Inverse Proportion (Inverse Variation):
The relationship between two variables in which the product is a constant.
Speed is inversely proportion to time:
The distance from home to work is 5 miles. If I travel 30 mph, then it only takes me 10 min. to get home. But if I want to home sooner so I can catch my favorite show on tv, say in 6 min., then I would need to increase my speed to 50 mph.
For sound, the product of the frequency of a wave and it's wavelength is the speed of sound. The speed of sound is a constant. Therefore, the frequency and wavelength of a given tone are inversely proportional. (This is why higher notes have really short wavelengths and low notes have very long wavelengths.)