3.4 Sound
- Syllabus
- 0625–2026–2027
- Topic
- 3.4
- Level
- —
Learning objectives
3.4.1Production of sound by vibrating• Describe the production of sound by vibrating sources3.4.2Longitudinal nature of sound waves• Describe the longitudinal nature of sound waves3.4.3Approximate range of frequencies• State the approximate range of frequencies audible to humans as 20 Hz to 20 000 Hz3.4.4A medium is needed to transmit sound• Know: a medium is needed to transmit sound waves3.4.5Speed of sound in air is approximately• Know: the speed of sound in air is approximately 330–350 m/s3.4.6Method involving a measurement of• Describe a method involving a measurement of distance and time for determining the speed of sound in air3.4.7Changes in amplitude and frequency• Describe how changes in amplitude and frequency affect the loudness and pitch of sound waves3.4.8An echo as the reflection of sound• Describe an echo as the reflection of sound waves3.4.9Ultrasound as sound with a frequency• Define ultrasound as sound with a frequency higher than 20 kHz3.4.10Compression and rarefaction• Describe compression and rarefaction3.4.11That, in general, sound travels faster• Know that, in general, sound travels faster in solids than in liquids and faster in liquids than in gases3.4.12Uses of ultrasound in non- destructive• Describe the uses of ultrasound in non- destructive testing of materials, medical scanning of soft tissue and sonar including calculation of depth or distance from time and wave speed
Explain how a vibrating source produces sound
A sound starts when a source vibrates: it moves repeatedly backwards and forwards about an equilibrium position.
| Stage | What happens |
|---|---|
| source | an object such as a loudspeaker cone, tuning fork or vocal cord vibrates |
| nearby medium | the source pushes and pulls neighbouring particles |
| outward transfer | each disturbed region affects the next, so the sound disturbance travels away |
Touching a sounding tuning fork lightly to water can reveal its vibration by making splashes. Stopping the fork from vibrating stops the sound it produces.
The source must vibrate, but the source itself does not travel to the listener. It transfers a disturbance into the surrounding medium.
Describe why sound waves are longitudinal
A sound wave is longitudinal: particles of the medium vibrate parallel to the direction in which the wave and its energy travel.
| Direction | In a sound wave |
|---|---|
| wave travel | from the source towards the listener |
| particle vibration | backwards and forwards along the same line |
A particle oscillates about its own equilibrium position as the disturbance passes. It transfers energy to neighbouring particles rather than being carried from source to listener.
A graph of sound may be drawn with peaks and troughs, but that drawing does not make sound transverse. Classify the wave by the direction of particle vibration relative to wave travel.
Use the human audible-frequency range
A healthy human with normal hearing can hear approximately 20 Hz to 20 000 Hz. The upper value is also written as 20 kHz.
| Frequency | Relative to the approximate human range |
|---|---|
| below 20 Hz | below the audible range |
| 20 Hz to 20 000 Hz | audible range |
| above 20 000 Hz | above the audible range |
Convert before comparing: 1 kHz = 1000 Hz, so 15 kHz = 15 000 Hz and lies inside the approximate range.
The limits are approximate and vary between people and with age. A sound outside the human range may still be detected by another animal or by equipment.
Explain why sound needs a medium
Sound is a mechanical wave, so it needs a material medium to transmit its disturbance.
| Region | Can sound be transmitted? | Reason |
|---|---|---|
| solid, liquid or gas | yes | particles can interact and pass on the vibration |
| vacuum | no | there are no particles to pass on the vibration |
An astronaut outside a spacecraft cannot hear machinery through the vacuum between them, even if the machinery is loud and its frequency is in the human audible range.
A vacuum does not merely make sound quieter: it prevents sound transmission. Electromagnetic waves such as light can cross a vacuum, but sound cannot.
Use the approximate speed of sound in air
The speed of sound in air is approximately 330–350 m/s; 340 m/s is a useful typical value.
| Statement | Meaning |
|---|---|
| 340 m/s | sound travels about 340 m through air in 1 s |
| 0.34 km/s | the same typical speed expressed in kilometres per second |
For one-way travel, use speed = distance ÷ time. A sound that travels 680 m through air in 2.0 s has speed 680 ÷ 2.0 = 340 m/s.
Do not use 3.0 × 10^8 m/s: that is the speed of electromagnetic waves in a vacuum. The sound value is approximate, so a value within 330–350 m/s may be appropriate.
Measure the speed of sound using distance and time
Choose a long, measured distance between a sound source and one observer. A starting pistol is useful because its puff of smoke and bang are produced together.
| Step | Measurement action |
|---|---|
| 1 | measure the source-to-observer distance with a long tape or trundle wheel |
| 2 | the observer starts the stopwatch on seeing the smoke |
| 3 | the same observer stops the stopwatch on hearing the bang |
| 4 | calculate speed = measured distance ÷ measured time |
Use a long distance so the sound delay is much larger than the timing resolution. Repeat the measurement and average the times; keep the source and observer positions fixed.
The light-travel time is negligible over this distance, so the smoke gives the start time. Do not double the measured distance in this direct one-way method; doubling applies to an echo that travels out and back.
Relate amplitude and frequency to loudness and pitch
Amplitude controls loudness, while frequency controls pitch.
| Wave change | Heard effect |
|---|---|
| greater amplitude | louder sound |
| smaller amplitude | quieter sound |
| greater frequency | higher pitch |
| smaller frequency | lower pitch |
The two properties can change independently: a note can become louder at the same pitch, or higher-pitched at the same loudness.
Do not use amplitude to rank pitch or frequency to rank loudness. A high-pitched sound is not necessarily loud, and a loud sound is not necessarily high-pitched.
Describe an echo as reflected sound
An echo is a sound heard after a sound wave reflects from a surface and returns to the listener or detector.
| Stage | Path |
|---|---|
| outward | source → reflecting surface |
| return | reflecting surface → receiver |
| measured delay | time for both outward and return journeys |
If the receiver is beside the source, distance to the reflector = speed × echo time ÷ 2. At 330 m/s, an echo after 0.60 s comes from a surface about 99 m away.
Reflection changes the direction of the sound path. The measured echo time is round-trip time, so using speed × time without dividing by 2 gives twice the distance to the surface.
Define ultrasound using the 20 kHz boundary
Ultrasound is sound with a frequency higher than 20 kHz, which is higher than 20 000 Hz.
| Sound | Frequency condition | Human with normal hearing |
|---|---|---|
| audible sound | approximately 20 Hz to 20 kHz | normally audible |
| ultrasound | greater than 20 kHz | not audible |
Ultrasound is still a sound wave: it is longitudinal and needs a material medium. Its name describes frequency, not a different type of wave motion.
The definition is frequency greater than 20 kHz, not great loudness, small amplitude or high speed. A value exactly at 20 kHz is the approximate upper hearing boundary, not higher than it.
Describe compressions and rarefactions
A longitudinal sound wave consists of alternating compressions and rarefactions in the medium.
| Region | Particle spacing | Density and pressure |
|---|---|---|
| compression | particles closer together than normal | higher density and pressure |
| rarefaction | particles farther apart than normal | lower density and pressure |
One wavelength is the distance from the centre of one compression to the centre of the next compression, or from one rarefaction centre to the next.
Compressions are not individual particles moving all the way through the medium. The pattern travels while each particle vibrates backwards and forwards about its own equilibrium position.
Compare the speed of sound in solids, liquids and gases
In general, sound travels fastest in solids, more slowly in liquids and slowest in gases.
| State | General relative speed | Typical scale |
|---|---|---|
| gas | slowest | air about 340 m/s |
| liquid | faster | water about 1500 m/s |
| solid | fastest | many solids: several thousand m/s |
For equal path lengths, the sound travelling through the solid generally arrives first, followed by the liquid path and then the gas path.
This is a general order, not a claim that every solid has one fixed speed or that density alone determines speed. Always use a supplied material value when a calculation provides one.
Explain ultrasound uses and calculate echo distance
Ultrasound pulses can travel through a material and reflect at a boundary. The returning echo reveals where a boundary, flaw or object is located.
| Use | How ultrasound provides information |
|---|---|
| non-destructive testing | echoes from cracks, bubbles or internal boundaries locate defects without cutting open the material |
| medical scanning of soft tissue | different tissue boundaries return echoes that are processed into an image |
| sonar | echoes from the seabed or an underwater object give depth or distance |
For a pulse that returns to its source, depth or distance = wave speed × total echo time ÷ 2. In seawater at 1500 m/s, a return time of 0.080 s gives 1500 × 0.080 ÷ 2 = 60 m.
Use the wave speed in the actual medium and convert time to seconds. The factor of 2 is required because the measured time includes the outward and return journeys.