Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound. The production of ultrasound is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium. The most well known application of this technique is its use in sonography to produce pictures of fetuses in the human womb. There are a vast number of other applications as well.Novelline, Robert. Squire's Fundamentals of Radiology. Harvard University Press. 5th edition. 1997. ISBN 0674833392.rp: 34-35

Ability to hear ultrasound

The upper frequency limit in humans (approximately 20 kHz) is caused by 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 without passing through the middle ear. Carefully-designed scientific studies have been performed supporting what the authors call the hypersonic effect - that even without consciously hearing it, high-frequency sound can have a measurable effect on the mind.

It is a fact in psychoacoustics that 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 claim to be able to hear it, which is likely given the considerable variation of age-related deterioration in the upper hearing threshold.

Some animals – such as dogs, cats, dolphins, bats, and mice – have an upper frequency limit that is greater than that of the human ear and thus can hear ultrasound.

Diagnostic sonography

Sonography is used routinely in obstetric appointments during pregnancy, but the FDA discourages its use for non-medical purposes such as fetal keepsake videos and photos, even though it is the same technology used in hospitals.

Obstetric ultrasound is primarily used to:

• Date the pregnancy (gestational age)
• Confirm fetal viability
• Determine location of fetus, intrauterine vs ectopic
• Check the location of the placenta in relation to the cervix
• Check for the number of fetuses (multiple pregnancy)
• Check for major physical abnormalities.
• Assess fetal growth (for evidence of intrauterine growth restriction (IUGR))
• Check for fetal movement and heartbeat.
• Determine the sex of the baby