What Are the Best Materials for Reflecting Low-Frequency Sound?

[leka]

Active/Passive noise control

hi everyone,
I am currently working on a project to reduce noise in AC/Heating systems in airplanes. As reference am using some literature in Engineering Noise Control and some research papers on Active/Passive noise control. I am currently searching for some information on; if there are materials that are good reflectors of sound at low frequencies, between 500Hz and 1KHz. The current aplication that have in mind calls for a sort of a muffler(closed ends pipe, from outside). Geometry and other parameters are not defined yet.
If anyone has any sources or knows any information related to mechanisms for noise reduction or have any ideas feel free to elaborate.

cheers

 

[Artman]

In your research you have probably already come across this, but I'll mention it just the same. Phase cancellation is an active form of noise reduction. You may want to check into that.