Big picture
Sound is produced by vibrating objects and travels through a material medium as a disturbance carrying energy. A ringing bicycle bell can be heard across a street because its vibrating surface disturbs the surrounding air. The air particles do not travel from the bell all the way to your ear; neighbouring particles pass the disturbance onwards.
Understanding sound explains differences between a shrill whistle and a deep drum, why an echo takes time to return and how reflected ultrasound can reveal objects we cannot see. The chapter connects wave quantities, hearing, reflection and applications through definitions, activities and worked calculations.
Learning goals
Explain production and propagation of sound through a material medium.
Describe compressions, rarefactions and longitudinal wave motion.
Calculate frequency, period, wavelength and wave speed using SI units.
Distinguish loudness, pitch and quality and their physical connections.
Explain reflection, echoes, reverberation and ultrasound applications.
Trace sound through the human ear and interpret standard experiments.
Core ideas
1. Demonstrate that vibrations produce sound
A vibration is a repeated to-and-fro motion about a mean position. A plucked string, stretched drum membrane and vibrating vocal cords are sound sources. In a tuning-fork activity, gently strike the fork on a suitable rubber pad and bring its prongs near water without unsafe handling. The moving prongs disturb the water, showing visible evidence of vibration.
A vibrating object transfers energy to its surroundings. If its vibration stops, the sound production stops. The vibrating source need not move bodily towards the listener: a bell fixed in place can still produce sound because its material oscillates. Avoid assuming that all vibrations are audible; frequency and intensity also matter.
2. Show why sound needs a medium
A medium is the material through which the disturbance travels. Sound travels through gases, liquids and solids, but cannot propagate through a perfect vacuum. In the classic bell-jar demonstration, an electric bell remains visibly active while air is gradually removed. Its sound becomes fainter because less air is available to transmit the disturbance. The observation supports the requirement of a medium, not the claim that the bell has stopped vibrating.
Within a medium, particles oscillate near their mean positions and transfer energy through interactions. There is no sustained bulk transport of air from the source to the listener. Sound generally travels faster in solids than in liquids and gases in familiar examples, but speed depends on the material's elastic properties, density and temperature, not solely on its state.
3. Describe longitudinal waves and pressure variations
In a longitudinal wave, particle oscillations are parallel to the direction of wave propagation. Sound in air is longitudinal. A compression is a region of relatively high pressure and density; a rarefaction is a region of relatively low pressure and density. As a source moves forward, it compresses nearby air; its backward motion helps create a rarefaction.
Diagram description. Draw a horizontal propagation arrow. Below it, represent air particles by dots: dense clusters alternate with sparse groups. Label the dense regions C and sparse regions R. Mark small back-and-forth arrows parallel to propagation. The distance between consecutive compression centres, or consecutive rarefaction centres, is one wavelength.
A pressure-against-distance graph looks wave-shaped, but its vertical axis represents pressure variation, not air moving up and down. In a transverse wave, the oscillation is perpendicular to propagation, as illustrated by a stretched string moving vertically while a disturbance travels horizontally.
4. Define wavelength, frequency, period and speed
Wavelength λ is the distance between nearest points in the same phase, such as successive compression centres; its SI unit is metre. Frequency f is the number of complete oscillations per second, measured in hertz, where 1 Hz means one oscillation per second. Time period T is the time for one complete oscillation, in seconds. They satisfy f = 1 ÷ T.
The wave travels one wavelength in one period, so v = λ ÷ T = fλ, where v is wave speed in metres per second. Use the frequency of the source, not the number of compressions shown in an arbitrary drawing, unless the diagram supplies enough timing information.
Worked example. A source makes 660 oscillations in 3 s. Frequency = 660 ÷ 3 = 220 Hz. Period = 1 ÷ 220 = 0.004545 s, approximately 4.55 ms. If sound speed is 330 m/s, wavelength = 330 ÷ 220 = 1.5 m. These are different quantities with different units; 220 m/s would not be a valid statement of this frequency.
5. Separate amplitude, loudness, pitch and quality
Amplitude is the maximum magnitude of a particle's displacement from its mean position, or the maximum variation of the quantity shown on a particular wave graph. Larger amplitude generally corresponds to greater sound energy and intensity. Loudness is the ear's response and depends on intensity and sensitivity; it is not identical to a purely physical amplitude reading.
Pitch is associated with frequency: higher frequency produces a higher pitch. Quality, or timbre, distinguishes sounds with the same pitch and loudness because their waveforms contain different mixtures of component frequencies. A flute and violin playing the same note can sound different for this reason.
Feature heard | Main physical connection | Example |
|---|---|---|
High or low pitch | Frequency | Whistle versus deep drum |
Greater loudness | Usually greater amplitude and intensity | Striking the same drum more strongly |
Distinctive quality | Waveform and component frequencies | Same note on different instruments |
For otherwise comparable waves in the same medium, intensity is proportional to amplitude squared. Doubling amplitude gives four times the intensity, not necessarily four times the perceived loudness. The decibel is used to express sound level; do not treat it as the SI unit of frequency.
6. Relate speed to conditions and source distance
For a given medium under fixed conditions, changing frequency changes wavelength rather than independently changing speed. At v = 330 m/s, a 330 Hz sound has wavelength 1 m, while a 660 Hz sound has wavelength 0.5 m. Higher pitch is not automatically faster propagation.
Sound speed in air generally increases with temperature. Use the value given in a numerical problem rather than substituting a memorised approximation. Light travels much faster than sound, explaining why lightning is seen before thunder is heard.
Worked example. If thunder is heard 4 s after lightning and sound speed is 340 m/s, neglect the very small light travel time. Distance = speed × time = 340 × 4 = 1360 m, or 1.36 km. This is a one-way travel problem, so do not divide by two.
7. Apply reflection and echo timing
Sound undergoes reflection at a suitable surface. The incident wave direction, reflected direction and normal lie in one plane, and the angle of incidence equals the angle of reflection. A large hard surface generally reflects sound more strongly than a soft absorbent material under comparable conditions.
An echo is a separately heard reflected sound. Human hearing typically needs about 0.1 s between the original and reflected sound to distinguish them clearly. At 340 m/s, a round trip of 0.1 s covers 34 m, so the reflector must be about 17 m away under these conditions. This distance changes with sound speed.
Worked example. An echo returns 0.8 s after a pulse. At 340 m/s the total path is 340 × 0.8 = 272 m. The reflector distance is half this: 136 m. The formula d = vt ÷ 2 uses the measured out-and-back time. Compare with thunder timing, which involves no return trip.
8. Distinguish reverberation and useful multiple reflection
Reverberation is persistence of sound due to repeated reflections that overlap rather than form a clearly separate echo. Excessive reverberation makes speech unclear in a hall. Curtains, upholstered seating, suitable acoustic panels and other absorbent surfaces reduce unwanted reflections.
Multiple reflection can also be useful. Megaphones direct sound, stethoscopes convey body sounds through a tube, and suitably designed curved surfaces can distribute sound towards listeners. An auditorium must balance useful sound distribution with absorption; making every surface highly reflective does not guarantee good acoustics. A case describing blurred speech after each sentence points towards reverberation rather than a change in the speaker's frequency.
9. Explain audible ranges and ultrasound applications
The approximate human audible range for a young healthy person is 20 Hz to 20,000 Hz. Infrasound is below 20 Hz; ultrasound is above 20,000 Hz. Hearing ranges vary with age and individual condition. Some animals detect frequencies outside the human range; “inaudible to humans” does not mean no animal can hear the sound.
Ultrasound can clean intricate parts, reveal cracks in metal and produce diagnostic images from reflections at tissue boundaries. Some applications use focused ultrasonic energy, such as breaking certain stones under specialist supervision. Echolocation uses emitted sound and returning reflections; bats and dolphins provide examples.
SONAR uses sound pulses and their reflected returns to find underwater objects or depth. Worked example. A pulse returns from a seabed after 1.6 s. With water sound speed 1500 m/s, depth = 1500 × 1.6 ÷ 2 = 1200 m. The return time includes both downward and upward travel. Applications included here explain the broader topic; teachers should align assessed scope with the prescribed course.
10. Trace the human hearing pathway
The pinna collects sound, and the auditory canal directs it to the eardrum. Pressure variations make the eardrum vibrate. The middle-ear ossicles, the hammer, anvil and stirrup, transmit and amplify these vibrations towards the inner ear. The cochlea contains fluid and sensory cells that convert mechanical stimulation into electrical signals. The auditory nerve carries these signals to the brain, where sound is interpreted.
Diagram description. Label pinna, auditory canal and eardrum in sequence from outside inward. Behind the eardrum label the three ossicles; deeper inside label the cochlea and auditory nerve. The Eustachian tube connects the middle ear towards the throat and helps equalise pressure. Do not describe the auditory nerve as carrying sound waves through air: it carries nerve signals. Avoid placing objects in the ear during any classroom activity.
A reliable approach to questions
Identify whether the problem concerns source vibration, propagation, a wave quantity, reflection or hearing.
Write the given values with SI units and convert milliseconds or kilohertz first.
Select f = 1 ÷ T, v = fλ or distance = speed × time as appropriate.
Decide whether the measured time covers one-way travel or a reflected round trip.
For explanations, connect the observed effect to particle motion, frequency, amplitude or reflection.
Check units and distinguish physical intensity from perceived loudness.
Connections that matter
Motion provides speed, distance and time relationships used in sound numericals.
Work and energy explains how a wave transfers energy without bulk transport of matter.
Biology connects mechanical vibration to sensory cells and nerve signalling in the ear.
Common mistakes
Drawing air particles travelling continuously from bell to ear; they oscillate near mean positions.
Calling a pressure wave graph transverse because it has peaks; inspect what the vertical axis represents.
Dividing thunder distance by two; only reflected round-trip timing needs halving.
Treating higher pitch as higher speed; under fixed conditions higher frequency means shorter wavelength.
Revision checklist
I can explain sound production, the medium requirement and longitudinal propagation.
I can define and calculate frequency, period, wavelength and speed.
I can distinguish pitch, loudness, intensity and quality.
I can solve echo and SONAR problems and explain reverberation.
I can trace hearing through the ear and describe ultrasound applications.