Sound travels through the air by producing pressure waves - areas of high pressure and areas of low pressure. The rate at which these waves reach our ears is called frequency. It is measured in cycles per second, where a cycle is the distance between waves, or wavelength. The higher the frequency, the higher the pitch of the sound.
The range of human hearing is approximately from 20 cycles per second, to 20,000 cycles per second. An engine produces sounds across a wide range of frequencies. The mufflers and resonators in the engine exhaust system reduce these sounds to an acceptable level.