Humans
The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear.Auditory sensation can occur if high‐intensity ultrasound is fed directly into the human skull 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 decrease with age.An American 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 the signals,which may be because of the considerable variation of age-related deterioration in the upper hearing threshold.The Mosquito is an electronic device that uses a high pitched frequency to deter loitering by young people.



Animals
Bats use 4D 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,some insects will make evasive manoeuvres to escape being caught.Ultrasonic frequencies trigger a reflex action in the noctuid moth that cause it to drop slightly 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 with normal hearing can hear ultrasound.A dog whistle exploits this by emitting a high frequency sound to call to a dog.Many such 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,including dolphins,can hear ultrasound and use such 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.

Ultrasound generator/speaker systems are sold as electronic pest control devices,which are claimed to frighten away rodents and insects,but there is no scientific evidence that the devices work.

Ultrasound Definition
Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz."

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