What this chapter is about
Sound is a form of energy that travels as waves through a medium. In this chapter, you study how sound is produced, how it propagates, and what characteristics describe different sounds. You learn why you can hear someone calling from across a field, why thunder sounds different from a flute, and why you hear an echo in a large empty hall.
This chapter builds on your understanding of motion and waves to explain everyday phenomena involving sound. You will explore how the speed of sound changes in different media, what determines pitch and loudness, and how the human ear perceives sound. You will also study practical applications such as SONAR and ultrasound imaging, along with understanding why some sounds are audible while others are not.
After studying this chapter, you should be able to describe sound as a longitudinal wave, calculate quantities like wavelength and frequency, explain how characteristics of sound depend on wave properties, and understand applications of sound waves in technology and medicine.
Key ideas
- Sound is produced by vibrating objects and travels as a longitudinal wave, where particles of the medium vibrate back and forth along the direction of wave propagation.
- Sound requires a material medium (solid, liquid or gas) to travel; it cannot travel through a vacuum.
- The speed of sound depends on the medium: sound travels fastest in solids (about 5000 m/s in steel), slower in liquids (about 1500 m/s in water), and slowest in gases (about 340 m/s in air at 20°C).
- Frequency is the number of vibrations per second, measured in hertz (Hz). Pitch is how the ear perceives frequency — higher frequency means higher pitch.
- Amplitude is the maximum displacement of a vibrating particle from its rest position. Loudness depends on amplitude — larger amplitude means louder sound.
- The audible range for humans is 20 Hz to 20,000 Hz. Sounds below 20 Hz are called infrasound; sounds above 20,000 Hz are called ultrasound.
- Echo is the reflection of sound. To hear a distinct echo, the reflecting surface must be at least 17 metres away (so total path is at least 34 m).
- Ultrasound has applications in medical imaging (sonography), cleaning, and SONAR (Sound Navigation and Ranging) used to measure ocean depth and detect underwater objects.
Formulas and facts to remember
Wave equation: v = f × λ Speed of sound (v) equals frequency (f) multiplied by wavelength (λ).
Relation between time period and frequency: T = 1/f Time period (T) is the time for one complete vibration.
Minimum distance for echo: d = v × t / 2 where t is the time between producing a sound and hearing its echo; divide by 2 because sound travels to the surface and back.
Speed of sound in air at 20°C: approximately 340 m/s (or 344 m/s).
Audible frequency range for humans: 20 Hz to 20,000 Hz.
Intensity of sound is measured in decibels (dB). Normal conversation is about 60 dB; sounds above 80 dB can cause hearing damage over time.
Reverberation is the persistence of sound due to multiple reflections in an enclosed space.
Worked examples
Example 1: Finding wavelength
A tuning fork produces a sound of frequency 512 Hz. If the speed of sound in air is 340 m/s, find the wavelength of the sound wave.
Solution: Using v = f × λ λ = v / f λ = 340 / 512 λ = 0.664 m (approximately 66.4 cm)
The wavelength of the sound wave is about 0.66 metres.
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Example 2: Calculating echo distance
A person standing in a valley claps and hears an echo after 1.5 seconds. If the speed of sound is 340 m/s, how far is the reflecting cliff?
Solution: Total distance travelled by sound = v × t = 340 × 1.5 = 510 m This is the distance to the cliff and back. Distance to cliff = 510 / 2 = 255 m
The cliff is 255 metres away.
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Example 3: Finding frequency from time period
A sound wave has a time period of 0.002 seconds. What is its frequency? Is it audible to humans?
Solution: f = 1 / T = 1 / 0.002 = 500 Hz
Since 500 Hz lies between 20 Hz and 20,000 Hz, the sound is audible to humans.
Common mistakes
Thinking sound can travel through vacuum → Sound needs a medium; it cannot travel where there are no particles to vibrate.
Confusing pitch with loudness → Pitch depends on frequency (how fast vibrations are), while loudness depends on amplitude (how large vibrations are).
Using total echo distance as the distance to reflector → Sound travels to the surface and back, so divide the total distance by 2.
Believing speed of sound is same in all media → Speed is highest in solids, then liquids, then gases; it depends on the medium's properties.
Thinking ultrasound is harmful because it is inaudible → Ultrasound is simply above human hearing range and is safely used in medical imaging.
Quick revision
- Sound is a longitudinal wave requiring a medium; it cannot travel through vacuum.
- v = f × λ connects speed, frequency and wavelength.
- Pitch depends on frequency; loudness depends on amplitude.
- Human audible range: 20 Hz to 20,000 Hz.
- Echo requires minimum 17 m distance to reflecting surface (34 m total path).
- SONAR uses ultrasound to measure depths and detect objects underwater.