How sound waves travel for reverse parking sensors?
The sensors are not covered with plastics case, they are potted in epoxy resin. The resin conducts sound waves better than the air, so there is no problem about that. Similar case is installing the echosounder on boats, you can bore the hull and install the sensor to be directly exposed to water, or glue the sensor directly in the inner part of the hull. There will be a slight attenuation compared to the direct water mount, but it still work good. In a car where the medium is the air, potting the sensor into epoxy has no influence, since the air is a way worse sound conductor.