At 10:08 AM 4/24/02 -0400, you wrote:
>Combine this with the mini-
>mized turbulence, and you have a formula to favor evolution of acoustic
>communication systems which, for long distance efficacy, use low
>frequencies and do so primarily at dusk and/or dawn.
Excellent point, Randy!
Sound trapping in a layer happens in many instances. I guess most have
read about the SoFar layer in the ocean.
Sound energy dissipating in a hemisphere grows to 1/4 of intensity as the
radius (distance from point source) doubles, whereas sound travelling in a=
disc, in the same distance becomes 1/2 the power ( 6 dB lost, vs 3 dB
lost). Periods of low turbulence make this doubly apparent. I believe
there is actual differential velocity for different frequencies, as in a
pipe, where the blow of wood on pipe at one end of a culvert, produces
first the low sound, followed by the higher sounds.
Here in Hollis, we discovered the sound absorption (screening) of a young
white pine forest (20 - 30 years old) becomes a sound pipe for loud sounds=
in older stands (40 to 60 years old), apparently because the lowest 50 feet=
of branches fall off, and the canopy above forms an acoustic ceiling. The=
trunks offer little echo or dispersion at low frequencies, so loud low
sounds travel much "further".
This all makes evening rod-and-gun-club shooting at skeet 1/2 mile away
(but with #20 and #12 gauge shotguns facing my house) register peaks of 82=
- 84 dBA on my Richter scale at my doorway, and many home owners 2 miles
further away than us register complaints with town hall. Their rate now
approaches 40 shots per minute for several hours every Tuesday, Sunday, and=
whenever they feel like it. I will gladly email to anyone a good fidelity=
WAV file of our typical "evening meal in the patio".
Fortunately for my AM bird recordings, these "hunters", obviously great
masters of woodsy lore and stealth, never seem to wake up until about 10 AM=
my very best,
MIST Software Associates
75 Hannah Drive, Hollis, NH 03049
coming soon : EnjoyBirds bird identification software.