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Homework Help: Reassurance for Physics Coursework (Sound)

  1. Apr 13, 2006 #1
    Hello :rolleyes:

    I just wanted to make sure I have these ideas right before I right them up in my physics coursework! I recently did some tests and got these relationships out, but didn't really have enough time to double-check them.

    Ok. I took a simple test involving sound and distance. Is the distance proportional to the inverse square of sound intensity? So, as the distance increases a bit the intensity decreases by a lot?

    Also, if I had a case around a speaker (say made from cardboard), but it still had the speaker exposed, would it increase the volume as opposed to no case? (I got this shown in my results - I think the case was called a "baffle" by my teacher)

    I'm not too good around this area of physics as you may be able to tell!

  2. jcsd
  3. Apr 13, 2006 #2


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    Staff: Mentor

    "Is the distance proportional to the inverse square of sound intensity? So, as the distance increases a bit the intensity decreases by a lot?"

    You mixed up your words a bit in there. The sound power is proportional the the inverse square of the distance. Basically think of concentric spheres centered on an isotropic sound source (like a firecracker). As the sound waves radiate out from the source at the center, the same power is going to go through each concentric sphere. But the surface area of each sphere is bigger and bigger, going as r^2, right? So the same sound power is going through a bigger surface area, so the sound intensity (something like power/area ?) will fall off as 1/r^2.

    Your second question is not worded well enough to answer, IMO. You would need an accurate diagram of what you are trying to ask. If the "speaker" is already enclosed and you put a 5-sided box around the back of it, nothing will happen. If by "speaker" you mean an open-frame speaker cone assembly, then putting an enclosure behind it can definitely increase the volume that you get out in front of the speaker.
  4. Apr 13, 2006 #3

    Thanks for the reply! When I just re-read what I wrote I noticed I was using the wrong word, I meant amplitude rather than intensity. I think :rofl:

    Ok. I did word the second question completely uselessly and I didn't even mean a box. lol. I'm in a bit of a rush you see! :rolleyes:

    I mean like a speaker with a card square around it, with no depth (except for the thickness of the cardboard). Like a fram with a picture, except a large piece of card then a cut out piece for the speaker to fit in.

  5. Apr 13, 2006 #4


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    Staff: Mentor

    So if the 2nd question is really whether adding a wall where the cone frame mounting points are changes the volume in front of the speaker cone, then I would guess the answer is no. If you have an open-frame speaker, with the metal frame around it and the magnet and coil to the rear, the putting a wall or frame on it shouldn't do much of anything. The sound waves being pushed out the front still go out the front, and the sound waves being pushed out the back (through the big openings in the metal frame) still go out the back.

    The main change happens if you put an enclosure of some kind in the back. That can change the amount and character of sound that you get out the front.
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