A loudspeaker changes electrical energy into sound by making air vibrate. In a dynamic speaker, the changing audio signal drives a coil of wire attached to a cone. As the cone moves back and forth, it creates pressure variations that travel outward as sound waves.
This engineering idea matters because nearly every phone, headphone, car stereo, and public address system depends on it.
Key Facts
- Magnetic force on a current-carrying wire: F = BIL, where B is magnetic field strength, I is current, and L is wire length in the field.
- Sound wave speed in air at room temperature is about v = 343 m/s.
- Wave relationship: v = fλ, where f is frequency and λ is wavelength.
- Higher audio frequency makes the cone vibrate more times per second, producing a higher pitch.
- Greater cone displacement usually produces larger pressure changes, which are heard as louder sound.
- Electrical power delivered to a speaker can be estimated by P = V^2/R for a sinusoidal RMS voltage across resistance R.
Vocabulary
- Cone
- The cone is the lightweight diaphragm that pushes and pulls air to create sound waves.
- Voice coil
- The voice coil is a coil of wire that carries the audio current and experiences magnetic force.
- Permanent magnet
- The permanent magnet provides a steady magnetic field for the voice coil to push against.
- Diaphragm
- A diaphragm is any vibrating surface that transfers mechanical motion to the surrounding air.
- Frequency
- Frequency is the number of vibration cycles per second, measured in hertz.
Common Mistakes to Avoid
- Thinking the magnet turns on and off with the music is wrong because the permanent magnet usually provides a constant magnetic field while the changing current in the voice coil changes the force.
- Assuming the cone only moves outward is wrong because sound requires back-and-forth vibration that creates alternating compressions and rarefactions in air.
- Confusing loudness with pitch is wrong because loudness depends mainly on wave amplitude, while pitch depends mainly on frequency.
- Ignoring impedance when calculating speaker power is wrong because the current and power depend on the speaker load, often 4, 6, or 8 ohms.
Practice Questions
- 1 A speaker plays a 440 Hz tone in air where the speed of sound is 343 m/s. What is the wavelength of the sound?
- 2 An 8 ohm speaker has an RMS voltage of 4.0 V across it. Using P = V^2/R, what electrical power is delivered to the speaker?
- 3 A speaker cone moves forward when the voice coil current flows one direction and backward when the current reverses. Explain how this motion creates a traveling sound wave in the air.