Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I would like to start a discussion involving how sound works.

  1. Feb 18, 2011 #1
    I would like to start a discussion involving how sound works. I've read plenty of websites but none can satisfy the level of curiosity I have. To get started I would like to know what it means for low frequency to travel farther than a high frequency all sound waves travel the same speed? The explanations Ive gotten from websites is just that they are lower that doesnt tell me why it goes farther I feel that there is more to it than that?
  2. jcsd
  3. Feb 18, 2011 #2
  4. Feb 18, 2011 #3
    Re: Sound

    Thats not what Im asking Ive read plenty of websites on this subject but none explain it in a way I can understand. I know how fast sound travels and that it depends on the air pressure and temperature. I want to know why low frequencies travel farther I dont see any reason why they would it just seems that more pressure is required to get a low frequency started and so it travels farther because their is more energy involved is this correct?
    Last edited: Feb 18, 2011
  5. Feb 18, 2011 #4
    Re: Sound

    Here's a start for you:


    If you read through here:


    You will note that as the frequency of the sound increases, the attenuation also increases (by the square of the frequency).

    Put simply, the higher the frequency of the sound, the more attenuation there is as it travels through the medium (in this case air) and as such the quicker the intensity drops over distance.
  6. Feb 18, 2011 #5
    Re: Sound

    That site helped alot. I would also like to know how reflections on walls work, for example If you have a sound in free air, pretend there is no floor or celing, just a wall behind you how do you determine how much increase in pressure there is with the intial wave and the one reflecting from the wall?Im not talking about the exact formula im talking about if there is any increase at all?
  7. Feb 18, 2011 #6
    Re: Sound

    Actually the part about the 100hz wave traveling four times as far as the 400hz wave wouldnt a 100hz wave require more energy to travel the same distance as 400hz given any level?
  8. Feb 18, 2011 #7
    Re: Sound

    Energy of a sound wave isn't related to its frequency. So no, it wouldn't require more energy to travel the same distance.

    What the article refers to is that for the level of attenuation a 400hz wave encounters over a set distance, a 100hz wave would have to travel four times as far to encounter the same level of attenuation.

    Not that a 100hz wave travels four times further than a 400hz wave.
  9. Feb 18, 2011 #8
    Re: Sound

    This is what I can't wrap my mind around lets take a 12200hz sound wave which is about 1inch and say it when it has reached 20 feet away from the source did it produce the same amount of pressure to reach that distance as a 20hz wave did to reach that same distance?
  10. Feb 18, 2011 #9
    Re: Sound

    This is where the equations from the second link I gave you come in.

    The 12200hz wave has massively higher attenuation than the 20hz wave. So the intensity of the first wave (and as such the pressure) after a set distance will be less than that of the second wave. (The first wave will have a lower decibel reading than the second after a set distance.)
  11. Feb 18, 2011 #10
    Re: Sound

    Dont you mean the first wave is the strongest and the second wave will have a lower decibel reading.
  12. Feb 18, 2011 #11
    Re: Sound

    No. Have you not read the links I posted?

    The higher the frequency, the more attenuation the wave encounters.

    This means that after a set distance, the higher frequency wave will have a lower intensity (and as such lower decibel reading).

    A low frequency wave can have a decibel level equal to a high frequency wave.
  13. Feb 18, 2011 #12
    Re: Sound

    Tell me if this is right, when a bass speaker moves forward the air in front is compressed and the air behind the speaker rarefacts(whatever the opposite of compression is called) so the air in front will travel towards the back to achieve equilibrium, thus not producing a very loud sound not matter how fast its moving is this correct?
  14. Feb 18, 2011 #13
    Re: Sound

    Ah, I see your problem. You're not understanding what a decibel is and what it means in relation to sound pressure. Read up on these first.

    Pressure and volume are not the same thing. A bass speaker can send out a wave that is almost inaudible, but the pressure of that wave can be damaging to the ear. This means the wave has a low volume and high pressure.

    The opposite of compression is expansion. I'm not sure about your explanation of a bass speaker. All speakers operate on this principle, so the volume hypothesis above cannot be correct.
  15. Feb 19, 2011 #14
    Re: Sound

    You know thats why they put the woofers in an enclosure to have the back wave work with the front wave.
  16. Feb 19, 2011 #15
    Re: Sound

    Again, you need to understand the difference between the pressure and the perceived volume.

    Your description didn't mention the enclosure, hence it wasn't strictly correct and could apply to all speakers.

    I'm not going to sit and argue the technicalities. You've received the answer as to why low frequencies travel further than high frequencies. Any further questions on that are welcome.
  17. Feb 19, 2011 #16
    Re: Sound

    I got on here to learn about sound, it seems youre saying I can only ask one question, if this forum doesnt allow this kind of back and forth could you recommend a place I can go for that kind of exchange. I wasnt going to ask just one question and it would be a waste to start another thread for different questions.
  18. Feb 20, 2011 #17
    Re: Sound

    You are free to ask as many questions as you like and I will gladly answer (see the last part of that post you quoted).

    However, it is not uncommon for people to ask posters to read up on a few things before continuing to ensure they fully understand what they are saying. All I have asked is that you read up and understand the various terms you have thrown around erroneously.

    I have asked this as you are bringing up small technicalities which would be answered if you understood the terms correctly. I don't have the time to sit and type up every little thing I'd like to to help you learn, so I can only point you in the right direction.

    Again, if you have further questions please do post them. But ensure you follow any advice I give (re reading materials) before continuing - it might just solve the problem for you.
  19. Feb 20, 2011 #18
    Re: Sound

    Just to come back to this, energy is related to amplitude not frequency. If both waves have the same amplitude they have the same energy.

    Decibels are related to the pressure level, which is related to the energy of the wave.

    So a 20hz wave can have a higher pressure reading than a 122000hz wave.

    Have a look at these two:

    Sound Intensity: http://en.wikipedia.org/wiki/Sound_intensity
    Sound Energy Density: http://en.wikipedia.org/wiki/Sound_energy_density

    They should help you with the energy / pressure of a sound wave and with your above problem.

    Just note that a higher frequency doesn't mean a higher decibel reading.
  20. Feb 20, 2011 #19
    Re: Sound

    I understand what you mean. I mistakenly thought when you said the second wave was stronger that the first wave was the second wave once it proceeded past a certain point. You meant the very next wave coming up from the source, correct?
  21. Feb 20, 2011 #20
    Re: Sound

    I am trying to understand how sound waves add to together how come if I take a bucket and hum some notes certain notes will sound louder than others same thing with a trash can but if I take a dresser shelf or a coffee mug nothing happens no matter what note I play?Also when I play a subwoofer in my room its not as loud as when I put in my truck but when I put it a two door car its not as loud as is was when it was in the room this doesnt add up if the if the truck is louder than the room the car should be some where in between but its the least in volume?(because my room is twice as big a space as the car.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook