Sound is a small fraction of the pressure wave spectrum. In terms of wavelength, we can hear far more than we can see: while deep red light has waves about twice as long as those of deep violet, the lowest-pitched sound waves we can hear are about ten times longer than the highest. ‘Sound waves’ explains infrasound, the lower frequencies detectable by touch rather than hearing, and the audio range, describing the fundamental differences between sound power and loudness, and frequency and pitch. It also considers sound underwater. Sonar systems can be passive systems that simply detect sounds, while active sonar transmits sounds and detects those that are reflected.