A loudspeaker turns electrical signals into moving air so that we hear music, speech, and sound effects. The signal from an amplifier changes rapidly in voltage and current, matching the shape of the original sound wave. Inside the speaker, this changing current drives a coil of wire in a magnetic field.
The result is a vibrating cone that pushes and pulls the surrounding air to make pressure waves.
Key Facts
- A loudspeaker converts electrical energy into mechanical motion and then into sound energy.
- The magnetic force on a current-carrying wire is F = BIL when the wire is perpendicular to the magnetic field.
- The voice coil moves because the current direction and size change with the audio signal.
- Sound wave speed in air is approximately v = 343 m/s at room temperature.
- Wave speed, frequency, and wavelength are related by v = fλ.
- Higher frequency means faster cone vibration, while greater amplitude usually means louder sound.
Vocabulary
- Voice coil
- A coil of wire attached to the speaker cone that experiences a magnetic force when electric current flows through it.
- Permanent magnet
- A magnet in the speaker that provides a steady magnetic field for the voice coil to push against.
- Cone
- The lightweight surface that moves back and forth to push air and create sound waves.
- Suspension
- The flexible support system that keeps the cone centered while allowing it to vibrate.
- Frequency
- The number of vibrations or wave cycles per second, measured in hertz.
Common Mistakes to Avoid
- Thinking the magnet turns on and off, which is wrong because the permanent magnet usually provides a steady field while the changing current in the voice coil changes the force.
- Saying the cone creates sound by moving air in only one direction, which is wrong because sound is made by repeated compressions and rarefactions as the cone moves back and forth.
- Confusing loudness with pitch, which is wrong because loudness is mainly related to wave amplitude while pitch is mainly related to frequency.
- Ignoring the suspension and spider, which is wrong because these parts center the voice coil and prevent rubbing while still allowing controlled motion.
Practice Questions
- 1 A speaker produces a 686 Hz tone in air where the speed of sound is 343 m/s. What is the wavelength of the sound?
- 2 A voice coil segment has length 0.050 m in a magnetic field of 0.80 T and carries a current of 2.0 A perpendicular to the field. What magnetic force acts on that segment using F = BIL?
- 3 Explain why reversing the direction of current in the voice coil reverses the direction of cone motion.