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Motor effect of electric current: 11.6 How a loudspeaker works - an application of the electric motor effect

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INDEX physics notes: motor effect of an electric current and applications

11.6 The loudspeaker - an application of the motor effect

 How does a loudspeaker work?

A loudspeaker works in the opposite way to a microphone.

Also I've added a comparison of headphones and loudspeakers.

diagram showing how a loudspeaker works electromanetic coil vibrating cone diaphragm creating sound waves

Diagram showing how a loudspeaker works

The motor effect of a magnetic field and conducting coil is used in the function of loudspeakers, often of the small headphone variety these days!

In loudspeakers, an a.c. current conducting coil is moved by in a magnetic field to convert electrical energy into sound energy by way of a vibrating cone (diaphragm).

The cone vibrates the air and the oscillations produce the sound waves you hear.

An a.c. (must be an alternating) current is passed through a coil of insulated copper wire attached to the base of a paper-cardboard cone (or plastic cover).

The coil is wrapped around one pole of a permanent magnet and is itself surrounded by the other pole - you can see this in the diagram where the magnet is specially shaped to allow the coil/cone base to fit in.

The cylindrical magnet produces a strong radial magnetic field and at right angles to the coil - both contribute to the maximum vibration affect.

(You can't see that the south pole continues to the left into the hollow base tube of the cone - diaphragm).

When the current passes through the coil, it produces a magnetic field and around the coil wire which interacts with the magnetic field of the permanent magnet.

This causes a force to move the cone (to which the coil is attached).

The larger the current 'signal' the larger the movement of the cone.

In this case we are not dealing with rotation as in an electric motor, but with a mechanical 'to and fro' vibration effect, but it still involves what we call the 'motor effect'.

When the ac current reverses, the force acts in the opposite direction, so the cone moves in the opposite direction too.

Therefore the variation in the a.c. current signal makes the cone vibrate (oscillate) in the same varied way, which in turn makes the air particles vibrate and it is these variations in pressure that causes sound waves to emanate from the speaker.

Therefore the frequency of the sound waves produced by the cone vibrations is the same as the frequency of the a.c. signal.

In reality, the a.c. signal is very complex enabling us to hear a full sound picture of e.g. music generated from a complex music wave superimposed on a carrier wave.

The diagram below is a reminder of a simplified sound wave model.

Headphones and loudspeakers

They both work in the same way, converting electrical wave signal into a sound signal.

However, as well as the difference in size, the power input and output are much lower for a headphone, particularly if it consists of small earbuds.

A loudspeaker needs to fill a much larger space e.g. room or dance hall, whereas a headphone only has to fill your ear lobe!

With the onset of mobile phones, they have enough power to allow you to use a pair of earbud headphones for several hours.

If you want to power some loudspeakers from your mobile phone, you will need some extra battery power!


Note that a microphone is a bit like loudspeaker working in reverse - illustrated below,

diagram explaining how a microphone works magnet coil diaphragm

Keywords, phrases and learning objectives on the motor effect of an electric current

Be able to interpret a diagram to explain how a loudspeaker works and know it is an application of the electric motor effect.


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INDEX physics notes: motor effect of an electric current and applications