Russell,
Tackling these in reverse order:
lambda =3D v / f therefore the higher the velocity the longer the wavelengt=
h.
See
https://en.wikipedia.org/wiki/Wavelength
so we are recording wavelengths in water 4.4 times longer than they would b=
e
in air.
Phase of reflections: Please bear with me.
Phase can only be described in comparison with a reference point or wave.
Sound is a longitudinal wave (light being a transverse wave) and when it is=
reflected from a heavy solid surface, the highs and lows of the incoming an=
d
outgoing waves have to be equal at the instant of reflection.
What causes reflection? In short, an abrupt change in impedance. This used=
to be important in long distance telephone circuits which standardised at
600 Ohms impedance.
With sound which travels through different media, any step in acoustic
vibration impedance or "characteristic impedance" will cause a reflection o=
f
the sound energy. In water, sounds can be reflected by, say, the sides of a=
swimming pool which are of higher impedance than water. At the surface, the=
air has a lower impedance and there will be a reflection back down into the=
water.
Ignoring sound or vibration absorption, the total energy of a sound path
must be accounted for.
Bouncing from medium impedance to high impedance (water to concrete) divide=
s
the sound energy into a small part penetrating the concrete where the
surface molecules store the incoming energy and reflect most of it back int=
o
the water. A moment of high pressure is reflected as high pressure.
At a water/air boundary, there is a mismatch the other way and the air
cannot temporarily store the absorbed energy so it is returned to the water=
as a reflection. This means, at the moment of reflection, a negative
antiphase reflected pressure is formed in the water as the sum total energy=
has to be constant.
What results is a reflection below the air/water surface. This energy has t=
o
go somewhere at the speed of sound in water, and it becomes a reflection
downwards. With zero net energy stored at the surface, this refection has t=
o
be in antiphase. QED.
The part frequency plays is in the path length when positive and or negativ=
e
reflections interact. Underwater, you are in a maze of interfering paths.
As soon as you dip a hydrophone mic rig into water, you are listening to a=
different world. The antiphase surface reflection tends to cancel out the
wanted underwater sounds. The wavelenght at 1 KHz becomes 1.5 metres, makin=
g
interference extinctions and maxima much more prominent. The binaural mic
separation changes from 170 mm in air to 750 mm in water.
Add in-phase and antiphase reflections and it is not surprising that
underwater stereo is not as straighfrward as in air.
Listen out in a swimming pool and see if the water borne noise seems to
rise in frequency after a dive. That's the surface reflections and the
environment the underwater stereo mic rig has to contend with.
David Brinicoombe
Thank you for taking the time to respond in such detail, David.
Of course I had the wavelength vs propagation speed relationship backwards =
-
a couple of moments' more thought would have been useful!
As to the phase relationship to the challenge of recording stereo underwate=
r,
especially the antiphase nature of the reflection from the surface, I'll ne=
ed
to study that when I have more time at my disposal.
I still don't see why whether the reflected wave, either from the surface o=
r
from a solid reflector, should have any more effect than the same in an air=
environment.
I can certainly see why a spacing of 170 mm in air would translate to 750 m=
m in water, though.
Russell Dawkins
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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