Re: [LAD] Attenuation of sounds in 3D space

On Thursday, July 22, 2010 01:07:57 pm fons@email-addr-hidden did opine:

> On Thu, Jul 22, 2010 at 10:35:15AM -0400, Paul Davis wrote:
> > On Thu, Jul 22, 2010 at 10:29 AM, Chris Cannam
> >
> > <cannam@email-addr-hidden-day-breakfast.com> wrote:
> > > Question that just occurred to me. I'm very ignorant about spatial
> > > audio, and although I'm sure several of my colleagues could tell me
> > > this, I thought it might be sort of on-topic here. Is it possible,
> > > or easy, or sensible, or worthwhile, to reduce a B-format recording
> > > into stereo in multiple different ways in order to achieve
> > > different subjective "listener position" results when using
> > > headphones?
> >
> > my limited understanding is this: the B-format data encodes the
> > source position relative to some defined point in space. the decoder
> > can map the "origin" used to define the positional space however it
> > wants to. whether or not any decoders actually offer any control over
> > this is another matter.
>
> The first order B-format consists of four signals:
>
> W: equivalent to an 'omni' microphone,
> X,Y,Z: equivalent to figure-of-eight microphones
> pointing forward/back, left/right, and up/
> down respectively.

And interesting scenario, Fons. But it leads this simple minded broadcast
engineer with 45 years experience to ask a question.

1. How are the signals brought into phase such that electronically, all mic
ribbons or diaphragms seem to occupy the same space, just facing in
different directions?

If this is not addressed, then this will lead to some interesting comb
filter effects if the signals are not kept from mixing, which they will of
course do in the ear.

Granted, the PV of sound in normal air would require separations of inches
till the stuff above high C comes into play, but at the snares and cymbals
frequencies, I would have to assume some coloration of the sound from this
effect alone. And of course the same concern comes into play at the
speakers since they are generally placed around the listener which in no
way approximates the nearly single point reception these mics will hear.

In my own mind, the placement of a PZ microphone in each of the places one
would place the playback speakers would seem to be a superior method, at
least for a listener sitting in the nominal center, who will be so
overwhelmed by (supposedly not important sonically we are told) the phasing
errors that he cannot single out a single largest cause for the lack of
realism.

In my history of electronic repairs for a living over the last 60+ years,
one instance of truly hair raising realism took place when I was about 21,
and working one of the service benches at Woodburn Sound in Iowa City IA,
USA. I had bought some car parts at noon, and when I left about 6 for
dinner, I forgot & left them on the corner of the bench. Having a key to
the back door I let myself into the back door about 8, which was pretty
dark by then as only one 25 watt bulb out in the display area was on, and
half way to the door to my bench area & right in the door to the front,
display room, the Dukes of Dixieland marched by, going right over me. It
seems that Woody and Saul Marantz were out in front, had pulled a 2nd JBL
Hartzfield speaker out of Saul's econoline van, setting it just inside the
front door opposite to ours in the other front corner of the display floor,
along with a Berlant/Concertone tape deck capable of running at 15 and 30
IPS. And the tape was the master that had cut the Dukes then current hit
record, running at 30 ips. SNR was a good 70+ db, and there was no tape
hiss audible unless you walked directly in front of the JBL 075 ring
radiator tweeters that had been added to both our Hartzfield and to the one
Saul was carrying around. No tone controls, and only a 30 watt Marantz
stereo amp., those Hartzfields were then, and may be yet, the most efficient
speakers ever made, never used more than 3 or 4 watts/channel to get SPL's
that would have done Joshua's trumpets at Jericho proud.

Truly a total immersion in the sound, from about 35hz to nearly 30khz.
Those tweeters could do a fairly good job of reproducing a 25khz square
wave.

It took till I had been introduced to Saul Marantz and shook hands, and for
that tape (on 14" NAB reels) to run out before the hair on the back of my
neck was truly relaxed. Saul it turned out was an endless source of
technical knowledge sprinkled with BTDT stories. And needless to say, I
did not manage to get that hydromatic transmission I had just stick shifted
back together till a day later. Yeah, I'm a JOAT. :)

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The cost of living is going up, and the chance of living is going down.
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