Physical System for High Frequency Acoustic Attenuation

Discussion in 'Sound Science' started by Yardstick, May 23, 2018.
  1. Yardstick
    Anybody else think that title sounds like something from a research paper rather than a forum post? :D 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?
     
  2. pinnahertz
    The application of materials for deliberate high frequency loss is something that has not been extensively studied, as generally the goal is to protect a mic from wind with the least possible loss of HF response. This Study details some test of various materials, but you likely won't find anything that does what you're looking for. The change you describe is best done electronically.
     
  3. Yardstick
    Thank you for the response and link. It gives me some ideas to try. I may look into an electronic solution as well. I might have an idea if that's an option when I get access to the microphone.
     
  4. castleofargh Contributor
    high freqs are easy to block, anything blocking the direct path will remove more high freqs than low freqs. the problem is about where the mic is relatively to your mouth and the other sources of noise.
    for the mid range it might be different, but I'm guessing that your problem comes from your device doing it's best to capture voices. perhaps your mic uses the lowest connection standard(the one meant for basic speech instead of music), perhaps there is some processing done to increase the mids where voices usually are, and it just happens to also boost some other loud sound in the car leading to distorted sound or at least crap audio?
     
  5. Glmoneydawg
    Sounds like overdriving....too much signal into amp section?
     
  6. Yardstick
    I thought high frequencies would be easier to block independent of the rest of the frequency range, but it turns out it's a little more complicated when you want to retain lower frequencies as much as possible. It might work to block the sound to the mic in the vehicle though. Putting a hand over the mic slots in the overhead console area seems to help as long as the person talking doesn't push too hard. They must be over-amping the mid to high range. I do hear some background noise when the driver is talking but it doesn't sound like it is the source of the distortion. I'll try something simple like taping a piece of plastic over the mic slots to see if that works.

    That could be. As I said above, I'll try something simple first. Maybe bringing the level at the mic down across the frequency range will help.
     
  7. ev13wt
    Place an EQ between mic pre and car?
    Cover the slots with something plastic
     

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