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Pitch and loudness are the two words Class 8 students swap most often in sound questions. This post explains how sound is made, what frequency and amplitude mean, works through two short calculations, and tells you where the topic now sits in the NCERT books.
Is sound still in the Class 8 book?
Not in the new one. NCERT's current Class 8 book, Curiosity, is marked "Reprint 2026-27", and none of its 13 chapters is about sound. Sound was Chapter 10 of the older Class VIII book, which some schools and boards may still follow. NCERT's new Class 9 book, Exploration (first edition April 2026), has a full chapter, "Sound Waves: Characteristics and Applications". Check with your teacher which book your exams follow. The ideas below come from the older Class 8 chapter and prepare you for Class 9.
How is sound produced?
Every sound comes from something vibrating: a stretched rubber band, a drum skin, a bell, the strings of a guitar. Touch a ringing bell and you feel it shaking. Often the vibration is too small to see.
In humans, sound comes from the voice box, or larynx. Two vocal cords stretch across it with a narrow slit between them. Air from the lungs makes them vibrate. The older chapter says the vocal cords are about 20 mm long in men and about 15 mm in women, and shorter in children, which is one reason their voices sound different.
Why does sound need a medium?
Sound travels through solids, liquids and gases, but not through empty space. The older Class 8 chapter has you suck air out of a tumbler holding a ringing phone: the ring grows fainter. The new Class 9 chapter describes the classic version, an electric bell in a jar with the air pumped out, which becomes almost silent even though you can see it ringing. With no air, nothing carries the vibrations. That is why, as the Class 9 chapter notes, astronauts on a spacewalk cannot directly hear each other speak.
We hear when the vibrations reach the eardrum, which vibrates too and passes the signal to the inner ear and then the brain.
What decides pitch and loudness?
Two properties of a vibration matter.
Frequency is the number of vibrations (oscillations) per second, measured in hertz (Hz). One hertz is one oscillation per second. Higher frequency means higher pitch. A whistle is high-pitched; a drum is low-pitched.
Amplitude is how far the vibrating object moves from its rest position. Larger amplitude means louder sound. Loudness is measured in decibels (dB).
The older chapter's table puts normal conversation at about 60 dB and an average factory at about 80 dB, and says noise above 80 dB becomes physically painful. Humans hear roughly 20 Hz to 20,000 Hz. Sounds outside that range are inaudible to us, although dogs can hear above 20,000 Hz and ultrasound scanners work there.
Worked example 1: frequency and time period
A tuning fork's prong makes 2,560 vibrations in 10 seconds. Find its frequency and time period.
Frequency = vibrations ÷ time = 2,560 ÷ 10 = 256 Hz.
Time period = time for one vibration = 1 ÷ frequency = 1 ÷ 256 s, about 0.0039 s.
Check: 256 vibrations per second × 0.0039 s per vibration ≈ 1 s. The trap is dividing the wrong way round. Frequency is "how many per second", so divide the count by the time.
Worked example 2: how far away was the lightning?
This one previews Class 9. You see lightning and hear thunder 3 seconds later. The new Class 9 chapter uses 344 m/s for the speed of sound in air in one of its examples, so use that.
Light reaches you almost instantly, so the 3 seconds is the time sound takes.
Distance = speed × time = 344 × 3 = 1,032 m, about 1 km.
Which mistakes cost marks?
Swapping pitch and loudness. Pitch goes with frequency, loudness with amplitude.
Writing frequency per minute. Hertz means per second.
Saying sound travels through a vacuum. It needs a medium.
Calling every unpleasant sound "high-pitched". Noise can be low-pitched too.
A practice routine for this chapter
Day 1: Without notes, define frequency, amplitude, time period, pitch and loudness.
Day 2: Stretch a rubber band over a box. Pluck it hard and softly, then tighten it. Note which change affects loudness and which affects pitch.
Day 4: Solve: a pendulum makes 20 oscillations in 40 s. Find its time period and frequency (answers: 2 s, 0.5 Hz).
Day 7: Explain to someone why a bell in a jar goes quiet when the air is removed.
If the link between a vibration and the wave it makes is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. When you move up, our guide to sound in Class 9 is the next step.
Tonight, do the rubber band test and write two sentences: one on pitch, one on loudness.
