Loudspeakers and Mircophones
(Triple Only)
Brook Edgar & Hannah Shuter
Teachers
Explainer Video
Microphones
Microphones work using the generator effect.
When sound waves reach the microphone, they cause the diaphragm to vibrate. The diaphragm is a thin, flexible piece of plastic that moves easily when sound waves hit it. The vibrating diaphragm causes a coil of wire attached to it to vibrate as well. The coil is placed in a magnetic field produced by a permanent magnet. As the coil vibrates, it moves through the magnetic field and cuts the magnetic field lines. This induces a potential difference across the ends of the coil. The pd induced is alternating (changes direction) because the wire is vibrating, moving back and forth.
If the circuit is complete, a current will flow. Loud sounds cause larger vibrations (higher amplitudes), producing a larger potential difference as the coil moves more, so it cuts more magnetic field lines. The frequency of the alternating potential difference induced is the same as the frequency of the sound wave.
Worked Example:
The image below shows a simple microphone.
Explain the purpose of the diaphragm in the microphone.
A loud sound is made close to the microphone. Explain how this affects the potential difference produced by the microphone.
Answer:
The diaphragm is a thin, flexible membrane that vibrates when sound waves hit it.
A louder sound causes the diaphragm to vibrate with a larger amplitude. This means the coil will cut through more magnetic field lines, so a larger potential difference is induced.
Worked Example:
A microphone is used to convert sound waves into an electrical signal.
Explain how a microphone produces a potential difference from sound waves.
Answer:
Sound waves cause the diaphragm to vibrate.
The vibrating diaphragm causes a coil of wire attached to it to vibrate.
The coil cuts the magnetic field lines of the permanent magnet.
This induces a potential difference across the coil.
Loudspeakers
Loudspeakers work using the motor effect.
An alternating current is passed through the loudspeaker. This current flows through a coil of wire, which induces a magnetic field around the coil. The coil is placed in the magnetic field of a permanent magnet and the interaction between the magnetic field around the coil and the magnetic field of the permanent magnet causes a force to act on the coil. As an alternating current is used, this means the current changes direction. When the direction of the current reverses, the direction of the force also reverses.
Because the current is constantly changing direction, the force on the coil keeps changing, causing the coil to vibrate. The vibrating coil is attached to a cone, so the cone vibrates as well. As the cone moves backwards and forwards, it causes the air molecules around it to move. This movement creates pressure variations in the air. These pressure variations travel through the air as a sound wave. A larger current produces a larger force and therefore a louder sound, and a higher frequency alternating current will cause the cone to vibrate more frequently, producing a higher pitched sound wave.
Worked Example:
A loudspeaker is connected to an alternating current source.
The force on the coil is directly proportional to the current. The current in the coil is and produces a force of . Calculate the force on the coil when the current is .
Explain how increasing the frequency of the alternating current affects the sound produced by the loudspeaker.
Answer:
Because force is directly proportional to current, we need to work out what the current was multiplied by to get from -> , then multiply the force by the same factor:
Increasing the frequency of the a.c. signal makes the coil (and cone) vibrate more times each second. This produces a higher frequency sound wave and therefore a higher pitch sound.
Worked Example:
A loudspeaker is connected to an a.c. signal.
Explain how the loudspeaker produces sound waves from an alternating current.
The current in the loudspeaker coil is increased. Explain how this affects the force acting on the coil.
Answer:
An alternating current flows through the coil, which is in a permanent magnetic field.
This causes a force on the coil (motor effect).
When the current reverses, the force reverses, so the coil vibrates backwards and forwards.
The coil is attached to a cone, so the cone vibrates and makes the air particles move, causing pressure variations that travel as a sound wave.
Increasing the current increases the force on the coil. This is because the force is proportional to the current, so a larger current gives a larger force (and therefore a bigger vibration / louder sound).
Practice Questions
A moving-coil loudspeaker uses the motor effect to produce sound. Describe how a loudspeaker uses the motor effect to create sound waves.
-> Check out Hannah's video explanation for more help.
Answer:
A current flows through the coil in a magnetic field.
The coil experiences a force due to the motor effect, causing it to move.
Reversing or varying the current causes the coil and attached cone to vibrate, producing pressure variations (sound waves) in the air.
Microphones use the generator effect to detect sound. A moving-coil microphone converts sound waves into electrical signals.
Describe how sound waves cause a potential difference to be induced in the coil of a microphone.
Explain why louder sounds produce a larger electrical signal.
-> Check out Hannah's video explanation for more help.
Answer:
Sound waves cause the diaphragm to vibrate.
The diaphragm is attached to a coil, so the coil vibrates in a magnetic field, cutting the field lines.
This induces a potential difference across its ends.
Louder sounds have larger amplitudes, so the diaphragm and coil move further.
This increases the rate at which the coil cuts magnetic field lines, producing a larger induced p.d.