Last Friday we tried three different types of sound absorbent materials. The first one is plastic foam. We placed a bunch of foam at the small funnel and tested the sensor. Until 2.5m everything went well, but at larger distances the foam does not absorb enough. But, the combination of plastic foam with the big funnel worked excellent.Nevertheless we would like to use as less material as possible, so we tried a second material, which was craft foam. This did not work at all. So we immediately tried a third one: felt. This turned out be the best material for our purposes. The size of the funnel does not matter. However, the possibility of accretion is a big disadvantage.
After looking for the right absorbent material, we tested the influence of a temperature gradient to the sensor. We used a hairdryer to warm up the air inside the pipe. As we expected, when the temperature rose, the measured distance was smaller than it should be. This is because of the higher speed of sound at higher temperature. In order to eliminate this error we added a temperature and humidity sensor. We are now trying to write a code which implement the data of the temperature/humidity sensor into the distance measurement to tackle the temperature problem.