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Some students picture sound as something that flies from a speaker to their ear, like a ball. Physics education researchers have a name for that picture, and it is behind many wrong answers in this chapter. This post explains what sound actually is, works through the calculations the syllabus asks for, and ends with a practice routine.
What does the 2026–27 chapter cover?
In the new NCERT book *Exploration*, this is Chapter 10, Sound Waves: Characteristics and Applications. The CBSE Class IX syllabus for 2026–27 lists production and propagation of sound as a longitudinal wave, graphs of sound waves, wavelength, frequency, time period, amplitude, intensity and speed, pitch and loudness, sound in solids and liquids, echo, reverberation and echolocation. It also asks you to derive and use the expression for the speed of sound. Check the syllabus for your exam year.
How does sound actually travel?
A vibrating object pushes on the air next to it, squeezing it into a region of higher density, a compression. As it moves back, it leaves a region of lower density, a rarefaction. These regions pass from particle to particle. Each particle only vibrates back and forth about its own position, parallel to the direction the wave travels. That makes sound a longitudinal wave. Because it needs particles to pass the disturbance along, it is a mechanical wave and cannot travel through a vacuum, which NCERT shows with the bell-jar experiment.
What travels is energy, not air. Hrepic, Zollman and Rebello (2010) studied introductory physics students and found that, besides the wave model, students used an "entity" model, in which sound is a self-standing thing separate from the medium, or a hybrid of that and the wave model. If you catch yourself saying "the sound particles reach my ear", that is the entity model talking.
How are wavelength, frequency and speed related?
Wavelength (λ): distance between two consecutive compressions (crests on the density graph), in metres.
Frequency (ν): number of oscillations per second at a point, in hertz.
Time period (T): time for one oscillation, so ν = 1/T.
Speed: one wavelength travels in one time period, so v = λ/T = λν.
The speed depends on the medium, not on the frequency. NCERT's table gives about 340 m/s in air, 1500 m/s in water and 5000 m/s in steel, at 15 °C. In air it also rises with temperature: about 331 m/s at 0 °C and 344 m/s at 22 °C.
Example 1: a musical note. A note of 440 Hz travels through air at 344 m/s. λ = v/ν = 344/440 ≈ 0.78 m. Its period is 1/440 ≈ 0.0023 s.
Example 2: the limits of hearing. NCERT works out that at 344 m/s, 20 Hz has a wavelength of 17.2 m and 20 kHz has 1.72 cm.
What can humans hear?
The audible range is roughly 20 Hz to 20,000 Hz (20 kHz), and NCERT notes it varies between people and shrinks with age. Below 20 Hz is infrasound, which elephants can detect; above 20 kHz is ultrasound, which dogs, cats, bats and dolphins can detect. Pitch is how we perceive frequency; loudness is how we perceive amplitude. They are separate: a whistle can be quiet and high, thunder loud and low.
How do echoes and sonar work?
We hear two sounds separately only if they reach us at least 0.1 s apart. In 0.1 s, sound in air at 340 m/s goes 34 m. That is a round trip, so the wall must be at least 17 m away for an echo. Shorter delays, under about 0.05 s, blur into reverberation.
Example 3: an echo. You clap and hear the echo after 0.4 s. Distance to the wall = (340 × 0.4) ÷ 2 = 68 m.
Example 4: sonar. A ship's ultrasonic pulse returns from the seabed after 2.4 s. With sound at 1500 m/s in seawater, depth = (1500 × 2.4) ÷ 2 = 1800 m.
Example 5: two sounds through a fence. Two friends stand 340 m apart along a steel fence. A knock reaches the listener through the steel in 340/5000 = 0.068 s and through the air in 340/340 = 1 s. The gap is about 0.93 s, well over 0.1 s, so she hears two knocks.
Which mistakes come up most?
Forgetting to halve the echo or sonar distance.
Thinking frequency changes the speed. In the same medium, a higher frequency means a shorter wavelength, not a faster wave.
Mixing pitch and loudness, or loudness and intensity. Intensity is measurable; loudness depends on the listener.
Saying sound travels through space. It needs a medium.
Labelling particles as moving with the wave in assertion-reason questions.
Waves are much easier to understand when you can watch compressions move while each particle stays put. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For a guided activity, try our post on learning waves and sound with simulations.
A practice routine
Redo NCERT Examples 10.3 (lightning 1.7 km away) and 10.6 (sonar, 688.5 m) without looking.
For every echo question, draw the path out and back before you divide.
Use a free frequency app, as NCERT suggests, to find the frequency of your own whistle, then calculate its wavelength.
Write one assertion-reason question about particles in a sound wave and answer it.
Tonight, time the gap between a lightning flash and its thunder, or between a clap and its echo, and turn it into a distance.
