Sound is a longitudinal mechanical wave — it travels by squeezing and stretching the particles of a medium, so it cannot move through a vacuum. Master a handful of ideas here (speed in different media, the echo formula, SONAR and reverberation) and you collect some of the most predictable, formula-light marks in CDS Science.
Why this topic matters in CDS
Sound is a guaranteed scorer in CDS Science. Questions are usually one-line concept checks — which medium carries sound fastest, what an echo needs, what SONAR measures — with the occasional single-step numerical built on one clean formula.
Because the chapter sits on a few solid facts, a small amount of clarity converts directly into marks. You almost never need long calculations; you need to know how sound behaves and why.
Examiners love comparisons and real situations — bell jar experiments, a thunderclap heard after lightning, bats hunting in the dark, a ship measuring sea depth. Learn the reasoning behind each example; options are often a scene, not a formula.
What sound actually is
Sound is a form of energy produced by a vibrating body. The vibrating object pushes the particles of the surrounding medium back and forth, and this disturbance passes from particle to particle as a wave, carrying energy without the particles themselves travelling along.
Sound is a longitudinal wave: the particles of the medium vibrate parallel to the direction in which the sound travels. This produces alternate regions of:
- Compression — particles crowd together, pressure and density are high.
- Rarefaction — particles spread apart, pressure and density are low.
It is this travelling pattern of compressions and rarefactions, not the particles themselves, that carries the sound energy from the source to your ear. A guitar string, a tuning fork, your vocal cords and a ringing bell all make sound the same way — by vibrating and setting up these pressure waves in the surrounding air.
Sound needs a material medium (solid, liquid or gas) to travel because it moves through particles. In a vacuum there is no sound — this is shown by the classic bell-jar experiment, where a ringing bell falls silent as the air is pumped out.
Key wave quantities
Every sound wave can be described by a small set of quantities that the exam returns to again and again.
- Frequency (f) — number of vibrations per second, measured in hertz (Hz). It decides the pitch of the sound.
- Wavelength (λ) — distance between two consecutive compressions (or rarefactions).
- Time period (T) — time for one complete vibration; T = 1 ÷ f.
- Amplitude — the maximum displacement of a particle; it decides the loudness.
v = f × λ
(speed = frequency × wavelength)
Also f = 1 ÷ T. Loudness is measured in decibels (dB).
Human hearing spans roughly 20 Hz to 20,000 Hz (the audible range). Below 20 Hz is infrasound, produced by sources such as earthquakes and large animals like whales and rhinoceroses; above 20,000 Hz is ultrasound, which bats, dolphins and porpoises use for navigation and which powers medical scans and industrial testing. A wave with a higher frequency therefore has a shorter wavelength, since their product (the speed) stays fixed in a given medium.
Speed of sound in different media
The speed of sound depends on the medium, not on the loudness or pitch of the source. It travels fastest in solids, slower in liquids and slowest in gases, because tightly packed, tightly bound particles pass the disturbance along far more quickly than the loosely spaced particles of a gas.
Order of speed: solids > liquids > gases.
Approximate values: in air ≈ 343 m/s (at 20°C), in water ≈ 1480 m/s, in steel ≈ 5960 m/s.
In air at 0°C the speed is about 331 m/s. A handy fact for the exam: sound speeds up by roughly 0.6 m/s for every 1°C rise in temperature. The exam also expects you to know what else raises or lowers the speed in air:
- Temperature: higher temperature → higher speed, so sound travels a little faster on a hot day.
- Humidity: more moisture → higher speed, because water vapour is lighter than dry air.
- Pressure: at constant temperature, a change in pressure has almost no effect on the speed.
Thinking sound travels faster simply because the source is louder, or because the gas is denser by weight. In gases, speed depends on temperature and humidity, not on loudness, frequency or amplitude. Change the temperature or the medium — not the volume knob. Note too that sound is far slower than light, which is why thunder always lags behind a lightning flash.
Reflection of sound and echoes
Like light, sound reflects off hard surfaces, obeying the law that the angle of incidence equals the angle of reflection. A reflected sound heard distinctly after the original is an echo.
To hear a clear echo, the reflected sound must reach the ear at least 0.1 second after the original; otherwise the brain merges the two (persistence of hearing).
Minimum distance for an echo (sound goes to the wall and back):
2d = v × t, so d = (v × t) ÷ 2
With v = 343 m/s and t = 0.1 s:
d = (343 × 0.1) ÷ 2 ≈ 17 m.
Memorise the magic figure: the reflecting surface must be at least about 17 metres away (often rounded to 17.2 m using 344 m/s) for a distinct echo. This single number answers several CDS questions outright.
Reverberation and how it is controlled
When sound is reflected repeatedly from the walls, floor and ceiling of a closed hall, the many overlapping reflections cause the sound to persist even after the source stops. This prolonged, blurred sound is called reverberation.
Excess reverberation muddies speech and music, so large halls are designed to control it:
- Covering walls and ceilings with sound-absorbing materials such as fibreboard, heavy curtains and perforated panels.
- Using compressed fibreboard or carpets to soak up extra reflections.
- Seats and audiences themselves absorb a good deal of sound.
Do not confuse echo with reverberation. An echo is a single, distinct repetition of sound from one distant surface; reverberation is the continuous overlapping of many reflections in an enclosed space.
SONAR, ultrasound and applications
The echo principle has powerful real uses, all favourites in CDS.
- SONAR (Sound Navigation And Ranging): a ship sends ultrasonic pulses into the sea; the echo bouncing off the seabed or an object is timed to find the depth or distance. This is called echo-ranging.
- Ultrasound scans: high-frequency waves (above 20 kHz) form images of the heart, kidneys and unborn babies, and break kidney stones.
- Echolocation: bats and dolphins emit ultrasound and judge distance from the returning echoes.
- Industry: ultrasound detects flaws and cracks inside metal blocks (non-destructive testing) and is used to clean delicate parts.
In every case the idea is the same: send out a known sound pulse, time the echo and convert that time into a distance. The very short wavelength of ultrasound also lets it travel in a sharp, well-defined beam, which is why it gives clearer images and tighter ranging than ordinary audible sound.
SONAR depth formula:
2d = v × t, so d = (v × t) ÷ 2,
where v is the speed of sound in water (≈ 1480 m/s) and t is the total to-and-fro time.
Supersonic speed and the sonic boom
When an object such as a fighter jet moves faster than the speed of sound in air, it is said to travel at supersonic speed. The ratio of an object's speed to the speed of sound is its Mach number — Mach 1 is exactly the speed of sound.
A supersonic aircraft pushes the air into a sharp shock wave. When this shock wave reaches the ground, it is heard as a sudden, sharp, thunder-like sound called a sonic boom, which can even rattle windows.
Mach 1 = speed of sound; above Mach 1 = supersonic. The sonic boom is caused by the shock wave, not by the engine noise.
Worked example: finding depth with SONAR
A ship sends an ultrasonic pulse straight down towards the seabed. The echo returns after 3 seconds. If the speed of sound in sea water is 1480 m/s, find the depth of the sea. Also find the wavelength if the pulse frequency is 40,000 Hz.
Notice the key step: the time given is the round trip, so we use 2d. Forgetting the factor of 2 is the single biggest error in echo numericals.
Quick traps examiners set
A few recurring confusions are worth nailing down before the exam.
- Vacuum: sound cannot travel through it; light can. Astronauts on the Moon need radios because there is no air to carry sound.
- Speed order: solids > liquids > gases, the reverse of what many expect.
- Echo factor of 2: the sound travels to the surface and back, so always use 2d.
- Pitch vs loudness: frequency sets pitch, amplitude sets loudness — keep them separate.
- Echo vs reverberation: one clear repeat vs continuous overlapping reflections.
If a numerical mentions an echo, SONAR or depth, write 2d = v×t first. If it mentions pitch or shrillness, think frequency; if it mentions loudness, think amplitude.
Previous-year style question
Q. A person standing between two parallel cliffs claps and hears the first echo after 2 s and the next after 3 s. If the speed of sound is 340 m/s, what is the distance between the two cliffs?
Answer: For the first cliff, 2d1 = v × t1 = 340 × 2 = 680 m, so d1 = 340 m. For the second cliff, 2d2 = 340 × 3 = 1020 m, so d2 = 510 m. The distance between the cliffs = d1 + d2 = 340 + 510 = 850 m.
When echoes come from two surfaces, find each distance separately using 2d = v×t, then add them. The clap stands between the surfaces, so the gap is the sum of the two distances.
Quick revision
- Sound — a longitudinal mechanical wave; needs a medium, no sound in vacuum.
- v = f×λ; frequency sets pitch, amplitude sets loudness (in dB).
- Speed order — solids > liquids > gases; rises with temperature and humidity.
- Audible range 20 Hz–20,000 Hz; below is infrasound, above is ultrasound.
- Echo — needs ≥ 0.1 s gap, reflecting surface ≈ 17 m away; use 2d = v×t.
- SONAR uses echo-ranging in water; reverberation is overlapping reflections in a hall.
- Supersonic — faster than sound (above Mach 1); shock wave gives a sonic boom.
Frequently asked questions
Why can sound not travel through a vacuum?
Sound is a mechanical wave that travels by vibrating the particles of a medium. A vacuum has no particles to pass the vibration along, so sound cannot move through it. This is why the bell in a bell-jar experiment falls silent once the air is removed.
In which medium does sound travel fastest and why?
Sound travels fastest in solids, then liquids, and slowest in gases. In solids the particles are very closely packed and tightly bound, so they pass the disturbance from one to the next far more quickly than the loosely spaced particles of a gas.
What is the minimum distance needed to hear a clear echo?
The reflected sound must reach the ear at least 0.1 second after the original. Using d = (v × t) / 2 with the speed of sound in air, the reflecting surface must be about 17 metres away for a distinct echo to be heard.
What is the difference between an echo and reverberation?
An echo is a single, distinct repetition of sound reflected from one distant surface. Reverberation is the continuous, overlapping persistence of sound caused by repeated reflections from the walls, floor and ceiling of a closed hall.
How does SONAR measure the depth of the sea?
SONAR sends an ultrasonic pulse downwards and times how long the echo takes to return from the seabed. Using 2d = v × t, where v is the speed of sound in water, the depth d is found by halving the round-trip distance.
What causes a sonic boom?
When an aircraft flies faster than the speed of sound (supersonic, above Mach 1), it creates a shock wave in the air. When this shock wave reaches the ground it is heard as a sudden sharp, thunder-like sound called a sonic boom, not the engine noise.
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