Normal modes in a acoustic chamber

[Myrddin]

Have a project to do on an acoustic resonance chamber, a loudspeaker attached to a perspex box with a copper pipe, there was a microppohne inside the chamber connected to oscillloscope. We investigating a few variables in most detail was route [1]

[1]damping by chainging materials in the chamber carpet foam etc.
[2] using a birch plywood as the ceiling and seeeing the effects
[3] changing pipe size

in [1] the resonant frequencies were recorded for different materials, then reonance curve plotted to find quality factor, reverberation time, abosorption coefficent, we found the carpet was best absorber. But the resonance posistions changed slight WHY is this? can't find any detail explination anywhere, is it simply due to the change in dimensions caused by introducing the damping material?

[2] Found the birch plywood had quich high aborbtion coefficent, have deduced this is beceause the energy was transferred out of the system by causing the plywood to vibrate thus transferring sound outside? is this the principle on some musical instruments work

[3] This is the trickiest bit that i don't understand and our results are very inconsistent. Seem to find that the changing pipe doesn't add or removed and resonant frequency but seems to change the amplitude. Such that the smaller pipe has a section of frequencies of very high amplitude resonance, and the longer pipe produces the same resosnances but a different section of the resonance have very high amplitude?

Any help/explination/ guidance will be grateful! otherwise will end up doing this on xmas eve

 

[Myrddin]

still not done !

 

[Myrddin]

Have worked on part 1) and 2) hopefully enough for the project, going to do detail of how our results of damping are shown in use of special materials in theatres and qudiotoriums etc, and talk about musical instruments for 2).

Any help with 3) would still be grateful as the material I am finding is basic nodes in a pipe are similar to that on a string etc but can't find anything for the explination of pipes of different length have difference sections of high amplitude resonances : /

 

[Myrddin]

Seem as though there's no acousticians in the house !