The last paragraph is the meat, if this is too long. I have belatedly gotten
around to reading the AES paper by Sunish George, Francis Rumsey et al entitled
"Development and Validation of an Unintrusive Model for Predicting the
Sensation of Envelopment Arising from Surround Sound Recordings" Dec 2010.
This paper defines what envelopment sounds like quite well, But to paraphrase
the US Supreme Court on pornography, I know envelopment when I hear it. For me
it is a sense of "You Are There" concert hall realism. For some time now, I
have been using Ambiophonics and the RACE program to generate a sense of
envelopment by using two speakers behind the listening position at an angle of
about 20 degrees and only two meters or so away from the main listening family
size area.. This rear speaker pair is fed a duplicate of the front pair and is
just crosstalk cancelled using somewhat different parameters than used for the
front 2.0 pair. Neither the level nor the delay of the rear pair is critical
within reason which is a rather remarkable result.
I have had hundreds of listeners here and elsewhere in the world confirm the
effect. I call it Envelophonics and there is a tutorial on the details on the
Ambiophonics website. I wish I had the resources to get a 50 man listening
panel and produce an AES paper but perhaps NYU will produce a thesis on
Envelophonics like the one Roginska et al did on frontal Ambiophonics. (Link
on the Ambiophonics.org home page)
Basically when you turn on the rear crosstalk cancelled speakers, you have an
enhanced spatial sense of being there at the recording site. The stage gets
wider and it is easier to localize instruments and sense depth. The home
listening area also gets larger and, if way off the center line or walking
about the room, you still get a feeling of space and can hear both channels
clearly. In brief what most auditors hear and describe matches what the
referenced paper and other earlier sources describe as envelopment. There is
no sense of ASW (Apparent Spource Width) changing so that factor is not an
issue. Now if you turn off RACE, so you just have a rear stereo pair behind
you, this envelopment effect collapses entirely. I had originally used the
rear speakers for surround sound purposes so as to achieve a full circle of
direct sound in the horizontal plane from just ordinary 4.0 media. They were
also used to correct a 2.0 pinna localization cue distortion.. So I was
pleased that the rear speakers were additionally valuable when auditioning 2.0
media. Note that head tracking has never been required for Ambiophonics and is
similarly not needed when Envelophonics is added. You can move your head all
you wish, lean, nod, stand, move forward or back, rotate the head, etc. with
little affect.
Why is crosstalk cancellation fort the rear pair essential to producing this
kind of envelopment? Subject to a PHD project someday by someone, I believe
the effect is easy to explain. In normal hearing the pattern of very very
early reflections from the half circle of seats or heads behind you changes in
directional cues with the location of a sound source in front of you. But if
you are listening to a 60 degree stereo loudspeaker triangle in a listening
room, all the reflections from nearby rear surfaces have essentially the same
level and time differences at the ears no matter where the virtual image in
front is. The brain senses this as a flat earth effect. If you then provide a
rear reflection pattern that does change normally with the frontal direct sound
source location, you get early reflections that come from all over including
the sides which are known to stimulate envelopment. This rich early reflection
collection now also swamps the later lower level room reflections so their
static nature is less significant and even desirable. Again, if you just have
ordinary stereo at the rear it does not work and worse, you might then have
front to rear reversals. With RACE on at the rear, the level can be quite high
before anything behind you becomes audible. List members are welcome to visit
to hear the effect for themselves or just use one of the RACE apps or
components and DIY. Ralph Glasgal, [email protected]
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