Re: Mixing it...

Subject: Re: Mixing it...
Date: Tue May 25 1999 - 07:39:58 EDT

Lonce came to almost the same conclusion I did, which I believe may be

I think the key elements were the large lake, the time of night, and the
season. It seems to me that the 'warm' lake water (in comparison to the
cooler night air) had created an atmospheric temperature inversion (and a
light mist). The changes in the density of the air caused the sound to be
refelected back to the ground.

The frequency of a passing sound is the same as the source when the
listener is at right-angles (at the 'norm' in physics parlance) to the
sound. With the train approaching, the sound heard at ground level is
doppler-shifted up.

If one imagines that the sound could travel (almost) straight up, and be
reflected back down, the frequency of the sound that goes 'up', is the
frequency at right-angles to the train, which is lower than the doppler
shifted sound at ground level. This would cause a single echo, where the
echo is lower in frequency than the direct signal.

At a 45 degree vertical angle, the freqeuncy would be halfway between the
direct doppler-shifted sound, and the 'normal' (90 degree sound). It's a
simple matter to extend this and understand how the echo would be a
continuously descending frequency as the train approaches, and ascending
as the train left.

This was checked by listening to the echoed sounds next to each other.
The original horn was at different freqeuncies, but the echoes were
always on a trajectory back to the (normal) frequency of the train horn.

> I am a bit confused by the "followed by" in your description, but
> roads and lakes are great reflectors that create a secondary path
> from the sound generator to your ears that is slightly longer than
> the direct path. This creates a periodicity (rather like a comb
> filter) in what you hear that is related to the difference in
> arrival times for the two paths.

This is also evident when standing next to (very) tall brick walls. A
clap is followed by a 'zing', as part of the sound wave is successively
reflected back downwards by the gap (c 1 cm) that exists between the
bricks. It is also particularly pronounced if the surface below the wall
is smooth granite.

> If the sound source is moving toward you, the difference between
> the lengths of the two paths is getting longer and the pitch seems
> to drop. As the source passes you and moves away, the difference
> between the two paths shrinks and the pitch rises.
> I don't know that this is what you were hearing, but what I
> describe is easy to hear as a jet or airplane passes by
> overhead. In fact, for simulating some sounds such as the jet
> fly-by, the secondary reflection path is key for realism.
>- lonce

And this was more than just a single secondary reflection.



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