What Is Ultrasound? (Above 20 kHz)

Ultrasound is sound that has a frequency greater than 20,000 hertz (20 kHz), the approximate upper limit of human hearing. It obeys exactly the same physical laws as audible sound — it is still a mechanical pressure wave that needs a medium to travel through — but its very short wavelength lets it do things audible sound cannot: form narrow beams, resolve tiny details, and reflect crisply off small objects. The American National Standards Institute (ANSI/ASA S1.1) formally defines ultrasound as “sound at frequencies greater than 20 kHz,” and the term covers an enormous range, from just above the hearing threshold up to several gigahertz in laboratory settings.

Because ultrasound cannot be heard, most people only encounter the word through its applications: a prenatal sonogram, a jewelry-cleaning tank, a bat detector, or a dog whistle. This article explains what ultrasound physically is, how it is generated and measured, where the frequency bands used in medicine, industry, and nature actually sit, and why — despite being inaudible — it still carries real safety limits.

How Is Ultrasound Defined, and Where Does It Start?

Ultrasound begins exactly where human hearing ends. The audible range for a healthy young adult runs from about 20 Hz to 20 kHz; anything above that threshold is, by definition, ultrasound, and anything below 20 Hz is infrasound. This 20 kHz cutoff is not an arbitrary round number — it reflects the mechanical limits of the human middle ear, particularly the mass and stiffness of the ossicles and eardrum, which cannot respond efficiently to vibrations faster than about 20,000 cycles per second. The upper limit also declines with age (a form of hearing loss called presbycusis), which is why so-called “mosquito” ringtones pitched near the top of the audible range have been marketed as audible mainly to younger listeners — though in practice the age-related cutoff varies enough between individuals that many older adults can hear them too.

Ultrasound is not a separate physical phenomenon from audible sound — it is governed by the same wave equation, the same relationship between frequency, wavelength, and the speed of sound (λ = v/f), and the same behavior at boundaries described by acoustic impedance. What changes is scale: because frequency is so high, wavelength becomes very short. At 20 kHz in air (speed of sound ≈ 343 m/s at 20°C), the wavelength is already down to about 1.9 cm; at 1 MHz — a typical medical-imaging frequency — the wavelength in soft tissue (sound speed assumed ≈ 1,540 m/s) is roughly 1.5 mm. That short wavelength is precisely what gives ultrasound its practical value: shorter wavelengths resolve smaller objects and can be focused into narrower, more directional beams than long-wavelength audible or infrasonic waves.

How Is Ultrasound Generated and Detected?

Almost all practical ultrasound is produced by the piezoelectric effect: certain crystals and ceramics (originally quartz, now usually lead zirconate titanate, PZT) mechanically deform when an alternating voltage is applied across them, and that deformation launches a pressure wave into the surrounding medium at the same frequency as the driving voltage. The same crystal works in reverse as a receiver, converting an incoming pressure wave back into a measurable electrical signal — which is why a single ultrasonic transducer can both “ping” and “listen.”

This piezoelectric approach traces back to French physicist Paul Langevin, who during World War I built an ultrasonic transducer from a thin quartz sheet sandwiched between steel plates to detect submarines — the direct ancestor of modern sonar. Decades earlier, in 1893, Francis Galton had built the Galton whistle, an adjustable whistle used to map the upper hearing limits of humans and animals, showing for the first time that many animals could hear well beyond the human range.

Other generation methods exist for specialized frequencies: magnetostrictive transducers (used in industrial ultrasonic impact treatment), capacitive micromachined ultrasonic transducers (CMUTs) in modern medical probes, and pulsed lasers for the extreme gigahertz frequencies used in materials research. Detection, correspondingly, ranges from simple piezoelectric microphones to laser interferometers for picosecond-scale ultrasonic pulses.

What Frequency Ranges Does Ultrasound Cover?

“Ultrasound” is not one single frequency — it is a band that spans roughly six orders of magnitude, from 20 kHz to several GHz, and different applications cluster in very different parts of that band. The table below summarizes verified frequency ranges by application.

Sources: American National Standards Institute (ANSI/ASA S1.1); International Atomic Energy Agency, Diagnostic Radiology Physics: A Handbook for Teachers and Students (2014); Wikipedia “Ultrasound” and “Medical ultrasound” (cross-checked against cited peer-reviewed sources).

What Is Ultrasound Used For?

How Does Ultrasound Work in Medical Imaging?

In diagnostic sonography, a handheld probe sends short ultrasound pulses (typically 1–18 MHz, most commonly 2–15 MHz) into the body and listens for echoes reflecting off boundaries between tissues of different acoustic impedance — for example, the boundary between soft tissue and bone, or between amniotic fluid and a fetus. The machine measures the round-trip time of each echo and, assuming a constant sound speed of about 1,540 m/s in soft tissue, converts time-of-flight into depth, building up a real-time cross-sectional image. Lower frequencies (3–5 MHz) travel farther into the body before being absorbed, so they are used for deep abdominal or obstetric scans; higher frequencies (7–15 MHz) attenuate faster but resolve much finer detail, so they are used for superficial structures like thyroid, tendons, or small blood vessels. Doppler ultrasonography adds a further trick: it measures the frequency shift of echoes off moving structures (the same Doppler effect that changes a passing ambulance siren’s pitch) to visualize blood flow, most famously in Doppler echocardiography of the heart. Unlike X-ray or CT, medical ultrasound uses no ionizing radiation, and diagnostic power densities are kept below roughly 1 W/cm² specifically to avoid tissue heating or cavitation.

How Do Bats and Other Animals Use Ultrasound?

Echolocating bats emit ultrasonic calls, typically above 20 kHz and reaching frequencies beyond 100 kHz — possibly up to about 200 kHz in some species — then interpret the returning echoes to build a real-time map of insects, obstacles, and terrain in complete darkness. This was first demonstrated experimentally in 1794 by Italian scientist Lazzaro Spallanzani, who showed that blinded bats could still hunt and navigate normally, proving they relied on something other than vision (though the ultrasonic mechanism itself wasn’t confirmed until the 20th century). Many nocturnal insects — moths, certain beetles, praying mantises, and lacewings — have evolved ultrasonic hearing purely to detect and evade hunting bats; some tiger moths even emit their own ultrasonic clicks to jam bat echolocation or advertise that they are toxic. Toothed whales and dolphins use ultrasonic biosonar for navigation and hunting, and harbor porpoises hold the highest confirmed upper hearing limit of any marine mammal tested, at around 160 kHz. Domestic dogs hear up to roughly 45 kHz and cats up to about 64 kHz — both well into the ultrasonic range, which is why dog whistles (typically tuned between 23 and 54 kHz) are inaudible to humans but clearly audible to dogs.

How Is Ultrasound Used in Industry and Engineering?

Industrially, ultrasound has two broad families of application: sensing/ranging and power/processing. On the sensing side, ultrasonic non-destructive testing (NDT) sends pulses (commonly 2–10 MHz for metals and composites, or 50–500 kHz for less dense materials like wood and concrete) into a manufactured part; internal cracks or voids reflect part of the pulse back, revealing flaws without damaging the part or requiring ionizing radiation, and it has been a standard alternative to weld radiography since the 1960s. Non-contact ultrasonic sensors — the kind behind many automatic door openers, parking-assist sensors, and handheld room-measuring tools — use time-of-flight ranging in exactly the same way sonar does underwater. On the power side, ultrasonic cleaners (20–40 kHz) use collapsing cavitation bubbles to blast contaminants off jewelry, dental and surgical instruments, and optical parts; ultrasonic welding (15–40 kHz) joins plastics using frictional heat; and sonochemistry (roughly 20 kHz to several hundred kHz) uses cavitation-driven extremes of local temperature and pressure to speed up chemical reactions, break up solids, and disperse particles.

How Loud Can Ultrasound Be, and Is It Safe?

Ultrasound is inaudible, but that does not make it automatically harmless — sound pressure level (measured in decibels) still applies to ultrasonic waves exactly as it does to audible ones, and very intense ultrasound can transfer real energy into tissue. According to a review by the UK’s Advisory Group on Non-ionising Radiation (AGNIR), published by the UK Health Protection Agency in 2010, occupational exposure to ultrasound in excess of about 120 dB may contribute to hearing loss, exposure above roughly 155 dB may cause harmful heating effects, and exposures above roughly 180 dB have been calculated as potentially lethal. That same review recommended general-public exposure limits for airborne ultrasound at 70 dB (at 20 kHz) and 100 dB (at 25 kHz and above) — figures far below what causes tissue damage, reflecting a cautious, protective margin. For comparison, everyday diagnostic and consumer ultrasound (medical scanners, humidifiers, cleaners) operates at power densities that are orders of magnitude below industrial cavitation thresholds, which is one reason properly performed diagnostic ultrasound carries no established risk to patients.

Source: AGNIR (Advisory Group on Non-ionising Radiation), “Health Effects of Exposure to Ultrasound and Infrasound,” UK Health Protection Agency, 2010.

How Does Ultrasound Differ from Infrasound and Audible Sound?

Ultrasound and infrasound sit on opposite ends of the same audible spectrum, bracketing the roughly 20 Hz–20 kHz band that defines human hearing. Both are physically identical to audible sound — mechanical pressure waves obeying the same wave equation — the only difference is frequency and, as a direct consequence, wavelength and how efficiently each travels or is absorbed.

Myth vs Fact

Myth: “Ultrasound and supersonic mean the same thing.”
Fact: They describe entirely different physical quantities. Ultrasound refers to a frequency — sound waves pitched above 20 kHz, the upper edge of human hearing. Supersonic refers to speed — an object or aircraft moving faster than the speed of sound (about 343 m/s in air at 20°C) — and has nothing to do with pitch or frequency. A jet can be supersonic while making a very audible, low-pitched sonic boom; it is not “ultrasound” simply because it travels fast. Confusing the two terms (Wikipedia’s ultrasound entry explicitly flags this mix-up) is one of the most common everyday misconceptions in acoustics.

Frequently Asked Questions

Can humans hear ultrasound at all?
Not through normal air-conducted hearing, because the middle ear’s mechanics cap out around 20 kHz. However, very high-intensity ultrasound delivered directly to the skull can be perceived via bone conduction, bypassing the middle ear and stimulating the cochlea directly — a documented but unusual pathway, not something that happens with ordinary ultrasonic devices.

What is the highest frequency that counts as ultrasound?
There is no fixed upper boundary — ultrasonic devices have been built operating from just above 20 kHz up to several gigahertz in specialized acoustic microscopy and materials research. In practice, “ultrasound” almost always refers to the 20 kHz–20 MHz range used in medicine and industry; frequencies above that shade into the specialized field of acoustic microscopy.

Is ultrasound dangerous to humans?
At the intensities used in diagnostic medical imaging (kept below about 1 W/cm² specifically to avoid heating or cavitation), no established risk to patients exists. At much higher industrial power levels — used deliberately for cleaning, welding, or chemical processing — ultrasound can heat tissue, and documented occupational safety thresholds exist (hearing-loss risk above ~120 dB, heating effects above ~155 dB).

Why do dog whistles work if humans can’t hear anything?
A dog whistle is deliberately tuned to emit in the roughly 23–54 kHz range, above the human hearing ceiling of about 20 kHz but well within a dog’s hearing range, which extends to around 45 kHz. The whistle isn’t silent — it is simply pitched above what human ears can register.

How is ultrasound different from an X-ray for medical imaging?
Ultrasound uses non-ionizing sound-wave pulses (typically 1–18 MHz) and measures echo timing and strength to build an image; it uses no ionizing radiation, which is why it is preferred for repeated monitoring and prenatal care. X-rays use ionizing electromagnetic radiation absorbed differently by different tissue densities. The two rely on entirely different physical principles and produce different diagnostic information.

Do all animals hear more ultrasound than humans?
No. Birds, for instance, have not been shown to be sensitive to ultrasound at all, while dogs, cats, bats, dolphins, and porpoises all hear well above the human 20 kHz ceiling — porpoises up to around 160 kHz, among the highest confirmed limits recorded in any animal.

Conclusion

Ultrasound is simply sound above the frequency our ears can register — the same physics of sound that governs a spoken word or a musical note, just compressed into much shorter wavelengths. That short wavelength is the whole story behind its usefulness: it lets engineers build narrow, controllable beams for imaging tissue, testing welds, cleaning jewelry, and ranging distances, while nature had already discovered the same trick millions of years earlier in bats, dolphins, and moths. Understanding where ultrasound sits on the broader spectrum — bounded by infrasound on one side and the edge of human hearing on the other — makes clear that “inaudible” does not mean “without physical effect,” which is exactly why verified decibel safety limits exist even for sound nobody can hear.

For a broader grounding in these concepts, see the pillar guide to sound and acoustics fundamentals, or continue exploring related terms below.