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I Directing Low Frequencies

  1. Feb 26, 2017 #1
    Hey everyone.
    I have a couple of questions in regards to the direction of sound. I am wanting to direct lower frequencies (30-60Hz). When I say direct, I mean more of a beam rather than radiating out from the front of the speaker in all directions like a traditional wave.

    I have been looking at placing the speaker in the focal point of a reflector, however to do this with lower frequencies you need a fairly large reflector from what I have read. I am trying to do this with a single speaker.

    So my first question is in regards to the size of reflector for these lower frequencies, can it be half or quarter wave length? Is there a way to get around this limitation of the size of the reflector?

    My second question is what is the ideal size of speaker for lower frequencies? Is there a relationship between the size/diameter of the speaker and a specific wavelength?

    Does anyone else have ideas for directing low frequencies with single speakers?

    Any thoughts or input would be greatly appreciated.

    Cheers
     
  2. jcsd
  3. Feb 26, 2017 #2

    berkeman

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    Welcome to the PF.

    Yeah, I would use a single speaker at the focus of a large parabolic reflector that is steerable, or maybe several speakers driven as a phased array. What is the application?
     
  4. Feb 26, 2017 #3

    berkeman

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  5. Feb 26, 2017 #4

    sophiecentaur

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    The size of reflector needed to direct 30-60Hz sound effectively would be pretty big. (The wavelength is around 10m so the aperture would need to be comparable with (significantly bigger than) that.
    That would be a bulky arrangement and you could produce simple directivity (a 'doughnut' pattern) by using two speakers with anti phase connection. It would give a good null in a direction at right angles to the line between them. Unfortunately, any directive sound emitter can suffer from reflections from nearby objects. 30Hz would be very hard.
     
  6. Feb 26, 2017 #5

    berkeman

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    Good point!
     
  7. Feb 26, 2017 #6

    Nidum

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    Horn tube .

    Horn tube loudspeakers are commercially available .
     
    Last edited: Feb 26, 2017
  8. Feb 26, 2017 #7
    I forgot to add, that I was wondering if you could use a 1/2 or 1/4 wavelength width parabolic reflector which would reduce the size. Are there any rules or theory's around sound waves and reflectors?
     
  9. Feb 27, 2017 #8

    davenn

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    yes, you already asked that ... did you read the responses ?
    you didn't acknowledge any of them

    you also didn't answer berkeman's question as to what it is you are trying to achieve

    Do you realise that sound at 30 - 60 Hz is just a buzz ?

    here is an example of 30 Hz .... the signal from the crab nebula pulsar


    wavelength of a sound at 30Hz is 11.5 metres .... at 60 Hz = 5.7 metres
    again ... knowing what you are trying to do will determine how well a 1/4 or 1/2 wave diameter dish will perform


    Dave
     
  10. Feb 27, 2017 #9

    sophiecentaur

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    This is a diffraction problem. To produce a 'fair' directivity, your 'aperture' (reflector in this case) needs to be at least several wavelengths across or the launched wave will turn up in all directions. Such a small dish is virtually 'hardly there', as far as the waves are concerned. You can do the experiment by standing behind a wall and judging whether it makes any difference to the rumble of traffic going past. The wall may cut down the high frequencies a bit but not the low frequencies.
     
  11. Feb 27, 2017 #10
    I remember there was a company making a special kind of speaker that used nonlinear phenomena in air to produce sound at the listener's ear. I think it had a higher frequency carrier wave (which could be directed easily), and I guess it used some kind of amplitude modulation to generate lower frequency waves. I think it was intended for stuff like museum exhibits where only the person in front of the exhibit could hear the sound. I can't remember what it was called though.
     
  12. Feb 28, 2017 #11
    Firstly, thank you everyone who has replied here.
    The other day when I tried to see the thread all that would load was the 'Something to add box', and I couldn't see my original post, hence why i have mentioned the same thing twice! I also couldn't see any replies, hence not acknowledging anyone. So thank you.

    In regards to what we are doing with it or its application, we dont have a specific application in mind, more curiosity than anything.
    We are trying to see if we could mount the arrangement on a bearing that is standing straight up, allowing the contraption to turn in a full 360 degree circle. Which would allow you to have a beam that you could 'wash' over people if you know what I mean.

    Sorry I could not be more specific, this is literally just curiosity and extra time were filling in.

    Essentially I am reading here that for a frequency that low we would need a reflector that is massive, too massive for what we are doing. Half or quarter wavelengths wont work?
     
  13. Feb 28, 2017 #12

    berkeman

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    For an effect at a concert or something? Why?
     
  14. Feb 28, 2017 #13
    As I said, we dont really have a purpose in mind at the moment, more of a if we could do it, then I am sure we could find something to use it for. As you mentioned, in a concert or theatre could potentially be a use, if we could get it to work.
     
  15. Mar 1, 2017 #14

    sophiecentaur

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    A lot of the problems would be solved if you could use multiple sources. Not a problem, these days with wireless networking. Just expensive!!!! :frown:
     
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