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How to lower SPL by 30 - 40 dB

Discussion in 'Microphones' started by audionoob, Sep 14, 2016.

  1. audionoob

    audionoob Active Member

    Hi all, I am not sure if this is the right place to ask, but I want to take a shot: I have a circuit with small silicon microphone that has acoustic overload at 128db, however environment where I am going to monitor sound can get louder than that so I need to reliable lower incoming sound pressure by approximately 30db, kind of like putting protective headphones over ears when you do construction or lawn work. What's the best way to achieve that? Is there any acoustic putty or foam I can use to construct protective dome around mic or something?
     
  2. Boswell

    Boswell Moderator Distinguished Member

    By "silicon microphone" do you mean a MEMS device or something else? Since your post implies a single type of sound source, what does the acoustic spectrum look like?
     
  3. dvdhawk

    dvdhawk Well-Known Member

    There are inline Pads, (devices to attenuate the electrical signal produced by the microphone) you can connect between the microphone and a mixer or record. But as far as reducing the physical sound pressure by restricting exposure and airflow to the capsule, that seems certain to have a lot of negative effects on the sound quality (e.g. muffled). You can seal it in a Mason jar, or put it in an airtight box made of lead if you want to, but to what end? If sound quality doesn't matter and your goal is to document the SPL level, any measurements would be pointless unless you could accurately test and measure the exact amount the enclosure reduced SPL. And it would need to be a complete assembly, and be airtight to do much good. Different materials have different levels of noise reduction throughout the audio spectrum. Silicone microphones are common in hearing aids and are probably not the best option for artillery/gun range, jet engine, Cape Canaveral type decibel levels - but that may be a key component of your project. Not knowing more about your 'environment', or your end needs, is going to make it difficult to get good suggestions.

    If you can share any more details, they would be useful.
     
    audiokid likes this.
  4. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    At what frequency?
     
  5. audionoob

    audionoob Active Member

    Thanks for reply all, yes this is a MEMS mic, and it is to monitor noise level, I can't answer question about frequency and spectrum because I simply don' t know yet, but what it is - basically it is an experiment that I am doing, I want to monitor noise levels and record them when they get over certain amount that can damage hearing, basically I will set threshold and and listen to the noise at approximately 10 - 15khZ whenever it gets over that level I will measure how long it was there, all data is stored to SD card and I will later analize it. So the sound quality is not really that important since I only monitor voltage spikes on the mic's output. I do not really care about accuracy in dB measurement, 30dB number came from earmuffs that I have, it says 30dB so I assumed it lowers environmental SPL by approximately 30dB and that's why I assume if I lower incoming airwave by 30dB that will guarantee mic's safety. So error threshold is fine, what I do need though, is stability over himidity, temparetaure, etc. Currently my best plan is to simple cut small square peice of dense foam and glue it around microphone's input port, that seem to attenuate incoming SPL by some decent amount, but I am concerned now that it will decay under the sun or wind, or rain moisture, etc... So I was wondering if there any industry standard for tricks like that maybe. The best other idea I came up with is literally just glue small dome shaped plastic object over microphone to simulate earmuffs over ear but that just sound to cheesy.
     
  6. Lofty Whitaker

    Lofty Whitaker Active Member

    Sounds like it will identify what noise is causing potential hearing damage. Question though, if you reduce the SPL getting to the mic how will you set the threshold it records at? Might need a MEMS mic with higher SPL handling. Is there such a thing?
     
  7. audionoob

    audionoob Active Member

    That part is easy, its outputting voltage, so you send it to DAC and instead of Vpp of let's say 1v you trigger at Vpp 0.7V for example. But that's theory. While i was waiting for replies to my post I did some more research and came across mems mic that has SPL limit of 160dB with guaranteed linear responce up to 130dB, I think that's what I will go with :) I think 160dB is so loud that I will never come across anything close to it in my experiment.
     
  8. DonnyThompson

    DonnyThompson Distinguished Member

    Let's hope not. You are using ear protection, though, right?
     
  9. paulears

    paulears Well-Known Member

    It overloads at 128dB and you want to be with it in a space working at over 150dB? Crazy, and dangerous!
     
  10. Lofty Whitaker

    Lofty Whitaker Active Member

    I'm hoping the idea is that you can leave this gadget in place and it will record only when the db exceeds a certain threshold. Thus identifying the noise that is dangerous. Audionoob will probably end up with snippets of AC/DC, ask Brian Johnson ;)
     
  11. paulears

    paulears Well-Known Member

    Ah - I get it now. He wants to physically reduce the SPL entering the microphone. Either absorption by something in the way of the flow, as in foam, or by cleverer tricks like venturi devices - which can step up or down pressure depending on which way around they are used. All these devices impact on frequency response, so some experimentation is needed.
     

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