Sound

When the school bell rings, when a friend's voice comes through a phone, when a mosquito's whine keeps you awake, or when firecrackers rattle your windows on a festival evening — every one of these is the same physical event wearing a different costume: something is vibrating, that vibration is shaking a medium, and the shaking is reaching your ear. Sound is not a "thing" that instruments store up and release; it is a to-and-fro motion travelling as a disturbance through a gas, a liquid or a solid, and it cannot cross empty space — a vacuum — at all. This chapter builds one coherent story out of every sound you have ever heard. It starts with the fact that every sound source is vibrating — a plucked rubber band, a struck bell, your own vocal cords — and shows how that vibration must travel through a medium to reach your ear, where a stretched membrane called the eardrum picks it up and passes the signal to the brain. Along the way you will learn to describe any vibration with three numbers: how far it swings (amplitude), how long one swing takes (time period), and how many swings happen every second (frequency, measured in hertz). Those three numbers explain why a drum sounds different from a whistle, why a woman's voice differs from a man's, and why some sounds are too high or too low for human ears to detect at all. The chapter closes on a practical question: when does a sound stop being pleasant and start being harmful? You will meet the decibel scale, the difference between music and noise, and the real health costs of noise pollution, plus what can be done to control it. By the end, you should be able to explain any sound-producing situation, from a jaltrang made of water-filled bowls to a busy city street, in terms of vibration, medium, amplitude, frequency and loudness.

Key takeaways

Sound is produced by a vibrating body — the to-and-fro motion of an object; once vibration stops, sound stops too.
Even tiny, invisible vibrations, as in a struck bell, can be felt or detected, for example by a thermocol ball bouncing off a vibrating tumbler.
In humans, sound is produced by two vocal cords stretched across the larynx (voice box), which vibrate as air from the lungs is forced through the narrow slit between them.
Sound needs a material medium — solid, liquid or gas — to travel; it cannot travel through a vacuum, which is why a ringing phone in an evacuated jar cannot be heard.
Sound makes the eardrum, a thin stretched membrane, vibrate; the vibrations pass to the inner ear and then the brain, and that whole process is hearing.
A vibration is described by its amplitude (how far it swings), time period $T$ (time for one oscillation) and frequency $f$ (oscillations per second, unit hertz, Hz), related by $f = 1/T$ .
Loudness depends on amplitude — loudness is proportional to the square of the amplitude — measured in decibels (dB); pitch depends on frequency, higher frequency giving a shriller, higher-pitched sound.
The human ear detects frequencies roughly from 20 Hz to 20,000 Hz (20 kHz), the audible range; sounds outside it exist but cannot be heard by us, though some animals and ultrasound machines use them.
Unwanted sound is noise; pleasant sound is music — but even music turns unpleasant if too loud, and excessive sound causes noise pollution, which can bring hypertension, anxiety, sleep loss and hearing impairment.
Key idea: every sound begins as a vibration that must travel through a medium (never a vacuum) to reach the eardrum, and its amplitude and frequency completely fix what we perceive as loudness and pitch.

The core insight

Sound is produced by vibrating bodies, must travel through a material medium (solid, liquid or gas) and never through a vacuum, is received by the eardrum and passed to the brain, and is fully characterised by its amplitude (loudness, proportional to amplitude squared) and its frequency (pitch), with the human ear audible only from 20 Hz to 20,000 Hz.

What this chapter covers

The chapter builds outward from a single idea, in this order:

Vibration as the origin of sound — every sound source, from a rubber band to a struck metal plate, is vibrating; stop the vibration and the sound stops.
How the human body makes sound — the larynx and vocal cords, and why voices differ by age and sex.
How sound travels — the need for a medium (gas, liquid or solid) and the impossibility of sound in a vacuum.
How we hear — the outer ear, the eardrum, and the path of a signal to the brain.
Describing a vibration precisely — amplitude, time period and frequency, connected to loudness and pitch.
The limits of hearing — the 20 Hz to 20,000 Hz audible range, and sound beyond it (ultrasound).
Sound in society — music versus noise, and the causes, effects and control of noise pollution.

The core ideas

Vibration: the origin of every sound

Every sound, without exception, starts with a vibration — the to-and-fro motion of an object. Pluck a tightly stretched rubber band and it swings rapidly while producing sound; hold it still and the sound stops instantly. Strike a hanging metal plate with a stick and you can feel it trembling even when the motion is too small to see. Strike the edge of a water-filled dish with a spoon and ripples spread across the surface, showing the vibration directly. The rule is always the same: a vibrating object produces sound, and when the vibrations die out, so does the sound. This holds for musical instruments too, and the vibrating part is not always obvious. In the veena or sitar it is a stretched string; in the tabla or mridangam a stretched membrane; in the flute a vibrating air column; in the manjira, ghatam and kartal, which are simply struck, the body of the instrument itself. When a sitar string is plucked, what you hear is not just the string — the whole wooden body vibrates along with it.

How humans produce sound

Speak loudly or hum with a hand on your throat, and you feel a distinct vibration. In humans, sound is produced by the voice box, or larynx, at the upper end of the windpipe. Two vocal cords are stretched across it, leaving a narrow slit between them. When the lungs force air through this slit, the cords vibrate and produce sound; muscles attached to them can pull the cords tight and thin, or loose and thick, changing the voice produced. Vocal cord length differs by person: about 20 mm in men, about 15 mm in women, and shorter still in children. Shorter, tighter cords vibrate faster, which is why children's and women's voices are typically higher-pitched than men's — a link made precise once frequency is defined below.

Sound needs a medium for propagation

A ringing phone inside a sealed glass jar is heard clearly — until the air is pumped out, and the ringing fades as it goes. If every trace of air were removed, creating a true vacuum, no sound would be heard at all, even though the phone is still ringing as before. This is one of the most important facts in the chapter: sound needs a material medium to travel, and it cannot travel through a vacuum. The medium need not be air. An ear placed gently at the water's surface can hear a bell shaken underwater — how whales and dolphins communicate. Holding a long metal rod or wooden table to the ear while a friend scratches the other end shows sound also travels through solids. A string-and-cup toy telephone works the same way, the taut string carrying the vibration. So sound travels through gases, liquids and solids — never through empty space.

Hearing sound: the human ear

The outer, funnel-shaped part of the ear collects sound and channels it down a canal to a thin, tightly stretched membrane, the eardrum. Sound vibrations reaching this membrane make it vibrate; the eardrum passes the vibrations to the inner ear, and a signal then travels to the brain. This whole process, from vibration reaching the ear to a signal reaching the brain, is hearing. Because the eardrum is so thin and delicate, it must never be probed with a sharp or hard object — doing so can tear it and permanently impair hearing.

Figure

A labelled cross section of the human ear showing sound entering the outer ear funnel and travelling down the ear canal to the eardrum which vibrates and sends a signal through the inner ear to the brain

Amplitude, time period and frequency of a vibration

To compare two vibrations precisely, three quantities are used. The amplitude is the maximum displacement of the vibrating object from its rest position — how far it swings. Strike a tumbler gently and then harder, and a small thermocol ball touching its rim is displaced a small distance and then a much larger one; that displacement measures amplitude directly. The time period, symbol $T$ , is the time for one complete oscillation. The frequency, symbol $f$ , is the number of oscillations completed in one second, measured in hertz (Hz); a frequency of 1 Hz means exactly one oscillation per second.

f = \frac{1}{T}

where: $f$ is frequency in hertz and $T$ is time period in seconds. This single relationship connects the two ways of describing the same vibration — how long one swing takes, and how many swings happen every second — and it is used repeatedly in the worked examples below.

Figure

Two oscillation graphs of displacement against time comparing a slow large amplitude vibration with a fast small amplitude vibration and marking one time period and the amplitude on each

Loudness and pitch

Loudness and pitch describe two completely different aspects of a sound, and confusing them is the single most common mistake in this chapter (see the Warning below). Loudness depends on amplitude: greater amplitude means louder sound. The relationship is a square law, not a simple proportion:

L \propto A^2

where: $L$ is loudness and $A$ is amplitude, so doubling the amplitude of a vibration increases the loudness not by two times but by four times. Loudness is measured on the decibel (dB) scale: normal breathing about 10 dB, a soft whisper about 30 dB, normal conversation about 60 dB, busy traffic about 70 dB, an average factory about 80 dB — above about 80 dB, sound becomes physically painful.

Pitch, by contrast, depends on frequency: higher frequency gives a higher, more shrill pitch; lower frequency gives a lower pitch. A drum vibrates at low frequency and sounds low-pitched, while a whistle vibrates at high frequency and sounds shrill. A bird's call is high-pitched while a lion's roar is low-pitched — even though the roar is far louder, showing loudness and pitch vary independently. This is also why a woman's or child's voice, from shorter, tighter vocal cords, is usually higher-pitched than a man's.

Audible and inaudible sounds

Not every vibration produces a sound the human ear can detect. Frequencies below about 20 Hz, and above about 20,000 Hz (20 kHz), are inaudible to humans, so the human audible range is roughly 20 Hz to 20,000 Hz. Some animals, such as dogs, hear well above 20,000 Hz, which is why police whistles pitched above human hearing still work on trained dogs. Sound above 20,000 Hz is called ultrasound, widely used in medicine to investigate and track medical conditions.

Noise, music and noise pollution

Sound pleasing to the ear, such as a harmonium or sitar, is music; sound causing discomfort, such as construction work or a blaring horn, is noise — but even a musical sound becomes noise if played too loud. The presence of excessive or unwanted sound is noise pollution, caused chiefly by vehicles, firecrackers, machines and loudspeakers, plus household sources like loud televisions, some kitchen appliances and air conditioners. It is not merely annoying: prolonged exposure can cause lack of sleep, hypertension, anxiety and other disorders, and continuous exposure to loud sound may cause temporary or even permanent hearing impairment. Noise pollution is controlled at its source and along its path: fitting silencing devices to engines, vehicles, machines and appliances; keeping noisy industries away from residential areas; using horns less; running televisions and music systems at lower volumes; and planting trees along roads and around buildings to cut the sound reaching homes.

Derivation AFinding time period and frequency of a vibration from a count of oscillations

The goal here is to turn a simple classroom observation — a certain number of oscillations counted over a measured stretch of time — into the two quantities that fully describe a vibration: its time period $T$ and its frequency $f$ . This is the single most repeated calculation in this chapter, so the reasoning is worked through once in general terms before being applied to specific numbers in the worked examples that follow, using only the definitions already stated.

Step 1. Identify what is actually measured: the total number of oscillations, call it $n$ , occurring in a measured total time, call it $t$ . Neither $n$ nor $t$ alone is the time period or the frequency — they must be combined.

Step 2. Find the time period. Since $n$ identical oscillations happen in total time $t$ , each single oscillation takes an equal share of that time: $T = t/n$ , the total time spread evenly across every oscillation counted.

Step 3. Find the frequency directly. Frequency is oscillations completed in one second; if $n$ happen in $t$ seconds, the number happening in one second is $f = n/t$ .

Step 4. Check the two routes agree: taking the reciprocal of $T = t/n$ gives $1/T = n/t$ , exactly the expression for $f$ in Step 3, so $f = 1/T$ always. This identity is checked with real NCERT numbers in Worked examples 1 and 2 below (a pendulum swinging 40 times in 4 seconds, and a mosquito's wings vibrating 500 times a second). Verified: for any $n$ oscillations in time $t$ , $T = t/n$ and $f = n/t$ satisfy $f = 1/T$ by direct substitution.

Go deeper · Derivation B · 1 minWhy thunder is heard after the lightning flash is seen

Derivation BWhy thunder is heard after the lightning flash is seen

The goal is to explain, using only the fact that sound needs a medium and travels far slower than light, why a lightning flash is always seen before its thunder is heard, even though both are produced at the same place and the same instant high in the storm cloud. This is exactly the reasoning the chapter's own exercises ask for, worked here as an explicit chain rather than a one-line answer, since each step draws on a fact already established above.

Step 1. Fix what happens at the source. Lightning is an electrical discharge that produces an intense flash of light and, by violently heating and expanding the air along its path, a loud crack of thunder — both produced at essentially the same instant and place.

Step 2. Compare how each disturbance reaches an observer. Light can travel through a vacuum and moves at about $3\times10^8$ metres per second. Sound is a mechanical vibration needing a medium — the air — and travels far slower, roughly $340$ metres per second at room temperature.

Step 3. Compare travel times over the same distance $d$ : light takes $d/(3\times10^8)$ , sound takes $d/340$ . Since the speed of light is roughly a million times the speed of sound, light's travel time is utterly negligible in comparison — the flash effectively arrives the instant it is produced.

Step 4. Read off the conclusion: because the flash arrives almost instantly while sound takes an appreciable, measurable time over the same distance, an observer always sees lightning first and hears thunder afterwards, with a gap that grows with distance — exactly what Worked example 3 below calculates. Verified: light's travel time $d/(3\times10^8)\ \text{s}$ is smaller than sound's travel time $d/340\ \text{s}$ by a factor of about $8.8\times10^5$ for any $d$ , confirming the flash is effectively simultaneous with the discharge while the thunder lags.

Worked examples

Board level - time period and frequency of a swinging pendulum

A pendulum oscillates 40 times in 4 seconds. Find its time period and its frequency.

Step 1. Use Derivation 1: $T = t/n = 4\ \text{s} / 40 = 0.1\ \text{s}$ .

Step 2. Find frequency: $f = 1/T = 1/0.1 = 10\ \text{Hz}$ . Checking directly, $f = n/t = 40/4 = 10\ \text{Hz}$ — the same answer, confirming Derivation 1. Answer: time period $T = 0.1$ s and frequency $f = 10$ Hz.

Board level - time period of a mosquito's wingbeat

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, and is this sound within the range of human hearing?

Step 1. The vibration rate given, 500 vibrations per second, is exactly the frequency, so $f = 500\ \text{Hz}$ .

Step 2. Time period is the reciprocal of frequency, $T = 1/f = 1/500 = 0.002\ \text{s}$ , which can also be written as $2\ \text{milliseconds}$ .

Step 3. Check audibility: the human audible range is roughly 20 Hz to 20,000 Hz, and $500\ \text{Hz}$ lies comfortably inside this range, which matches everyday experience — the mosquito's high-pitched whine is very much audible, if annoying. Answer: time period $T = 0.002$ s (2 ms), and yes, at 500 Hz the sound is well within the audible range for humans.

Higher-order thinking (HOTS / olympiad-style) - how far away is a thunderstorm

A student sees a flash of lightning and, using a stopwatch, counts a gap of exactly 3 seconds before hearing the thunder. Taking the speed of sound in air as 340 m/s and the speed of light as effectively instantaneous over this distance, estimate how far away the storm is.

Step 1. Apply Derivation 2: light's travel time is negligible, so the flash is effectively seen the instant the lightning strikes, and the whole measured gap of 3 seconds belongs to the sound alone.

Step 2. Distance equals speed times time: $d = v \times t = 340\ \text{m/s} \times 3\ \text{s} = 1020\ \text{m}$ .

Step 3. Convert units: $1020\ \text{m} = 1.02\ \text{km}$ . Answer: the storm is approximately 1.0 km (1020 m) away. Timing note: under two minutes once the key simplification is spotted — light's travel time is ignorable, so the whole stopwatch reading belongs to the sound. The trap is reaching for the speed of light instead of sound, or trying to correct for light's travel time (about three-millionths of a second here), which is far too small to matter against the 3-second gap.

Common traps

Mixing up time period and frequency. $T$ (seconds) is how long one oscillation takes; $f$ (hertz) is how many oscillations happen per second — reciprocals, not the same quantity.
Treating loudness as depending on frequency, or pitch on amplitude. Loudness depends only on amplitude; pitch depends only on frequency. A lion's roar is loud and low-pitched; a bird's chirp can be soft and high-pitched.
Getting the amplitude-loudness relation wrong. Loudness is proportional to the square of the amplitude — doubling amplitude quadruples loudness, it does not merely double it.
Reversing the frequency-pitch direction. Higher frequency means higher, more shrill pitch — never the reverse.
Assuming sound can cross a vacuum, like light or radio waves. Sound is a mechanical vibration and absolutely needs a medium; a bell in an evacuated jar produces no sound outside it.
Treating noise and music as fixed categories. Even a musical instrument's sound becomes noise if played too loudly for too long.
Blurring noise pollution with air pollution. They are parallel but separate ideas, and noise pollution has its own health effects — hypertension, anxiety, hearing damage — not just "annoyance."

Common trapLoudness and pitch are not the same thing

The single most common mistake in this chapter is treating loudness and pitch as interchangeable, or assuming a "big" sound is automatically both loud and high-pitched. They are governed by two different properties of a vibration. Loudness depends only on amplitude, following a square law, $L \propto A^2$ , so doubling amplitude makes a sound four times as loud, not twice. Pitch depends only on frequency, with higher frequency giving a higher, shriller pitch. The two vary independently: a lion's roar is loud but low-pitched (large amplitude, low frequency), while a bird's call can be soft yet high-pitched (small amplitude, high frequency). For loudness, think amplitude; for pitch, think frequency — never substitute one for the other.

You should be able to…

Explain why every sound source must be vibrating, with the vibrating part in at least four musical instruments.
Describe how the larynx and vocal cords produce sound, and why voices differ in pitch by age and sex.
State why sound needs a medium and cannot cross a vacuum, with a demonstrating experiment.
Trace the path of sound from the outer ear through the eardrum and inner ear to the brain.
Define amplitude, time period and frequency, and calculate one from the other using $f = 1/T$ .
Explain how loudness depends on amplitude squared and pitch on frequency, keeping the two separate.
State the human audible range (20 Hz to 20,000 Hz) with an inaudible-sound and an ultrasound example.
Distinguish noise from music, and list three sources and three health effects of noise pollution.
Suggest three practical measures to control noise pollution, at the source and along its path.

Ask the tutor: if you would like, I can walk through more pendulum-style time-period and frequency problems, or explain how a jaltrang or toy telephone works in terms of the ideas in this chapter.

If sound cannot travel through a vacuum, how do astronauts in space talk to each other?

They do not rely on sound waves travelling through space at all — the space around a spacecraft is essentially a vacuum, so sound genuinely cannot cross it. Astronauts instead use radio, carried by electromagnetic waves, the same family as light. Unlike sound, electromagnetic waves need no medium and travel fine through a vacuum, which is also how sunlight reaches Earth. Inside a spacecraft, where there is air, astronauts speak and hear each other normally, since the cabin air acts as a medium exactly as on Earth.

Why does my voice sound different from my friend's or my teacher's?

The difference comes mainly from the length and tightness of your vocal cords, stretched across the larynx. Longer, looser cords vibrate more slowly, giving a lower-pitched voice — typically why adult men, with vocal cords around 20 mm long, have deeper voices. Shorter, tighter cords, as in women (about 15 mm) and especially children, vibrate faster and give a higher-pitched voice. The exact shape of a person's throat, mouth and nasal passages also colours the sound, which is part of why you recognise a familiar voice instantly, even without seeing who is speaking.

Is a loud sound always high-pitched, and a soft sound always low-pitched?

No — this is one of the most important distinctions in the chapter. Loudness depends on amplitude (how big the swing is), while pitch depends on frequency (how many swings per second), and the two combine independently. A lion's roar is loud but low-pitched, with a large amplitude but low frequency. A bird's chirp can be relatively soft yet high-pitched, with a high frequency but smaller amplitude. Ask separately "how big is the amplitude" for loudness and "how fast is the vibration" for pitch.

Can loud sound really damage my hearing, and is it permanent?

Yes. Sound above roughly 80 decibels starts to become uncomfortable, and sustained exposure — from traffic, machinery, loud music or fireworks — can cause temporary or, with repeated exposure, permanent hearing impairment, along with poor sleep, high blood pressure and anxiety. This is also exactly why a sharp, pointed or hard object must never go into the ear: it can tear the thin eardrum directly, a separate and immediate risk from noise exposure. Keeping volumes moderate, limiting time in very loud environments, and using ear protection near loud machinery genuinely help protect hearing over a lifetime.