Doppler effect

[abr95]

On page 174 of physics chapter 8, it says that when a a sound source is moving towards an observer, by holding the wavelength constant, we see the frequency increases as the velocity increases? Don't the wavelengths get smaller as the source moves closer to the observer? Why is it saying wavelength is constant?

[NS_Tutor_Phil]

The actual wavelength doesn't change, but the perceived wavelength does. It's a bit tricky, so I hope that that makes sense.