29 Jul
2010
29 Jul
'10
3:59 p.m.
On 2010.07.29. 15:20, Gene Heskett wrote:
On Thursday, July 29, 2010 08:52:04 am Jörn
Nettingsmeier did opine:
I am fairly sure 'doppler' *is* what I meant. I work on spatialization
project where sound sources can and will move around on arbitary
trajectories in virtual space.
Well as for 'distance law'. Microphone receives and registers sound
pressure instead of sound intensity.
intensity != pressure
intensity ~ pressure^2
While true that energy at 2x distance is 1/4, the pressure excerted is
1/2 neverheless (pressure value is independent of area - i.e. it uses
constant nominal area).
So you end up with a linear correlation instead of square one.
pressure ~ 1/distance
intensity ~ 1/distance^2
There is also the thing with acoustic impendance. For simplicity I
didn't include it into "equations" and that's the reason they're not
equalities, but are proportionalities.
john,
On 07/29/2010 02:35 PM, JohnLM wrote:
This use of 'doppler' I would call incorrect, because the doppler shift is
a shift in the apparent frequency of the sound rising at it approaches, and
decreasing as it leaves. I'm sure you have a term for what you mean, but
doppler isn't it. That police officers radar gun measures your speed by
listening to the echo from your vehicle, and comparing it to the signal its
sending, which to simplify, results in a beat frequency which is exactly
your speed if approaching or departing exactly to or from the radar guns
position. That is why it is often called doppler radar& the weather guys
us it also. Because they don't stand directly in front of you to take a
reading, there is some small vector error in your favor.
Someone else was trying to describe the distance vs square law change in
the apparent volumes. So let me try from the broadcast engineering field to
explain that better. Imagine a point src of energy, be it light, sound,
or other radiation such as a radio or tv signal. Measured at distance x,
you will get your reference signal, call it 0 db in this case. Now,
without changing anything else move your measuring instrument to a point
that is now at a distance of 2x. You don't get half the signal, but 1/4 of
it, because the same energy that was hitting a square of any arbitrary
measurement, say a square inch, has in addition to being spread twice as
wide at distance 2x, it is also twice as high. So the new reading will be
-6 db compared to the original '0' db.
That is why we call it the square law. The only way to get that back is to
make the receptor itself 4 times bigger. But while I have observed that
there are quite wide variations in ears, I have not seen an individual with
expandable ears (yet) :)
> when you hand-craft distance cues, you should not expect wonders for
> sounds originating inside your sphere of speakers. travelling through
> the center quickly can be made to work, though. So from what I can understand, if I apply doppler
effect, distance
attenuation and other pre-process filters to a mono input and then
push it through the panner plugin I get fairly correct representation
of sound in 3D space. Right?
depending on what you mean by "fairly correct", "representation",
and
"3D space" :-D
doppler would come into play only if you change the distance of the
sound, not when you move it on the sphere, as the distance remains
constant. >> I failed to find anything discussing mixing in
AMB. Can I just sum the
>> channels of all sounds, like I would do it to any
>> "direct-speaker-to-channel" formats?
>
> yes. that's a fundamental property of all linear systems, and
> independent of the signal representation.
>
> i've written a little howto for ambi mixing in ardour a while ago, maybe
> you'll find it useful:
> http://cec.concordia.ca/econtact/11_3/nettingsmeier_ambisonics.html
> for lac2010, i tried to find out how ambisonic mixing can be applied to
> a pop production:
> http://stackingdwarves.net/public_stuff/linux_audio/lac2010/Field%20Repo
> rt-A_Pop_production_in_Ambisonics.pdf
>
> best,
>
> jörn