Loudspeakers transmit sound through unique structure

Credit: Maegha Singh/ Credit: Maegha Singh/

You constantly see loudspeakers, commonly referred to as ‘speakers,’ at parties and concerts. They’re in every sports stadium, music studio, and movie theater. They are even found in every fire truck. They are an integral part of the world around us, but what exactly are they, and how do they work? Despite their multitude of drastically different usages, all loudspeakers are fundamentally the same: they use mechanical energy to propagate surrounding air molecules in waves that the ear detects as sound. Generally, this process is always the same, yet a single loudspeaker is capable of producing a plethora of different sounds at different frequencies and decibels. The ability of a speaker to produce all forms of sound, including tones that range from EDM to classical music, stems from its unique structure.

Speakers are composed of several distinct parts that work together to produce sound. To start, a loudspeaker contains at its center a voice coil, which is essentially just a coil of wire. Surrounding the voice coil is a ring-shaped magnet. When an electric current is transmitted through the speaker, such as when it is connected to an iPod, the current runs through the voice coil and induces a magnetic field around the coil. This magnetic field then interacts with the magnetic field of the magnet, causing the coil to oscillate back and forth. Attached to this voice coil is a cone called the diaphragm, which oscillates in synchrony with the voice coil. The movement of the diaphragm is what propagates molecules in the air and produces the sound we hear.

To make sure that the voice coil and diaphragm are properly aligned, a small ring of flexible material called a spider is placed between the diaphragm and the voice coil. The spider also works with the suspension, an outer ring of material that surrounds the diaphragm, to ensure the cone and coil are aligned properly. A dust cap is placed at the center of the diaphragm to protect the coil from the outside environment. In summary, speakers transmit sound when current flows through the coil, the coil oscillates with the diaphragm, and the diaphragm moves air molecules to create sound.

This process sounds relatively straightforward, but if all speakers work this way, why does a guitar sound different from a violin on a speaker? How is a loudspeaker able to clearly deliver a unique pitch that distinguishes one instrument from another, one artist’s voice from the next? The answer has to do with the nature of sound, and how sound waves are produced. When a sound wave is created, it contains two important features: a frequency and an amplitude. The frequency of a sound wave determines how high or low the pitch will register to our ears; the amplitude measures how loud the sound will be.

Theoretically, a pitch with the same frequency and amplitude should sound the same, regardless of the instrument or device producing the sound. Because sound is a propagation of air molecules, however, frequencies and amplitudes are never perfect. Instead, they contain small unintentional hiccups and movements that give each sound its unique tone. This is why a piano and a violin sound different, even when playing the same exact note. Each instrument creates movement of air molecules in a unique way, which gives each instrument — and more generally, each and every sound — its defining quality. These distinct notes are then transmitted through speakers, which are able to distinguish between these slightly varied tones by mimicking the imperfect frequencies and amplitudes of the sound.

As noted earlier, a speaker requires a connection to an electric current from an external source such as an iPod. The current from this external source is called an audio signal, which is simply a representation of sound in an electric current. Just as sound waves can have frequencies, so can electric currents, which is what allows the electric current to transmit the frequency of the sound wave to the coil. When the audio signal is transmitted through the coil, the coil oscillates with the frequencies that will produce the sound represented in the electric current. This is what allows the speaker to transmit sound.

Through this unique structure, loudspeakers are able to produce the sounds that we are surrounded by every day. So the next time you’re on the bus and you plug your earphones in, make sure you think about everything that has to happen in the earbud that allows you to hear your favorite band.