usual, give credence to the
expression "there's nothing new..." Of course, if one looks closely at the
cited patents, you'll see that the microphones and associated circuitry mix
down to single-channel outputs (or provide a meter reading of energy density of
sound waves). Gerzon, Craven, and colleagues didn't steal anything from their
predecessors: They just took ideas a step or two further. (I can say the same
with my sole patent.) It is amazing that after studying audiology and hearing
science that I continue to find much valuable information in the older books by
Harry F. Olson and Leo L. Beranek (among others), or vintage articles from
Proceedings of the IRE, etc. Although audio has been a life-long hobby,
Ambisonics is relatively new to me. As with many things in life, one idea leads
to another, and pretty soon you become immersed in a plethora of literature and
potentially useful ideas. My earliest
introduction to Ambisonics was the articles that appeared in The Audio Amateur
(circa 1970s), but I was quite young and too much into conventional stereo to
take quadraphony seriously (but, hey, I was smart enough to keep all of my
hardbound issues of TAA for future reference). A little over one year ago, I
had communicated with Bengt-Inge Dalenback of CATT-Acoustic. I was trying to
build a surround system for testing cochlear implant patients in controlled
but real-world scenarios, and auralization was one way of improving current
test protocols. Bengt-Inge was very helpful, and he is the person who
re-introduced me to Ambisonics and the sursound list.
At present, a surround system known as R-Space is being used to study cochlear
implant (CI) efficacy in noise. Larry's system offers advantages to research
scientists and audiologists, but its original scope was mainly limited to
hearing aid studies and being small enough to fit in an audiometric test booth
(this puts the loudspeakers a mere 2 feet from the listener). If you'd like to
see an actual R-Space install, I took a snapshot of one and it appears in a
PowerPoint available thru my website (cochlearconcepts.com). Use of the R-Space
is a step in the right direction, but I sincerely believe an Ambisonic system
(or a high-order ambisonic system) provides greater flexibility and larger
listener sweetspot. Oticon has a fancy surround setup that used HOA, but this
is more elaborate than what I need or can afford (I'm an independent researcher
who does this as a hobby). After reading a long list of articles, to include
most of Michael Gerzon's articles
on Ambisonics and psychoacoustics, I have a couple of questions. Questions
follow:
1. Is there any preferred method of calibrating speakers used in an Ambisonic
setup? Options at hand include swept sine, pink noise, MLS, and IR measures.
There are articles in the AES (and elsewhere) that compare these methods, but
has does anyone have a particular preference of calibrating speakers when it
comes to Ambisonics? My current setup is a circular array of eight speakers (r
= 1.4 m) in an average sized living room. I have an Earthworks calibration
microphone at the listening position (ear-level). Side note: I have considered
adding gobos between speakers (thus enclosing the space) in addition acoustic
absorbers and diffusers throughout the room.
2. Has anyone compared or noted differences between the Virtual Visual
Microphone (VVM) software and offline processing using MATLAB? My speaker setup
lends itself to the code outlined in the article Using Matlab/Simulink as an
implementation tool for Multi-Channel Surround Sound by P. Schillebeeckx, I.
Paterson-Stephens, and B. Wiggins. If I were to use VVM to do the same
(starting with B-formatted files), any thoughts as to how the mic directivity
should be set (cardiod being 1, subcardiod being approx. 0.7) when using eight
or more loudspeakers?
3. I have seen discussion and articles regarding Ambisonics and shelving
filters. Any recommendations as to "best" filter settings based on
speaker-to-listener radius? For example, the aforementioned R-Space has a
radius = 2 ft. (0.61 m). If I were to use a recording made from a Soundfield
mic with the R-Space, what sort of filtering would be required for such a tiny,
8-speaker arrangement? Would this system even lend itself to Ambisonics? The
arrangement I have at home has a radius of 1.4 meters and is what I'll be using
for my research. I anticipate adding more speakers to make it more of a
periphonic system. To date, my background noise recordings were made using a
TetraMic. The speech stimuli are "dry" (semi-anechoic room) recordings that can
be auralized to match the background room noise reverb characteristics. The
speech stimuli, as is probably obvious, are used to measure speech
comprehension ability. I try to keep signal-to-noise ratios
reastic, which is a weakness of many studies. To date, studies have not shown
significant improvement with binaural CIs over a single CI when speech stimuli
are presented in noise. It is my belief that spreading the noise about (i.e.
periphony) will make the tests more valid, regardless of outcome.
Many thanks to anybody who can provide insight or links to articles regarding
any of the questions (by the way, as an AES member, I can download articles
from the AES library--I just need title/author in order to access articles).
Happy Holidays to Everyone!
Eric
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