Sound is a form of energy that travels in waves and is created when an object vibrates. These vibrations move through a medium (like air, water, or solid objects) and reach our ears. Any vibrating body sets the particles of the medium around it into motion. This creates a sound wave. For example; a drum skin vibrates when hit by an object causing the air around it to vibrate. There are many scientific concepts that needs to be understood in this lesson.
Vibration motion
Vibrational motion is also called oscillatory motion, a type of motion in which an object moves back and forth around an equilibrium position. An equilibrium is a fixed position or the mean position around which the movement of objects happen. The motion of object repeats at regular intervals. Some examples of vibratory motion are:
| Example | Description |
|---|---|
| Guitar string | When plucked, it vibrates rapidly and produces sound. |
| Ringing bell | The metal of the bell vibrates to create sound waves. |
| Tuning fork | When struck, its arms vibrate and make a humming sound. |
| Swing | A swing going back and forth is a slow example of vibration. |
Sound from human body
In humans, sound is produced in the larynx or the voice box. It is situated at the top of the windpipe or trachea. The trachea has flexible pair of vocal cords which are two muscular folds with a narrow slit between them. As air passes through the slit between the vocal cords, they vibrate. These vibrations produce sound waves. The sound then travels through the mouth and nose and comes out as speech or other vocal sounds. Magnitude/pitch of sound depends upon the tightness and length of the vocal cords determine the pitch of the sound. Tight & thin vocal cords produces high-pitched sound whereas loose & thick vocal cords produce low-pitched sound (like an adult male’s voice). Tongue, lips, and teeth help shape the sound into words.
Propagation of Sound
Propagation of sound means how sound travels from the source (where it's produced) to the listener (like your ears). Sound is a mechanical wave, which means it travels by making particles of the medium vibrate therefore, it definitely requires one of the medium such as solid, liquid or a gas. It cannot travel through a vacuum (like outer space) because there are no particles to carry the vibrations. Sound travels in sequence of steps from source to the ears as shown in the table below.
|
Step |
What Happens |
|
1 |
A vibrating object (e.g., tuning fork) creates disturbances in the air. |
|
2 |
These disturbances cause the air particles to vibrate back and forth. |
|
3 |
Vibrations move from one particle to another in the form of longitudinal waves. |
|
4 |
The energy reaches your ear, and you hear the sound. |
Structure of a Sound Wave
Sound travels in the form of longitudinal waves, which move parallel to the direction of the vibration. These waves consist of compressions and rarefactions. A sound wave is a pattern of disturbance caused by the movement of energy through a medium (like air) due to a vibrating source. In a sound wave, particles of the medium vibrate back and forth in the same direction resulting in a zone of compression and rarefaction alternatively. A compression is the region where particles are close together (high pressure) while rearefaction happens as particles are spread out (low pressure).A series of compression and rarefaction causes wavelength. Wavelength (λ) is the distance between two compressions or two rarefactions. Amplitude of a sound wave is the maximum distance particles move from their rest position. It relates to loudness. Frequency is the number of waves passing a point in one second. It determines the pitch.
Speed of Sound
The speed of sound is the distance that a sound wave travels in one second through a particular medium. Speed of Sound (v)=Wavelength (λ)×Frequency (f). Where, v = speed (in meters per second, m/s), λ (lambda) = wavelength (in meters, m) and f = frequency (in hertz, Hz). The sound waves differently in different medium . Their speed in three media is as shown in the table below
|
Medium |
Approximate Speed (at room temperature) |
|
Air (gas) |
343 m/s |
|
Water (liquid) |
1,480 m/s |
|
Steel (solid) |
5,960 m/s |
|
Vacuum |
? No sound (0 m/s) |
Loudness of sound
Loudness is the intensity of a sound received by the human ear. It tells us how loud or soft a sound is. Example: A whisper is soft (low loudness), while a drumbeat is loud (high loudness). Loudness is determined by the amplitude of the Sound Wave. Larger amplitude produces louder sound while the smaller amplitude produces softer sound. To produce a larger sound a harder strike on a bell makes it vibrate more, increasing the amplitude and making the sound louder. Loudness is measured by the unit called Db ( Decibel). Sounds below 30 Db are generally very soft (e.g., whisper) whereas the sounds above 100 Db sounds very loud and may be harmful (e.g., loud music or jet engine). The sounds above 120 dB can cause hearing damage. Some examples of different Db of sounds is mentioned in the table below:
|
Source of Sound |
Approx. Loudness |
|
Rustling leaves |
20 dB |
|
Normal conversation |
60 dB |
|
Heavy traffic |
85 dB |
|
Rock concert |
110 dB |
|
Jet engine |
140 dB |
Noise pollution
Noise pollution is the presence of unwanted, disturbing, or harmful sounds in the environment that can affect human health and well-being, as well as animals and nature.
| Source | Example |
|---|---|
| Traffic | Honking, engine noise, road construction |
| Industries & Factories | Machinery, generators, grinding equipment |
| Construction Work | Drilling, demolition, concrete mixers |
| Household Appliances | Loudspeakers, TV, vacuum cleaner |
| Public Events & Celebrations | Loud music, DJ systems, firecrackers |
| Airports & Railways | Aircraft takeoff, train whistles |
| Area Type | Daytime Limit | Nighttime Limit |
|---|---|---|
| Residential | 55 dB | 45 dB |
| Industrial | 75 dB | 70 dB |
| Commercial | 65 dB | 55 dB |
| Silent Zone* | 50 dB | 40 dB |
*Silent zones include hospitals, schools, and courts (within 100 m).
| Method | Description |
|---|---|
| Planting trees | Trees act as sound barriers |
| Using silencers | In vehicles, machines, and generators |
| Avoiding loudspeakers | Especially near hospitals, schools, and at night |
| Soundproofing buildings | Using noise-absorbing materials |
| Strict laws and awareness | Rules to limit noise in public and residential areas |
Human ear
The human ear is a sense organ that helps us hear sounds and also maintains our balance. It is divided into three main parts: Outer ear, middle ear and inner ear. Outer Ear (External Ear) has Pinna (ear flap) and ear canal. External ear collects sound waves from the surroundings. And directs the sound waves into the ear canal. The sound travels through the canal and reaches the eardrum which is the middle ear portion. Middle Ear has eardrum (Tympanic membrane) supported by 3 tiny bones hammer (Malleus), anvil (Incus) and stirrup (Stapes). The eardrum vibrates when sound waves hit it. These vibrations pass to the three bones, which amplify the sound. The bones pass the vibration to the inner ear. Stapes is the smallest bone in the human body.Inner Ear makesup Cochlea (spiral-shaped), Auditory nerve and semicircular canals (for balance). The cochlea converts vibrations into electrical signals and send these to the brain through the auditory nerve.
CHAPTER SOLUTIONS
Question 1. Choose the correct Answer: Sound can travel through
Answer: (d) Solids, liquids, and gases.
Question 2.
Voice of which of the following is likely to have a minimum frequency?
Answer: (a) Baby girl
Question 3.
In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false:
Answers:
(a) Sound cannot travel in a vacuum. (T)
(b) The number of oscillations per second of a vibrating object is called its time period. (F)
(c) If the amplitude of vibration is large, the sound is feeble. (F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
(e) The lower the frequency of vibration, the higher is the pitch. (F)
(f) Unwanted or unpleasant sound is termed as music. (F)
(g) Noise pollution may cause partial hearing impairment. (T)
Question 4.
Fill in the blanks with suitable words:
Answer:
Question 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
Time taken by pendulum to complete 40 oscillations = 4 seconds
Time taken by pendulum to complete 1 oscillation = 440 = 110 = 0.1 sec.
Time period of pendulum = 0.1 sec.
Frequency of pendulum = oscillation time = 404 = 10Hz.
Hence the frequency of the pendulum is 10 Hz.
Question 6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Answer:
For making 500 vibrations time taken is 1 second. For making 1 vibration time taken is = 1500 second = 0.002 second.
Hence time period of the vibration is 0.002 seconds.
Question 7. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak (b) Sitar (c) Flute
Answer:
|
Instrument |
The part which vibrates to produce sound |
|
(a) Dholak |
Stretched membrane |
|
(b) Sitar |
Stretched string |
|
(c) Flute |
Air column |
Question 8. What is the difference between noise and music? Can music become noise sometimes?
Answer:
Noise: Unpleasant sounds are called noise. Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.
Music: Pleasant sound is called music. Music brings about a soothing effect.
Yes, music can become noise sometimes when the musical instruments produce very high volume sounds.
Question 9. List sources of noise pollution in your surroundings.
Answer:
Question 10. Explain in what way noise pollution is harmful to humans.
Answer:Answer: I would suggest my parents buy the house which is three lanes away from the roadside. This would protect us from noise pollution which is maximum at the roadside building. This would safeguard our health and peace of mind.
Question 12. Sketch the larynx and explain its function in your own words.
Answer: The other name of the larynx is the voice box. It is present at the upper end of the windpipe. The function of the larynx is to produce sound.
Question 13. Lightning and thunder take place in the sky and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Answer: The light travels at the speed of 3 × 108 ms-1 which is very large in comparison to the speed of sound which travels at the rate of 330 ms-1 in the air. That is why lightning is seen earlier and thunder is heard later.