Sound Waves: Characteristics and Applications · Lesson 13 of 14
Ultrasonic and Infrasonic Waves, and their Applications
“Frequencies outside human hearing reveal natural events, image the body, detect defects and locate objects through reflected sound.”
• Distinguish infrasonic audible and ultrasonic ranges. • Describe applications of infrasonic waves. • Describe medical and industrial applications of ultrasound. • Define echolocation. • Explain how bats use ultrasonic echoes. • Explain the principle of sonar. • Solve sonar distance problems. • Describe the audio-surveillance example.
Human hearing covers only a small part of the possible sound-frequency range. Frequencies below and above this range can still carry information and energy, and they have important scientific, medical, industrial and biological applications.
Ultrasonic and Infrasonic Waves, and their Applications
Infrasonic Waves
Infrasonic waves have frequencies below 20 Hz. The chapter lists applications including detecting natural events such as earthquakes and volcanic eruptions, and detecting severe storms because such low-frequency waves can travel long distances through air and through the Earth.
Ultrasonic Waves
Ultrasonic waves have frequencies above 20 kHz. Their applications include ultrasonography for imaging internal organs without surgery, breaking kidney stones into smaller pieces, ultrasonic welding, cleaning delicate machine parts and detecting internal defects in metal blocks.
Echolocation
The ability to locate objects by emitting sound and analysing reflected sound waves.
Most bats emit short bursts of ultrasound. These waves reflect from obstacles and prey. By sensing the returning echoes, the bat can estimate where objects are located and navigate in darkness. Dolphins, whales and some birds also use forms of echolocation.
Sonar
Sound navigation and ranging: a method that sends ultrasonic waves through water and analyses returning reflections to determine properties such as distance and direction of underwater objects.
The measured sonar time normally represents the complete outward-and-return journey. Therefore the one-way distance to the object is found by dividing the total sound path by two.
Problem
A sonar pulse returns after 0.90 s in seawater where sound speed is 1530 m s⁻¹. Find the distance to the object.
- 1.Round-trip time = 0.90 s.
- 2.One-way time = 0.90 ÷ 2 = 0.45 s.
- 3.Distance = speed × one-way time.
- 4.d = 1530 × 0.45 = 688.5 m.
- 5.The object is 688.5 m away.
Problem
A sonar signal sent downward returns after 4 s. If sound speed in seawater is 1500 m s⁻¹, find the depth.
- 1.The 4 s represents down-and-back travel.
- 2.One-way time = 4 ÷ 2 = 2 s.
- 3.Depth = 1500 × 2.
- 4.Depth = 3000 m.
Audio Surveillance
The chapter also describes how characteristic low-frequency sounds from drones and aircraft engines can be detected with sensitive sound sensors even when the objects are difficult to see. This type of monitoring is described as audio surveillance.
The Quest Continues
Sound is increasingly used as a scientific probe: recordings from planetary missions, earthquake sounds used in climate-related measurements, mosquito buzzing used for identification and tiny sounds from soil organisms all illustrate how acoustic information can reveal otherwise hidden processes.
Echolocation and sonar use the same broad reasoning as echo calculations: emit a sound, detect its reflection, measure the round-trip time and infer distance.
Quiz
Which description best matches Echolocation?
Which description best matches Sonar?
Which term matches this description: The ability to locate objects by emitting sound and analysing reflected sound waves.
Which term matches this description: Sound navigation and ranging: a method that sends ultrasonic waves through water and analyses returning reflections to determine properties such as distance and direction of underwater objects.
Which statement is a key takeaway from this lesson?
Practice Problems
- Classify 15 Hz, 200 Hz and 40 kHz.
- Give two applications of infrasonic waves from the chapter.
- Give three applications of ultrasonic waves from the chapter.
- Explain how a bat can locate prey without seeing it.
- A sonar echo returns after 5 s in seawater at 1525 m s⁻¹. Find the one-way distance.
Key Takeaways
• Ultrasonic waves have frequencies above the upper limit of human hearing, while infrasonic waves have frequencies below the lower limit. • Some animals can produce or detect frequencies outside the human audible range. • Ultrasound is used in applications such as medical imaging, SONAR, cleaning and detecting internal defects. • Reflected ultrasonic waves can provide information about the position, distance or structure of objects.