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Name: Linda L.
Status: other
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Question:
In a dynamic microphone, when the voice coil moves in relation to the stationary magnet,
1)how is the electrical current created?
2)how would the computer recognize what has been sent by the microphone?



Replies:
Dear Linda,

A dynamic microphone, like all electrical generators, produces electric signals in accordance with Lenz's Law. Lenz's Law says that if a wire is moved through a magnetic field, an emf (voltage) is induced in the wire. The voltage is proportional to the strength of the magnetic field, the speed of motion of the wire and is affected in a complicated way by the angle between the wire and the magnetic field and the angle between the direction of motion and the magnetic field and wire. To give you an idea of the angular dependence, the induced voltage is a maximum if the wire is perpendicular to the magnetic field and the wire motion is perpendicular to the magnetic field and the wire; it is zero if the wire is parallel to the magnetic field or if the wire motion is parallel to the magnetic field or to the wire. Complicated!

However, in the usual case, a voltage is generated which is proportional to the speed of the coil. Thus, if the coil is moved back and forth in a motion that can be described by a sine wave, an electrical sine wave will be generated. This can then be sent to a computer which can digitize it (measuring the voltage many times per second and storing the numbers).
The computer can later change these numbers back into voltages (produce an analog signal) and regenerate the music.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Linda,

One primary principle of electromagnetism is that a moving magnet creates an electrical field and a moving electrical field (current) creates a magnetic field. Even though it is the voice coil that moves, it is moving through a magnetic field. Relativity dictates that there is no difference between a moving magnet and a stationary coil versus a stationary magnet and a moving coil. Either way, the coil sees a moving magnetic field, which induces a current in the coil. The quicker and longer the motion of the coil, the larger and longer the current that is produced. This current moves through a wire, where the voltage of the current is read by a meter. I belive this meter can be mechanical (analog) or digital though I am not sure exactly how the meter works at the molecular level. The current is proportional to the frequencies being inputted. It is in this way that the original signal can be reproduced by transforming the electrical signal back into a sound wave via the voice coil in a speaker. If the original sound wave is an analog signal, a ADC (analog-to-digital converter) converts the signal to digital (binary), which a computer can interpret.

Here is a link to an overview of how several different types of microphones work:

http://electronics.howstuffworks.com/question309.htm

If you are looking for very detailed information on microphones:

http://en.wikipedia.org/wiki/Microphone

And if you are interested on how ACD or DAC work:

http://en.wikipedia.org/wiki/Analog-to-digital_converter

http://en.wikipedia.org/wiki/Digital-to-analog_converter

Matt Voss


For the first part of your question, moving a conductor across a magnetic field is what causes the current. This is the same effect that is used in a generator or alternator. Unfortunately, I do not believe I could give a more detailed explanation and remain technically accurate.

For the second part of your question, "how would the computer recognize...", It is a matter of sampling the electrical impulse coming from the microphone. Sampling just means measuring, repeatedly. The computer really only reads the strength of whatever vibration is on the microphone at that instant. (kind of like making a single dot on a graph) by taking many, rapid samples, that 'dot' is extended into a longer, wavy line that represents the sound waves striking the microphone.

Ryan Belscamper



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