0625 · 3.4Sound

Sound as a longitudinal wave

0:00A shout near a cliff can return as an echo. The sound travels away from you, reaches the rock face, and comes back. That returning sound carries information about how far away the cliff is. By the end of this video, you will understand how sound travels, what controls its pitch and loudness, and how echoes reveal hidden objects. We need one starting idea about waves. A wave transfers energy from one place to another without transferring matter overall.

0:25For sound, particles vibrate around their usual positions while the disturbance travels onwards. Keep the movement of the particles separate from the movement of the wave, because they are not the same thing. Sound is produced by vibrating sources. A guitar string moves backwards and forwards, a drum skin vibrates, and a loudspeaker cone moves repeatedly. These vibrating objects disturb the material surrounding them. In air, that means they make nearby air particles move, passing energy from the source into the surrounding air.

0:55Imagine a loudspeaker cone moving outwards. It pushes nearby air particles together, creating a region of increased pressure. When the cone moves inwards, the nearby particles become more spread out. Repeated vibrations produce a travelling pattern of crowded and spread-out regions, which eventually reaches your ear and makes your eardrum vibrate. The names for these regions are part of the Supplement content for Extended students. A compression is a region where particles are closer together, with higher pressure than the undisturbed air.

1:24A rarefaction is a region where particles are further apart, with lower pressure. Compressions and rarefactions alternate as the sound wave travels through the medium. Sound waves are longitudinal waves. This means that particle vibrations are parallel to the direction in which the wave travels. If sound travels horizontally to the right, the particles vibrate horizontally backwards and forwards. They do not travel all the way from the loudspeaker to your ear.

1:49The disturbance passes through them instead. A useful comparison is a line of people passing a gentle push along. Each person moves slightly and affects the next person, but nobody walks along the whole line. Air particles similarly pass a disturbance onwards through their interactions. The comparison is only a model of energy transfer. Real air particles simply vibrate and collide with their neighbours. Sound needs a medium to travel through.

2:15A medium is the material carrying the wave, such as a gas, liquid or solid. In a vacuum, there are no particles to pass on the vibrations. A vibrating source can therefore continue moving without transmitting sound across an empty space.

Speed of sound measurement

2:29The bell jar demonstration makes this idea visible. An electric bell rings inside a sealed jar while a pump removes the air. As the air is removed, the sound becomes quieter even though the bell mechanism still moves. Any faint sound still heard travels through the supports, not through the vacuum inside the jar. The next comparison is also Supplement content for Extended students. In general, sound travels faster in solids than in liquids, and faster in liquids than in gases.

2:57The particles interact differently in these materials, so disturbances are passed on at different speeds. Strong interactions in many solids allow vibrations to pass through them particularly quickly. Do not explain this pattern using particle spacing alone. How strongly the particles interact also matters, so density alone does not determine sound speed. For this syllabus, remember the general order of solids, then liquids, then gases. In air, the approximate speed of sound is three hundred and thirty to three hundred and fifty metres per second.

3:28We can measure the speed of sound using distance and time. Place one student with two wooden blocks a measured distance from an observer. The student strikes the blocks together, creating a visible movement and a sharp sound. The observer times the delay between seeing the blocks meet and hearing the sound. Light travels much faster than sound, so its travel time is negligible in this experiment. The measured delay is therefore approximately the time taken by sound to cross the separation.

3:55Use a large, clear distance so that the delay is easier to measure. Repeat the measurement and calculate a mean time to reduce the effect of random variation. Human reaction time is an important limitation of this method. Starting and stopping a stopwatch introduces uncertainty, especially when the sound takes only a short time to arrive. A longer distance makes that uncertainty a smaller fraction of the measured time. Repeating measurements helps, but it does not make reaction-time uncertainty disappear completely.

4:23Suppose the measured distance is one hundred and seventy metres, and the mean delay is zero point five zero seconds. Speed equals distance divided by time, because speed tells us the distance travelled each second. Divide one hundred and seventy by zero point five zero. The calculated speed is three hundred and forty metres per second, within the expected range for sound in air.

Loudness, pitch and echoes

4:45Students often mix up three properties. These are speed, loudness and pitch. Speed describes how quickly the disturbance travels through the medium. Loudness describes how loud or quiet the sound seems, while pitch describes how high or low it sounds. A higher-pitched sound does not automatically travel faster through the same air. Amplitude is the maximum displacement from the equilibrium position. The equilibrium position is where a particle would be without the vibration.

5:11A larger amplitude means larger vibrations and a louder sound, when other conditions remain the same. Smaller vibrations produce a quieter sound, so amplitude affects loudness rather than pitch. Frequency is the number of complete vibrations per second. Its unit is the hertz, so a frequency of one hundred hertz means one hundred complete vibrations each second. Higher frequency produces higher pitch, while lower frequency produces lower pitch. A guitar string vibrating more rapidly therefore produces a higher note.

5:43Striking a drum harder usually makes its vibrations larger, so the sound becomes louder. That does not necessarily make the drum vibrate more frequently. Similarly, a sound can be quiet and high-pitched, or loud and low-pitched. Always connect amplitude with loudness and frequency with pitch, rather than treating loudness and pitch as the same property. The approximate range of frequencies audible to humans is twenty hertz to twenty thousand hertz.

6:08Twenty thousand hertz is also called twenty kilohertz. This is an approximate range, not a guarantee that everyone hears every frequency within it. Hearing varies between people, and the upper limit commonly decreases with age. A sound wave may be represented by a wavy graph, but that does not make sound transverse. The graph can show how particle displacement changes with time. Its up-and-down shape is a way of displaying measurements, not the direction of particle movement through the air.

6:34In the air itself, the particles vibrate parallel to the wave’s direction. An echo is the reflection of sound waves. Sound reaches a surface, such as a wall or cliff, and some of it returns towards the source. If the returning sound arrives sufficiently later, you hear it separately from the original sound. The delay contains information about the distance travelled, which makes echoes useful for measurement.

Ultrasound and echo applications

6:57Ultrasound is sound with a frequency higher than twenty kilohertz. It is beyond the approximate upper limit of human hearing, but it still consists of mechanical vibrations travelling through a medium. Its applications are Supplement content for Extended students. We will use reflected ultrasound to understand sonar, medical scanning and non-destructive testing. Sonar uses sound pulses and their echoes to locate objects or measure depth. A device on a boat sends a pulse down through the water.

7:25The pulse reflects from the seabed and returns to the device. Measuring the time between sending and receiving the pulse gives the time for the complete outward and return journey. Distance travelled equals speed multiplied by time. However, that calculation gives both parts of the journey when the measured time is an echo time. The depth is only the downward distance, so we divide the total distance by two. Depth therefore equals sound speed multiplied by echo time, divided by two.

7:52Suppose ultrasound travels through seawater at one thousand five hundred metres per second, and the echo returns after zero point zero eight zero seconds. Multiply one thousand five hundred by zero point zero eight zero to find the total distance travelled. That gives one hundred and twenty metres for the outward and return journey. Divide by two, giving a water depth of sixty metres. The same reasoning applies when sonar measures the distance to an underwater object.

8:19Use the sound speed in the actual medium, not automatically the speed in air. Also check whether the stated time is one-way travel time or echo time. Only an outward-and-return time requires the distance to be divided by two. In non-destructive testing, ultrasound checks materials without cutting them open or damaging them. A probe sends ultrasound pulses into a metal block, for example. A crack inside the block can reflect a pulse before it reaches the far surface.

8:45An unexpectedly early echo therefore provides evidence of an internal defect. In medical scanning, a probe sends ultrasound pulses into soft tissue. Some ultrasound reflects at boundaries between different tissues and returns to the probe. The return times help locate those boundaries, and the collected echoes build an image. This allows internal soft-tissue structures, including a developing fetus, to be examined without cutting into the body.

Exam question and recap

9:10Here is an exam-style question linking several of these ideas. Describe the particle motion in a sound wave travelling through air, and describe a compression. Then state what happens to loudness when amplitude increases. Finally, explain why an echo time cannot be used directly to calculate the one-way distance. Air particles vibrate backwards and forwards parallel to the direction of wave travel. A compression is a region where particles are closer together and pressure is higher than in undisturbed air.

9:39Increasing amplitude increases loudness. An echo time includes the outward and return journeys, so speed multiplied by that time gives twice the one-way distance. Let us bring the ideas together. Vibrating sources produce sound, and a medium passes the vibrations onwards as a longitudinal wave. Compressions and rarefactions travel through the material while individual particles vibrate around their equilibrium positions. Amplitude controls loudness, frequency controls pitch, and the medium affects the speed.

10:08Human hearing extends approximately from twenty hertz to twenty thousand hertz, while ultrasound has a frequency above twenty kilohertz. Sound in air travels at about three hundred and thirty to three hundred and fifty metres per second. Echoes are reflected sound waves, and their travel times reveal distances. Ultrasound uses this principle in sonar, soft-tissue scanning and non-destructive testing.