Sound Waves | Famous Books for UPSC Exam (Summary & Tests) PDF Download

Introduction

Longitudinal waves are characterized by the movement of particles in the medium parallel to the direction of wave propagation. Instead of drifting, the particles oscillate back and forth around their equilibrium positions. Sound waves behave in a similar manner, making them longitudinal waves.

On the other hand, transverse waves involve individual particles of the medium moving perpendicular to the direction of wave propagation while approximately maintaining their original positions. Rather than oscillating along the wave's path, the particles oscillate up and down around their equilibrium positions as the wave travels.

Sound waves possess certain characteristics

• Crest and trough: The highest point in a wave is known as the crest, while the lowest point is called the trough.
• Wavelength: It refers to the distance between successive compressions or rarefactions and is denoted by λ (lambda) in meters (m).
• Oscillation: It represents the change in density from high to low values, completing one full oscillation.
• Frequency: Denoted by ν (nu), it measures the number of cycles per second and is expressed in hertz (Hz).
• Wave period: The time taken for consecutive compressions or rarefactions to pass a fixed point, denoted by T in seconds (s).
• Frequency and period are related as follows: ν = 1/T.
• The pitch of a sound is determined by the frequency, with higher frequencies corresponding to higher pitches. The amplitude of a sound wave represents the maximum disturbance or variation in the medium around its average value, and it affects the loudness or softness of the sound. As a sound wave travels away from its source, the amplitude and loudness decrease, resulting in a decrease in the perceived sound intensity.
• The quality or timbre of a sound helps differentiate it from others with the same pitch and loudness. A rich quality is associated with pleasant sounds. A sound consisting of a single frequency is called a tone, while a note is produced by a mixture of several frequencies. Noise, characterized by unpleasantness, is undesirable to the ear.
• The intensity of sound refers to the amount of sound energy passing through a unit area per second. It can be measured by considering the number of frequency cycles passing through the area.

Speed of sound in different media

• The speed of sound varies depending on the medium it travels through. Sound waves propagate through a medium at a finite speed. The speed of sound is lower in gases, such as air, compared to solids (including metals and non-metals). Additionally, within a medium, the speed of sound increases with an increase in temperature. Higher temperatures result in increased kinetic energy of molecules, causing them to collide more rapidly and forcefully, thus increasing the speed of sound.
• When an object exceeds the speed of sound, it is said to be traveling at supersonic speed. Sound traveling at supersonic speed creates shock waves in the air, which carry significant energy and can cause damage to buildings and break window glass. The sudden change in air pressure associated with these shock waves produces a sharp and loud sound known as a "sonic boom."

Reflection of Sound

• The law of reflection of sound states that the angles of incidence and reflection are equal, measured with respect to the normal to the reflecting surface. The incident, reflected, and normal lines all lie in the same plane.
• An echo refers to the reflection of sound that reaches the listener with a delay after the direct sound. Our brain processes and sends the sensation of sound for about 0.1 seconds. To hear an echo distinctly, the time interval between the original sound and the reflected sound should be at least 0.1 seconds. The total distance covered by the sound from the source to the reflecting surface and back should be at least 34.4 meters, assuming the speed of sound is 344 m/s.
• For clear echo perception, the distance between the obstacle and the source of sound should be half of this total distance, which is 17.2 meters. This distance can vary with changes in the temperature of the air. Multiple and successive reflections can result in the perception of multiple echoes. The sound of thunder is heard due to multiple successful reflections on various surfaces.
• Reverberation refers to the repeated reflections of sound, which enhance its impact. It can be observed in spaces such as musical halls, conference halls, and cinema halls, where the ceilings are designed to allow sound to reach all corners of the hall through reflection.
• Multiple reflections of sound have various applications, such as the use of a stethoscope in medicine to listen to internal body sounds like the heartbeat or lungs. Additionally, the design of twisted ceilings in halls ensures sound reflection for optimal acoustics.

In conclusion, sound waves are mechanical waves characterized by the movement of particles within a medium. Sound cannot propagate through a vacuum because there are no particles to vibrate. Understanding sound waves is important for UPSC aspirants as questions related to sound waves often appear in UPSC prelims and mains exams. Air is the most common medium through which sound travels. Vibrating objects compress the air in front of them, creating regions of high pressure called compressions (C), and as they move backward, regions of low pressure called rarefactions (R). This creates a series of compressions and rarefactions in the air, forming a sound wave that propagates through the medium. The density of particles in the medium determines the pressure variations and visual representation of sound propagation through density or pressure fluctuations.