ABSTRACT
This chapter introduces infrasound, sounds at frequencies below human hearing, and ultrasound, sounds above human hearing. Sonar, which historically was within audible hearing ranges now primarily uses ultrasound. Since sonar is a specialized area widely used in surveillance, it is covered separately in Chapter 4. Since they are somewhat related to sound waves, devices that detect shock waves are also discussed here. Sound waves and shock waves are not the same, but devices to detect them are very similar. For an explanation of the distinctions between these phenomena, see Section 5 (Description and Functions).
