Chapter 13: Sound

Sound is Produced by Vibrating Bodies

Introduction

Sound helps us to communicate with one another and plays an important role in our life. The vibrating body produces sound.
What is sound?

• A sound is a form of energy that is produced when air molecules vibrate in a particular pattern called waves. Hence, the sound is a wave.
• Vibration can be described as a back and forth motion of an object.
• Depending upon the vibrations, a sound is produced. Sound cannot be produced without any vibration.

Sound is important as it helps us to communicate with one another. A variety of sounds are generated in the surroundings.

Sound is Produced by a Vibrating Body:

Production of sound waves:-

• Vibration: The back-and-forth motion of an object is called vibration.
• A sound is produced by a vibrating body. The to and fro or back and forth motion of an object is termed as vibration.
• Vibrating objects produce sound. Example: Sound is produced when a string or band is plucked.
• The sounds are produced due to these vibrational energies.
• As the amplitude of the sound waves is quite small, the vibrations are not visible. But it can be felt.
• Sound or sound waves are generated or produced by different sources.
• When a body vibrates, it produces sound in the surrounding medium.
• The vibrations are transferred to the medium surrounding the sound source.
• When the vibrations propagate away from the source, they form a sound wave.
• The speed at which they moved is called the speed of sound.
• In most cases, the amplitude of vibrations is so small that we cannot hear them. However, we can feel the vibrations if we come in contact with the vibrating body.

Examples:

• You can feel the vibrations of the diaphragm stretched over speakers.
• A tightly stretched band when plucked, vibrates and produces sound.

Sound Produced by Humans

How do Humans Produce sound?

In humans, the sound is produced by the voice box or the larynx.

• The voice box is at the upper end of the windpipe.
• Two vocal cords are stretched across the larynx in such a way that it leaves a narrow slit between them for the passage of air.
• When the lungs force air through the slit, the vocal cords vibrate and produce sound.
• The type and quality of voice produced differ in each human being. It depends on the tightness and the thickness of the vocal cords.
• Adult male vocal cords are larger than a female’s and are between 17 mm to 25 mm in length. Hence, male voices are usually lower-pitched.
• Female vocal cords are between 12.5 mm to 17.5 mm in length. Hence, the women’s voices are higher in pitch.
• Children’s vocal cords have very short vocal cords.
• The human voice is used for talking, singing, laughing, crying, screaming, whistling, clicking and whispering
• Human beings have a voice box or larynx which is present in their throat on the upper side of the windpipe.
• The larynx has two vocal cords which have a narrow slit between them so that air can pass through them.
• As the lungs throw the air out of the windpipe, it passes through the slit and hence allows the production of sound as the vocal cords start vibrating.
• The vocal cord muscles also play a role in the production of sound.
• Their thickness and tightness describe the quality or type of voice a person has.
• The vocal cords in males are of length 20 mm and females have 15mm long vocal cords. Children, on the other hand, have very short-length vocal cords. Hence, the voices, their quality, and their type are always different in women, men, and children.

SOUND NEEDS A MEDIUM TO PROPAGATE

• The traveling of sound is called the propagation of sound.
• Sound cannot propagate in the absence of a medium. The place where there is no air or air is removed is called vacuum.
• Sound does not propagate (travels) through a vacuum. It travels through solid, liquid and gas.

WE HEAR SOUND THROUGH OUR EAR

• The shape of the outer part of the ear is like a funnel.
• The tympanum is like a stretched membrane; which vibrates when sound waves strike it.
• Then the sound waves reach the eardrum or tympanum.
• The eardrum is like a stretched rubber sheet.
• From the eardrum, sound waves reach the inner ear; through the middle ear. From the inner ear, sound waves are sent to the brain.

How do we hear?

• We know that sounds are produced as waves in the air or any other medium.
• As these sound waves travel to our ears they convert them into electrical signals or messages that our brain can understand.
• Our ears have a special structure that allows this function.
• There are three major parts of the human ear:

 

• The outer ear (Pinna): It catches the sound waves and forwards them to the next part of the ear, that is, the middle ear.
• The middle ear: It converts the sound waves into vibrations that then travel to the inner ear. It can do this with the help of the eardrum. The eardrum is a thin rubber-like sheet present in the Middle ear. As the sound waves reach the eardrum, it vibrates and these vibrations propagate to the inner ear.
• The inner ear (cochlea): It receives the vibrations sent by the eardrum. It contains a liquid substance and the vibrations that enter the inner ear move through this liquid. There are tiny hairs present inside the inner ear that turn these vibrations into signals for the brain and pass them to the brain through the hearing nerve. As the brain receives the signal it interprets the sound. However, this whole process is so quick that we cannot notice it.

 

AMPLITUDE, TIME PERIOD AND FREQUENCY OF A VIBRATION

• The to and fro motion of an object is also known as oscillatory motion.
• When a pebble is dropped in pond water, it produces ripples in water. The ripple is called a wave.
• Sound travels producing similar waves.

Amplitude and frequency are two important characters of sound.

Sounds produced by the different objects are differentiated by the amplitude and frequency of sound.

Frequency: The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz.

Amplitude: The distance from normal to the peak is called amplitude. Since sound travels in the form of waves, the sound has amplitude.

Time period: The time required to produce one complete oscillation is called the time period.

 Oscillatory motion - When an object travels in a ‘to and fro’ motion, that is, when an object vibrates it is said to have an oscillatory motion.

• An oscillation is said to be the movement of the object from one point to another in a periodic time.
• One oscillation is said to be the movement between the two endpoints or extreme points of the motion of the object.

The number of oscillations an object takes per second is called its frequency.

The SI unit of frequency is Hertz (Hz).

1 Hz = 1 oscillation per second

20 Hz = 20 oscillations per second

Time Period - The time taken for one complete oscillation of a sound wave is called the time period of the sound wave.

• The number of molecules that a sound wave displaces from the mean position creates the amplitude of the sound wave.
• The amplitude of a sound wave can be defined as the maximum displacement of the particles from their mean position due to the vibrations.

SHRILLNESS OR PITCH

• Frequency determines the shrillness and pitch of the sound.
• Sound with greater frequency is shriller and has a higher pitch. Sound with lower frequency is less shrill and of lower pitch.

Pitch of the sound

• Every person has a different sound quality.
• Also, every musical instrument vibrates to produce a different kind of sound. This quality of sound is characterized by its Different quality of sounds that may have the same pitch and loudness.
• The pitch of a sound depends upon the frequency of the sound wave.
• The pitch would be higher if the frequency of the sound is high.

Different organisms and objects have different types of sound because of varied pitch:

LOUDNESS

• The loudness of sound depends on the amplitude of the sound wave.
• When the amplitude is large, the sound produced is loud.
• The loudness of sound is measured in decibel (dB).

The Loudness of a Sound

• The loudness of a sound depends upon the amplitude of the sound.
• The higher the amplitude, the higher is the displacement of the particles and the higher is the loudness of the sound.
• The loudness of the sound is directly proportional to the square of its amplitude.
• The SI unit for measuring the loudness of a sound in decibels (dB).

AUDIBLE AND INAUDIBLE SOUND

• Sounds frequency between 20 Hz to 20,000 Hz are called audible sounds.
• Sound with a frequency below 20 hertz and above 20,000 hertz is called the sound of inaudible range.
• The hearing range of human beings is between 20 hertz to 20,000 hertz.
• Humans cannot hear the sound of inaudible range.
• Many animals, such as dogs, cats, etc. can hear the sound with a frequency above 20,000 hertz

How Loudness and Pitch are different from each other?

Audible and Inaudible sounds

• Sounds that can be heard by human ears are called audible sounds. The range of audible sounds is from 20 Hz to 20 KHz.
• Sounds that cannot be heard by human ears are called inaudible sounds. Any sounds having a frequency less than 20 Hz or greater than 20 KHz are categorized as inaudible sounds.

The inaudible sounds can be divided into two categories as infrasound and ultrasound.

NOISE AND MUSIC

• The sound that is unpleasant to us is called
• Example sound of the horn, sound near the site of construction work, etc.
• The sound that is pleasant to our ear is called musical sound, such as the sound of a musical instrument, the song of a good singer, etc.

• Noise can be regarded as an unpleasant sound. Noise consists of sound waves of varied frequencies which have no particular periodic pattern of repetition. Therefore, noise is regarded as a mixture of sound waves with irregular frequencies.
• Music, on the other hand, is a pleasant sound that has a clear pitch. Musical sound can be created by arranging and combining different sounds in a particular order. The frequencies of a musical sound are harmonious.

NOISE POLLUTION

Noise pollution can be defined as the presence of undesirable and unpleasant sounds in the earth's environment.

• Unwanted and excessive sound in our environment creates Noise Pollution.
• Sounds of crackers, factories, vehicles, desert coolers, air conditioners, aeroplane, transistors or television with high volume, loudspeakers, etc. create noise pollution.
• Noise pollution can create many types of health-related problems, such as lack of sleep (insomnia), hypertension, loss of hearing, anxiety, etc. Sound above 80 dB is very painful to hear.
• A person who is exposed to loud sound continuously may get permanent or temporary loss of hearing (impairment of hearing).
• Human beings can bear sounds ranging up to 85 decibels only. Above that, any noise can damage our hearing power.
• Generally, any sound that has a frequency of more than 30 dB is considered noise.
• The unwanted noise causes an adverse effect on the health of the organisms present on the earth.
• According to the World Health Organization, the maximum sound limit that is ideal for cities is 45 dB only.
• However, it has been found that the sound range in many big cities of the world lies up to 90 dB.
• Hence noise pollution is common in many cities today and even rural areas to a great extent.

Causes of Noise Pollution

1. Transport noise: The sound of the traffic on roads, railways and aircraft leads to noise pollution. As the number of vehicles such as cars, motorcycles, buses and trucks is increasing in the cities, especially the Metropolitans, noise pollution is extremely high there.
2. Industrial noise: Industries, factories and other commercial businesses cause high-intensity sounds that pollute the environment.
3. Neighborhood noise: The noise from the radios, televisions, air conditioners, coolers, kitchen applications and other electrical appliances used in houses leads to noise. Not only this, commercialization around the residential areas often leads to unwanted sounds due to small-scale industries such as printing, car repairing, etc.
4. Construction noise: Construction of houses, industries and various architectures also lead to sound pollution.
5. Political activities: Noise pollution is also created due to the rallies and demonstrations conducted by various political parties in cities and rural areas.
6. Bursting of crackers and fireworks: People burst crackers on several occasions such as festivals and ceremonies that lead to sound pollution in the neighborhoods.
7. Natural sounds: The environment of the earth also sometimes leads to unpleasant sounds due to lightning, thunderstorms, earthquakes, volcanic eruptions, sounds of various animals etc.

Effects of Noise Pollution

1. Excessive noise in the surroundings can lead to serious health problems such as hypertension, lack of sleep or insomnia, anxiety, lack of memory, stress, irritation and even nervous breakdown.
2. It can lead to temporary or permanent hearing loss in human beings as well as animals.
3. Excessive noise leads to increased blood pressure. It increases cholesterol levels in the blood and therefore increases the chances of cardiovascular diseases in a person.
4. If the sound intensity is more than 180 dB it can lead to the death of a person.
5. Noise pollution also leads to a decrease in the production of digestive juices by our body.
6. It can affect animals as well and can lead to their death and loss of habitats. Excessive noise decreases a person's ability to concentrate especially on children and they are unable to concentrate well on their studies due to neighborhood noises.
7. Unwanted sounds in the environment can hinder the animals from finding their prey or their direction of motion.

Prevention of Noise Pollution

MEASURE TO LIMIT NOISE POLLUTION

Noise pollution can be limited by controlling the noise coming from the source. Noise pollution can be limited by taking some measures like:

1. Factories and other industries should be set up at a distance from residential areas.
2. Silencing devices should be incorporated into heavy vehicles such as aircraft, industrial equipment, machinery and other home appliances.
3. We should always play television, radio and other music systems at a low sound so that it does not harm the neighborhood.
4. The use of horns should be minimized especially near public places such as hospitals, religious places and schools.
5. Soundproofing systems should be installed in industries, party halls and other buildings that produce a high amount of unwanted sounds.
6. Trees should be planted in huge numbers as they can absorb unwanted noise from the environment.
7. People who work in noisy conditions such as industries and mines should be provided earplugs to protect their ears.
8. People should be made aware of noise pollution its adverse effects so that they can take an active involvement in preventing it.
9. TV or radio should be played at low volume.
10. Minimum use of vehicle horns.
11. By installing high-quality silencers in vehicles.
12. Plantation of trees along the roadsides because trees absorb sound.