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.
Understanding How Speakers Create Sound
The moving parts of a speaker must be light enough to respond quickly, yet strong enough to stay controlled. The voice coil sits in a narrow gap beside a permanent magnet. When the audio current reverses direction, the magnetic push on the coil reverses too.
This makes the coil move in step with the changing signal. A suspension called the surround lets the cone move while keeping it centred.
Another flexible support, often called the spider, prevents the coil from rubbing on the magnet. Rubbing causes distortion, heat, and sometimes permanent damage.
A speaker cone does not create one perfectly flat pressure pulse. Different parts of the cone can flex at high frequencies. This behaviour is called cone breakup.
It can add unwanted peaks or roughness to the sound. Engineers choose materials such as paper, plastic, metal, or woven fibres to balance low mass, stiffness, and damping. Damping means reducing unwanted vibration after the signal changes.
The speaker enclosure matters too. Without a box, sound from the front of the cone can mix with sound from the back. At low frequencies these waves can partly cancel because the cone creates opposite pressure changes on its two sides.
Most full-range speakers are divided into drivers with different jobs. A large woofer moves enough air for bass notes. A small tweeter can move rapidly and reproduce high notes.
A crossover circuit sends low-frequency electrical signals to the woofer and high-frequency signals to the tweeter. This reduces strain on each driver. Headphones use the same basic principle in a much smaller space.
Their tiny diaphragms sit close to the ear, so they need to move far less air than a room speaker. A subwoofer uses a larger driver and enclosure because long-wavelength bass needs large air movement.
Efficiency is an important engineering limit. Much of the electrical energy entering a speaker becomes heat in the coil rather than useful sound. If the coil gets too hot, its electrical resistance rises and the speaker becomes less effective.
Very large signals can push the cone beyond its safe travel. This is called over-excursion. It may tear the suspension or make the coil strike parts of the magnetic system.
Distortion can happen before visible damage. It occurs when cone motion no longer matches the electrical signal accurately.
Students should notice that volume, pitch, sound quality, and power are related but not identical. A high-power speaker is not automatically louder at every frequency, and a louder sound is not automatically clearer.
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.