Sound Waves: Characteristics and Applications
Class 09 ScienceSound is produced by vibrations. Vibration refers to the periodic to and fro motion (oscillations) of an object. Pulling a stretched string or striking a metal object makes them vibrate, which produces sound.
Propagation of Sound
Sound can travel or propagate through solids, liquids and gases. The material through which sound propagates is called a medium. A space where there is no medium (matter) is referred to as vacuum.
Sound cannot propagate in vacuum. Sound needs a medium to propagate. The medium can be a solid, a liquid, or a gas.
Sound Waves
The disturbance consisting of a series of alternating compressions and rarefactions propagating through a medium, without the actual flow of the particles of medium, is called a sound wave. The direction in which the wave travels is known as the direction of propagation of the wave.
In sound waves, the particles of the medium vibrate back and forth parallel to the direction of propagation of disturbance. Such waves where the particles vibrate in a direction parallel to the direction of the wave propagation are known as longitudinal waves.
Waves that require a material medium for propagation are called mechanical waves. Sound wave is a type of a mechanical wave.
Wavelength
As a sound wave propagates, at any given instant of time the density of the medium varies periodically with distance from the source. In the region of a compression, the density of the medium increases above the average density and the highest point represents the maximum density. Thereafter, the density decreases and eventually falls below the average density in the region of rarefaction and the lowest point represents the minimum density. The highest point is called the crest and the lowest point is called the trough of the wave.
The distance between the two consecutive crests or two consecutive troughs is called the wavelength of a wave. The wavelength is usually represented by λ (Greek letter lambda). Its SI unit is the metre (m).
Frequency
The density of the medium at the given position changes alternately between a maximum density (crest) and a minimum density (trough). The change in the density of the medium at a fixed point, from maximum to minimum and then again to maximum (or vice versa), makes one complete oscillation.
The number of density oscillations at a fixed point per unit time is the frequency of the sound wave. It is usually represented by ν (Greek letter nu). Its SI unit is per second (s-1) also called a hertz (symbol Hz).
Time Period
The time taken for one complete density oscillation at a fixed point is defined as the time period of the wave. The time period of the wave is represented by the symbol T. Its SI unit is second (s).
The time period and frequency are inversely related; a shorter time period corresponds to a higher frequency.
$$ \nu = \frac{1}{T} $$
Amplitude
Sound propagates as density oscillations of the medium via compressions and rarefactions. The amplitude of a sound wave is the maximum change in the density of air in a compression (or a rarefaction) compared to the average density. A larger change in density corresponds to a larger amplitude.
The amplitude and the amount of energy a wave carries are related. A wave with a larger amplitude carries more energy than the one with a smaller amplitude.
Intensity
The amount of sound energy passing through a unit area perpendicular to the direction of the propagation of sound wave in a unit time is called the intensity of sound.
As sound wave travels away from its source, it spreads over a larger area. Since the energy must be conserved, the same amount of energy is now spread over a larger area. Hence, the intensity decreases with distance from the source. Sounds produced with initial larger amplitude carry more energy and can travel a larger distance before intensity reduces to zero.
Speed of Sound
The speed of sound describes how fast sound waves propagate through a medium. The speed of sound can be defined as the distance which a point on a wave, such as a crest (or a trough), travels in unit time.
For a sound wave of given frequency, the distance between two consecutive crests (or two consecutive troughs) is one wavelength (λ). This distance is covered by the disturbance in one time period (T). Therefore, the speed of sound (v) is
speed = wavelength × frequency
$$v = \lambda \times \nu$$
The speed of sound depends on the medium through which it travels. It travels fastest in solids, slower in liquids, and slowest in gases.
The speed of sound in air also depends on the temperature and humidity. As we increase the temperature or humidity, the speed of sound increases.
Human Perception of Sound
Human perception of sound is described by terms, such as loudness and pitch which do not have simple relations with the physical properties.
Pitch
How frequency is perceived by humans is called pitch. Sounds perceived to be shrill, such as a whistle or a siren are said to have high pitch, while deep sounds like thunder or an aircraft rumble have low pitch. In general, high pitch sounds have higher frequency and low pitch sounds have lower frequency.
Humans can only hear a limited range of sound frequencies. The audible range or the human hearing range is from 20 Hz to 20,000 Hz (20 kHz). Sound waves with frequency below 20 Hz are called infrasonic waves, while sound waves with frequency above 20 kHz are called ultrasonic waves. Humans cannot hear infrasound or ultrasound but some animals can. Dogs, cats, bats, dolphins can detect ultrasound, while elephants can detect infrasound.
Loudness
Humans perceive the amplitude of a sound wave as loudness. Sounds with larger amplitude are heard louder, while those with smaller amplitude sound softer. The loudness decreases as we move farther away from the source.
Reflection of Sound
Sound waves can bounce off obstacles like solids or liquids and this is known as the reflection of sound. Sound follow the same laws of reflection as those of light.
The directions in which the sound is incident and is thereafter reflected make equal angles with the normal to the reflecting surface at the point of incidence, with all three lying in the same plane. One of the most common examples of the reflection of sound is an echo.
Echo
If the time gap between two sounds reaching us is at least 0.1 s, we can hear them as separate sounds. If it is less than 0.1 s, then we cannot clearly distinguish between them. This time duration of 0.1 s can help us estimate the minimum distance of a reflecting surface from which we can hear an echo.
Echoes are stronger from hard and smooth surfaces that reflect sound. Soft surfaces like curtains tend to absorb sound, while rough surfaces scatter it in different directions, and in such cases it is not possible to hear the echoes clearly.
Reverberation
Sometimes the emitted sound can undergo multiple reflections from the walls in a large hall or auditorium. Multiple reflections make sound persist after the source stops emitting sound, a phenomenon called reverberation. This occurs when sound reflections from surfaces arrive with a time difference less than 0.05 s.
Ultrasonic and Infrasonic Waves
Sound waves with frequencies outside the human audible range have important applications in science, medicine, and technology.
Echolocation
Bats are nocturnal creatures that fly and search for their prey in the dark without colliding into objects. Most bats emit short bursts of ultrasonic waves which are reflected from nearby objects. By sensing the echoes the bat can determine the position of obstacles and prey.
This ability to locate objects using reflected sound waves is called echolocation. Besides bats, animals such as dolphins, whales, and some birds also use echolocation for navigation and hunting.
Humans have adapted the same principle in underwater exploration through sonar (sound navigation and ranging). In sonar, ultrasonic waves are sent into water and the reflected waves are analysed to determine the distance, direction, and speed of underwater objects, such as submarines or shipwrecks.