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Author Topic: Material Transparent to Ultrasonic  (Read 2630 times)
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I'm using an HC-SR04 ultrasonic sensor in a gadget that is going to sit outside for an appreciable length of time.  I am looking for some sort of material that is "transparent" to ultrasonic to act as a barrier against bugs, dirt, water and the environment in general.

Can anyone recommend a product or material? 

Thanks.
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I'm using an HC-SR04 ultrasonic sensor in a gadget that is going to sit outside for an appreciable length of time.  I am looking for some sort of material that is "transparent" to ultrasonic to act as a barrier against bugs, dirt, water and the environment in general.

Can anyone recommend a product or material? 

Thanks.

Anything you use is likely to attenuate the sound level somewhat; what effect this will have on the sensor is anyone's guess (most likely it will make the sensor either less accurate, or lower the range).

So - in short - you want something that is thin so it won't block the sound, but won't deteriorate appreciably over long periods of time with exposure to the elements.

First make sure the mounting/enclosure is such that only the elements stick out the enclosure, and that you seal around the elements with something (silicone, perhaps). This will keep the electronics from being exposed.

For the elements themselves, I would try a couple of things:

1. Small pieces of nylon pantyhose - the thinner the better - stretched over the elements.
2. Earbud headphone cushions (not sure if these are still made anymore, though) stretched over the elements.

For both, you might need to use some kind of glue to keep them in place...
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Or you could try using special thin latex or rubber that is commonly sold as "something for the weekend"
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The basic problem with ultrasound is that it doesn't pass from air to solids or vice versa - so solid objects act as efficient reflectors, any barrier that's solid is going to do this unless its very thin and light (clingfilm might be worth trying). 

You can get transducers where the external surface is the radiating membrane and thus can be sealed.  In fact I got some from Rapid Electronics a while back.
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Such as car reversing sensors
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@rchasan what did you finally go with?
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A MaxBotix HRXL-MaxSonar-WRC series weatherproof sensor.  The cost was around $100 at that time.  Though it is designed for harsh environments - a plus for this project - it turned out not to have a fast enough response time for my purposes: to count people at a busy location.  Also, the output signal was sometimes unreliable.  On the plus side, the staff at MaxBotix was very helpful.

Ultimately I purchased an IR based Seco-Larm Enforcer (model # E-931-S35RRQ).  It is easily mounted in a suitable outdoor case, takes a wide range of input voltages and provides quick and accurate responses. It is a pretty slick little unit.  It uses a remote reflector, which is precisely what I was trying to avoid, but I could not find anything ultrasound-based that would do the job at a reasonable price.

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They sell cheap sets of after-market car reversing sensors - 4 waterproof transducers
for a plausible cost - you can even use it as is and connect to the LED outputs for
some applications.

With an ultrasound reflection sensor you need as thin a membrane as possible
to reduce reflection, a few microns in fact, which is too tiny to be robust.  So in practice
you have to go with a layer that reflects a bit, which has to be right up close to the
transducer so its reflection can be gated out by arrival time.  You get reduced sensitivity
as the true reflected signals are attenuated.  The membrane mustn't be resonant at
the operating frequency or it will ring for ages.
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Or you could try using special thin latex or rubber that is commonly sold as "something for the weekend"

I imagine one should not go for the french ticklers but just the old plain ones.
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