Speaker is a device that converts an electrical
signal into an acoustic signal. Speaker consists of mainly three parts: 1) A
permanent magnet, 2) A diaphragm, 3) A coil of wire/voice coil. Speaker works
on a principle of electromagnetism and motor effect. The sound emits by a
speaker is due to movement of a diaphragm, which compresses and expand of air
surrounding to it. This movement of a diaphragm is due to electromagnet
attached to it. An electromagnet is a magnet which is created by current
carrying coil often known as a solenoid. In the speaker, a permanent magnet is
placed inside the coil. When current passes through the coil, it creates a
magnetic field around the coil. This magnetic field has north and south polarity
same as a permanent magnet. Electromagnet feels the force due to a permanent
magnet and this force makes electromagnet and diaphragm move. Back and forth
movement of a diaphragm according to a different range of frequencies, it
produces sound.

           
According to Motor effect, “when an electric current
flow through a wrapped coil that is placed in an electric field, the coil will
experience a force and move.” The movement of diaphragm depends upon the
direction of current flow through the coil. This can be shown by two cases: 1)
as shown in fig-1 when current flows in the upward direction and according to
right-hand rule, it creates a clockwise magnetic field around the coil and
direction of the force is in the direction of thumb means a direction of the
current. The direction of a magnetic field by a coil is opposite to the
direction of a magnetic field by a permanent magnet. This cause repulsion
between these two magnets, so coil move right attached to a diaphragm. 2)as
shown in fig-2 when current flows in a downward direction and according to
right-hand rule, it creates an anti-clockwise magnetic field around the coil
and direction of the force is in the direction of thumb means the direction of
the current. The direction of a magnetic field by a coil is in the direction of
a magnetic field by a permanent magnet. This cause attraction between these two
magnets, so coil move left attached to a diaphragm.

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As applying alternating current to the coil, the diaphragm moves back and forth
according to the direction of current and at a specific frequency of the
applied alternate current.