Because of compressions and rarefactions,  pressure fluctuations accompany the sound wave. The diagram represents an instantaneous snapshot of the sound wave, and the lower part of the diagram illustrates the oscillating pressure. Pressure is high at compressions and low at points of rarefaction.  

 The wavelength is the distance between 2 peaks, valleys, or any other corresponding points on the wave.  Wavelength is inversely related to the frequency. If frequency doubles, wavelength halves. When you switch the frequency setting on your scanner from, say 2.5 MHz to 5 MHz, the wavelength is cut in half! Vice versa, when you change frequency from 6 MHz to 3 MHz, the wavelength doubles.

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