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A theory question!

Discussion in 'Subwoofers' started by cosaw, Jun 15, 2004.

  1. cosaw

    cosaw
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    Just doing some research and dowloaded a function generator to my PC.

    http://www.marchandelec.com/ftp/fg_lite.exe

    How is it that with the volume up on my fairly mediocre PC speakers I can hear a sine wave quite easily down to 12HZ?

    I thought this would be imposible without a high end sub. Am I missing something? Can someone enlighten me. I'm only just learning about subs so I'm sure this one can be cleared up quite easily.

    How low can anyone else here and on what sort of setup?

    Simon
     
  2. ShinObiWAN

    ShinObiWAN
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    You may be able to hear the 12hz tone - I personally doubt it, because I use 2 x 12" Ruark subs and can only hear/feel content down to 16hz when using a test tone generator, after this the output is unuseable.
    When you say you can hear this, do you have your ear right up to the speaker? Or can you really feel a strong trum thoughout the room, as if everything is shaking? Thats real gut wrenching bass.
     
  3. MuFu

    MuFu
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    What you're hearing is probably harmonic distortion. In attempting to play a 12Hz tone, your speakers are producing audible, higher frequencies.
     
  4. Nimby

    Nimby
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    With a true 12Hz you would be thinking you could actually count the beats.
    But as it's inaudible you can't. But the furniture vibrates silently too at that frequency. So you should be able to feel something if it was there at all.

    You'd need a pretty good sub to reach any sort of decent volume at that low frequency. It might be there with your speakers but at a very low level.
    That's 2 Hz below my 15Hz roll off on my 16-46 PCi. The roll-off is like a cliff. One Hz difference and there's nothing there at all!

    Any sort of volume at 12Hz is probably going to make you sick! It's somewhere around the body's resonant frequency. I believe 7-8Hz is the real killer though. They scrape you up with a shovel after a high dose of 7-8Hz! :(

    Do a Google for <infrasonic weapons> . Interesting reading.

    Nimby
     
  5. chrisgeary

    chrisgeary
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    in my room, a 12hz sine wave is similar to the buffeting feeling you get when you have certain windows of your car open at speed (although that is probably nearer 7-10hz). it is certainly audible to my ears in that it manifests itself as pulses of pressure; i can feel that pressure and it makes the background noise of the room come and go. this is at -30db as well (although since that is the internal sine wave generator of my 561, i'm not sure how meaningful -30db is. my point is its not loud, but the sub can pressurise the room effectively enough that i can feel it in my ears without cranking it up to insane coil melting volumes).
    Simon, if you can feel pressure in your ears like in the car, or maybe when a helicopter is flying over head, then that would be a reasonable comparison. either way, we are talking about shifting a lot of air, something that small pc speaker drivers aren't designed to do.
     
  6. deckard

    deckard
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    I'm pretty sure that's the explanation, your speakers are probably also producing output at 24, 36, 48, 60Hz etc so you may well hear something as you get to the lower extension of your speakers.
     
  7. avanzato

    avanzato
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    What speakers do you have?

    There is a DSP circuit by Waves that adds higher harmonics to simulate lower bass response. It's possible you have something like that on the computer.
     
  8. micb3rd

    micb3rd
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    Something also to bear in mind is that below 20 hz cone excursion inceases a lot, often you can hear subwoofers driver motor making noise which is not actually the sound of the bass note being played.

    Trust me you PC speakers can't play 12hz, hell I doubt they hit 30hz with any authority at all.
     
  9. cosaw

    cosaw
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    Thanks for the replies guys.

    From what people have said it would seem that it must be some sort of distortion or harmonic. I did not expect to hear anything below around 50Hz and so was suprised when I kept taking the frequency down. What I would say is that in order to hear this lower frequency "sound effect" as you decrease the frequency you need to increase the volume in tandem in order to hear the sound.

    The speakers are really fairly inexpensive, small, typical pc speakers. If I keep the volume at reasonable level for music (a lot lower) the sound rolls off sharply bellow 50Hz. It's only when whacking the volume up can I hear any noise, and it is not loud, room shaking or sickening but is quite evident.

    I'll have to plug my pc up to my sub-less stereo equipment downstairs and see what happens.

    If any of you want to try the program I used on my pc speakers it's only around a 50k download there's a link directly to it in my original post. Perhaps this is an effect that is common in cheap pc speakers.

    Thanks again

    Simon
     
  10. Londondecca

    Londondecca
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    Even at 50Hz, I would not expect a small speaker to be able to produce such a low frequency.
     
  11. cosaw

    cosaw
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    It would seem then I can't trully tell at what frequency my speakers offer genuine sound. Oh well the research continues!

    Simon
     
  12. Nimby

    Nimby
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    The PC speaker cones could simply be bottoming out! :blush:

    The excursion (cone movement) is way too much at low frequencies. But many speakers still try to do it. So the coil or cone hits the end stops. Which probably means something is whacking against the magnet on the inward excursion.

    Turning the volume up is a bit unkind at low frequencies on any speaker. Or even a sub! They thrash away, producing no audible noise and you think a bit more volume will help. But a glance at the graph of the sensitivity of the human ear will show you that you need an extra 30dB just to get the loudness of deep bass the same as the midrange!

    Throw in the problem of moving enough air to reproduce a low bass note and you're just asking for a damaged driver. Bass guitar doesn't go all that low. (~40Hz) But they still use large drivers to make loud noises and often use great stacks of them at a concert. It's all about being able to move enough air at low frequencies. The bigger the cone the more air is compressed for every cone movement and the less distance they have to move just to move the same amount of air as a smaller unit.
    I hesitate to think how many 2-3" drivers (or PC speakers) they'd need for bass guitar at a concert.

    Nimby
     
  13. ~Kev H~

    ~Kev H~
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    For 56Kb that really is a great little program! with my sub only being connected to the left channel it is interesting to compare left/right ie with/without sub without moving from your seat. It sounds to me like a pure sine wave that is being generated so I dont think it would be harmonics that have been put there by the program that you are hearing.

    My hearing of what i would describe as a sound definately stops just a couple of Hz above 20Hz, I think your PC speakers are likely going to their maximum excursions in and out and you are hearing some kind of "noise" that is generated as the cone hits max/min excursions, this would obviously be produced at twice the frequency.. so 24Hz.. and you could well be able to hear that.
     
  14. Londondecca

    Londondecca
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    The input maybe a sine wave but the speaker will still produce distortion and harmonics.

    20Hz is difficult for a sub woofer at say -6db. For this they used typically an 8 or 12 inch speaker with a high powered amp. 20 - 40 or even 60 Hz is a very low frequency for an average bookshelf sized speaker.

    Human hearing is not a good way to measure such things
     

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