Perception of ultrasound
Humans The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear, which acts as a low-pass filter. Ultrasonic hearing can occur if ultrasound is fed directly into the skull bone and reaches the cochlea through bone conduction without passing through the middle ear. Children can hear some high-pitched sounds that older adults cannot hear, because in humans the upper limit pitch of hearing tends to become lower with age. A cell phone company has used this to create ring signals supposedly only able to be heard by younger humans; but many older people can hear it, which may be due to the considerable variation of age-related deterioration in the upper hearing threshold. See also The Mosquito (an electronic device used to deter loitering by young people). Animals Many animals—such as dogs, cats, dolphins, bats, and mice—have an upper frequency limit that is higher than that of the human ear and thus can hear ultrasound. Bats use ultrasounds to navigate in the darkness. Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. They can detect frequencies beyond 100 kHz, possibly up to 200 kHz. Many insects have good ultrasonic hearing and most of these are nocturnal insects listening for echolocating bats. This includes many groups of moths, beetles, praying mantids and lacewings. Upon hearing a bat the i
sects will make evasive manoeuvres to escape being caught by the bat. Ultrasonic frequencies trigger a reflex action in the noctuid moth that cause it to drop a few inches in its flight to evade attack. Tiger moths also emit clicks which may disturb bats' echolocation, but may also in other cases evade being eaten by advertising the fact that they are poisonous by emitting sound. Dogs can hear sound at higher frequencies than humans can. A dog whistle exploits this by emitting a high frequency sound to call to a dog. Many dog whistles emit sound in the upper audible range of humans, but some, such as the silent whistle, emit ultrasound at a frequency in the range 18–22 kHz. Toothed whales (Odontocetes), including dolphins can hear ultrasound and use ultrasonic sounds in their navigational system (biosonar) to orient and capture prey. Porpoises have the highest known upper hearing limit, at around 160 kHz. Several types of fish can detect ultrasound. In the order Clupeiformes, members of the subfamily Alosinae (shad), have been shown to be able to detect sounds up to 180 kHz, while the other subfamilies (e.g. herrings) can hear only up to 4 kHz. Pest control Main article: Electronic pest control Ultrasound generator/speaker systems are sold with claims that they frighten away rodents and insects, but there is no scientific evidence that the devices work.