Bandpass noise filtering

  1. I am looking to filter the human voice from enviornmental noise (engine/machinery noise). I was doing some digging and the human voice is between 300 and 10,000 Hz, unfortunatly some of the engine noise is in the same frequency.

    I could build a bandpass filter and at least eliminate some of the low and high frequency noise but how do you filter noise that is in the same frequency as the signal that you want?

    Would you have to hook up a mic to a network analyser to see what the specific frequency of the machinery is and then apply a notch filter? And accept that certian tones of voice (no pun inteneded) would be truncated?

    I have heard there are advanced statistical filters but I have no idea how these work and how well they really work as an active filter.
  2. jcsd
  3. berkeman

    Staff: Mentor

    Sounds like a good candidate for Noise Cancellation:

  4. sophiecentaur

    sophiecentaur 14,697
    Science Advisor
    Gold Member

    I agree. But it isn't a trivial exercise. It is relatively easy to make noise cancelling headphones because the noise can be identified (it's outside and not inside the cans) and the source of the anti-noise is right next to each ear. It is much harder to produce an extensive region in which the noise from an engine is significantly cancelled because the phases change with position.

    What is the actual setup; i.e. where is the voice you want to hear and where is the noise source?
    One possibility would be to have an adaptive filter that could lower the level of received engine noise - by introducing strong notches in the regions of the worst of the engine frequencies. This would leave spaces (hopefully) through which sufficient voice energy would get to make sense of it. You would need to let the filter 'learn' the noise environment before the voice is introduced. I think this is doable with DSP, assuming the signal to noise ratio is suitable.
  5. This is for Aviation head sets. This is when the transmit button is pressed and NOT from ambient noise piercing the head set. When the transmit button is pushed all of the engine noise in addition to your voice is being transmitted through the mic. You can put little guards on there that muffle the noise a bit but its still really loud. This is partly do to the fact that I fly a very noisy nearly open cockpit plane.

    I have a bachelors level knowlage of filter design (analog), I will have to look up how adaptive filters work. Are these commercially avalible?

    Bose makes noise canceling head sets but that does not apply to transmissions.
  6. sophiecentaur

    sophiecentaur 14,697
    Science Advisor
    Gold Member

    Ok. That's a clearer idea of what you want. The mikes (lip mikes?)that commentators use, in noisy crowds, are very effective and they work on the basis of directivity I believe. Just a passive design in the old days. Also, a vox operated switch will stop the background noise when you are not talking. Less fatiguing to listen to but It doesn't help intelligibility.
    I'd guess that, as the aircraft engine speed doesn't change often and many of the engine noise frequency components are harmonically related, an adaptive 'comb' filter could really help.
  7. sophiecentaur

    sophiecentaur 14,697
    Science Advisor
    Gold Member

    These guys seem to have done the job for you - at a price. It could be a bit heavy and bulky to fit on a flying helmet, though.
    It does make me think that a hardware approach may be better. You will be dealing with wind noise as well as engine noise and that is a universal problem 'cos you can't filter out broadband noise that lies in the wanted (voice) spectrum.
  8. Commercial aviation headsets such as David Clark brand, already do as pretty good a job at noise rejection.

    If you want better performance than that, go back to the WWII solution, the throat microphone.
  9. berkeman

    Staff: Mentor

    Yeah, noise cancelling microphones on headsets are very common. Why don't you just use one of the standard commercially-available headsets with noise cancelling microphones? They are common in aviation and in noisy mobile applications (like motorcycle police officers).
  10. That is cool, I will contact them and see if they make something compatable with aviation head sets. The key is I have to be able to wire it into the planes radio and intercom or figure out how to retrofit it onto my existing head set.

    I was hoping for a filter because that way it would filter out other pilots transmissions into my head set as well.
    Last edited: Jul 8, 2014
  11. A true noise canceling mic is not common in the aviation circles that I am apart of. There are alot of other pilots that have garbled transmissions as well. The avionics store in my area also could not help me.

    Do you know of any commercially avalible mics that are compatable with aviation head sets or active filters that could be wired into the radio/intercom system? I contacted david clark and Bose and they could not help me either.

    The aviation head sets are great for eliminating sound when there is no radio transmission going on but its the incoming and outgoing transmission that is hte issue. It would also be nice to filter other pilots transmissions as well.
  12. sophiecentaur

    sophiecentaur 14,697
    Science Advisor
    Gold Member

    It would be very difficult to do that in your receiver as the signal and noise are already added together. The way to deal with the problem is at source - eliminating a significant amount of acoustic noise getting to the transducer.

    Did you read the comment, above, about the throat microphone - as in WW2? Possibly a way forward. Otherwise you may need to take a step up in your budget and look at the headsets that were also referred to. No one said that Flying was a cheap hobby. I find Sailing is quite expensive enough but it's not in the same league as your passion. Commiserations.
  13. The price of the mic does not bother me. I just need to know if they can make it compatable.

    I will have to email them as it looks like its a forign company.
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?

Draft saved Draft deleted