Sound Notes

Sound

Introduction to Sound

Sound is a form of energy that produces a sensation of hearing.
It originates from various sources such as humans, birds, bells, machines, vehicles, televisions, and radios.
Sound is one form of energy among others like mechanical and light energy.
The principle of conservation of energy applies to sound: energy can neither be created nor destroyed, but only transformed.
Clapping produces sound, requiring the use of energy.
This chapter covers the production, transmission, and reception of sound.

Production of Sound

Sound is produced by vibrating objects.

Activity 11.1

A tuning fork is used to demonstrate sound production.
- Striking the tuning fork on a rubber pad sets it vibrating.
- Bringing the vibrating tuning fork near the ear allows one to hear the sound.
- Touching the prongs of the vibrating tuning fork with a finger allows one to feel the vibration.
- Suspending a table tennis ball near the vibrating tuning fork demonstrates the vibration visually.

Activity 11.2

Water is used to visualize the vibration.
- Touching the surface of water with a vibrating tuning fork creates disturbances in the water.
- Dipping the prongs of a vibrating tuning fork in water amplifies the effect.

General Methods of Sound Production

Sound can be produced by plucking, scratching, rubbing, blowing, or shaking objects.
These actions set the objects into vibration, which leads to sound production.
Vibration is a rapid to-and-fro motion of an object.
Human voice is produced by vibrations in the vocal cords.
The buzzing sound of a bee is also a result of vibration.
A stretched rubber band produces sound when plucked due to its vibration.

Activity 11.3

Musical instruments produce sound through vibrating parts.

Propagation of Sound

Sound requires a medium to travel, which can be solid, liquid, or gas.
The medium transmits sound from the source to the listener.
When an object vibrates, it causes the particles in the surrounding medium to vibrate.
The particles themselves do not travel the entire distance; instead, they displace adjacent particles, which then return to their original positions.
This process continues until the sound reaches the ear.
The disturbance, not the particles, travels through the medium; this disturbance is called a wave.
A wave is a disturbance that moves through a medium by setting neighboring particles into motion.
Sound waves are mechanical waves, characterized by the motion of particles in the medium.
Air is a common medium for sound transmission.
When a vibrating object moves forward, it compresses the air in front, creating a high-pressure region called a compression (C).
When the object moves backward, it creates a low-pressure region called a rarefaction (R).
Rapid back-and-forth motion produces a series of compressions and rarefactions, which constitute the sound wave.
Propagation of sound can be visualized as propagation of density or pressure variations in the medium.

Longitudinal Waves

Activity 11.4

A slinky is used to demonstrate longitudinal waves.
- Stretching a slinky and pushing it sharply shows compressions and rarefactions.
- Marking a dot on the slinky shows that the particles move back and forth parallel to the direction of the disturbance.
Sound propagates in the medium as a series of compressions and rarefactions, similar to the slinky demonstration.
In longitudinal waves, particles of the medium move parallel to the direction of propagation of the disturbance.
Particles oscillate back and forth about their position of rest.
Sound waves are longitudinal waves.
There is another type of wave called a transverse wave, where particles oscillate perpendicular to the direction of wave propagation.
Light is a transverse wave, but not a mechanical wave because the oscillations are not of the medium particles, nor their pressure or density.

Characteristics of a Sound Wave

Sound waves are described by their frequency, amplitude, and speed.
Density and pressure change as the sound wave moves through the medium.
Compressions are regions where particles are crowded, representing high density and pressure.
Rarefactions are regions where particles are spread apart, representing low density and pressure.
A peak on a graph represents maximum compression (crest), and a valley represents rarefaction (trough).
Wavelength $(\lambda)$ is the distance between two consecutive compressions or rarefactions and is measured in meters (m).

Frequency

Frequency indicates how often an event occurs.
When sound propagates, the density of the medium oscillates between maximum and minimum values.
The number of oscillations per unit time is the frequency $(\nu)$ of the sound wave, measured in hertz (Hz).
Counting the number of compressions or rarefactions that cross a point per unit time gives the frequency.
Time period (T) is the time taken for one complete oscillation and is measured in seconds (s).
Frequency and time period are related by the formula: $v = {1 ewline T}$
Pitch is how the brain interprets the frequency of sound; higher frequency corresponds to higher pitch.
Objects of different sizes and conditions vibrate at different frequencies, producing sounds of different pitch.

Amplitude

Amplitude (A) is the magnitude of the maximum disturbance in the medium.
It determines the loudness or softness of a sound; larger amplitude corresponds to louder sound.
The amplitude depends on the force with which an object is made to vibrate.
A soft sound has less energy (smaller amplitude), while a loud sound has higher energy (larger amplitude).
As a sound wave spreads out, its amplitude and loudness decrease.
Quality or timbre distinguishes sounds with the same pitch and loudness.
A pleasant sound has a rich quality.
A tone is a sound of single frequency, while a note is a mixture of several frequencies and is pleasant to hear.
Noise is unpleasant to the ear, while music is pleasant and of rich quality.

Speed of Sound

Speed of sound (v) is the distance a point on a wave travels per unit time.
The formula for speed is: $v = {distance ewline time} = {\lambda ewline T}$
Since $v = {\lambda ewline T}$ , and $v = \lambda \nu$ , therefore $v = \lambda \nu$
Speed equals wavelength times frequency.
The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

Example 11.1

A sound wave has a frequency of 2 kHz and a wavelength of 35 cm. How long will it take to travel 1.5 km?
Given:
- Frequency, $v = 2 kHz = 2000 Hz$
- Wavelength, $\lambda = 35 cm = 0.35 m$
Speed, $v = \lambda \nu = 0.35 m \times 2000 Hz = 700 m/s$
Time, $t = {d \newline v} = {1.5 \times 10^3 m ewline 700 m/s} = 2.1 s$
Thus, sound will take 2.1 seconds to travel a distance of 1.5 km.

Intensity and Loudness

The amount of sound energy passing each second through a unit area is the intensity of sound.
Loudness is the ear's response to the sound and is not the same as intensity.
Even if two sounds have equal intensity, they may be perceived differently in loudness.

Reflection of Sound

Sound bounces off solids or liquids, similar to a rubber ball bouncing off a wall.
Sound follows the laws of reflection.
- The angle of incidence equals the angle of reflection.
- The incident direction, reflected direction, and the normal to the reflecting surface are in the same plane.

Activity 11.5

Two pipes can be used to demonstrate reflection of sound.

Speed of Sound in Different Media

Sound travels at a finite speed that depends on the properties of the medium.
The speed of sound is much less than the speed of light.
Thunder is heard later than the lightning flash is seen.
Speed of sound also depends on the temperature of the medium; as temperature increases, speed increases.
The speed decreases when moving from solid to a gaseous state.
Example: In air, the speed of sound is 331 m/s at 0°C and 344 m/s at 22°C.

Echo

An echo is the sound heard after reflection from a suitable object.
The sensation of sound persists in the brain for about 0.1 seconds.
To hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds.
At 22°C, the speed of sound is about 344 m/s, so the total distance covered by the sound should be at least 34.4 meters.
The minimum distance of the obstacle from the sound source should be half of this distance, i.e., 17.2 meters.
Multiple echoes can occur due to successive reflections, like the rolling of thunder.

Example 11.2

A person clapped near a cliff and heard the echo after 2 seconds. What is the distance of the cliff if the speed of sound is 346 m/s?
- Given:
  - Speed of sound, $v = 346 m/s$
  - Time, $t = 2 s$
- Distance traveled by sound = $v \times t = 346 m/s \times 2 s = 692 m$
- Distance to the cliff = $692 m \newline 2 = 346 m$

Reverberation

Reverberation is the persistence of sound due to repeated reflections.
Excessive reverberation is undesirable in auditoriums or big halls.
To reduce reverberation, sound-absorbent materials like compressed fiberboard, rough plaster, or draperies are used on walls and roofs.

Uses of Multiple Reflection of Sound

Megaphones, horns, trumpets, and shehanais are designed to direct sound in a specific direction.
Stethoscopes use multiple reflections to transmit the sound of a patient's heartbeat to a doctor's ears.
Curved ceilings in concert halls and conference halls help distribute sound evenly.
Curved soundboards behind stages can also spread sound across the hall.

Range of Hearing

The audible range for humans is about 20 Hz to 20,000 Hz.
Children and some animals can hear up to 25 kHz.
Sensitivity to higher frequencies decreases with age.
Frequencies below 20 Hz are called infrasonic or infrasound.
Some animals use infrasound for communication.
Frequencies above 20 kHz are called ultrasonic or ultrasound.
Animals such as dolphins, bats, and porpoises produce ultrasound.
Hearing aids are electronic devices that amplify sound for people with hearing loss.

Applications of Ultrasound

Ultrasound waves have high frequencies and travel in well-defined paths.

Cleaning: Used to clean hard-to-reach places by detaching dust and dirt particles.
Detecting Flaws: Used to detect cracks in metal blocks by reflecting off defects.
Echocardiography: Used to create images of the heart.
Ultrasound Scanner: Used to image internal organs and detect abnormalities like stones or tumors.
Ultrasonography: Used for examination of the fetus during pregnancy.
Breaking Kidney Stones: Used to break small kidney stones into fine grains.