Sound Is Produced By A Vibrating Body

Sound is a crucial part of our everyday lives, enabling communication and providing information about our surroundings. We hear various sounds, from doorbells and footsteps to musical instruments.

The fundamental principle behind sound production is vibration. Whenever an object vibrates, it produces sound. Vibration is defined as the to and fro, or back and forth, motion of an object. If you touch a school bell while it's ringing, you can feel it vibrating. When the vibration stops, the sound also stops.

Many examples illustrate this connection:

• Striking a metal plate or pan causes it to vibrate and produce sound. Holding it tightly stops the vibration and the sound.
• Plucking a stretched rubber band makes it vibrate and create sound.
• Striking a metal dish filled with water causes vibrations, which are visible as waves on the water surface, simultaneously producing sound.

While the vibrations of some objects are large enough to be seen, the vibrations of many sound-producing objects are too small in amplitude to be visible but can still be felt.

Musical instruments produce sound through the vibration of specific parts. For example, in a sitar or guitar, the stretched strings vibrate. In a drum or tabla, the stretched membrane vibrates. In instruments like manjira (cymbals) or ghatam (mud pot), sound is produced by striking them.

S.No.	Musical Instrument	Vibrating Part Producing Sound
1.	Veena	Stretched string
2.	Tabla	Stretched membrane
3.	Sitar / Guitar	Stretched string
4.	Flute	Air column inside the flute
5.	Harmonium	Reeds vibrating with air
6.	Dholak / Drum	Stretched membrane (head)
7.	Manjira / Ghatam / Noot / Kartal	The instrument itself (beaten/struck)

It's important to note that when parts of an instrument vibrate (like strings or membranes), the entire instrument's body often vibrates as well, contributing to the overall sound produced. Creating a "jaltrang" by filling bowls with water at different levels and striking them gently demonstrates how different volumes of water can produce varied pitches.

Sound Produced By Humans

In humans, sound (our voice) is produced by the voice box, also called the larynx. You can feel vibrations in your throat when you speak, sing, or hum, because the larynx is located in the upper part of the windpipe (trachea).

Inside the larynx, there are two folds of tissue called vocal cords. They are stretched across the voice box, leaving a narrow gap or slit between them. When we speak, the lungs force air up through this narrow slit. As the air passes, it causes the vocal cords to vibrate, and these vibrations produce sound.

Muscles attached to the vocal cords can adjust their tension and thickness. The quality or type of voice (e.g., pitch) changes depending on whether the vocal cords are tight and thin or loose and thick.

Differences in voice pitch between men, women, and children are due to the differing lengths of their vocal cords. Men generally have longer vocal cords (about 20 mm), while women have shorter ones (about 15 mm). Children have very short vocal cords, which is why their voices typically have a higher pitch.

Sound Needs A Medium For Propagation

For sound to travel from its source to a listener, it needs a substance or material to pass through. This substance is called a medium. Sound cannot travel through a vacuum, which is a space completely empty of matter.

Sound can travel through different types of media:

• Gases: Sound travels through air, which is a mixture of gases. When someone speaks at a distance, the sound travels through the air to reach the listener's ears. An experiment with a cell phone ringing inside a vacuum-sealed tumbler demonstrates that as air is removed from the tumbler, the sound becomes fainter, proving that air (a gas) is needed for sound to travel. If all the air were removed (creating a vacuum), no sound would be heard.
• Liquids: Sound can travel through liquids like water. Shaking a bell underwater in a bucket allows the sound to travel through the water to reach your ear placed on the surface. This explains how aquatic animals like whales and dolphins communicate underwater.
• Solids: Sound also travels efficiently through solids. You can hear sounds by placing your ear against a solid object like a long metal rod, a wooden table, or even a stretched string (like in a toy telephone) and having someone tap or scratch the other end.

In summary, sound requires a medium (solid, liquid, or gas) to propagate. It travels outwards in all directions from the vibrating source within that medium.

We Hear Sound Through Our Ears

Our ears are the organs responsible for hearing sound. The outer part of the ear is shaped like a funnel, which collects sound waves from the surroundings. These sound waves travel down a passage called the ear canal.

At the end of the ear canal is a thin, stretched membrane called the eardrum. The eardrum is like a sensitive diaphragm. When sound waves reach the eardrum, they cause it to vibrate.

These vibrations of the eardrum are then transmitted to the inner ear, which contains structures that convert the mechanical vibrations into electrical signals. These electrical signals are sent to the brain via nerves. The brain interprets these signals as sound, allowing us to hear and understand what the sound is.

It is important to be very careful with our ears and never insert any sharp, pointed, or hard objects into the ear canal, as this can easily damage the eardrum and impair hearing.

Amplitude, Time Period And Frequency Of A Vibration

A vibration is a type of motion called oscillatory motion, which is a repetitive to and fro movement. The characteristics of these vibrations determine the properties of the sound produced.

• Time Period: The time taken for one complete oscillation or vibration is called the time period.
• Frequency: The number of oscillations or vibrations that occur in one second is called the frequency of oscillation. Frequency is a key property of sound.

The standard unit for frequency is hertz, symbolised as Hz. One hertz corresponds to one oscillation per second ($1 \text{ Hz} = 1$ oscillation/second). If an object vibrates 20 times in one second, its frequency is 20 Hz.

The properties of sound that help us distinguish between different sounds are primarily its amplitude and frequency.

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Loudness And Pitch

The loudness and pitch are two important characteristics of sound, determined by the amplitude and frequency of the vibrations, respectively.

• Loudness: The loudness of a sound is determined by the amplitude of the vibration that produces it. A larger amplitude means the vibrating object moves further from its resting position, resulting in a louder sound. A smaller amplitude produces a feeble (soft) sound. The loudness of sound is directly proportional to the square of the amplitude of the vibration ($L \propto A^2$). This means if the amplitude doubles, the loudness becomes four times greater. Loudness is measured in a unit called decibel (dB).

Examples of sounds at different decibel levels:

Sound Source	Loudness (dB)
Normal breathing	10
Soft whisper (at 5m)	30
Normal conversation	60
Busy traffic	70
Average factory	80

Sounds above 80 dB can become physically painful and potentially cause hearing damage.

• Pitch (Shrillness): The pitch or shrillness of a sound is determined by its frequency. A higher frequency vibration produces a higher-pitched, shriller sound (e.g., a whistle, a bird's chirp). A lower frequency vibration produces a lower-pitched sound (e.g., a drum beat, a lion's roar).

Comparing voices, a child's voice typically has a higher frequency (and thus higher pitch) than an adult's. Similarly, a woman's voice usually has a higher frequency than a man's, even if the loudness is the same.

While a bird's sound might be high-pitched but quiet (low loudness), a lion's roar can be low-pitched but very loud (high loudness). Loudness and pitch are independent properties.

Audible And Inaudible Sounds

Humans cannot hear sounds of all frequencies. The range of frequencies that the average human ear can detect is called the audible range. This range is approximately from 20 hertz (20 Hz) to 20,000 hertz (20 kHz).

• Sounds with frequencies below 20 Hz are called infrasound and are inaudible to humans.
• Sounds with frequencies above 20,000 Hz (20 kHz) are called ultrasound and are also inaudible to humans.

Some animals, however, have different audible ranges. Dogs, for example, can hear sounds at frequencies higher than 20,000 Hz. This ability is utilised in high-frequency dog whistles that humans cannot hear.

Medical equipment like ultrasound machines operate at frequencies above the human audible range (greater than 20,000 Hz) to create images for diagnosis.

Noise And Music

Sounds in our environment can be broadly categorised based on how they are perceived:

• Music: Sounds that are pleasant, harmonious, and pleasing to the ear are generally termed as music. Sounds produced by musical instruments like a harmonium, sitar, or flute are typically considered musical sounds.
• Noise: Sounds that are unwanted, unpleasant, or harsh to the ear are called noise. Noise is often irregular and lacks a pleasing pattern. Examples include sounds from construction sites, vehicle horns, shouting in a classroom, or loud machinery.

While music is generally pleasant, even musical sounds can become unpleasant and be perceived as noise if they are too loud or played inappropriately.

Noise Pollution

Just as unwanted substances pollute the air (air pollution), the presence of excessive or unwanted sounds in the environment constitutes noise pollution. Noise pollution is a significant environmental concern in many areas.

Common sources of noise pollution include:

• Sounds from transportation (vehicles, aircraft engines).
• Explosions, particularly bursting firecrackers.
• Operation of machines in industries and at home.
• Loudspeakers and public address systems.

Within homes, common sources contributing to noise pollution can be loud televisions or radios, certain kitchen appliances, desert coolers, and air conditioners.

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What Are The Harms Of Noise Pollution?

Exposure to excessive noise can have serious detrimental effects on human health and well-being:

• It can cause various health problems, including lack of sleep, increased blood pressure (hypertension), anxiety, and stress-related disorders.
• Continuous exposure to loud noise, especially above 80 dB, can lead to temporary or even permanent loss of hearing (hearing impairment).

Total hearing impairment from birth is rare; partial hearing loss is more commonly caused by disease, injury, or age-related factors. Children with hearing loss may also have difficulty developing speech due to the direct link between hearing and speaking. Technological devices and learning sign language can significantly improve the quality of life for hearing-impaired individuals, and societal support is vital.

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Measures To Limit Noise Pollution

Controlling noise pollution requires tackling the sources of excessive noise and implementing measures to reduce its impact:

• Install silencing devices in engines of aircraft, vehicles, and industrial machinery.
• Use mufflers in automobiles.
• Locate noisy operations and industries away from residential areas.
• Minimise the use of automobile horns, especially in sensitive areas like hospitals and schools.
• Reduce the volume of televisions and music systems at home.
• Plant trees along roadsides and around buildings. Trees act as sound barriers, absorbing sound and reducing the amount of noise that reaches residential areas.

Implementing these measures helps to create a quieter environment and mitigate the harmful effects of noise pollution on human health.

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