Sound is a form of energy that travels as a wave produced by vibrating objects. A loudspeaker cone, a guitar string, vocal cords — all vibrate and set the air around them vibrating, carrying the sound to our ears. CSEC Physics treats sound as the everyday example of a longitudinal wave.
How sound travels
When an object vibrates it pushes on the air particles next to it, creating regions where the particles are squeezed together (compressions) and regions where they are spread apart (rarefactions). These regions move outward as a longitudinal wave — the particles vibrate back and forth parallel to the direction the sound travels, but they do not move along with it; they only pass the energy on.
Because sound needs particles to pass the vibration along, sound cannot travel through a vacuum. A ringing bell inside a jar from which the air is pumped out becomes silent — a classic demonstration. Sound travels fastest in solids, slower in liquids, and slowest in gases, because the particles are closer together in solids and pass on the vibration more readily. In air it travels at about 340 m/s.
Properties of a musical note
Two features of a sound wave relate to what we hear:
- Pitch depends on frequency — a higher frequency gives a higher-pitched note. Humans can hear roughly 20 Hz to 20,000 Hz; sound above this range is ultrasound.
- Loudness depends on amplitude — a larger amplitude gives a louder sound.
Echoes and the speed of sound
An echo is a reflection of sound from a hard surface. Echoes can be used to measure the speed of sound or distance:
speed = total distance ÷ time
If a sound takes time t to travel to a wall and back, the distance to the wall is (speed × t) ÷ 2, because the sound covers the gap twice. For example, if an echo returns in 0.4 s and the speed of sound is 340 m/s, the wall is (340 × 0.4) ÷ 2 = 68 m away.
This "send a pulse and time the echo" method is the basis of sonar (measuring sea depth and detecting objects underwater) and of ultrasound scanning in medicine.
Resonance and vibrating columns
Objects have a natural frequency at which they vibrate most easily. When a sound of that frequency is applied, the object vibrates with large amplitude — this is resonance. Musical instruments use resonance in vibrating strings (guitar, violin) and vibrating air columns (flute, organ pipe) to produce loud, clear notes. Shortening a string or air column raises the frequency and the pitch.
Uses of ultrasound
Ultrasound (frequency above 20,000 Hz) has several uses you should know:
- Medical scanning, including imaging an unborn baby — it is safer than X-rays because it is not ionising;
- cleaning delicate equipment and jewellery;
- sonar for measuring depth and detecting shoals of fish or submarines;
- detecting flaws in metals.
Measuring the speed of sound
A classic experiment measures the speed of sound in air using an echo. A person stands a measured distance (say 100 m) from a large wall and claps; a helper times the gap between the clap and the echo returning. Because the sound travels to the wall and back, the total distance is 200 m. If the echo returns in 0.6 s, the speed is 200 ÷ 0.6 ≈ 333 m/s, close to the accepted 340 m/s. Repeating and averaging reduces the large reaction-time error. An alternative method times the gap between seeing a distant event (such as a hammer strike) and hearing it, since light arrives effectively instantly.
Displaying sound on an oscilloscope
A cathode-ray oscilloscope (CRO) connected to a microphone turns a sound into a visible waveform on a screen, and CSEC questions often show such traces. From the trace you can compare sounds:
- a louder sound has a taller wave (greater amplitude);
- a higher-pitched sound has waves that are closer together (higher frequency / shorter period);
- the same note from different instruments has a different wave shape, which is why a guitar and a flute playing the same note sound different (this is called quality or timbre).
Being able to read amplitude and frequency from an oscilloscope trace — and link them to loudness and pitch — is a reliable source of marks and ties this topic back to the general properties of waves.
Common exam mistakes
- Saying sound is a transverse wave — sound is longitudinal (compressions and rarefactions).
- Claiming sound can travel through a vacuum — it cannot; it needs a medium.
- Forgetting to halve the distance in echo problems (the sound travels there and back).
- Mixing up pitch (frequency) and loudness (amplitude).
The range of hearing and noise
Humans can hear sounds roughly between 20 Hz and 20,000 Hz; this range shrinks with age, so older people often cannot hear the highest frequencies. Sound below 20 Hz is infrasound and above 20,000 Hz is ultrasound — both inaudible to us, though some animals (dogs, bats, dolphins) can hear or use them. Very loud sound is a form of noise pollution that can damage hearing permanently if the ear is exposed for long periods; this is why ear protection is worn near machinery and aircraft. Loudness is often described in decibels (dB), a scale on which prolonged exposure above about 85 dB is considered harmful. Knowing the audible range and the idea of noise pollution rounds out the topic and links it to health and the environment.
Key terms to remember
- Sound — energy produced by vibrations, travelling as a longitudinal wave.
- Compression / rarefaction — regions where particles are squeezed together / spread apart.
- Pitch — how high or low a note is; depends on frequency.
- Loudness — how loud a sound is; depends on amplitude.
- Echo — a reflection of sound from a hard surface.
- Ultrasound — sound above 20,000 Hz; used in scanning, cleaning and sonar.
- Resonance — large-amplitude vibration when a sound matches an object's natural frequency.
- Audible range — about 20 Hz to 20,000 Hz for humans.
Quick recap
- Sound is produced by vibrations and travels as a longitudinal wave of compressions and rarefactions.
- It needs a medium — it cannot travel through a vacuum — and travels fastest in solids.
- Pitch ↔ frequency, loudness ↔ amplitude; the audible range is about 20 Hz–20 kHz.
- Echoes are reflected sound; remember to halve the distance in calculations.
- Ultrasound is used in medical scans, cleaning, sonar and flaw detection.