Sound
What is Sound?
Sound is a form of energy that travels as a mechanical wave — it requires particles to vibrate in order to propagate. Unlike electromagnetic waves (light, radio waves), sound cannot travel through a vacuum. This is why space is silent.
Sound is produced when an object vibrates, causing the surrounding medium (air, water, or solid) to vibrate in turn, passing the disturbance outward as a wave.
Sound as a Longitudinal Wave
Sound is a longitudinal wave — the particles vibrate back and forth parallel to the direction the wave travels. This creates alternating regions of:
- Compressions — regions where particles are pushed together (higher pressure)
- Rarefactions — regions where particles are spread apart (lower pressure)
This is different from transverse waves (like light or water surface waves) where particles vibrate at right angles to the wave direction.
Properties of Sound Waves
Frequency:
- Number of complete waves per second, measured in Hertz (Hz)
- Determines pitch: high frequency = high pitch; low frequency = low pitch
- Human hearing range: approximately 20 Hz to 20,000 Hz
- Below 20 Hz = infrasound (felt rather than heard; elephants communicate this way)
- Above 20,000 Hz = ultrasound (cannot be heard by humans)
Amplitude:
- The maximum displacement of particles from their rest position
- Determines loudness: large amplitude = loud; small amplitude = quiet
- Loudness is measured in decibels (dB)
- Normal conversation: ~60 dB; pain threshold: ~130 dB
Wavelength:
- Distance from one compression to the next (or one rarefaction to the next)
- Related to frequency: higher frequency = shorter wavelength (at a given speed)
Wave speed:
- Determined by the medium, not the source
- The wave equation: v = f x lambda (speed = frequency x wavelength)
Speed of Sound
Sound travels at different speeds depending on the medium:
| Medium | Approximate speed |
|---|---|
| Air (20°C) | 340 m/s |
| Water | 1,500 m/s |
| Steel | 5,000 m/s |
| Diamond | 12,000 m/s |
Why sound travels faster in denser, more rigid materials: particles are closer together and more strongly connected, so vibrations pass between them more efficiently. Sound travels fastest in solids, then liquids, then gases.
Sound speed also increases with temperature — warmer air has more energetic particles that transfer vibrations faster.
The speed of sound in air (approximately 340 m/s) is much slower than the speed of light (300,000,000 m/s). This is why you see lightning before you hear thunder — you can estimate the distance by counting seconds after the flash and dividing by 3 (to get km).
Worked Example — Wave Equation
A sound wave has a frequency of 500 Hz and travels at 340 m/s in air. Find its wavelength.
v = f x lambda → lambda = v / f = 340 / 500 = 0.68 m
Now find the frequency of a sound wave with wavelength 0.02 m travelling at 340 m/s:
f = v / lambda = 340 / 0.02 = 17,000 Hz (within human hearing range)
Reflection of Sound — Echoes
When sound waves strike a hard surface, they are reflected. A reflected sound is called an echo. For an echo to be heard separately from the original sound, the reflecting surface must be at least 17 m away (the minimum distance for the echo to return after the original sound has ended).
Reverberation occurs in enclosed spaces where multiple reflections create a prolonged sound — desirable in concert halls but can reduce speech clarity.
Ultrasound and Its Applications
Ultrasound — sound with frequency above 20,000 Hz — is used because it can be directed as a precise beam and reflects off boundaries between materials:
- Medical imaging: ultrasound scans (prenatal scans, soft tissue imaging) — safe alternative to X-rays; reflections from different tissue boundaries are processed into images
- Sonar (Sound Navigation And Ranging): ships and submarines emit ultrasound pulses to measure ocean depth or detect objects; time for echo to return gives distance
- Echolocation: bats and dolphins emit ultrasound and interpret echoes to navigate and locate prey — extraordinary biological sonar
- Industrial cleaning: high-frequency vibrations in liquid remove dirt from delicate instruments (watches, medical equipment)
- Non-destructive testing: ultrasound detects cracks or flaws inside materials without damaging them
The Human Ear
Sound waves travel through the ear canal and cause the eardrum (tympanic membrane) to vibrate. This mechanical vibration is amplified by three tiny bones:
- Hammer (malleus)
- Anvil (incus)
- Stirrup (stapes)
The stirrup transmits vibrations to the fluid-filled cochlea, where specialised hair cells convert mechanical vibrations into electrical signals sent to the brain via the auditory nerve.
High frequency sounds stimulate hair cells near the base of the cochlea; low frequency sounds stimulate cells near the apex.
Prolonged exposure to loud sounds damages or destroys hair cells — this damage is permanent and causes hearing loss.
Key Terms
- Longitudinal wave — wave where particle vibration is parallel to wave travel direction
- Compression — region of high particle density in a longitudinal wave
- Rarefaction — region of low particle density in a longitudinal wave
- Frequency — number of waves per second (Hz); determines pitch
- Amplitude — maximum particle displacement; determines loudness
- Decibel (dB) — unit of loudness
- Ultrasound — sound with frequency above 20,000 Hz
- Echo — reflected sound wave
Common Mistakes
- Saying sound travels fastest in air — it travels fastest in solids
- Confusing frequency (pitch) with amplitude (loudness)
- Thinking sound travels through space — sound cannot travel through a vacuum
- Saying ultrasound is "louder" — it has a higher frequency, not necessarily greater amplitude
Tips and Tricks
- Pitch = Frequency; Loudness = Amplitude — remember P-F and L-A
- Sound needs a medium: solid > liquid > gas for speed (denser/more rigid = faster)
- Wave equation triangle: v = f x lambda; cover the unknown to get the formula
- Thunder distance: count seconds after lightning flash, divide by 3 to get approximate distance in km