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?
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...