> The NB assumption isn't based on the bandwidth of the signal you are
> transmitting but rather the receive bandwidth (doppler spread) wrt to
> your centre frequency. Just to remind those who forget that a common
> speed of sound in water is 1500 m/s, thus relatively small velocities
> can produce a large frequency spread.
Just to give an example of the type of doppler spreads, a relatively
fast ship runs at 30 knots, or roughly 15 m/s. That's 1% of the wave
velocity. 1% of the speed of light is 3000 km/s. I would be surprised
to see a solid object travelling at that sort of speed anywhere in
The speed of sound in air is even slower (330 m/s) but no one except
bats use acoustics for ranging and navigation in air. I wouldn't be
the least surprised if extreme Doppler shifts are essential to the
bats' ability to navigate by acoustics.
Reply by Steve Underwood●January 14, 20052005-01-14
Rune Allnor wrote:
> The speed of sound in air is even slower (330 m/s) but no one except
> bats use acoustics for ranging and navigation in air. I wouldn't be
> the least surprised if extreme Doppler shifts are essential to the
> bats' ability to navigate by acoustics.
It seems most bats only hear the doppler. They have signal processing,
before the audio gets to their brain, which brings the signal down to
baseband. That way the can see the bugs for the trees :-). Some of the
big eared bats can wiggle their ears to create some doppler, and defeat
their own signal processing. That way they have the possibility of
selectively making the clutter visible to them.