We rely heavily on sound in our daily lives. It helps us communicate and stay aware of our surroundings. For instance, you know when a school period ends by hearing the bell, or you detect someone at your door by their knock or the doorbell sound. Even footsteps can alert you to someone's approach.

In games like hide and seek, a blindfolded person often uses sound clues to locate others.

Sound is a fundamental aspect of our environment, enabling us to interact with each other and experience a variety of auditory sensations, from conversations to the sounds of musical instruments like flutes, tablas, and harmoniums.

Understanding sound involves exploring several questions: How is sound created? How does it travel from its source to our ears? How do we perceive sound? And why do some sounds seem louder or different from others?

Sound Is Produced By A Vibrating Body

The primary source of sound is a vibrating object. Vibration is a rapid to-and-fro or back-and-forth motion of an object.

When you touch a school bell while it is ringing, you can feel it vibrating. Once the sound stops, the vibration also stops.

Simple experiments demonstrate this principle:

• Striking a metal plate or pan suspended freely causes it to produce sound and vibrate. Holding the plate tightly stops the vibration and the sound.
• Plucking a stretched rubber band causes it to vibrate and produce sound. The sound ceases when the vibration stops.
• Striking a metal dish filled with water causes the dish to vibrate, producing sound. The vibrations of the dish disturb the water surface, creating ripples or waves. If you touch the dish to stop the vibrations, the sound and the water waves disappear.

These observations confirm that vibrating objects are the source of sound. While sometimes the vibrations are visible, often they are too small to see with the naked eye, but they can still be felt.

In musical instruments, sound is produced by the vibration of specific parts:

S.No.	Musical Instrument	Vibrating Part Producing Sound
1.	Veena / Sitar	Stretched string
2.	Tabla / Dholak / Mridangam	Stretched membrane (drumhead)
3.	Flute	Air column inside the pipe
4.	Harmonium	Reeds (vibrating metal strips)
5.	Manjira (Cymbals)	Metal plates
6.	Ghatam / Noot (Mudpot)	Body of the pot
7.	Kartal	Wooden/Metal pieces
8.	Ektara	Stretched string

Sometimes, the sound produced by an instrument like a sitar or a drum is not just from the primary vibrating part (string or membrane); the vibration is transmitted to the entire body of the instrument, which also vibrates and contributes to the sound we hear.

Creating a 'jaltrang' by striking bowls filled with water to different levels demonstrates how changing the vibrating body (water level/bowl) can produce different sounds.

Sound Produced By Humans

In humans, sound is produced in the voice box, also known as the larynx. You can feel the voice box by placing your hand on your throat while speaking, singing, or buzzing; you will feel vibrations.

The voice box is located at the upper end of the windpipe (trachea). Inside the larynx are two structures called vocal cords. These vocal cords are stretched across the voice box in a way that leaves a narrow space or slit between them.

When we speak or produce sound, the lungs force air through this narrow slit between the vocal cords. As air passes through, it causes the vocal cords to vibrate, and these vibrations produce sound.

Muscles connected to the vocal cords allow them to be made tighter or looser, and thinner or thicker. The quality or type of voice (e.g., pitch) depends on the tension, thickness, and length of the vocal cords.

For instance, the vocal cords in adult males are typically longer (about 20 mm) than in adult females (about 15 mm). Children have very short vocal cords. These differences in vocal cord length lead to the distinct voice qualities observed in men, women, and children.

Sound Needs A Medium For Propagation

Sound travels from the source that produces it to the listener through a medium. A medium is the substance or material through which sound waves can pass. Sound needs a medium to propagate (travel).

You can hear a friend calling from a distance because sound travels through the air between you.

The need for a medium can be demonstrated:

• Place a ringing cell phone inside a dry glass tumbler. As you suck air out of the tumbler while listening, the sound of the ring becomes fainter. If all the air could be removed (creating a vacuum), the sound would not be heard at all. This shows that sound cannot travel through a vacuum (space with no air or matter).
• Sound can travel through liquids. If you shake a bell underwater in a bucket and place your ear gently on the water surface, you can hear the sound of the bell. This is how marine animals like whales and dolphins communicate underwater.
• Sound can travel through solids. If you place your ear at one end of a metre scale or a long metal rod while a friend gently scratches the other end, you can hear the scratching sound clearly through the solid material. This also works with wooden or metallic tables.
• Sound can even travel through strings, as demonstrated by making a toy telephone using cups and a stretched string.

So, sound is produced by vibrating objects and travels outwards in all directions through a medium, which can be a gas (like air), a liquid (like water), or a solid.

We Hear Sound Through Our Ears

Our ears are the organs responsible for detecting sound. The outer part of the ear is shaped like a funnel, which collects sound waves and directs them down a passage called the ear canal. At the end of the ear canal is a thin, stretched membrane called the eardrum.

When sound waves reach the eardrum, they cause it to vibrate. These vibrations are then transmitted to the inner ear. From the inner ear, electrical signals are sent to the brain via nerves. The brain interprets these signals as sound, allowing us to hear.

A model using a tin can with a stretched rubber balloon and grains of cereal demonstrates how vibrations affect the eardrum. Sound spoken into the can causes the rubber membrane to vibrate, making the cereal grains jump.

It is very important to protect our ears and eardrums. Never insert sharp, pointed, or hard objects into the ear canal, as this can damage the eardrum and impair hearing.

Amplitude, Time Period And Frequency Of A Vibration

As we know, sound originates from vibrations, which are oscillatory movements. These vibrations have specific characteristics:

• 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 measured in units of Hertz (Hz). One Hertz (1 Hz) means one oscillation per second. If an object vibrates 20 times in one second, its frequency is 20 Hz.

Different sounds can be distinguished based on their properties, primarily amplitude and frequency.

Loudness And Pitch

Two key characteristics that help us differentiate sounds are loudness and pitch.

• Loudness: The loudness of a sound is related to the amplitude of the vibration that produces it. The amplitude is the maximum displacement of a vibrating object from its central resting position. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound is feeble (soft).

  Loudness is proportional to the square of the amplitude of the vibration ($L \propto A^2$). This means doubling the amplitude increases the loudness by a factor of four.

  Loudness is expressed in a unit called the decibel (dB).

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

  Sound levels above 80 dB are considered physically painful and harmful.

• Pitch: The pitch of a sound is determined by its frequency. It describes how "shrill" or "grave" a sound is. A higher frequency of vibration results in a higher pitch and a shriller sound. A lower frequency results in a lower pitch.

  For example, a drum vibrates at a low frequency, producing a low-pitched sound. A whistle vibrates at a high frequency, producing a high-pitched sound. A bird's chirping is high-pitched, while a lion's roar is low-pitched, although the lion's roar is much louder (greater amplitude) than the bird's sound.

  Generally, the voice of a child or a woman has a higher frequency (and thus higher pitch) compared to the voice of an adult man.

Audible And Inaudible Sounds

While vibrating objects produce sound, not all sounds are detectable by the human ear. Our ability to hear is limited to a specific range of frequencies.

• Sounds with frequencies below approximately 20 Hertz (20 Hz) are called inaudible sounds for humans.
• Sounds with frequencies higher than approximately 20,000 Hertz (20 kHz) are also inaudible to the human ear.

The typical range of frequencies that humans can hear, known as the audible range, is roughly from 20 Hz to 20,000 Hz (or 20 kHz).

Some animals have different hearing ranges. For example, dogs can hear sounds at frequencies much higher than 20,000 Hz. This is why special high-frequency whistles are used to call dogs, which are inaudible to humans.

Technology like ultrasound equipment, used in medicine for imaging and diagnosis, operates at frequencies above the human audible range ($> 20,000 \text{ Hz}$).

Noise And Music

Sounds can be classified based on how they affect us.

• Music refers to sounds that are pleasant or pleasing to the ear, often produced by musical instruments.
• Noise refers to sounds that are unpleasant or unwanted. Examples include sounds from construction sites, loud vehicle horns, the cacophony of many people talking at once, or loud noises from machines.

However, the distinction can sometimes be subjective, and even musical sound, if played at a very high loudness level, can become unpleasant and be considered noise.

Noise Pollution

Similar to air pollution (the presence of unwanted substances in the air), noise pollution is defined as the presence of excessive or unwanted sounds in the environment.

Common sources of noise pollution include:

• Sounds from vehicles (traffic noise).
• Explosions, including firecrackers.
• Running machines in industries.
• Loud loudspeakers.
• Noisy appliances at home, such as televisions and radios played at high volume, some kitchen equipment, desert coolers, and air conditioners.

What Are The Harms Of Noise Pollution?

Prolonged exposure to excessive noise can have significant negative impacts on human health, leading to various health-related problems such as:

• Lack of sleep (insomnia).
• Hypertension (high blood pressure).
• Anxiety and stress.
• Other health disorders.

Continuous exposure to loud noise can also cause damage to hearing, resulting in either temporary or even permanent hearing impairment.

Measures To Limit Noise Pollution

Controlling noise pollution requires reducing the sources of noise. Several measures can be taken:

• Using silencing devices in aircraft engines, transport vehicles, industrial machinery, and home appliances.
• Locating noisy operations and industries away from residential areas.
• Minimising the use of automobile horns.
• Keeping the volume of televisions and music systems low, especially in homes and public places.
• Planting trees along roadsides and around buildings. Trees act as barriers that absorb sound, reducing the amount of noise reaching people in residential areas.

Hearing Impairment

Total hearing loss is uncommon and often congenital (from birth). Partial hearing impairment can result from illness, injury, or the natural aging process. Children with hearing loss need special support, such as learning sign language for effective communication. Since speech development is closely linked to hearing, hearing-impaired children may also have speech difficulties.

Modern technology provides various devices to help hearing-impaired individuals improve their quality of life. Society can also contribute by creating more supportive living environments and promoting inclusivity for those with hearing difficulties.

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