4.7.2.4 Loudspeakers (physics only) (HT only)

Question 1

What do loudspeakers and headphones use?

Answer 1

loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves

Question 2

Description of moving coil loudspeaker:

Answer 2

• cone which has a coil of wire wrapper around one end
• the coil of wire is connected to an AC electrical supply
• permanent magnet which goes inside the coil of wire

Question 3

How does a moving-coil loudspeaker work?

Answer 3

• the current in the electrical circuit is varying
• as the current passes through the coil it generates a magnetic field
• the magnetic field from the coil interacts with the magnetic field from the permanent magnet - the magnetic fields exert a force on each other - either attract or repel each other
• produces a resultant force which coil experiences (inwards or outwards) which causes the cone to move
• when the current switches direction/reverses the direction of the (resultant) force on the coil reverses - causes the force on the cone to reverse so cone move in and out
• the varying current causes the coil to vibrate - the vibrating coil causes the cone to vibrate (the size of the current affects the size of the force)
• the vibrating cone causes air molecules to move
• the movement of air molecules produces the pressure variations in the air needed for a sound wave
• the air molecules bunch together forming compressions and spread apart forming rarefactions

Question 4

How does changing the frequency of the AC supply affect the loudspeaker?

Answer 4

• changing the frequency of the AC supply can change the frequency which the cone vibrates at
• higher frequency will produce a higher pitched sound
• lower frequency will produce a lower pitched sound

Question 5

How does changing the size of the current affect the loudspeaker?

Answer 5

• if size of current is increased then the amplitude of the vibration is increased
• this increases the volume of the sound

Question 6

How do headphones work?

Answer 6

similar to moving coil loudspeaker just much slower