Question about sound absorption/reflection

[ft92]

Hello everyone! I would like to share with you the results of an experiment we did in the first year physics lab and ask for your point of view to understand why we got the results that I'm about to tell you. The experiment was nothing too fancy, we were trying to test how well materials absorbed and reflect sound by measuring the intensity first with no medium between the speaker and the microphone and then with different material in between like foam carpet wood ecc.. Now i don't understand why when the sound source was very tightly close with the material we registered a smaller drop in intensity than we did when a little gap between source, material was left, sometimes it got even magnified a little! Instinctively I would say that the sound intensity should drop proportionally to the distance, but maybe the little gap allowed for some reflections to bounce back and forth, hence more sound wave made it through the material. I post the picture of the set up case1/case2 to help you understand what we did.( I tried to add photos but it doesn't work :()

 

[sophiecentaur]

I can't find any picture on your post but I suggest that the reason for the variation in results will be due to reflections between soundproofing and the loudspeaker source and the spacing. You have an example of a transmission line and the Impedance of the 'load', seen by the loudspeaker, will change as the distance along the transmission line changes. A standing wave is being formed. This will allow different amounts of sound power to pass into the material as the distance cnanges. Your suggestion about echos in the system is on the right lines and you need to reduce / eliminate the effect.

I would say that the sound intensity should drop proportionally to the distance,

You would expect that the variations due to the standing wave to be present 'on top of' a general trend towards a lower level as the distance increases.
If you use microphones on both sides of the material, you will most likely find that the ratio of sound levels measured will be the same but that they vary in absolute terms as the position changes.
I don't know what actual set up you used but, if the measurements were done in an open lab then all the above would be worse. If your arrangement was inside a sealed tube with sound damping in the walls, you would expect more regular results.

 

[tech99]

I am careful about measurements of absorption when the wavelength of the sound is large compared to the size of the absorbing sheet. In such a case we do not have "optical" conditions. In optical conditions we have two free space paths with an attenuator in between them, a simple situation. When the path is very short, there are two effects possible,
1) The beam from the loudspeaker is not yet diverging but is parallel, because the absorber is in the radiation near field, closer then the Rayleigh distance D^2/(2*lambda).
2) The absorber is so close it is in the reactive near field of the speaker, so it receives additional excitation.