Ron Capik
08-26-2003, 10:13 AM
Jerry Steiger wrote:
> "Ron Capik" <r.capik@worldnet.att.net> wrote in message
> news:3F4A63C7.39307AD5@worldnet.att.net...
> Snip
> > One thing to note is that thermal gradients can cause large
> > refractive effects and winds (especially near water) can
> > produce lots of odd thermal gradients.
>
> Ron,
>
> Thanks! I was aware of thermal gradients causing diffraction, though I don't
> know what kind of gradients are required and over what size scale they need
> to operate. Does a variation in the wind have a similar effect to a
> variation in temperature/density? If you have, for example, wind moving
> toward you on your right and away from you on your left, would the sound
> waves coming from your front move faster on the right and slower on the left
> and hence bend towards your left?
>
> Jerry Steiger (whose been out of school way too long)
The real world is a bit more complex than that. For some odd reason I also
get the feeling that you are thinking in two dimensions. Sound fields are
dynamic
three dimensional things that may exist in non homogeneous media, thus the
sound may be deflected left, right, up, or down. Thus the sound may be deflected
over your head. On top of that, the sound from the two speaker stacks will
experience
different path dependent delays resulting in comb filtering effects. Thermal
effects
(per degree F) are more than an order of magnitude larger than wind velocity
effects
(per MPH) and they all work as vector sums. [Density has a negligible effect on
velocity but average molecular weight of the medium has a large effect.]
Then to further confuse, we can add in Psychoacoustic like the precedence
effect...
OK, now that your head is spinning I'll throw out the dual: how does one
simulate
these odd effects in a recording? I needed this effect for a play I was working
on.
Here's part of that thread:
================================================== ================
In article <381373C6.11B2@v5v.com>, Lorin David Schultz <Lorin@v5v.com> wrote:
>Perry R. Cheatham wrote:
>> Low frequencies will travel through the ground, the speed of sound is
>> faster through the earth
>You sure about that?
>Even if it's true, it will only work for ten or twenty feet before the
>mass of the ground dissipates the wave completely.
>
>Lorin
Very little acoustic energy enters the ground because of the very large
acoustic impedance mismatch between air and ground. Of the little sound
that gets into the ground most will be absorbed by friction in a few
inches. Seismic waves may be faster than acoustic waves (in air) but
a fife and drum is hardly a seismic wave generator.
However, drag at the air/ground interface can have an impact on the
speed of sound near the interface. The impact of the effect has a
frequency component associated with surface roughness related to grass,
bushes, trees, hills, etc. This drag effect will cause some temporal
dispersion.
A larger effect is caused by the temperature coefficient of the speed
of sound. The temperature variations along the acoustic path refract
the sound waves resulting in random variations in phase and magnitude
of the sound field. The larger the distance, the greater the variations.
Now, the audience doesn't care about the physics of the sound field or
how the sound was processed to create the effect. All the effect needs
to do is sound "right" to the audience.
What one does for a stage show may be very different from what they
do for a movie, video, or CD.
I needed an effect for a theatric performance and was able to build
something to meet my need.
The rest is academic, for now.....
Once again, thanks all for the hints.
It's been a fun thread.
Ron Capik
--
================================================== ===================
Enough for now....
Ron Capik
--
> "Ron Capik" <r.capik@worldnet.att.net> wrote in message
> news:3F4A63C7.39307AD5@worldnet.att.net...
> Snip
> > One thing to note is that thermal gradients can cause large
> > refractive effects and winds (especially near water) can
> > produce lots of odd thermal gradients.
>
> Ron,
>
> Thanks! I was aware of thermal gradients causing diffraction, though I don't
> know what kind of gradients are required and over what size scale they need
> to operate. Does a variation in the wind have a similar effect to a
> variation in temperature/density? If you have, for example, wind moving
> toward you on your right and away from you on your left, would the sound
> waves coming from your front move faster on the right and slower on the left
> and hence bend towards your left?
>
> Jerry Steiger (whose been out of school way too long)
The real world is a bit more complex than that. For some odd reason I also
get the feeling that you are thinking in two dimensions. Sound fields are
dynamic
three dimensional things that may exist in non homogeneous media, thus the
sound may be deflected left, right, up, or down. Thus the sound may be deflected
over your head. On top of that, the sound from the two speaker stacks will
experience
different path dependent delays resulting in comb filtering effects. Thermal
effects
(per degree F) are more than an order of magnitude larger than wind velocity
effects
(per MPH) and they all work as vector sums. [Density has a negligible effect on
velocity but average molecular weight of the medium has a large effect.]
Then to further confuse, we can add in Psychoacoustic like the precedence
effect...
OK, now that your head is spinning I'll throw out the dual: how does one
simulate
these odd effects in a recording? I needed this effect for a play I was working
on.
Here's part of that thread:
================================================== ================
In article <381373C6.11B2@v5v.com>, Lorin David Schultz <Lorin@v5v.com> wrote:
>Perry R. Cheatham wrote:
>> Low frequencies will travel through the ground, the speed of sound is
>> faster through the earth
>You sure about that?
>Even if it's true, it will only work for ten or twenty feet before the
>mass of the ground dissipates the wave completely.
>
>Lorin
Very little acoustic energy enters the ground because of the very large
acoustic impedance mismatch between air and ground. Of the little sound
that gets into the ground most will be absorbed by friction in a few
inches. Seismic waves may be faster than acoustic waves (in air) but
a fife and drum is hardly a seismic wave generator.
However, drag at the air/ground interface can have an impact on the
speed of sound near the interface. The impact of the effect has a
frequency component associated with surface roughness related to grass,
bushes, trees, hills, etc. This drag effect will cause some temporal
dispersion.
A larger effect is caused by the temperature coefficient of the speed
of sound. The temperature variations along the acoustic path refract
the sound waves resulting in random variations in phase and magnitude
of the sound field. The larger the distance, the greater the variations.
Now, the audience doesn't care about the physics of the sound field or
how the sound was processed to create the effect. All the effect needs
to do is sound "right" to the audience.
What one does for a stage show may be very different from what they
do for a movie, video, or CD.
I needed an effect for a theatric performance and was able to build
something to meet my need.
The rest is academic, for now.....
Once again, thanks all for the hints.
It's been a fun thread.
Ron Capik
--
================================================== ===================
Enough for now....
Ron Capik
--