# Ultrasonic Range Meter

Discussion in 'Electronic Basics' started by roots_of_culture, Mar 27, 2006.

1. ### BanGuest

Can I say my 2 cents:
atmospheric absorption at 40kHz is a whopping 150dB/100m (20C 70%H) at nasty
10kHz it would be only 18dB. So for larger distances go down with your
frequency.
Also your calculation seems off. 100m means -80dB (inverse square law). Then
there is absorption/diffusion at the reflector, let that be another 20dB.
This makes it -100dB. Your receiver needs 94-75= +19dB SPL, you have only
108 - 250 = -142dB available. Forget it.
You can concentrate your beam with an old satelite dish (not the offset
ones) by mounting the transceiver in there. This would maybe improve the
ratio by 30dB. But then any temperature gradients or air movement would
Figures are from "Sound System Engineering" by Davis&Davis, ISBN
0-240-80305-1

2. ### MartinGuest

Thanks Ban. I want to point out that you were thinking 100m instead of
50m. I'm using that application note from senscomp (link in my
previous post) and the calculations seem right. SPL reduction at 50m is
20*log(30/5000) ~ -45dB.
There's one thing that was confusing though. According to senscomp's AN
and http://www.earthworksaudio.com/tech/hf_sound.pdf air absorption is
about 15dB for 50m. However, according to numerous atmospheric
absorption (based on ISO 9613-1) calculators, this value should be
40kHz signal at 20 degrees C and 70%.
I suppose ISO 9613 must be right. So 50m seems a bride too far here.

3. ### MartinGuest

Thanks Ban. I want to point out that you were thinking 100m instead of
50m. I'm using that application note from senscomp (link in my
previous post) and the calculations seem right. SPL reduction at 50m is
20*log(30/5000) ~ -45dB.
There's one thing that was confusing though. According to senscomp's AN
and http://www.earthworksaudio.com/tech/hf_sound.pdf air absorption is
about 15dB for 50m. However, according to numerous atmospheric
absorption (based on ISO 9613-1) calculators, this value should be
40kHz signal at 20 degrees C and 70%.
I suppose ISO 9613 must be right. So 50m seems a bridge too far here.

4. ### BanGuest

That guy is an idiot, for 1kHz it is 0.7dB/100m, it is also not the frequncy
sqared, but has two plateaus, where the nitrogen(0.6dB/100m) and
oxygen(300dB/100m) levels are, which are then constant for all higher freq
in this gas alone.

5. ### Rich GriseGuest

I wouldn't even bother to try ultrasonics beyond about maybe 3M (10 feet),
for a whole bunch of reasons. But, laser rangefinders for maybe 10' and
farther are available off-the-shelf:

So I'd look at the minimum range on the lasers, and do the ultrasonics
for short-range sensors. ;-) (also pretty much off-the-shelf.)

Who's Chris?

Good Luck!
Rich

6. ### Jonathan KirwanGuest

Just as a note: Radio Shack has an off-the-shelf ultrasonic unit that
claims 3cm to 300cm (3m) range. Cost is under US\$15 until some time
in June (it's on sale -- half price.) It's a unit for the Vex robot
kit thing.

Jon