Generally the upper limit of human hearing is around 20kHz (20,000 hertz) This limit becomes lower with age and indeed most children can hear some high frequency sounds that older people (including teachers!) may not be able to hear. It was widely reported that a mobile phone ringtone maker created a ringing tone using high frequency sound which only young children could hear. Whether or not this was true we don't know but shows a creative example of a potential application for near ultrasonic frequencies!
Signal generators can be used in conjunction with a loudspeaker so students can hear the differences in frequency and these frequencies can be shown on the screen of an oscilloscope where the sound is shown as a waveform. Around the 20kHz point, the high pitched whistle becomes inaudible. This is the point at which ultrasonic frequencies begin.
Dogs, dolphins and bats have a greater upper limit to their hearing and so they can hear ultrasonic frequencies. Using a dog whistle, microphone and oscilloscope, a wave pattern can be displayed when the whistle is blown showing that sound is being produced even though it cannot be heard. A dog whistle emits ultrasound at a frequency in the range of 16 kHz to 22 kHz which dogs can hear
Applications for ultrasound include:
Sonography - uses ultrasonic frequencies to 'see' muscles, tissues and bones and is used for medical purposes.
Ultrasonic testing - is used to test the thickness of objects and test for weaknesses in metals during manufacture.
Ultrasonic cleaners - use frequencies from 20-40kHz to clean lenses, jewellery and tools.
Bats - use ultrasonic echo-location enabling them to find their prey.
Pest control - Units are available which claim to frighten off rats, mice and mosquitoes using ultrasonic frequencies.