Chapter Overview
Sound is an everyday sensory experience. This chapter explains how vibrating objects produce sound, how sound travels through a material medium, why sound is a longitudinal mechanical wave, and how wavelength, frequency, time period and amplitude describe a sound wave. It also explains reflection, echo, reverberation, ultrasound, infrasound, echolocation and SONAR.
Sound originates from vibrating objects.
Sound transfers energy through a material medium.
v = fλ and T = 1/f.
Echo, ultrasound, echolocation and SONAR.
1. Production of Sound
Sound is produced when an object vibrates. A stretched rubber band, tuning fork, drum membrane, musical instrument string and human vocal cords can produce sound through vibration.
Tuning fork
When a tuning fork is struck, its prongs vibrate. These vibrations disturb nearby air and produce sound.
2. Propagation of Sound
Sound travels from a source to a listener through a material medium. The medium can be a solid, liquid or gas. Sound does not travel through vacuum because there are no particles available to pass the disturbance along.
Mechanical wave
A wave that requires a material medium for propagation is called a mechanical wave. Sound is a mechanical wave.
Longitudinal wave
In air, particles of the medium vibrate back and forth parallel to the direction in which the sound disturbance travels. Therefore, sound is a longitudinal wave.
3. Compressions and Rarefactions
A longitudinal sound wave consists of alternating regions of compression and rarefaction.
| Compression | Rarefaction |
|---|---|
| Particles are closer together. | Particles are farther apart. |
| Higher pressure/density than the average. | Lower pressure/density than the average. |
| Part of the travelling disturbance. | Part of the travelling disturbance. |
The distance between the centres of two successive compressions, or two successive rarefactions, is one wavelength.
4. Sound Waves and Energy
A sound wave carries energy from the vibrating source through the medium. The particles of the medium oscillate about their mean positions; the disturbance passes from one group of particles to the next.
5. Graphical Representation of a Sound Wave
A sound wave can be represented by showing how pressure or displacement varies with position or time. In a longitudinal wave, closely spaced regions correspond to compressions and widely spaced regions correspond to rarefactions.
The graph helps us identify wavelength, amplitude and frequency. The exact shape of a real sound signal can be more complicated than a simple sine wave.
6. Characteristics of a Sound Wave
6.1 Wavelength (λ)
Wavelength is the distance between two successive points in the same phase, such as two consecutive compressions. SI unit: metre (m).
6.2 Frequency (f or ν)
Frequency is the number of complete vibrations or oscillations per second. SI unit: hertz (Hz).
6.3 Time Period (T)
Time period is the time taken for one complete oscillation.
6.4 Amplitude
Amplitude is the maximum displacement of the vibrating particles from their mean positions. Greater amplitude generally means greater energy/intensity and a louder sound for the same conditions.
7. Speed, Frequency and Wavelength
The speed of a wave depends on how quickly the disturbance travels. For sound waves:
Since T = 1/f, another useful form is:
Worked example
A sound wave has frequency 150 Hz and wavelength 2 m. Find its speed.
8. Speed of Sound in Different Media
Sound travels at different speeds in different media. Under ordinary conditions it generally travels fastest through solids, more slowly through liquids, and slowest through gases.
| Medium | Relative speed | Reason at this level |
|---|---|---|
| Solids | Generally highest | Particles are strongly connected and can transmit disturbances efficiently. |
| Liquids | Intermediate | Particles are closer than in gases but can move relative to each other. |
| Gases | Generally lowest | Particles are relatively far apart. |
In air near room temperature, the speed of sound is approximately 340 m/s. The exact value varies with conditions such as temperature.
9. Human Perception of Sound
Pitch
Pitch is the sensation related mainly to frequency. A higher-frequency sound is generally perceived as higher pitched.
Loudness
Loudness is the sensation related mainly to the strength/intensity of the sound and therefore to amplitude under comparable conditions.
Timbre / quality
The quality or timbre of a sound helps us distinguish sounds produced by different sources even when they have similar pitch and loudness. It depends on the waveform and its harmonic content.
| Perception | Mainly related to |
|---|---|
| Pitch | Frequency |
| Loudness | Amplitude / intensity |
| Timbre | Waveform / harmonic content |
10. Audible, Infrasonic and Ultrasonic Sound
For a normal human listener, the approximate audible frequency range is 20 Hz to 20,000 Hz (20 kHz).
| Type | Frequency | Examples / uses |
|---|---|---|
| Infrasound | Below 20 Hz | Very low-frequency sounds; produced by some natural phenomena and used in specialized monitoring. |
| Audible sound | About 20 Hz–20 kHz | Normal human hearing range. |
| Ultrasound | Above 20 kHz | Medical imaging, some treatments, industrial uses, SONAR. |
11. Reflection of Sound
Sound can be reflected from hard surfaces. The reflected sound can reach a listener after the original sound.
Echo
An echo is a reflected sound heard separately from the original sound. For a distinct echo, the reflected sound must reach the listener after a sufficient time gap; the commonly used minimum is about 0.1 s.
Example
Taking speed of sound as 340 m/s and a minimum time gap of 0.1 s:
12. Reverberation
Reverberation is the persistence of sound caused by repeated reflections from surfaces. It can occur in large rooms, halls and enclosed spaces.
| Echo | Reverberation |
|---|---|
| A distinct reflected sound is heard separately. | Multiple reflections overlap and prolong the sound. |
| Needs a sufficiently separated reflecting surface. | Often occurs when reflecting surfaces are relatively close. |
Excessive reverberation can make speech difficult to understand. Soft furnishings, curtains, acoustic panels and other sound-absorbing materials can reduce unwanted reflections.
13. Ultrasound and Its Applications
Ultrasound has frequencies above the upper limit of normal human hearing. Because of its short wavelength and ability to reflect from boundaries, it has many practical applications.
Ultrasound can be used to create images of structures inside the body.
High-intensity ultrasound can be used in certain medical procedures, including some treatments for kidney stones.
Ultrasonic techniques can help detect internal flaws or clean delicate objects in suitable applications.
Uses reflected sound waves to determine underwater distance or depth.
14. Echolocation
Some animals use reflected sound to locate objects or prey. This process is called echolocation. Bats are a well-known example. They emit high-frequency sounds and use returning echoes to obtain information about nearby objects.
15. SONAR
SONAR stands for Sound Navigation and Ranging. It uses sound waves, especially underwater, to detect objects and estimate distance or depth from the time taken for the reflected sound to return.
Here, v is the speed of sound in the medium and t is the total time for the sound to travel to the object and return.
Example
A SONAR pulse returns after 2 s. If the speed of sound in water is taken as 1500 m/s:
16. Applications Around Us
- Human speech through vibration of vocal cords.
- Musical instruments using vibrating strings, membranes or air columns.
- Echoes and reflection used to understand distances.
- Ultrasound used in medical imaging and selected treatments.
- SONAR used for underwater detection and ranging.
- Echolocation helps certain animals navigate and locate prey.
17. Quick Concept Comparisons
| Pair | Difference |
|---|---|
| Longitudinal vs transverse | Sound in air is longitudinal; particles vibrate parallel to wave propagation. |
| Compression vs rarefaction | Compression has relatively higher pressure/density; rarefaction has lower pressure/density. |
| Pitch vs loudness | Pitch relates mainly to frequency; loudness relates mainly to amplitude/intensity. |
| Echo vs reverberation | Echo is a distinct reflected sound; reverberation is persistence from overlapping reflections. |
| Infrasound vs ultrasound | Infrasound is below 20 Hz; ultrasound is above 20 kHz. |
18. Formula Sheet
19. Practice Questions
- What produces sound?
- Why is sound called a mechanical wave?
- Why can sound not travel through vacuum?
- Why is sound in air called longitudinal?
- Define compression and rarefaction.
- What is wavelength?
- Define frequency and state its SI unit.
- Define time period and write its relation with frequency.
- What is amplitude?
- Write the relation v = fλ and explain each symbol.
- How are pitch and frequency related?
- How are loudness and amplitude related?
- What is the normal human audible range?
- Differentiate between infrasound and ultrasound.
- Compare the speed of sound in solids, liquids and gases.
- What is an echo?
- Why is the distance divided by two in echo calculations?
- Calculate the distance of a reflector if an echo returns after 0.2 s. Take v = 340 m/s.
- What is reverberation?
- How can excessive reverberation be reduced?
- What is echolocation?
- Why do bats use high-frequency sounds?
- What does SONAR stand for?
- Explain the working principle of SONAR.
- Calculate the wavelength of a 200 Hz sound travelling at 340 m/s.
- Calculate the speed when f = 500 Hz and λ = 0.68 m.
- Find the time period of a 250 Hz sound.
- Why do sound waves carry energy but not carry medium particles continuously from source to listener?
- Give three applications of ultrasound.
- Differentiate between pitch, loudness and timbre.
20. Interactive Quiz — 30 Questions
Select one option for each question and click Check My Score.
21. Quick Revision — One Page
- Sound is produced by vibration.
- Sound is a mechanical wave and requires a material medium.
- Sound in air is longitudinal, consisting of compressions and rarefactions.
- Wavelength = distance between successive compressions or rarefactions.
- Frequency is measured in Hz; time period T = 1/f.
- Wave equation: v = fλ.
- Pitch depends mainly on frequency; loudness depends mainly on amplitude/intensity.
- Human audible range is approximately 20 Hz–20 kHz.
- Below 20 Hz = infrasound; above 20 kHz = ultrasound.
- Echo is reflected sound heard separately; reverberation is persistence due to multiple reflections.
- Distinct echo calculations commonly use a minimum time gap of about 0.1 s and therefore a reflector distance of about 17 m when v = 340 m/s.
- Ultrasound is used in medicine and industry; SONAR uses reflected sound for underwater ranging.