But ultrasound would be affected by the walls/partitions.  Let me =20
clarify ultrasound for this use though -- rather than send out a burst =20
that's reflected and re-detected, perhaps the second transducer (which =20
is really what it needs to measure distance from) will detect the =20
burst and re-transmit a modified version of that (expectedly with a =20
known time delay).  The problem is that I expect ultrasound won't go =20
through the obstacles, and reflections off other =20
walls/partitions/obstacles would give false distance readings.

Cheers,
-Neil.



Quoting Michael Watterson <mike@radioway.org>:

> Ultrasound pulses is really best for 6"
>
> Night and I'm tired, so excuse any mistakes...
>
> you are not limited to 40KHz or large transducers..
> A miniature electret can work to 70kHz
> Some small ceramic "beepers"/Speakers can be resonated at various
> ultrasound frequencies above 50KHz.
>
> There are miniature 40kHz transducers too
>
> You do need to short the input when you send the pulse or an analogue
> filter on the microphone  can take too long to recover. !
>
> 70kHz is about 4mm wavelength so you can make small light weight horns.
> The RF equivalent is 7GHz. In practice 10Ghz band is closest with cheap
> devices and licence free SRD spot frequencies. But the Ultrasound is
> simpler, cheaper and lower power and needs no "RF SRD" approval. The
> range isn't great (< 20m).
>
> You need pulses as then the receiver gain can be high. Any continuous
> wave system looking at phase shift tends to be deaf unless it's > 400GHz
> with very directional transmitter.
>
> A bat actually has a flap that momentarily closes its ear when it emits
> the "click".
>
> You measure time between sending pulse and hearing it.
>
> for 50mm minimum distance  using sound round trip is 100mm. Speed is
> 300m/s so time is 50/300 ms, approx 500/3 us =3D 166us, pulse wants to be
> about 80us
> but... 70kHz that's only about 6 cycles... Not much for a filter. 12
> cycles for 160us Pulse, limit is 100mm (about 4"). Or 18 cycles approx
> at 150mm / 6" about 240us pulse
>
> RF 10GHz?
> Speed is million times more. So you need 240 pico second pulse for
> minimum 6" and about 20 pico second measurement accuracy is about 0.5" I
> think
> BUT 1 cycle is 1 nano second at 10GHz... see the problem? At 18 cycles
> you are at 18 nanoseconds., about 500"  Even 10GHz RF is too low a
> frequency for 6" minimum distance I think.
>
> Car parking sensors are all Ultrasound I think.
>
>
>
>
>
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