Wave speed
For any wave, speed equals frequency times wavelength: v = f lambda.
Last updated
Analogy
Counting how many crests pass you each second and how far apart they are.
Example
lambda = v / flambda = 343 / 440lambda = 0.78 m
In practice
A 440 Hz note in air at 343 m/s has a wavelength of about 0.78 m.
Practice
Flashcards
- What is the formula for wave speed?v = f lambda.
Interview questions
- Explain how a standing wave forms on a guitar string and find the first three frequencies for a 0.65 m string with wave speed 260 m/s.
- Two waves moving opposite ways superpose
- Ends are nodes
- f_n = n v / 2L gives 200, 400, 600 Hz
Questions people ask
Does a wave's speed depend on its frequency?
For most waves in a given medium, no. Speed is set by the medium, such as air temperature for sound or tension for a string. Changing frequency changes wavelength instead, so v = f lambda stays the same. A higher note has a shorter wavelength but travels at the same speed.
How fast does sound travel?
In air at 20 degrees Celsius, sound moves at about 343 m/s. It is faster in warmer air, faster in water at about 1480 m/s, and faster still in steel at about 5000 m/s. Sound cannot travel in a vacuum because it needs a medium.