Sound as Energy

What is sound?

Sound is a form of energy that travels as a mechanical wave — a disturbance that moves through a material medium by causing particles to vibrate. Sound is produced whenever an object vibrates: a guitar string, a vocal cord, a loudspeaker cone, or a tuning fork all set nearby air particles oscillating, and those oscillations travel outward as sound.

Sound cannot travel through a vacuum. It requires a medium (solid, liquid, or gas) because it is the particles of the medium that carry the disturbance. This is why there is no sound in outer space.

Sound as a longitudinal wave

Sound is a longitudinal wave: the particles of the medium vibrate parallel to the direction in which the wave travels. This creates alternating regions of:

Compression: where particles are pushed close together (high pressure region).
Rarefaction: where particles are spread farther apart (low pressure region).

Compare this to a transverse wave (like light or a wave on a rope), where particles vibrate perpendicular to the direction of travel. Sound is always longitudinal in gases and liquids.

Properties of sound waves

Property	Definition	Effect on sound	Unit
Frequency (f)	Number of complete vibrations (cycles) per second.	Determines pitch. Higher frequency = higher pitch.	Hertz (Hz)
Amplitude (A)	Maximum displacement of particles from their rest position.	Determines loudness. Greater amplitude = louder sound.	metres (m) — loudness in decibels (dB)
Wavelength (λ)	Distance between two consecutive compressions (or rarefactions).	Related to pitch; shorter wavelength = higher frequency.	metres (m)
Period (T)	Time taken for one complete vibration.	T = 1/f. Lower frequency = longer period.	seconds (s)
Speed (v)	How fast the wave travels through the medium.	Depends on the medium, not the source.	metres per second (m/s)

The wave equation

v = f × λ | λ = v ÷ f | f = v ÷ λ

Where v = wave speed (m/s), f = frequency (Hz), λ = wavelength (m).

Example: Sound in air at 20°C, f = 440 Hz → λ = 340 ÷ 440 ≈ 0.77 m

Speed of sound in different media

Sound travels faster through denser, more elastic media because particles are closer together and can transfer the vibration more rapidly.

Medium	Speed of sound (approx.)
Air (20°C)	~340 m/s
Water	~1 480 m/s
Steel	~5 100 m/s
Wood	~3 800 m/s

Remember: Sound travels fastest through solids, slower through liquids, and slowest through gases. The speed of sound in a medium does NOT depend on the frequency or amplitude of the sound.

Human hearing range

The human ear can detect sounds in the frequency range of approximately 20 Hz to 20 000 Hz (20 kHz).

Infrasound: below 20 Hz — too low for humans to hear. Produced by earthquakes, elephants communicate using infrasound.
Audible sound: 20 Hz – 20 000 Hz — the range heard by a healthy human ear.
Ultrasound: above 20 000 Hz — too high for humans to hear. Dogs and bats can hear into this range.

Echoes and reflection of sound

Sound, like all waves, can be reflected. When a sound wave strikes a hard surface, it bounces back — this is called an echo. Echoes require the reflecting surface to be at least ~17 m away (so the reflected sound returns after the original sound has ended — at least 0.1 s later).

Applications of sound

SONAR (Sound Navigation And Ranging): Ships and submarines send out pulses of ultrasound and measure the time for the echo to return to calculate distance to the ocean floor or other objects.
Medical ultrasound: High-frequency sound waves (1–20 MHz) are used to produce images of internal organs and developing foetuses without harmful radiation.
Musical instruments: Use vibrating strings, columns of air, or membranes to produce sound at specific frequencies.
Noise-cancelling headphones: Use destructive interference — they produce a sound wave that is the exact mirror image (inverted amplitude) of incoming noise, cancelling it out.

Answer all questions in your exercise book. Show all calculations with units. Use v = 340 m/s for the speed of sound in air unless otherwise stated.

Question 1 · (2 marks)

Explain why sound cannot travel through outer space. In your answer, refer to the type of wave sound is and what it requires to travel.

Question 2 · (2 marks)

Distinguish between a compression and a rarefaction in a longitudinal sound wave. Which region has higher pressure?

Question 3 · (3 marks)

A sound wave has a frequency of 680 Hz and travels through air at 340 m/s. (a) Calculate the wavelength of the sound. (b) Calculate the period of the wave. (c) How would the wavelength change if the frequency doubled (assuming the same medium)? Explain.

Question 4 · (3 marks)

Compare the speed of sound in air, water, and steel. Which is fastest and which is slowest? Explain, in terms of particle spacing, why sound travels faster in solids than in gases.

Question 5 · (2 marks)

Define ultrasound and give TWO practical applications of ultrasound technology in modern life.

Question 6 · (2 marks)

A ship uses SONAR to measure the depth of the ocean. It sends out an ultrasound pulse and receives the echo 0.6 s later. If the speed of sound in seawater is 1 500 m/s, calculate the depth of the ocean at that point. Show all working.

Question 7 · (5 marks)

A tuning fork vibrates at a constant frequency of 500 Hz. The table below gives the speed of sound in three different media.

Medium	Speed of sound (m/s)	Wavelength at 500 Hz (m)
Air (20°C)	340	?
Water	1 480	?
Steel	5 100	?

(a) Use λ = v ÷ f to calculate the missing wavelength for each medium. Show your working for all three. (3)
(b) The frequency of the tuning fork (500 Hz) does not change as the sound moves between media. Explain, using your answers to (a), why the wavelength is so much longer in steel than in air. (2)

What is sound?

Sound as a longitudinal wave

Properties of sound waves

The wave equation

Speed of sound in different media

Human hearing range

Echoes and reflection of sound

Applications of sound

Sound Wave Visualiser

Transverse representation (displacement vs position)

Longitudinal view (compression & rarefaction)