For many waves it is important to remember these points:

• the frequency of the wave is set by whatever is driving the oscillation in the medium. Examples are a speaker that sets up a sound wave, or the hand that shakes the end of a stretched string.
  
  
• the speed of the wave is a property of the medium.
  
  
• the wavelength of the wave is then determined by the frequency and speed:

  λ = v/f

An important exception to these rules of thumb can be found in musical instruments. The musician generally excites a wide range of frequencies, but the frequencies that are sustained are those with wavelengths that are related to the length of the instrument. This is a resonance phenomenon, and the resonance frequency can be changed by changing the length of the instrument.

So, in a musical instrument:

• the wavelength is determined by the length of the instrument.
  
  
• the speed of the wave is a property of the medium.
  
  
• the frequency is then determined by the wavelength and speed.

  f = v/λ