Hi, David and Peter.
Nice to be in touch again.
Scaling the problem to the different sound speeds makes sense, but the
perceptual question that Peter asked is intuitively appealing. I was
leaning that way myself.
I scribbled a few triangles this morning and had to look up that darn Law
of Cosines for the 1000th time -can never remember it.
I took the idea of a point source sound 2M distant and 45=C2=B0 (to the lef=
t) of
the left ear, to keep the math simple.
That yields a scalene triangle with sides b=3D2.0, c=3D0.17, a=3D2.1236 and=
angle
A=3D 135=C2=B0 (Side c was drawn horizontally, at the bottom)
In air, the deltaT across c is about 360=C2=B5S.
In hard wood, using David's link, it's about 31=C2=B5S.
If we heard that 31=C2=B5S delay in air, I'm figuring we would expect the s=
ound
source to appear slightly less than 1=C2=B0 left of centre or A=3D90.94=C2=
=B0
That assumes we could consistently perceive the 2M distance in both wood
and air, which seems impossible at best, implausible at least.
If we scale up the inter-aural distance, 0.17M x (360/31) =3D 1.974M, and
keep our 2.0M distance to the sound source, Side a becomes 3.4254M
and Angle A becomes 119=C2=B0, or 29=C2=B0 left of centre. Still wrong.
If we take the 31=C2=B5S in air again, keep the 0.17 inter-aural distance, =
but
scale down the 2.0M sound source distance by 343/3960 for distance
perception:
b=3D 2*(343/3960)=3D 0.173, c=3D0.17, a=3D0.12276 A=3D41.93=C2=B0 Yikes. No=
w it's on the
wrong side!
We can see where this is going to end. The original triangle and any
derivations from it must remain similar. Duh?
Why bother with all of this? Isn't it the old issue of recording/measuring
some artifact in one medium and being forced to present it through another?
If I hear this 31=C2=B5S delay across my nice equilateral speaker triangle,
through air, can it possibly sound correct? Also, I start to doubt my own
perceptual abilities when numbers get that small.
I know that if I got the Schallers close enough to get any usable
correlation, I would also use a mid-side plugin and adjust to taste anyway,
rendering this part of the discussion rather moot ;)
I'm reminded of a CBC documentary I heard years ago about our perception of
time. One example I recall hearing was 2 beeps separated by a few mS. If I
recall correctly, the median of the people they tested identified 2 sounds
12mS apart as occurring at the same time. It seemed a huge number to me.
I've recorded a fair bit of music since then using digital equipment, and
(in the older gear) foldback through headphones was always a concern
because of the delay related to sampling and buffering time. Most singers
don't seem to care about 10 or 12 mS delay in their headphones, while very
good drummers start to complain bitterly at any delay over about 4mS. There
are lot's of reasons for this difference which go beyond time perception
specifically - I'm not 'dissing the vocal crowd, but I do find the
difference rather interesting.
While I can see it being worth some effort to use a 'sound transmitting
stereo bar' a la tongue depressor in some fashion on these ants, the sound
field would need to be much smaller than used in the video, which really
has no L/R correlation at all. I suspect the Schallers would be almost
touching and their casings would need to be discarded or sound insulated so
that all of the signal came through the 'bar'. Probably sandwich the
peizo's between 2 bars. Beyond that case or something similar, I doubt
there's much point in recording this kind of stuff in stereo.
Cheers,
--and apologies to Ben! I didn't intend to hijack the thread - just help it
along...
Keith
On Sat, Feb 14, 2015 at 2:20 AM, Peter Shute
[naturerecordists] <> wrote:
>
>
> When the speed of sound is faster, shouldn't the spacing be greater?
>
> Peter Shute
>
> Sent from my iPhone
>
> On 14 Feb 2015, at 12:01 pm, <mailto:
> > [naturerecordists] <
> <>> wrote:
>
>
>
> > I also tried mounting the Schallers on a tongue depressor to get a
> better
> > stereo image -needs more experimentation to get the spacing right.
>
> Keith,
>
> Just as a bit of theory, spaced stereo working on time differences will
> depend on the speed of sound in whatever medium you are recording. For
> instance the speed of sound in water is about 4.4 times the speed in air
> so
> you would expect the same time differences with mics spaced about 40 mm
> apart instead of 170mm for average ears.
>
> I've no idea of the speed of sound in damp earth, so there is an
> experiment
> for a start, like measuring the time difference from a thump with the mic=
s
> a
> metre apart..
>
> As for the woodpecker the speed of sound in wood is around 10 times the
> speed in air, which suggests s very close spacing might be worth trying.
>
> See:
> http://www.engineeringtoolbox.com/sound-speed-solids-d_713.html
>
> David Brinicombe
>
>
>
>
>
--
Keith Smith - KeithSmith.ca_Freelance Guitarist & Location Recording Servic=
e
<http://www.keithsmith.ca>
|