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Homework Help: Moving coil in a loudspeaker. help please.

  1. Nov 12, 2007 #1
    can anyone help with me understanding the physical mechanisms that determine the current that flows through a MOVING coil immersed in a magnetic field when a fluctuating voltage is applied across the coil?

    Also I need to find a Differential Equation that describes the behaviour of the current flowing through a STATIONARY coil when a voltage is applied across it?

    Any help would be greatly appreciated.

    Last edited: Nov 12, 2007
  2. jcsd
  3. Nov 12, 2007 #2


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    Staff: Mentor

    Welcome to the PF, belfunk. I've moved your post to the Homework Help forums, where homework and coursework questions should be placed.

    We discussed speaker cone movement a bit in this recent thread:


    The speaker coil on the back of the cone basically looks like an inductor with loss from the energy that gets imparted to the air to make the sound. The voltage across the inductor causes a current to flow, and this current generates a magnetic field that interacts with the DC bias magnet to cause forces. Can you show us some of your work on this question? You are required to show us your work in order for us to provide you with some tutorial help.
  4. Nov 14, 2007 #3
    well the 1st question of my essay was to describe the form and operation of an electromagnetically driven loudspeaker. This was my answer:

    "An electro-magnetically driven loudspeaker comprises of a moving coil. The moving coil is used to convert electrical signals into acoustic energy. The coil of the loudspeaker is located in an air gap in which there is a magnetic field produced by a permanent magnet. Then an amplified audio signal is applied to the coil. This voltage causes interaction between the current in the coil and the magnetic field from the permanent magnet, causing the coil to vibrate, and this motion is transmitted to a radiating element normally in the form of a conical diaphragm."

    For my own interest, not my assignment can someone tell me why the coil moves and not the permanent magnet?

    The next question is to describe the physical mechanisms that determine the current that flows through a moving coil immersed in a magnetic field when a fluctuating voltage is applied across the coil.

    I assume that the movement of the coil cutting through the magnetic field of the permanent magnet will determine the current.... is this true??? I am struggling to find anything about this on the net.

    I'm still trying to understand the differential equation question. I've been given some equations, but i'm trying to get my head around them.
  5. Nov 14, 2007 #4


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    Staff: Mentor

    The stationary magnet is fixed in the loudspeaker, so the only thing that can move is the speaker cone, with the coil attached to the cone. The force that moves the coil and cone comes from the current interacting with the stationary magnetic field.

    This wikipedia.org page is a pretty good introduction, with some good links for further reading:

  6. Nov 19, 2007 #5
    thanks for the help berkeman. much appreciated.
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