Riya stretches a thick rubber band between the legs of a chair. She pulls the rubber band down and releases it. The rubber band moves to and fro and a faint sound is heard. She observes that when she pulls the rubber band more and releases it, the sound produced is louder.

A tuning fork is struck gently on a rubber pad and then its prongs are brought near a thin paper strip. The paper strip starts vibrating and produces a rattling sound. When the tuning fork is touched by hand, both the fork and the paper strip stop vibrating and the sound stops.

During a science class, the teacher shows a model of the human voice box (larynx). She explains that inside the voice box there are two stretched tissues called vocal cords. When air from the lungs is forced through the region between the cords, they start vibrating and sound is produced. She also mentions that boys and girls often have different voices because their vocal cords are of different lengths and thickness.

Two friends, Aman and Yusuf, are standing near a railway line. Aman places his ear on the iron track and asks Yusuf to do the same. After some time, Aman says he can hear the sound of an approaching train through the track, even though the train is not yet visible and the sound in air is very faint.

In a school lab, an electric bell is fixed inside a glass bell jar. When the switch is turned on, the bell rings and students hear the sound clearly. Then the air inside the jar is slowly removed using a vacuum pump. As the air is pumped out, the sound becomes fainter, although the bell can still be seen vibrating. After most of the air is removed, the sound is hardly audible.

A diagram in the textbook shows air particles being pushed closer together in some regions and farther apart in other regions as a vibrating object moves. The close-packed regions are marked as compressions and the spread-out regions as rarefactions. The distance between two successive compressions is labelled as the wavelength of the sound wave.

During a school function, a student is asked to beat a drum softly and then with greater force. The audience clearly notices that the second sound is much louder than the first. However, the basic nature of the sound (its shrillness or pitch) appears the same in both cases.

In a family, a small child and his father speak the same word at almost the same loudness. Still, the family members can easily identify who is speaking without seeing them. They observe that the child’s voice is more shrill, while the father’s voice is deeper.

During a music practice, a flute and a drum are played together at the same loudness. The teacher asks students whether they can identify which sound is from the flute and which is from the drum. All students are able to do so easily, even with their eyes closed.

A chart in the classroom shows that the normal human ear can hear sounds only in a certain range of frequencies. Sounds below this range are marked as infrasonic and those above are marked as ultrasonic. The chart also shows that dogs and bats can hear some sounds which humans cannot.

At night, bats fly swiftly in complete darkness without colliding with objects. They produce very high frequency sounds which we cannot hear. These sounds reflect back from nearby objects and return to the bats’ ears. Based on this information, scientists explain how bats move safely at night.

Rohan visits a crowded market where many vehicles are honking, people are shouting and loudspeakers are playing advertisements. Later, he attends a music class where instruments like harmonium and flute are played softly in rhythm. He feels tired and disturbed in the market but relaxed in the music room.

Near a main road, many vehicles pass throughout the day and night. There is also a construction site where heavy machines are used. Loudspeakers are used occasionally for various functions. People staying nearby often complain of headache, disturbed sleep and difficulty in studying.

A school is located near a busy road. The principal notices that students in classrooms facing the road are easily distracted by noise from traffic and shops. The school management decides to plant trees along the boundary wall and requests the municipal authority to put “No Horn” signs near the school.

A diagram of the human ear in the textbook shows three main parts: outer ear, middle ear and inner ear. The outer ear consists of the pinna and ear canal. The middle ear has the eardrum and three small bones. The inner ear contains the cochlea and the auditory nerve which carries signals to the brain.

Meera’s friend listens to very loud music through earphones for many hours every day. After some months, she starts complaining of a ringing sound in her ears and difficulty in hearing faint sounds. The doctor advises her to reduce the use of earphones and avoid very loud sounds to protect her ears.

In a classroom, the teacher explains that a tuning fork makes 200 vibrations in 2 s. She asks the students to calculate the frequency and time period of the tuning fork. She also reminds them of the relation between frequency f and time period T, which is given by f = 1/T.

Two sound sources A and B produce sounds of the same loudness but different frequencies. Source A produces sound of frequency 300 Hz and source B produces sound of frequency 600 Hz. Students are asked to predict which sound will be shriller and which will be deeper.

A table in the textbook shows the approximate speeds of sound in three media at room temperature:

(i) Air: about 340 m s⁻¹
(ii) Water: about 1500 m s⁻¹
(iii) Steel: about 5000 m s⁻¹

Students compare these values to understand how sound travels in different media.

Anita wants to choose a place in her house for regular study. One corner is near the main road, where she can clearly hear traffic, horns and street vendors. Another corner is inside a room with thick curtains and a window opening to the backyard with trees. She observes that the second corner is much quieter.