Hi,
there is also a small study and listening test results that some
colleagues published on the 126th AES convention on the effect of
reverberation on ambisonic reproduction:
"The effect of listening room on audio quality in Ambisonics
reproduction" (2009)
Olli Santala, Heikki Vertanen, Jussi Pekonen, Jan Oksanen, and Ville Pulkki
The listening tests were done for ideal b-format signals, horizontal
decoding, with artificially changed reverberation, and the highest
listening test scores were found for direct-to-diffuse ratio of 1,
rather than anechoic conditions.
Regards,
Archontis
On 3/26/11 11:27 AM, Richard Lee wrote:
When I was the R&D teaboy at Wharfedale in the late 70's, I tried stereo in our
anechoic chamber; expecting great things w/o pesky room reflections. The results
were terrible; poor stereo sound as well as formal localisation which was the
reason for the experiment. Both much poorer than in a normal listening room.
Peter Fryer told me that BBC Research had tried the same thing with the same
results. They found putting a plywood floor down improved the sound and
localisation.[1]
MAG knew of this experiment and always advocated speakers close to room
boundaries rather than well away as the pseudo prophets would have it. His
reasoning was that if early reflections came from the same general direction,
they would not confuse.
I should also point out that simulating localisation w/o "some" reflections is
likely to be inaccurate.[3]
There are several reports of poor Ambi demos in Anechoics; including the false
prophet Floyd who repeats this ad nauseum. However, he may not have been using
a proper Ambisonic Decoder.[2]
I'm not convinced of the need for "semi(?) anechoic" listening rooms, ambisonic or
otherwise ... except for Joern's "... very convincing renderings of semi-anechoic or
free-field conditions. "
However, I'd urge that any purpose built listening room or studio have wonky walls,
floor& ceiling. Perfectly parallel surfaces have terrible flutter echoes and
you end up applying so much treatment to deal with these that your room becomes too
dead. Prof Peter Lord, Salford recommended at least 2 degrees wonkiness. Julian
Wright, Celestion finally managed to simulate and confirm this in the late 90's
when Patrick Macy, PAFEC developed the 1st successful acoustic boundary element (we
were Beta testers). Wonky walls give you much more flexibility to choose your room
reverb profile
[Richard, I know what you are thinking....yet again, I couldn't help but
mention the Hankel functions...sorry :-) ]
Duu...uh! Professor, does " ... substituting the spherical Bessel/Hankel (j_n / h_n)
functions with traditional Bessel/Hankel functions (J_n / H_n) ... " for NFC with "line
arrays" give a simple Minimum Phase 3dB/8ve filter? My calculator has no big B or H button.
[1] I've been trying to find the BBC Research report w/o success. If anyone knows it,
please post. It might be mentioned in "Stereophonic image sharpness" -
Harwood, WW vol74 1968 p207-11
The Kingswood Warren anechoic was rather bigger than ours.
[2] "Is my decoder Ambisonic?" - Heller et al, preprint 7753, AES San Francisco 08. This
is BLaH3 and includes "Designing Classic Ambi Decoders for Dummies".
[3] "Loudspeaker and the Stereo Image" - Millward, HFN 29, nov84
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