Any sound with a frequency above 20,000 Hz (or 20 kHz)—that is, above the highest audible frequency—is defined to be ultrasound.

In practice, it is possible to create ultrasound frequencies up to more than a gigahertz. (Higher frequencies are difficult to create; furthermore, they propagate poorly because they are very strongly absorbed.) Ultrasound has a tremendous number of applications, which range from burglar alarms to use in cleaning delicate objects to the guidance systems of bats.

The characteristics of ultrasound, such as frequency and intensity, are wave properties common to all types of waves. Ultrasound also has a wavelength that limits the fineness of detail it can detect. This characteristic is true of all waves. We can never observe details significantly smaller than the wavelength of our probe; for example, we will never see individual atoms with visible light, because the atoms are so small compared with the wavelength of light.

The most common use of ultrasound, creating images, has industrial and medical applications. The use of ultrasound to create images is based on the reflection and transmission of a wave at a boundary. When an ultrasound wave travels inside an object that is made up of different materials (such as the human body), each time it encounters a boundary (e.g., between bone and muscle, or muscle and fat), part of the wave is reflected and part of it is transmitted. The reflected rays are detected and used to construct an image of the object.

Sonar illustrates how a ship on the ocean utilizes the reflecting properties of sound waves to determine the depth of the ocean. A sound wave is transmitted and bounces off the seabed. Because the speed of sound is known and the time lapse between sending and receiving the sound can be measured, the distance from the ship to the bottom of the ocean can be determined. This technique is called sonar (originally an acronym for SOund Navigation And Ranging). Just as ships on the ocean, certain animals, like dolphins and bats, make use of sounds waves (sonar) to navigate or find their way. Ultrasound waves are sent out then reflected off the objects around the animal. Bats or dolphins then use the reflected sounds to form a “picture” of their surroundings (this is known as echolocation).

Practice Questions

Sample Test C/P Section Question 27

Practice Exam 3 C/P Section Passage 9 Question 49

Key Points

• When waves encounter a boundary between two materials, part of the wave is reflected and part is transmitted.

• By using high frequency sound waves, doctors can create images of parts of the body normally not visible.

• By transmitting sound waves and measuring the time between the transmission and receiving the reflection, ships can use sound waves to navigate. This is called sonar.

Key Terms

Ultrasound: Frequencies above the human detectable ceiling of approximately 20,000 Hz

Frequency: The quotient of the number of times n a periodic phenomenon occurs over the time $\text{t}$ in which it occurs: $\text{f}=\text{n}/\text{t}$.

Intensity: The average power that travels through a given area as the wave travels through space.

Sonar: A system for the detection of objects under water and for measuring the water’s depth by emitting sound pulses and detecting or measuring their return after being reflected.