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### Topic: ultrasonic sensor through thin plastic tube (Read 2297 times)previous topic - next topic

#### jbp7576

##### Nov 13, 2012, 02:46 pm
Hello !

I'm wondering about using an ultrasonic sensor, like maxbotix ones and I would need to make the sensor moving inside a plastic tube and take measure throught the plastic tube.

With some calculations and what I remember from my physics lessons, a sound wave can go through something if the wave length is longer than the obstacle thickness.
So this sensor emits 40khz soundwave, so if i'm not wrong a wave length of 8,2mm.

So a plastic tube with 1 or 2mm thickness could theorically let the soundwave pass.

But my question was : Does anyone ever tried this? Do I have to put something in the tube to avoir sound reflections? and these kind of sensors are enough powerful to be able to detect something outside the tube even after passing a 1 or 2mm plastic wall?

#### robtillaart

#1
##### Nov 13, 2012, 07:12 pm
Quote
So a plastic tube with 1 or 2mm thickness could theorically let the soundwave pass.

In theory there is no difference between theory and practice but in practice there is...

Can you explain the why you want to do this?

As far as I know you can expect reflections and the part of the signal that passes the wall will have to do that twice (when it comes back)
The question is if the signal is then still recognizable. I doubt but have never tested it so proof me wrong.

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### pylon

#2
##### Nov 14, 2012, 03:34 pm
Quote
With some calculations and what I remember from my physics lessons, a sound wave can go through something if the wave length is longer than the obstacle thickness.
So this sensor emits 40khz soundwave, so if i'm not wrong a wave length of 8,2mm.

This would mean normal sound (hearable) would go through obstacles of more than 2cm thickness. The glass of a window would be no problem for sound to pass, horrible vision. If sound is able to pass depends heavily on the material (obviously) and a damping of some factor is always to be expected. A 1mm plastic part is almost perfectly reflecting the ultrasonic sound my sensor is emitting, so your's won't be much different.

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