Name: Linda L.
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?
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
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
If you are looking for very detailed information on microphones:
And if you are interested on how ACD or DAC work:
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
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.
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Update: June 2012