Anybody else think that title sounds like something from a research paper rather than a forum post? I'm trying to figure out a good physical system or material to block high frequencies from getting to a microphone. I haven't found much when searching for a 'physical low pass audio filter' or 'sound frequency attenuation by material'. A lot of research is done on the opposite problem: blocking low frequency noise from traffic or building systems. The actual problem that I'm trying to fix is a vehicle's hands free audio that seems to over-amplify the mid to high frequency range of voices from inside the vehicle. The voice on the receiving phone sounds pitch-shifted slightly and quite distorted even at the lowest volume setting. I suspect the vehicle manufacturer used a cheap or inadequate noise reduction system to filter out low frequency traffic and vehicle noise then they amplify the remaining sound too much and it sounds distorted on the other end. I know that if I wanted to filter out wind noise, I'd probably use some kind of fluffy fuzzy thing in front of the microphone. If I wanted to filter out low frequency noise, I'd probably use some kind of porous foam. But what material/method would filter out the high frequency end of the audible spectrum without affecting the lower frequencies too much?