Sound Wave: Definition, Explanation and Examples

Sound Wave

Edited By Vishal kumar | Updated on Jul 02, 2025 06:19 PM IST

Sound waves are mechanical oscillations moving through a medium such as air, water, or solid materials; they’re created by any object that vibrates and makes the surrounding medium vibrate too. The sound spreads from continuously moving one thing creating regions of high density and low compression behind itself, bringing about a waveform which we hear as noise. In this article, we will discuss the sound waves which are important when studying oscillations, and physics courses that apply to NEET and JEE Main. Most times, understanding sound waves makes it easy to understand other things like wave propagation, frequency and also wavelength. Longitudinal waves are sound waves that transport power.

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What is Sound?
Solved Examples Based on Sound Wave
Summary

Sound Wave

What is Sound?

Sound is a type of energy that travels through a medium, typically air, as a wave. It is produced by the vibration of objects, which causes particles in the surrounding medium to oscillate. These oscillations create regions of compression and rarefaction, forming pressure waves that propagate through the medium. When these waves reach our ears, they are detected by the auditory system and interpreted by the brain as sound.

What is a Sound Wave?

Sound waves always travel through any elastic material medium with a speed that depends on the properties of the medium. As sound waves travel through the air, the molecules of air vibrate to produce changes in density and pressure along the direction of motion of the wave. If the source of the sound waves vibrates as a Sine wave, the pressure variations are also like Sine waves. Because of this, the mathematical description of sinusoidal sound waves is very similar to that of sinusoidal waves on strings.

There are two types of wave

Transverse waves	Longitudinal waves
1. In this particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave.	1. In this particles of a medium vibrate in the direction of wave motion.
2. Transverse waves travel in the form of crests and troughs.	2. Longitudinal waves travel in the form of compression and rarefaction
3. It can be transmitted through solids, they can be set up on the surface of liquids. But they can not be transmitted into liquids and gases.	3. It can be transmitted through solids, liquids, and gases because, for this wave propagation, volume elasticity is necessary.
4. Transverse waves can be polarised.	4. Longitudinal waves can not be polarised.

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Now, as the sound wave travels through the air, the element of air vibrates to produce a change in density and pressure along the direction of motion of the wave. So as we discussed in the table the movement of the sound waves is like compression and rarefaction, this is shown in the given image. The detail of this with an example will be discussed in the latter concept.

Here, $\lambda$ is the wavelength.

R = Rarefaction

C = Compression

Solved Examples Based on Sound Wave

Example 1: The amplitude of two waves is in a ratio of 5:2. If all other conditions for the two are the same, then what is the ratio of their energy densities?

1) 5:2

2) 10:4

3) 2.5:1

4) 25:4

Solution:

Energy density $\propto$ (amplitude)²

So the ratio of energies will be squared of amplitude ratio:

$\begin{aligned} \frac{E_{1}}{E_{2}} = {(\frac{A_{1}}{A_{1}})}^{2} \\ \frac{E_{1}}{E_{2}} = {(\frac{5}{2})}^{2} = (\frac{25}{4}) \\ E_{1} : E_{2} = 25 : 4 \end{aligned}$

Hence, the answer is the option (4).

Example 2: The following phenomenon cannot be observed for sound pulse

1) Refraction

2) Interference

3) Diffraction

4) Polarisation

Solution:

Pulse

When we flick one end of a long string that is under tension and has its opposite end fixed. In this manner, a single bump is formed and travels along the string this bump is called a pulse.

wherein

Pulse has a defined height and defined speed of propagation along the medium. The shape of the pulse changes very little.

Sound waves are longitudinal in nature so they cannot be polarised.

Hence, the answer is the option (4).

Example 3: A stone dropped from the top of a tower of height 200 m high splashes into the water of a pound near the base of the tower. When is the splash heard at the top? Given that the speed of sound in air is 332 m / s,(g=9.8 m /s²)

1) 1.634

2) 5.267

3) 2.622

4) 3.465

Solution:

Here, $h = 200 m g = 9.8 m / s^{2}$ and
velocity of sound $V = 332 m / s$
Let $t_{1}$ be the time taken by the stone to reach the surface of the pound

Then using
$S = u t + \frac{1}{2} a t^{2} h = 0 + \frac{1}{2} g t_{1}^{2}$
or $t_{1} = \sqrt{\frac{2 h}{g}}$
$\begin{aligned} ∴ t_{1} = \sqrt{\frac{2 \times 200}{9.8}} = \sqrt{\frac{2 \times 2 \times 10^{2}}{98}} = \frac{\sqrt{2 \times 10^{2}}}{\sqrt{49}} \\ = \frac{\sqrt{2} \times 10}{7} = \frac{1.414 \times 10}{7} = \frac{1414 \times 10^{- 2}}{7} \\ = 202 \times 10^{- 2} = 2.02 \sec . \end{aligned}$

Also, if $t_{2}$ is the time taken by the sound to reach a height of h then
$t_{2} = \frac{h}{v} = \frac{200}{332} = 0.602 \sec$
$∴$ Total time after which the sound of splash is heard
$= t_{1} + t_{2} = 2.02 + 0.602 = 2.622$

Hence, the answer is the option (3).

Example 4: A vibrating tuning fork of frequency n is placed near the open end of a long cylindrical tube. The tube has a side opening and is also fitted with a movable reflecting Poston. As the piston is moved through 4.65 cm, the intensity of sound changes from a maximum to a minimum. If the speed of sound is $400 m / s$ then $n$ .

1) 21.5054

2) 23.4506

3) 22.5056

4) 26.4534

Solution:

$\begin{aligned} Path difference = 4.65 \times 2 \\ Path difference = \frac{λ}{2} \\ ∴ 9 \cdot 3 = \frac{λ}{2} \\ λ = 18 \cdot 6 λ = 18.6 \\ v = n λ \\ n = \frac{u}{λ} \\ n = \frac{400}{18.6} \\ n = \frac{4000}{186} \\ n = 21.5054 \end{aligned}$

Hence, the answer is the option (1).

Summary

Sound waves are mechanical vibrations that travel through mediums like air, water, and solids. They are crucial in understanding wave phenomena such as frequency, wavelength, and energy density. Sound waves can be longitudinal, travelling as compressions and rarefactions. The provided examples illustrate key concepts like energy ratios, sound speed calculations, and the inability of sound waves to exhibit polarization, essential for NEET and JEE Main physics.

Frequently Asked Questions (FAQs)

1. What causes the Doppler effect in sound waves?

The Doppler effect is caused by relative motion between the sound source and the observer. When a sound source moves towards an observer, the perceived frequency increases (higher pitch), and when it moves away, the perceived frequency decreases (lower pitch). This effect occurs because the motion compresses or expands the wavelengths reaching the observer.

2. What is resonance in the context of sound waves?

Resonance is the tendency of a system to oscillate with greater amplitude at certain frequencies, called resonant frequencies. In the context of sound waves, resonance occurs when an object is forced to vibrate at its natural frequency, resulting in a larger amplitude of vibration and a louder sound.

3. What is the difference between constructive and destructive interference in sound waves?

Constructive interference occurs when two sound waves combine to create a wave with greater amplitude, resulting in a louder sound. Destructive interference happens when waves combine in a way that their amplitudes cancel each other out, resulting in a quieter sound or even silence. The type of interference depends on the phase relationship between the waves.

4. What is acoustic levitation, and how does it work?

Acoustic levitation is a method of suspending matter in air using sound waves. It works by creating a standing wave pattern between an emitter and a reflector. At certain points in this pattern, called nodes, the acoustic radiation pressure is strong enough to counteract gravity, allowing small objects to be suspended in mid-air.

5. What is the role of sound waves in seismology?

In seismology, sound waves play a crucial role in understanding the Earth's structure and composition. Seismic waves, which are essentially sound waves traveling through the Earth, come in different types (P-waves, S-waves, surface waves) and behave differently as they pass through various layers of the Earth. By studying how these waves travel and change, seismologists can infer information about the Earth's interior structure, composition, and properties.

6. What is a sound wave?

A sound wave is a type of mechanical wave that travels through a medium (such as air, water, or solids) by causing particles to vibrate. It is created by a vibrating source and propagates as a series of compressions and rarefactions in the medium.

7. How do sound waves differ from light waves?

Sound waves are mechanical waves that require a medium to travel, while light waves are electromagnetic waves that can travel through a vacuum. Sound waves are longitudinal, meaning particles vibrate parallel to the direction of wave travel, whereas light waves are transverse, with vibrations perpendicular to the direction of travel.

8. Why can't sound travel through a vacuum?

Sound cannot travel through a vacuum because it is a mechanical wave that requires a medium to propagate. In a vacuum, there are no particles to vibrate and transfer energy, so sound waves cannot form or travel.

9. What determines the speed of sound in a medium?

The speed of sound in a medium is determined by the medium's properties, primarily its density and elasticity. In general, sound travels faster in denser and more rigid materials. Temperature also affects sound speed, with higher temperatures typically resulting in faster sound travel.

10. How does frequency relate to pitch in sound waves?

Frequency is directly related to pitch in sound waves. Higher frequency sound waves are perceived as higher pitch sounds, while lower frequency sound waves are perceived as lower pitch sounds. The human ear can typically detect frequencies between 20 Hz and 20,000 Hz.

11. How do sound waves contribute to the greenhouse effect in planetary atmospheres?

While not typically associated with the greenhouse effect, sound waves can play a role in atmospheric heating. In some planetary atmospheres, acoustic waves generated by surface features or atmospheric turbulence can propagate upwards, carrying energy. As they dissipate in the upper atmosphere, this energy is converted to heat, contributing to atmospheric warming.

12. What is acoustic cloaking, and how does it work?

Acoustic cloaking is a technique to make objects "invisible" to sound waves. It works by designing materials or structures that bend sound waves around an object without scattering or reflecting them. This is typically achieved using metamaterials – engineered materials with properties not found in nature – that can precisely control the path of sound waves, making it appear as if the cloaked object isn't there.

13. What is acoustic streaming, and how does it occur?

Acoustic streaming is a steady fluid flow that can be generated by high-intensity sound waves. It occurs when sound waves create a non-zero average momentum flux in a fluid, resulting in a net flow. This phenomenon is particularly noticeable in situations with high-amplitude sound waves or near boundaries and obstacles. Acoustic streaming can be used in various applications, including mixing fluids and enhancing heat transfer.

14. What is the concept of acoustic holography, and how does it work?

Acoustic holography is a technique used to visualize sound fields by recording the amplitude and phase of sound waves. It works by using an array of microphones to capture the sound field at different points in space. This data is then processed to reconstruct the entire sound field, including its sources. This technique allows for the visualization and analysis of complex sound fields, useful in applications like noise source identification and acoustic imaging.

15. What is acoustic metamaterial, and how does it manipulate sound waves?

Acoustic metamaterials are engineered structures designed to control, direct, and manipulate sound waves in ways not possible with conventional materials. They typically consist of subwavelength structures that can bend, focus, or even block specific frequencies of sound. These materials can achieve negative refractive indices for sound, allowing for applications like super-resolution acoustic imaging and sound cloaking.

16. How do sound waves interact with Bose-Einstein condensates?

In Bose-Einstein condensates (BECs), a state of matter where atoms cooled to near absolute zero behave as a single quantum entity, sound waves take on unique properties. They can propagate as quantum excitations called Bogoliubov modes. Studying sound waves in BECs provides insights into quantum fluids and can be used to probe the properties of these exotic states of matter.

17. What is the difference between amplitude and loudness in sound waves?

Amplitude refers to the maximum displacement of particles from their equilibrium position in a sound wave. Loudness is the subjective perception of sound intensity by the human ear. While amplitude is directly related to loudness, the relationship is not linear, and other factors like frequency can affect perceived loudness.

18. How do sound waves interact with different materials?

Sound waves can be reflected, refracted, diffracted, or absorbed when interacting with materials. Reflection occurs when sound bounces off a surface, refraction happens when sound changes direction while entering a new medium, diffraction is the bending of sound waves around obstacles, and absorption is the conversion of sound energy into other forms of energy by a material.

19. How do musical instruments produce different notes?

Musical instruments produce different notes by creating sound waves of varying frequencies. This is achieved through different mechanisms depending on the instrument type. For example, string instruments vary string length or tension, wind instruments change the length of the air column, and percussion instruments use different sized resonating chambers.

20. What is the relationship between wavelength and frequency in sound waves?

Wavelength and frequency in sound waves are inversely related. As frequency increases, wavelength decreases, and vice versa. This relationship is described by the equation: wave speed = frequency × wavelength. Since the speed of sound is relatively constant in a given medium, as frequency changes, wavelength must change accordingly.

21. How do humans produce speech sounds?

Humans produce speech sounds by controlling the flow of air from the lungs through the vocal cords and mouth. The vocal cords vibrate to produce sound waves, while the shape of the mouth, position of the tongue, and other factors modify these waves to create different speech sounds.

22. What is the decibel scale, and why is it used for measuring sound intensity?

The decibel (dB) scale is a logarithmic scale used to measure sound intensity. It is used because human perception of loudness is logarithmic rather than linear, and the scale can represent the wide range of sound intensities we can hear (from barely audible to painfully loud) in a more manageable range of numbers.

23. How do sound waves travel through solids compared to gases?

Sound waves generally travel faster and more efficiently through solids than gases. This is because solid particles are closer together and more tightly bound, allowing for quicker energy transfer between particles. In gases, particles are further apart and move more freely, resulting in slower sound propagation.

24. What is acoustic impedance, and how does it affect sound transmission?

Acoustic impedance is a measure of how much a medium resists the flow of sound energy. It is determined by the density of the medium and the speed of sound in that medium. When sound waves encounter a boundary between media with different acoustic impedances, part of the wave is reflected and part is transmitted, affecting how sound travels between different materials.

25. How do noise-cancelling headphones work?

Noise-cancelling headphones work by using active noise control. They have microphones that detect ambient noise, then generate sound waves that are the exact opposite (180 degrees out of phase) of the detected noise. When these opposing waves combine, they cancel each other out, reducing the perceived noise.

26. How do echoes form, and what factors affect their intensity?

Echoes form when sound waves are reflected off a surface and return to the listener. The intensity of an echo depends on several factors, including the distance to the reflecting surface, the material properties of the surface (how much sound it absorbs or reflects), and the initial intensity of the sound. Atmospheric conditions can also affect echo intensity.

27. What is meant by the term "supersonic" in relation to sound waves?

"Supersonic" refers to speeds greater than the speed of sound in a given medium. When an object travels faster than the speed of sound, it is said to be moving at supersonic speed. This creates a shock wave, often heard as a sonic boom, because the sound waves from the object pile up and create a sudden, large pressure change.

28. How does the human ear detect and process sound waves?

The human ear detects sound waves through a complex process. Sound waves enter the ear canal and cause the eardrum to vibrate. These vibrations are transmitted through small bones in the middle ear to the cochlea in the inner ear. Here, hair cells convert the mechanical vibrations into electrical signals, which are then sent to the brain for interpretation as sound.

29. What is sound diffraction, and why is it important in acoustics?

Sound diffraction is the bending of sound waves around obstacles or through openings. It's important in acoustics because it allows sound to spread out and be heard around corners or behind barriers. Diffraction is more pronounced for lower frequency sounds (with longer wavelengths) than higher frequency sounds, which is why bass sounds often seem to travel further.

30. How does temperature affect the speed of sound?

Temperature directly affects the speed of sound, particularly in gases. As temperature increases, the average kinetic energy of particles in the medium increases, allowing sound waves to propagate faster. In air, the speed of sound increases by about 0.6 meters per second for each degree Celsius increase in temperature.

31. What is meant by the term "harmonics" in sound waves?

Harmonics are integer multiples of a fundamental frequency in a complex sound wave. They are produced when a vibrating object, like a string or air column, vibrates not only at its fundamental frequency but also at higher frequencies that are whole number multiples of the fundamental. Harmonics contribute to the timbre or quality of a sound.

32. How do sound waves behave differently in water compared to air?

Sound waves travel about 4.3 times faster in water than in air due to water's higher density and lower compressibility. They also travel farther in water before losing energy. However, when sound transitions between air and water, much of its energy is reflected due to the large difference in acoustic impedance between the two media.

33. How do animals like bats and dolphins use sound for echolocation?

Bats and dolphins use echolocation by emitting high-frequency sound waves and listening to the echoes that return after bouncing off objects. By analyzing these echoes, they can determine the location, size, and even texture of objects around them. This allows them to navigate and hunt effectively, even in dark or murky conditions.

34. What is the difference between near field and far field in acoustics?

In acoustics, the near field is the region close to a sound source where the sound pressure and particle velocity are not in phase, making sound behavior complex and unpredictable. The far field is the region farther from the source where sound behaves more predictably, with pressure and velocity in phase. The transition between these fields depends on the sound frequency and source size.

35. What is sound absorption, and how do materials like foam panels work to absorb sound?

Sound absorption is the process by which materials convert sound energy into heat energy. Materials like foam panels work by having a porous structure that allows sound waves to enter. As the waves travel through the material, they cause the air in the pores to vibrate, creating friction that converts sound energy to heat. The irregular surface of these materials also helps to scatter sound waves, further reducing reflected sound.

36. How does the shape of a room affect its acoustics?

The shape of a room significantly affects its acoustics by influencing how sound waves reflect, diffract, and interfere with each other. Parallel walls can create standing waves and flutter echoes. Curved surfaces can focus or disperse sound. The room's dimensions determine which frequencies resonate most strongly. Irregularly shaped rooms often have more balanced acoustics as they scatter sound more evenly.

37. What is the difference between longitudinal and transverse waves, and why are sound waves longitudinal?

Longitudinal waves have particle displacement parallel to the direction of wave travel, while transverse waves have particle displacement perpendicular to the direction of travel. Sound waves are longitudinal because they propagate through a medium by compressing and rarefying the medium in the same direction as the wave travels. This is due to the nature of pressure variations in fluids and solids that transmit sound.

38. How do shock waves differ from regular sound waves?

Shock waves are a type of propagating disturbance that moves faster than the local speed of sound in the medium. Unlike regular sound waves, which are typically sinusoidal, shock waves are characterized by an almost instantaneous jump in pressure, temperature, and density. They form when sound waves "pile up" due to an object moving faster than the speed of sound or from very intense explosions.

39. How do infrasound and ultrasound differ from audible sound waves?

Infrasound refers to sound waves with frequencies below the lower limit of human hearing (generally below 20 Hz), while ultrasound refers to frequencies above the upper limit of human hearing (generally above 20,000 Hz). Both infrasound and ultrasound behave according to the same physical principles as audible sound but interact differently with materials and organisms due to their different frequencies and wavelengths.

40. How do sound waves interact with phase changes in materials?

Sound waves can interact with phase changes in materials in several ways. They can induce phase changes by creating areas of high and low pressure, potentially causing localized vaporization or condensation. Conversely, phase changes can affect sound propagation by altering the medium's density and compressibility. For example, sound travels differently through water vapor than through liquid water.

41. How do sound waves contribute to the phenomenon of sonoluminescence?

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound. Intense sound waves create areas of high and low pressure in the liquid. In low-pressure regions, tiny bubbles form and then rapidly collapse when they enter high-pressure regions. This collapse is so violent that it generates extremely high temperatures for a brief moment, causing the emission of light.

42. How do sound waves interact with supercritical fluids?

Supercritical fluids are substances at temperatures and pressures above their critical points, where distinct liquid and gas phases do not exist. Sound waves in supercritical fluids can behave differently compared to normal fluids. The speed of sound and acoustic attenuation in supercritical fluids can change dramatically near the critical point, and these changes can be used to study the properties of supercritical fluids.

43. How do sound waves contribute to the formation of Chladni figures?

Chladni figures are patterns formed by particles on a rigid surface vibrating at resonant frequencies. When sound waves of specific frequencies are applied to a plate covered with fine particles, the plate vibrates in characteristic patterns. The particles move away from areas of maximum vibration (antinodes) and collect at points of minimum vibration (nodes), creating visible patterns that represent the vibrational modes of the plate.

44. What is the relationship between sound waves and quantum mechanics?

While sound waves are typically described classically, they also have quantum mechanical aspects. At the quantum level, sound waves are quantized into particles called phonons, analogous to photons for light. This quantum description becomes important in understanding phenomena like thermal conductivity in solids, superconductivity, and the behavior of sound in extremely confined spaces or at very low temperatures.

45. What is the role of sound waves in sonochemistry?

Sonochemistry is the study of chemical reactions induced or enhanced by sound waves