Russell Dawkins [naturerecordists] writes:
> David,
>
> It seems to be reflections from the surface (or any other reflective surf=
ace) would be antiphase only at certain frequencies for a given distance, o=
r at certain distances for any given frequency and cannot accurately be des=
cribed simply as 'in antiphase'. Moreover, given the speed of sound in wate=
r is roughly four times faster than the speed in air, the wavelengths would=
be roughly a quarter as long, not four times as long.
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
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
------------------------------------------------------------------------
Yahoo Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/naturerecordists/
<*> Your email settings:
Digest Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/naturerecordists/join
(Yahoo! ID required)
<*> To change settings via email:
<*> To unsubscribe from this group, send an email to:
<*> Your use of Yahoo Groups is subject to:
https://info.yahoo.com/legal/us/yahoo/utos/terms/
------------------------------------------------------------------------
|