P4.8 Loudspeakers & Microphones

• Loudspeakers and microphones are both similar in that they convert between sound waves and electrical signals.
• Loudspeakers take electrical signals and use the motor effect to convert them into vibrations, which then generate sound waves.
• Microphones detect vibrations caused by sound waves and use the generator effect to convert them into electrical signals.
• For both microphones and loudspeakers, the electrical signals involved are alternating currents (or alternating voltage).
• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet and connected to a cone. The coil of wire is permanently attached to the cone, but the coil and cone can slide back and fourth along the magnet. 

  
• What type of current flows through the wire in loudspeakers?

  Alternating current (AC)
• What does the AC create in the coil of a loudspeaker?

  An electromagnetic field
• How does the magnetic field of the coil interact with the permanent magnet?

  It interacts with the magnetic field of the magnet
• What effect does the interaction of the magnetic fields have on the coil?

  It exerts a force on the coil
• What is the result of the force exerted on the coil?

  The coil moves back and forth
• What happens to the cone as the coil moves?

  The cone changes shape
• What happens to the magnetic field and force as the current alternates?

  They also alternate
• What is the result of the cone and coil moving back and forth?

  They cause vibrations
• What do the vibrations of the cone create in the air?

  Pressure variations
• What are the pressure variations in the air called?

  Sound waves
• How do loudspeakers convert electrical energy into sound?

  • AC flows through wire, creating electromagnetic field
  • Magnetic field interacts with permanent magnet
  • Force on coil causes it to move
  • Cone changes shape, creating vibrations
  • Vibrations cause pressure variations in air (sound waves)
• Microphones have a very similar structure to loudspeakers, but are made to detect sound rather than create it. 

  
• How Microphones work:


  1. sound waves hit the diaphragm.
  2. This causes the diaphragm and coil of wire to move.
  3. As the wire is moving within the magnetic field of the permanent magnet, it will generate a current (electromagnetic induction / generator effect).
  4. The frequency and amplitude of the sound waves will determine how much the diaphragm vibrates, and therefore determine the frequency and amplitude of the current. 
• For both loudspeakers and microphones, you can call this part either the 'cone' or 'diaphragm'. BOTH terms are correct.

  
• What is the function of a loudspeaker?


  It converts electrical signals into sound waves
• What is the function of a microphone?


  It converts sound waves into electrical signals
• Do loudspeakers rely on the generator effect (electromagnetic induction), or the motor effect?


  Motor effect
• What 'effect' do loudspeakers use to convert electrical signals into vibrations (these vibrations then generate sound waves)?


  Loudspeakers utilise the motor effect.
• What 'effect' do microphones use to convert the vibrations caused by sound waves into electrical signals?


  Microphones utilise ethe generator effect.
• For microphones and loudspeaker, are the electrical signals involved direct or alternating currents?


  Alternating currents.

  
• 'cone' or 'diaphragm'

  
• 'cone' or 'diaphragm'

  
• Magnet

  
• Which two features of the sound waves hitting a microphones diaphragm determines the frequency and amplitude of the current produced?


  The frequency and amplitude of the sound waves determine the frequency and amplitude of the current produced.