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Ultrasonic Range Meter

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 carry your beam away. Figures are from "Sound System Engineering" by Davis&Davis, ISBN 0-240-80305-1 
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ciao Ban
Apricale, Italy
Reply to
Ban
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Thanks Ban. I want to point out that you were thinking 100m instead of

50m. :D 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
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air absorption is about 15dB for 50m. However, according to numerous atmospheric absorption (based on ISO 9613-1) calculators, this value should be about 1.2 - 1.3dB/m (about the same with your value 150dB/100m) for a 40kHz signal at 20 degrees C and 70%. I suppose ISO 9613 must be right. So 50m seems a bride too far here.
Reply to
Martin

Thanks Ban. I want to point out that you were thinking 100m instead of

50m. :D 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
formatting link
air absorption is about 15dB for 50m. However, according to numerous atmospheric absorption (based on ISO 9613-1) calculators, this value should be about 1.2 - 1.3dB/m (about the same with your value 150dB/100m) for a 40kHz signal at 20 degrees C and 70%. I suppose ISO 9613 must be right. So 50m seems a bridge too far here.
Reply to
Martin

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.

--
ciao Ban
Apricale, Italy
Reply to
Ban

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:

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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

Reply to
Rich Grise

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

Reply to
Jonathan Kirwan

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