Sound is a longitudinal wave that travels through a medium by creating alternating compressions and rarefactions. The human ear can detect sound waves with frequencies between about 20 Hz and 20 000 Hz. In this chapter you will explore what determines the pitch and loudness of a sound, and discover how ultrasound is used in medicine.
4.1 Sound as a Longitudinal Wave
When a tuning fork vibrates, it pushes and pulls the surrounding air molecules. This creates a series of compressions (high-pressure regions) and rarefactions (low-pressure regions) that radiate outward as a longitudinal wave. The speed of sound in air at room temperature (20°C) is approximately 343 m·s⁻¹.
PITCH is the quality of a sound that tells us whether it is 'high' or 'low'. A flute has a high pitch; a bass guitar has a low pitch. Pitch is determined entirely by FREQUENCY: the higher the frequency, the higher the pitch. A violin (high f) produces more compressions per second than a double bass (low f).
Note
HUMAN HEARING RANGE: Humans can hear sounds between about 20 Hz (very low pitch — deep bass) and 20 000 Hz (very high pitch — sharp whistle). This range decreases with age.
LOUDNESS is determined by AMPLITUDE. A loud sound has large amplitude — the air molecules are displaced a greater distance from their equilibrium positions, carrying more energy. A soft (quiet) sound has small amplitude. On an oscilloscope screen, a loud sound shows a tall waveform; a soft sound shows a short waveform.
Pitch vs Loudness
| Property | Property | Wave characteristic it depends on |
|---|---|---|
| Pitch | Frequency (f) | High pitch ↔ high f; low pitch ↔ low f |
| Loudness | Amplitude (A) | Loud sound ↔ large A; soft sound ↔ small A |
4.2 Echoes and Ultrasound
Sound, like all waves, obeys the LAW OF REFLECTION: when a wave strikes a surface, it bounces back. The reflected sound wave is called an ECHO. You hear an echo when sound reflects off a hard, distant surface (a cliff, a wall, an empty concert hall) and reaches your ear after a noticeable delay.
Definition
Echo
An echo is a reflected sound wave. It is heard when a sound wave strikes a surface and is reflected back to the listener.
Worked Example
A person shouts next to a cliff. They hear the echo 2 s after the shout. If the speed of sound in air is 340 m·s⁻¹, how far away is the cliff?
Given
- t = 2 s (total time for sound to travel to cliff and back)
- v = 340 m·s⁻¹
Find
Distance to cliff (d)
Solution
- 1The sound travels TO the cliff and BACK, so total distance = 2d.
- 2Total distance = v × t = 340 × 2 = 680 m
- 3Distance to cliff: d = 680/2 = 340 m
ULTRASOUND is sound with a frequency ABOVE the range of human hearing — above 20 000 Hz (20 kHz). While we cannot hear it, ultrasound has many important applications.
Note
ULTRASOUND FREQUENCIES used in medicine are typically between 1 MHz and 20 MHz (1 million to 20 million Hz) — far above the upper limit of human hearing.
MEDICAL ULTRASOUND IMAGING: A probe emits pulses of ultrasound. These pulses enter the body and are REFLECTED (echoed) at the boundaries between different tissues (e.g., muscle–bone boundary, or fluid–tissue boundary). The reflected pulses return to the probe at different times depending on the depth of the reflecting surface. A computer uses these timing differences to construct a cross-sectional image of the inside of the body. No radiation (X-rays) are used — ultrasound is safe for monitoring pregnancy.
Uses of ultrasound
- Prenatal (baby) scanning — monitoring fetal development safely without X-rays
- Echocardiography — imaging the heart in real time
- Detecting cracks in metal structures (non-destructive testing)
- Sonar — determining water depth and locating fish or submarines
- Cleaning delicate instruments using ultrasonic vibrations
BATS AND ECHOLOCATION: Bats emit pulses of ultrasound (20 kHz – 200 kHz) from their mouths or nostrils. The pulses bounce off objects (insects, trees, walls) and return to the bat's large ears. By measuring the time between emission and reception, the bat determines the distance, direction, and even the speed and size of the object. This process is called ECHOLOCATION.
DOLPHINS: Dolphins also use echolocation (they call it biosonar). They produce clicks and whistles in the range 200 Hz – 150 kHz. The sounds pass through the dolphin's melon (a fatty organ in its head) and are directed forward. Reflected sounds are received through the dolphin's lower jaw and conducted to the inner ear. Dolphins can detect objects 1–2 cm in diameter at distances of 100 m or more.
Real World
WHY USE ULTRASOUND (NOT AUDIBLE SOUND) FOR IMAGING? Higher-frequency (shorter wavelength) ultrasound can resolve finer details. A 5 MHz pulse has a wavelength of about 0,3 mm in tissue — fine enough to image structures inside the body at millimetre resolution.
Practice Question
A bat emits an ultrasound pulse and detects the echo 6 ms (0,006 s) later. If sound travels at 340 m·s⁻¹ in air, calculate the distance of the object from the bat.
(4 marks)