What is a wave, and what do wavelength and frequency mean?
Drop a stone in a pond and rings spread out over the water. The water itself does not travel across the pond: each bit of it just bobs up and down. What travels is the pattern, the disturbance. That is a wave: a pattern of motion that carries energy from place to place.
Sound is a wave too. When you speak, your voice pushes the air in front of you back and forth, and the pattern of squeezed and stretched air reaches the ear of whoever is listening.
Every wave has three numbers that describe it. The amplitude is how big the swing is: the height of the ripples, or how loud a sound is. The wavelength is the distance from one crest to the next. The frequency is how many crests pass a point every second, measured in hertz (Hz).
The three are tied together by the speed of the wave: speed = frequency × wavelength. Sound in air travels at about 343 metres per second, so a long wavelength means a low note and a short wavelength a high one.
You halve the wavelength of a sound wave. What happens to its frequency? The speed of sound does not change, and speed = frequency × wavelength. Half the wavelength means twice the frequency: the note goes up by an octave.
Hold on to these three words, amplitude, wavelength and frequency. In the next lesson we will find out that light is a wave too, an extremely fast and tiny one.
Speed v = f × λ, where f is the frequency and λ (the Greek letter lambda) the wavelength.
A wave at one moment looks like y = A sin(2π x / λ): A is the amplitude. As time passes the pattern slides along at speed v.
For sound, v ≈ 343 m/s: the note A above middle C, 440 Hz, has λ = 343 / 440 ≈ 0.78 m.
A wave is a travelling pattern described by amplitude, wavelength and frequency, tied together by speed = frequency × wavelength.