Published: 27 March, 2009

A speaker has the ability to create complex sounds

How do speakers produce more than one sound at a time (example: guitar and vocal)?

To help understand the complex sounds made by speakers, let's think about some properties of sound waves. Sound waves made by a guitar or a person's vocal chords are quite complex. Scientists usually think of complex sound waves as the addition of simple waves with different frequencies (or different wavelengths).

To help visualize the addition of waves, let's think about waves in water. If you throw a rock into water, it will create a ripple wave. The distance between the peaks of the wave is the wavelength. For a small rock the wavelength could be about one millimeter but for a large rock it could be several centimeters. What happens if you throw in a few rocks of different sizes at the same time? Each rock creates the same ripple wave as it would if it were thrown in alone, but the pattern you see in the water is much more complex; it is the addition of all the different waves.

Similarly, most of the sounds you hear are complex sound waves that can be thought of as the addition of dozens of "simple" or "pure" waves. Your brain interprets most complex sounds as a certain pitch with a certain "quality". For example, if a guitar plays a note and then you sing the same note, a person may recognize the two sounds as the same pitch because some of the simple waves that make up the two sounds have the same frequency. However, the relative amounts of these simple waves and the possible addition of some other frequencies allow us to distinguish the source of the sound. Our ears and brains are so good at this we can recognize subtle differences in people's voices or in guitars made with different materials.

The sound wave created by a guitar and voice played together is the addition of the sound waves from the two being played separately, so it is complex wave made up of two other complex waves. Thus it is not much harder for a speaker to play the combined sound than it is for the speaker to play the guitar or voice by itself. But we still haven't talked about how a speaker does it!

The Ask a Scientist question from 20 June, 2007, "How does a microphone work?", explains how a speaker is built and how the electric signal is converted to sound waves, so for now let's talk about how a speaker creates a complex sound wave. If the speaker moves back and forth at a single frequency it will move the air in front of it at that frequency; this is the creation of a simple sound wave. But just as a complex water wave involves water moving up and down at many frequencies at the same time, the speaker can move back and forth at many frequencies and create complex sound waves. If this sound wave contains all the components of a guitar and a voice, we'll hear both of the instruments.