Microphones, Loudspeakers & Headphones (HT only)
Dynamic Microphones
- A moving coil (dynamic) microphone also work using the principles of electromagnetic induction
- The convert the pressure variations in sound waves into variations in current in electrical circuits
A moving coil microphone
- When sound waves reach the microphone, the pressure variations cause the diaphragm to vibrate
- This in turn causes the coil to move back and forth, through the magnetic field
- As it does so, the coil cuts through the field lines, inducing an alternating potential difference in the coil and therefore an alternating current
Loudspeakers & Headphones
- Loudspeakers and headphones convert electrical signals into sound
- They work due to the motor effect
- They work in the opposite way to microphones
- A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet
Diagram showing a cross-section of a loudspeaker
- An alternating current passes through the coil of the loudspeaker
- This creates a changing magnetic field around the coil
- As the current is constantly changing direction, the direction of the magnetic field will be constantly changing
- The magnetic field produced around the coil interacts with the field from the permanent magnet
- The interacting magnetic fields will exert a force on the coil
- The direction of the force at any instant can be determined using Fleming’s left-hand rule
- As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction
- This makes the coil oscillate
- The oscillating coil causes the speaker cone to oscillate
- This makes the air oscillate, creating sound waves
Examiner Tip
Microphones and loudspeakers look very similar, but they do very different things. When tackling a question on either of them, make sure you are writing about the right one!
The explanation of the loudspeaker is very similar to the explanation of a motor, however direct current is used in a d.c motor and alternating current is used in a loudspeaker or headphone. You need to learn how both work.
When explaining how a loudspeaker works remember to refer to the alternating current and the changing magnetic field that it creates.
