Evidence That Sound Is a Wave
In 1787, German physicist Ernst Chladni in Wittenberg scattered sand on a metal plate and drew a violin bow across its edge — at 440 Hz the sand formed vivid geometric nodal patterns with exactly 4 nodal lines, providing the first quantitative, visual evidence that sound is a wave phenomenon. In 1808 Napoleon witnessed a Paris demonstration of these Chladni figures and awarded Chladni 6,000 francs to have the technique documented — because the patterns made the invisible wave structure of sound undeniably concrete.
List the four wave behaviours that should be demonstrable if sound is truly a wave. For each, describe a simple everyday observation that provides evidence. Write your predictions.
Warm-up — which property is evidence that sound undergoes superposition?
Know
- Four sound wave evidence phenomena: reflection, diffraction, resonance, superposition
- A real investigation or observation for each
- ACSPH071 — the syllabus focus for this lesson
Understand
- Why each phenomenon provides evidence for the wave model
- How acoustic engineering uses these properties
- Why sound cannot be described as a particle (lacks diffraction/interference)
Can Do
- Design or describe a simple investigation for each property
- Link each observation to the appropriate wave principle
- Write an extended response linking evidence to the wave model
Core Content
Clap your hands once near a brick wall and you will hear the echo fractions of a second later — sound has bounced back. Walk behind a speaker hidden behind a wall and you can still hear the music — sound has bent around the corner. Place a tuning fork near a wine glass of the same pitch and the glass sings back to you — sound has driven resonance. Hold two slightly-detuned tuning forks near your ear and the volume pulses at a slow, regular beat — two sound waves are superposing. Each of these everyday events is only possible because sound is a wave.
| Wave behaviour | Evidence for sound | Investigation / observation |
|---|---|---|
| Reflection | Echoes — sound bounces off surfaces; $\theta_i = \theta_r$ | Clap near a brick wall; record echo time; $d = vt/2$ |
| Diffraction | Hearing sound around corners; AM radio reception behind hills | Audio source behind a barrier; detects sound beyond geometric shadow |
| Resonance | Large amplitude when driving frequency = natural frequency | Humming near a wine glass; strike frequency vs. tap frequency; wine glass sings at its natural frequency |
| Superposition | Beats; constructive/destructive interference in rooms | Two speakers with slightly different frequencies; walk in room and hear loud/quiet regions |
Sound is a wave: (1) Reflection — echoes obey $\theta_i = \theta_r$; (2) Diffraction — sound bends around corners because $\lambda$ (≈0.02–20 m) is comparable to obstacle size; (3) Resonance — energy builds when driving frequency equals natural frequency; (4) Superposition — beats arise from two slightly different frequencies adding and cancelling (ACSPH071).
Pause — copy the four lines of evidence into your book before the check below.
The ability of sound to diffract around a corner is evidence that it is a wave.
Resonance in a wine glass occurs when the driving frequency is different from the glass's natural frequency.
Which phenomenon best demonstrates that sound obeys the superposition principle?
Activities
A student claps near a cliff and hears the echo after 0.6 s. Use $v_{sound} = 340$ m/s to calculate the distance to the cliff.
Design and describe an investigation that demonstrates resonance in a column of air. Include: apparatus, procedure, and what observation would confirm resonance has occurred.
An acoustic engineer wants to reduce unwanted echoes (reflections) in a concert hall but maintain resonance for the orchestra. Explain how these two design goals conflict, and suggest a design feature that addresses both.
Which wave behaviour does NOT provide evidence that sound is a wave?
A student holds a 256 Hz tuning fork above a closed-open air column. The resonant length is approximately ($v_{sound} = 340$ m/s):
The ACSPH071 syllabus dot point specifically requires investigations of sound's:
UnderstandBand 3(4 marks) 1. List and briefly describe four pieces of evidence that demonstrate sound behaves as a wave. For each, name the wave property it demonstrates.
ApplyBand 3(2 marks) 2. A student hears an echo from a building 0.4 s after clapping. How far away is the building? ($v = 340$ m/s)
AnalyseBand 5(4 marks) 3. A music teacher demonstrates that a tuning fork near a guitar string causes the string to vibrate. Identify the wave phenomenon demonstrated and explain the mechanism. Link to the concept of natural frequency.
Show all answers
Activity 2 — Echo
$d = 340 \times 0.6 / 2 = 102$ m
Short Answer — Model Answers
Q1 (4 marks): 1. Reflection — echoes: sound bounces off hard surfaces with angle of incidence = angle of reflection. 2. Diffraction — sound heard around corners; the wavelength of sound (~0.02–20 m) is comparable to everyday obstacles, so significant bending occurs. 3. Resonance — wine glass amplification or tuning fork + air column; driving frequency = natural frequency → large amplitude oscillation. 4. Superposition — beats produced by two nearby frequencies; the resultant displacement is the sum of both waves, causing alternating constructive and destructive interference.
Q2 (2 marks): $d = 340 \times 0.4/2 = 68$ m.
Q3 (4 marks): The phenomenon is resonance. The tuning fork emits sound waves at its natural frequency. The guitar string has the same (or very close) natural frequency. As the periodic sound waves from the fork strike the string, they drive it at its natural frequency. Energy builds up because input energy is always in phase with the string's oscillation. Amplitude increases dramatically — this is resonance. If the frequencies did not match, no sustained large oscillation would occur.
Ernst Chladni's 1787 Wittenberg experiment is the key: sand scattered on a metal plate bowed at 440 Hz formed 4 sharply defined nodal lines — a pattern that is impossible to explain unless sound is a wave that creates standing wave nodes. Napoleon's 1808 Paris demonstration spread this evidence widely. The same four wave properties Chladni exploited — reflection, diffraction, resonance and superposition — are what you have now systematically applied to sound in this lesson.