What you'll learn
Sound is the topic in CXC CSEC Physics that applies wave theory to something every student has direct experience of. Sound is a longitudinal wave produced by a vibrating source, and almost every property it has follows from that single fact — including why it cannot travel through a vacuum, why it travels faster in solids than in gases, and why a thunderclap is heard well after the lightning is seen. The topic also covers the distinction between the physical properties of the wave and the subjective qualities we perceive, which is examined regularly and frequently muddled. By the end of this guide you should be able to explain how sound is produced and transmitted, relate pitch and loudness to frequency and amplitude, describe how the speed of sound is measured, apply the echo method to distance calculations, explain the audible range and ultrasound, and describe resonance and its uses.
Key terms and definitions
Sound wave — a longitudinal wave produced by a vibrating source and transmitted through a material medium
Compression — a region where the particles of the medium are pushed close together and pressure is higher
Rarefaction — a region where the particles are spread apart and pressure is lower
Pitch — how high or low a note sounds, determined by frequency
Loudness — how loud a sound is perceived to be, determined by amplitude
Quality or timbre — the characteristic that distinguishes the same note played on different instruments
Echo — a reflected sound heard separately from the original
Audible range — the range of frequencies a human can hear, about 20 hertz to 20,000 hertz
Ultrasound — sound with a frequency above the upper limit of human hearing
Infrasound — sound with a frequency below the lower limit of human hearing
Resonance — the large-amplitude vibration produced when a body is driven at its natural frequency
Natural frequency — the frequency at which an object vibrates freely when disturbed
Core concepts
How sound is produced and transmitted
All sound originates from a vibrating source. A loudspeaker cone moves back and forth, a guitar string vibrates, a drum skin oscillates, and vocal cords vibrate as air passes over them.
As the source moves forward it pushes the air particles in front of it closer together, creating a compression, a region of higher pressure. As it moves back it leaves the particles more spread out, creating a rarefaction, a region of lower pressure.
These alternating compressions and rarefactions travel outwards through the medium, carrying energy away from the source. Each air particle merely oscillates back and forth about its own position and does not travel with the wave.
Because the oscillations are parallel to the direction of energy transfer, sound is a longitudinal wave.
Why sound needs a medium
Sound requires particles to transmit the compressions and rarefactions, so it cannot travel through a vacuum.
The standard demonstration is the bell jar experiment. An electric bell is suspended inside a sealed glass jar and switched on, and the sound is clearly heard. As a vacuum pump removes the air, the sound becomes progressively fainter until it can no longer be heard, even though the bell's hammer can still be seen striking. Letting air back in restores the sound.
The bell must be suspended rather than resting on the base, otherwise sound travels through the solid support and the demonstration fails — a detail examiners look for.
This explains why explosions in space are silent, and why astronauts must use radio, which is an electromagnetic wave and needs no medium.
The speed of sound in different media
Sound travels fastest in solids, more slowly in liquids, and slowest in gases. In air at room temperature it travels at about 330 to 340 metres per second; in water at about 1,500 metres per second; and in steel at about 5,000 metres per second.
The explanation is that the particles in a solid are closest together and most strongly bonded, so a disturbance is passed from one particle to the next most rapidly. In a gas the particles are far apart, so the disturbance takes longer to be transmitted.
The speed of sound in air increases with temperature, because the particles move faster and transmit the disturbance more quickly. It does not depend on the loudness or the pitch of the sound.
Sound travels far more slowly than light, which travels at about 3 times 10 to the power 8 metres per second. This is why lightning is seen before the thunder is heard, and why a distant cricket ball is seen to be struck before the sound arrives.
Pitch, loudness and quality
The physical properties of the wave map onto the perceived qualities of the sound, and keeping the two sets of terms separate is essential.
Pitch is determined by frequency. A higher frequency gives a higher pitch. Tightening or shortening a guitar string raises its frequency and therefore its pitch.
Loudness is determined by amplitude. A larger amplitude carries more energy and gives a louder sound. Striking a drum harder increases the amplitude without changing the frequency, so the note is louder but not higher.
Quality, also called timbre, distinguishes the same note played on different instruments. A note of the same pitch and loudness sounds different on a steel pan and a flute because each produces a different combination of additional frequencies alongside the main one.
On a waveform displayed on an oscilloscope, a higher-pitched note shows more waves across the screen, and a louder note shows taller waves. Being asked to sketch or interpret such traces is common.
The audible range, ultrasound and infrasound
A healthy young person can hear frequencies from about 20 hertz to about 20,000 hertz. The upper limit decreases with age and with exposure to loud noise.
Sound above 20,000 hertz is ultrasound. It has important applications. In medicine, ultrasound scanning produces images of a foetus or internal organs without the ionising radiation that X-rays involve, making it safer for this purpose. In industry, it detects cracks and flaws in metal castings. At sea, echo sounding measures the depth of water beneath a ship and locates shoals of fish.
Sound below 20 hertz is infrasound. It is produced by earthquakes and volcanic activity and can be detected before some natural disasters, and some animals communicate using it.
Echoes and measuring distance
An echo is a reflection of sound from a hard surface, heard as a separate sound after the original.
The key relationship is that the sound must travel to the reflecting surface and back, so the total distance covered is twice the distance to the surface.
Therefore distance to the surface equals speed multiplied by time, all divided by two. Forgetting to divide by two is the single most common error in this topic, and it doubles the answer.
This principle underlies echo sounding for water depth, ultrasound scanning, and one classroom method of measuring the speed of sound.
Measuring the speed of sound
Two methods are examinable.
The echo method requires a person to stand a measured distance from a large flat wall, at least 50 metres away, and clap. The time between the clap and the returning echo is measured. Because the sound travels to the wall and back, the speed is twice the distance divided by the time. To improve accuracy, the clapping is repeated in time with the returning echoes and the total time for many claps is measured and divided.
The direct timing method uses two observers separated by a large measured distance. One fires a starting pistol or strikes two blocks together, and the other starts a stopwatch on seeing the flash or the movement and stops it on hearing the sound. The speed is the distance divided by the time. The light travels effectively instantaneously over such a distance, so the delay is due entirely to the sound.
In both methods, a large distance is used to make the time interval long enough to measure reliably, since human reaction time introduces an uncertainty of about 0.2 seconds. Repeating and averaging reduces this random error.
Resonance
Every object has a natural frequency at which it vibrates freely when disturbed.
Resonance occurs when an object is driven by a periodic force at its natural frequency. Energy is transferred to it very efficiently and the amplitude of vibration becomes large.
Musical instruments depend on resonance: a column of air in a pipe, or a sounding board beneath strings, resonates and amplifies the sound.
Resonance can also be destructive. Bridges have been damaged by wind or by marching soldiers driving them at their natural frequency, which is why troops break step when crossing a bridge. Machinery can vibrate dangerously if a rotating part reaches a speed matching a natural frequency of the structure.
Worked examples
Example 1: An echo calculation (4 marks)
A student stands 85 metres from a cliff and claps. The echo is heard 0.50 seconds later. Calculate the speed of sound in air.
The sound travels from the student to the cliff and back, so the total distance covered is 2 × 85 = 170 metres.
The speed is the total distance divided by the time, which is 170 ÷ 0.50 = 340 metres per second.
This is a reasonable value for the speed of sound in air, which confirms the working. Omitting the doubling would have given 170 metres per second, which is clearly too low.
Example 2: Echo sounding (4 marks)
A ship sends an ultrasound pulse vertically downwards and receives the reflection 0.12 seconds later. The speed of sound in sea water is 1,500 metres per second. Calculate the depth of the water.
The total distance travelled by the pulse is speed multiplied by time, which is 1,500 × 0.12 = 180 metres.
Because the pulse travels down to the sea bed and back up again, this total distance is twice the depth.
The depth is therefore 180 ÷ 2 = 90 metres.
Example 3: Distinguishing pitch and loudness (3 marks)
A guitarist tightens a string and then plucks it harder than before. Describe the effect on the sound produced and on the waveform seen on an oscilloscope.
Tightening the string increases its frequency, so the note produced has a higher pitch. On the oscilloscope this appears as more waves fitting across the screen, since the wavelength of the trace is shorter.
Plucking harder increases the amplitude of the vibration, so the sound is louder. On the oscilloscope this appears as taller waves, since the amplitude of the trace is greater.
The two effects are independent: frequency determines pitch and amplitude determines loudness.
Common mistakes and how to avoid them
The most frequent error in this topic is forgetting to double the distance in an echo calculation, or forgetting to halve it when finding the distance to a surface. The sound always makes a return journey.
Students often describe sound as a transverse wave. It is longitudinal, travelling as compressions and rarefactions.
Another common slip is confusing pitch with loudness, or stating that a louder sound has a higher frequency. Frequency determines pitch; amplitude determines loudness.
Many candidates say sound travels faster in air than in water because air is less dense. The opposite is true: sound travels faster where particles are closer together and more strongly bonded.
In the bell jar demonstration, answers sometimes state that the bell stops working. The bell continues to vibrate, and the hammer can still be seen moving; what is missing is a medium to transmit the sound.
Finally, candidates sometimes claim that ultrasound is dangerous like X-rays. Ultrasound is not ionising, which is precisely why it is preferred for imaging a foetus.
Exam technique for "Sound"
In every echo calculation, write down whether the distance given is one way or a round trip before substituting. A single line noting total distance is twice the depth prevents the standard error.
State units at each stage, and check that a calculated speed of sound in air comes out near 330 to 340 metres per second. An answer far from this signals a mistake.
When describing an experiment to measure the speed of sound, include the reason for the large distance and the method of reducing reaction-time error. Both are separate marks.
Keep the physical and perceived terms paired in your answer: frequency with pitch, amplitude with loudness. Examiners look for the correct pairing rather than the terms alone.
For oscilloscope questions, describe both the number of waves across the screen and their height, since these correspond to frequency and amplitude respectively.
Quick revision summary
Sound is a longitudinal wave produced by a vibrating source, travelling as compressions of higher pressure and rarefactions of lower pressure, with particles oscillating about fixed positions. It requires a medium, as the bell jar experiment shows, so it cannot travel through a vacuum. It travels fastest in solids, slower in liquids and slowest in gases, at about 330 to 340 metres per second in air, increasing with temperature but independent of loudness and pitch. Frequency determines pitch and amplitude determines loudness, while quality distinguishes the same note on different instruments. The audible range is about 20 to 20,000 hertz, with ultrasound above it used for foetal scanning, flaw detection and echo sounding, and infrasound below it produced by earthquakes. In any echo calculation the sound travels to the surface and back, so the total distance is twice the distance to the surface. The speed of sound is measured by the echo method or by direct timing over a large distance, with repeats reducing reaction-time error. Resonance occurs when an object is driven at its natural frequency, amplifying musical instruments but also endangering bridges and machinery.