NCERT Solutions for Class 9th Science Chapter 10 Project work — The Journey Beyond

Book page 207 Updated on2026-09-08

Q1.

Many people use earphones extensively these days. Find out the research studies that might have been done to understand the impact of excessive use of earphones on hearing (if any). Also, find out how hearing is tested and what are the decibel ranges for defining mild, moderate and severe hearing loss. What are the government schemes for purchasing or fitting of aids or appliances and free cochlear implants? Write an article on your findings.

Answer

Method. This is a small research project, so work in three stages: gather, organise, then write.

Gather. Use reliable sources — WHO reports on safe listening, ICMR and AIIMS studies, and the Ministry of Social Justice and Empowerment website. Note the source and year against every fact you copy.
Understand how hearing is tested. An audiometer plays tones of known frequency at slowly increasing loudness; the softest level the listener can just hear is the hearing threshold at that frequency. Plotting threshold against frequency gives an audiogram.
Write a 400 – 600 word article: what earphones do to the ear, what the studies found, how loss is graded, and what help is available.

What a good article must contain

Grade of hearing loss	Hearing threshold (better ear)	What it means day to day
Normal	Below 20 dB	No difficulty
Mild	about 20 – 35 dB	Trouble following soft speech or conversation in a noisy place
Moderate	about 35 – 50 dB	Difficulty with ordinary conversation; a hearing aid usually helps
Severe	about 65 – 80 dB	Normal speech not heard; needs a powerful aid or an implant

Sample answer (opening of the article):

Sample answer: "Loudness is measured in decibels, and the decibel scale is deceptive: rustling leaves are a few dB, ordinary conversation about 60 dB, and firecrackers above 100 dB — but each small rise in dB means a large rise in sound intensity. Earphones sit inside the ear canal, so the sound reaching the eardrum is far louder than the same setting played on a speaker. The World Health Organization warns that listening above 80 dB for long periods puts hearing at risk, and studies of college students in India report measurable threshold shifts in heavy earphone users. The damage is to the hair cells of the cochlea, and those cells do not grow back — which is why this kind of hearing loss is permanent. The Government of India runs the ADIP scheme (Assistance to Disabled Persons for Purchase / Fitting of Aids and Appliances), which supplies hearing aids free or at subsidised cost, and free cochlear implants for eligible children are provided under the same scheme. The National Programme for Prevention and Control of Deafness supports screening and treatment."

The physics behind the advice: a wave with a larger amplitude carries more energy, and intensity is the energy delivered per unit area per unit time. Earphones put a small source very close to the eardrum, so the area is tiny and the intensity is high even at modest power. That is the reason for the '60/60 rule' often recommended — no more than 60% volume for no more than 60 minutes at a stretch.

Q2.

Make a cone using a poster paper or cardboard and adhesive tape. Cover a mobile phone that is playing music with the cone. Compare the loudness of the sound with and without the cone. You can also use another mobile phone with an app to measure the characteristics of the sound in both cases. Try experimenting with different shapes and record your observations. (This activity is to be facilitated by the teacher.)

Answer

Method. Roll the poster paper into a cone, tape it, and cut the narrow end so the phone's speaker fits snugly. Play a steady tone or a fixed piece of music at a fixed volume setting, and stand at a fixed distance with the measuring phone. Take a reading in dB without the cone, then with it, without changing the volume setting or your position.

Arrangement	Typical dB reading at 1 m	What you hear
Phone alone	about 62 dB	Thin, spreads in all directions
Narrow cone	about 68 dB in front, less at the sides	Louder ahead, sharply directional
Wide cone	about 66 dB in front, spread wider	Louder but less sharply aimed
Behind the cone	lower than the phone alone	Noticeably quieter

What a good answer must contain: the same volume setting in both cases, the same distance for every reading, several repeats averaged, and readings taken both in front of and behind the cone.

Why the cone makes it louder: the cone does not create energy. Without it, sound spreads out from the phone as roughly spherical waves and the same energy is shared over a rapidly growing area, so intensity falls quickly. The cone channels that energy into a narrower beam, so more of it passes through the small area in front of you each second. Intensity is energy per unit area per unit time, so a narrower beam means a higher intensity — and greater perceived loudness. The trade-off is that listeners to the side and behind hear less.

Did you know? This is the whole idea behind a megaphone, the flared bell of a shehnai and the horn of an old gramophone. The Whispering Gallery of the Gol Gumbaz in Bijapur uses the reverse trick — a curved dome that reflects and gathers a faint whisper so it can be heard right across the hall.

Q3.

How does the curved design of ceilings and walls behind the stage in concert and conference halls improve the quality of sound for the audience compared to flat surfaces? You may consult an architect or search it on the internet.

Answer

A curved surface behind and above the stage reflects sound outward and downward towards the audience, so more of the performer's energy reaches the listeners and it reaches all seats more evenly.

It gathers and redirects. Sound obeys the same laws of reflection as light: the incident and reflected directions make equal angles with the normal at the point of incidence. A curved reflector behind the stage catches sound that was heading up and back — energy that would otherwise be wasted — and turns it towards the hall.
It evens out the loudness. Intensity falls as sound spreads over a larger area, so seats at the back would be much quieter than seats at the front. Curved ceiling panels are shaped and tilted to send extra reflected sound to the far seats, so front and back rows hear a similar level.
It controls the delay. A reflection arriving within about 0.05 s of the direct sound blends with it and reinforces it. A reflection arriving after 0.1 s would be heard as a separate echo and would blur speech. Architects position the curved surfaces so that every reflected path is short enough to reinforce rather than confuse.
Flat parallel walls are worse because sound bounces back and forth between them many times, giving excessive reverberation and a garbled, smeared sound.

Why absorbers are needed too: reflection alone would make a hall ring. Sound-absorbing panels, upholstered chairs, curtains and other soft porous surfaces soak up the unwanted late reflections. The design target is a reverberation just long enough to make music sound rich, but short enough to keep speech crisp.

Did you know? Medieval Indian architects understood this well. The Whispering Gallery of the Gol Gumbaz at Bijapur in Karnataka is shaped so that even a faint whisper is reflected around the huge dome and heard several times over.

Q4.

Carry out a simple activity to measure the speed of sound, along with a friend in a large open ground of size 200 m or more. (This activity is to be facilitated by the teacher.) (i) Your friend stands at one end of the open ground with the balloons, while you stand at the other end with the stopwatch. (ii) Signal your friend to burst one balloon. When you see the balloon burst, start the stopwatch. As soon as you hear the ‘pop’ sound of the bursting balloon, stop the timer and note down the reading. (iii) Repeat this experiment multiple times and take the average value of the times noted. (iv) Note the approximate distance between you and your friend using a map application on a mobile phone. (v) Divide the distance measured with the average time to get the average speed of sound. What value of speed did you get from the experiment? Compare it with the speed of sound in air, which is typically about 346 m s–1 at 25 °C. (vi) Why did you measure the time between ‘seeing’ and ‘hearing’ the balloon burst?

Answer

(v) The experiment should give a speed of roughly 330 – 360 m s⁻¹, close to the accepted 346 m s⁻¹ at 25 ºC.

Sample readings for a measured distance of 250 m:

Trial	1	2	3	4	5
Time (s)	0.75	0.70	0.78	0.72	0.75

average time = (0.75 + 0.70 + 0.78 + 0.72 + 0.75) s ÷ 5
= 3.70 s ÷ 5 = 0.74 s

speed of sound = distance ÷ average time
= 250 m ÷ 0.74 s
= 338 m s⁻¹

Compared with 346 m s⁻¹, the error here is (346 − 338)/346 × 100 ≈ 2.3% — good for a stopwatch experiment. The main source of error is your own reaction time, which is about 0.2 s and is the reason many repeats must be averaged.

(vi) Why measure between 'seeing' and 'hearing': because light travels so much faster than sound that the flash of the burst can be treated as arriving instantly.

time for light to cross 250 m = 250 m ÷ (3 × 10⁵ m s⁻¹) ≈ 8 × 10⁻⁷ s
time for sound to cross 250 m ≈ 0.74 s
The light takes about a millionth of a millisecond — utterly negligible.

So the instant you see the burst is effectively the instant the sound was produced. Starting the stopwatch on the flash and stopping it on the pop therefore measures the travel time of the sound alone, with no need for any signal between you and your friend.

How to make the result better: use the longest ground available (a longer distance makes your fixed reaction-time error a smaller fraction of the total), take at least ten trials, and swap roles so that both partners' reaction times are averaged in. Note the air temperature too, since the accepted value changes from 331 m s⁻¹ at 0 ºC to 344 m s⁻¹ at 22 ºC.

Note on the Hindi edition: in Anveshan, part (v) of this project ends with 'जो लगभग होती है' — the comparison value has been left out. Take it from the English edition: about 346 m s⁻¹ at 25 ºC.

Q5.

Explore the internet resources to explore the effect of humidity and temperature on the speed of sound. Some such resources are: (i) https://phet.colorado.edu/en/simulations/sound-waves/ (ii) https://musiclab.chromeexperiments.com/Experiments (iii) https://phyphox.org/experiments

Answer

Both temperature and humidity increase the speed of sound in air, and temperature has by far the larger effect.

Condition	Approximate speed in air	Source
Dry air at 0 ºC	331 m s⁻¹	Section 10.6.3
Air at 15 ºC	340 m s⁻¹	Table 10.1
Air at 22 ºC	344 m s⁻¹	Section 10.6.3
Air at 25 ºC	346 m s⁻¹	The Journey Beyond, page 207
Humid air at the same temperature	a little higher than dry air	Section 10.6.3

What to do with each resource

PhET Sound Waves: vary the frequency and watch the compressions and rarefactions. Confirm for yourself that changing the frequency changes the wavelength but not the speed.
Chrome Music Lab: the Spectrogram and Oscilloscope experiments let you see the fundamental and overtones of your own voice — a direct look at timbre.
Phyphox: the 'Audio Spectrum' and 'Audio Scope' tools measure real frequencies. The app can also time an echo, so with a wall a known distance away you can measure the speed of sound yourself on a hot day and again on a cold morning.

Why warmer air carries sound faster: a compression is passed from one molecule to the next by collisions. Raising the temperature makes the molecules move about faster, so they reach their neighbours sooner and the disturbance is relayed more quickly. Humidity helps for a related reason: water molecules are lighter than the nitrogen and oxygen molecules they replace, so moist air is slightly less dense than dry air at the same pressure and carries the disturbance a little faster.

Try This: use the figures above to work out how far off a thunderstorm estimate can be. Counting 5 s and multiplying by 331 m s⁻¹ gives 1655 m; using 346 m s⁻¹ gives 1730 m — a difference of only about 75 m in 1.7 km, so the rough rule is quite safe.

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