Comprehensive theory, key formulas, diagrams, and memory aids for Sound.
Every day we hear sounds from various sources like humans, birds, bells, machines, vehicles, and televisions. Sound is a form of energy which produces a sensation of hearing in our ears. As we learned in the previous chapter, energy can neither be created nor destroyed. So, how is this specific form of energy produced and transmitted to our ears?
In this chapter, we are going to learn how sound is produced and how it is transmitted through a medium and received by our ears.
If you pluck the string of a guitar, it vibrates and produces sound. If you strike a tuning fork, its prongs vibrate and produce sound. If you touch your throat while speaking, you can feel vibrations. Sound is produced by vibrating objects. Vibration means a kind of rapid to-and-fro motion of an object. The energy required to make an object vibrate (and thus produce sound) is provided by some outside source (like your hand plucking a string or hitting a drum).
Sound needs a material medium to travel. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid, or gas.
When an object vibrates, it sets the particles of the medium around it vibrating. * The vibrating object pushes and compresses the air in front of it, creating a region of high pressure. This region is called a compression (C). * As the vibrating object moves backward, it creates a region of low pressure called a rarefaction (R). * As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium.
Note: The particles of the medium do not travel all the way from the vibrating object to the ear. A particle of the medium simply oscillates back and forth about its mean position and transfers its energy to the adjacent particle.
Because sound propagates by the collisions of particles in a medium, sound cannot travel through a vacuum. If you place a ringing bell inside a glass jar and pump all the air out using a vacuum pump, you will eventually not be able to hear the bell at all, even though you can see the hammer striking the gong.
Sound waves are longitudinal waves. In longitudinal waves, the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance.
A sound wave can be described completely by five characteristics: 1. Wavelength ($\lambda$): The distance between two consecutive compressions (C) or two consecutive rarefactions (R). Its SI unit is the metre (m). 2. Frequency ($\nu$): We know that when sound is propagated, the density of the medium oscillates. One complete oscillation is the change in density from maximum to minimum and back to maximum. The number of such oscillations per unit time is the frequency of the sound wave. * SI Unit: Hertz (Hz), named after Heinrich Rudolph Hertz. 3. Time Period ($T$): The time taken for one complete oscillation in the density of the medium. * Frequency and Time Period are related: $\nu = \frac{1}{T}$ 4. Amplitude ($A$): The magnitude of the maximum disturbance in the medium on either side of the mean value. It represents the "loudness" or "softness" of a sound. 5. Speed ($v$): The distance which a point on a wave, such as a compression or a rarefaction, travels per unit time. * $\text{Speed} = \text{Wavelength} \times \text{Frequency}$ ($v = \lambda \nu$)
Sound propagates through a medium at a finite speed. You hear thunder a little after seeing the lightning flash because the speed of light is much greater than the speed of sound. * The speed of sound depends on the properties of the medium (temperature, state of matter). * Sound travels fastest in solids, slower in liquids, and slowest in gases. * In air at $22^\circ\text{C}$, the speed of sound is about $344 \text{ m/s}$.
Just like light, sound gets reflected at the surface of a solid or liquid. The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface.
If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. * The sensation of sound persists in our brain for about $0.1 \text{ s}$. * To hear a distinct echo, the time interval between the original sound and the reflected one must be at least $0.1 \text{ s}$. * Because sound travels at $344 \text{ m/s}$, the total distance traveled by the sound must be $34.4 \text{ m}$ (forward and back). Therefore, the minimum distance to the obstacle must be half of that, $17.2 \text{ m}$.
In a big hall, a sound created is repeatedly reflected from the walls until it is reduced to a value where it is no longer audible. This repeated reflection that results in the persistence of sound is called reverberation. To reduce undesirable reverberation, auditorium roofs and walls are covered with sound-absorbent materials like compressed fibreboard.
Ultrasounds are high-frequency waves able to travel along well-defined paths even in the presence of obstacles. 1. Used to clean spiral tubes, odd-shaped parts, and electronic components. 2. Used to detect cracks and flaws in metal blocks. 3. Echocardiography: Used to get images of the heart. 4. Ultrasonography: Used by doctors to examine internal organs like the liver, gall bladder, uterus, or to monitor fetal growth during pregnancy. 5. SONAR (Sound Navigation And Ranging): A device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects, and to determine the depth of the sea.
Sound is a mechanical wave that requires a material medium for propagation, travelling through a series of compressions and rarefactions. It is defined by characteristics such as wavelength, frequency (which determines pitch), and amplitude (which determines loudness). While it travels faster in solids than in liquids or gases, it cannot travel in a vacuum at all. The principles of sound reflection allow us to hear echoes and use technologies like SONAR and medical ultrasonography to "see" using high-frequency, inaudible sound waves.