Ultrasound

FIELDS OF STUDY: Acoustics; Harmonics

ABSTRACT: Ultrasound is sound at a frequency higher than humans can hear. Hearing range varies across ages and individuals, but most young adults cannot hear sounds above twenty thousand hertz. Thus, any sound wave with a frequency above this is considered ultrasound. Ultrasound is widely used in medical imaging, sonar, industrial maintenance, and chemistry. It is also important for animals that use echolocation, such as bats and dolphins.

PRINCIPAL TERMS

• frequency: the number of cycles completed in a given unit of time.
• infrasound: a sound wave with a frequency below twenty hertz, the lower limit of human hearing.
• sonar: a method of using sound waves to "see," typically in an underwater environment, by sending and receiving pulses of sound; originally an acronym for "sound navigation ranging."
• sonography: the use of ultrasound in medicine to produce images of internal structures, such as tendons, muscles, and organs.
• ultrasonic: describes a sound, or a device that makes use of sound, with a frequency above twenty thousand hertz, the upper limit of human hearing.
• ultrasound identification: a real-time locating system that uses ultrasound to track the location of objects or individuals on small spatial scales.
• ultrasonic impact treatment: the use of ultrasonic vibrations to strengthen metals.
• ultrasonic testing: the use of ultrasound to test materials for internal flaws.

Above Human Hearing

The sounds that humans can hear are limited to the audible range of frequencies, between twenty and twenty thousand hertz. Within this range, higher frequencies are heard as higher pitched, and lower frequencies are heard as lower pitched. Anything above the audible range is considered ultrasonic, and any sound that falls below it is called infrasound. Some animals use ultrasound to navigate their environment, a technique known as "echolocation." The principle behind echolocation is one that scientists have co-opted to great effect.Ultrasonic acoustics follows the same principles as all other acoustics. The same physics is used to describe all waves, whether they be sound, light, or mechanical waves in the ocean. What people hear as sounds are vibrations caused by waves traveling through air, water, or some other medium.

Natural Ultrasound

The study of acoustics dates back at least to ancient Greece, when Pythagoras (ca. 580–ca. 500 BCE) wrote about the mathematics underpinning the music made by stringed instruments. Scientists would not come to appreciate the full spectrum of sound until much later. Yet millions of years before human civilization, some animals had evolved to use ultrasound as an important tool for survival. Many can hear sounds well outside the human range of hearing. They can communicate with one another at frequencies inaudible to the human ear.

Some species even use ultrasound to navigate. In 1794, Italian biologist Lazzaro Spallanzani (1729–99) demonstrated that bats can navigate in complete darkness using ultrasonic echolocation. Bats produce pulses of ultrasound, which bounce off the surrounding objects and return to their source, creating an echo. The time it takes for this echo to reach the bat’s ears tells it not only the distance but also the direction of any nearby obstacles—or prey. Many whales and dolphins use a similar technique underwater.

Low Range, High Resolution

When using sound for detection, in theory, any frequency can be used, whether infrasonic, audible, or ultrasonic. However, there is a trade-off. Infrasound tends to have very good range, but it has poor resolution, meaning that it fails to detect small objects. Ultrasound, in contrast, has limited range but excellent resolution. It is this ability to detect fine detail that makes ultrasound so useful in a variety of fields, including warfare, medicine, and industry.

The first human-made ultrasonic device was a dog whistle. It was invented in 1876 by famed English polymath Francis Galton (1822–1911), who was testing the hearing abilities of various animals. The first notable application of ultrasound was conceived in 1916 by French physicist Paul Langevin (1872–1946), who used waves of ultrasound to detect submarines underwater—an early precursor to sonar.

In addition to submarines, modern militaries also need to detect much smaller objects, such as divers, mines, and other such obstacles. Thus, military sonar uses a variety of both infrasonic and ultrasonic devices for detection. Some anglers, archaeologists, and ecologists also use underwater ultrasonic radar to search for fish, artifacts, or marine mammals.

On land, ultrasound identification (USID) uses constant pulses of ultrasound to track individuals or objects in small-scale environments. It is ideal for situations in which other locating systems, such as radio-frequency identification (RFID), would meet with too much interference. USID is commonly used in hospitals to track patients and make sure they do not wander.

Another common use of ultrasound in hospitals is medical sonography. Ultrasound is used to examine patients’ tissues and organs without causing harm or risking invasive surgery. Most famously, ultrasound is used by obstetricians to examine fetuses during pregnancy. It can also be used to detect tumors or other abnormalities.

Industrial Ultrasound

Ultrasonic testing is used in industry and materials science to search for problems that are otherwise invisible or difficult to detect. For example, by sending pulses of ultrasound through a pipe and measuring the echoes, engineers can determine how thick the pipe is and whether it is corroding.

The intense, high-frequency vibrations produced by focused ultrasound have other practical uses. They can be used to clean surfaces or mix substances. Ultrasonic impact treatment is a technique used in metallurgy to strengthen metals. For example, it can be used to restore the integrity of welds during bridge maintenance. In chemistry, ultrasound can be used to initiate or speed up chemical reactions by counteracting the forces that hold liquid molecules together. The frequency range of ultrasound is much larger than audible sound—there is no strict upper limit for ultrasound, after all—and its wide range of applications reflects this. Playing a role in everything from pregnancy to plumbing, ultrasonic technology is a silent but essential part of modern life.

Bibliography

Abu-Zidan, Fikri M., Ashraf F. Hefny, and Peter Corr. "Clinical Ultrasound Physics." Journal of Emergencies, Trauma, and Shock, vol. 4, no. 4, 2011, pp. 501–3.

Corcoran, Aaron J., Jesse R. Barber, and William E. Conner. "Tiger Moth Jams Bat Sonar." Science, 17 July 2009, pp. 325–27.

Demi, Marcello. "The Basics of Ultrasound." X-Ray and Ultrasound Imaging, edited by Daniele Panetta and Demi, Elsevier, 2014, pp. 297–322.

Hertzberg, Barbara S., and William D. Middleton. Ultrasound: The Requisites. 3rd ed., Elsevier, 2016.

Stiles, Timothy A. "Ultrasound Imaging as an Undergraduate Physics Laboratory Exercise." American Journal of Physics, vol. 82, no. 5, 2014, pp. 490–501.