Properties of Sound: Let's Make Some Waves!
From the MTR door chime to music in your earphones, sound is everywhere. But what actually is sound?
In this chapter you will learn that sound is made by vibrations, that it needs a medium, how its speed changes in solids, liquids and gases, how it reflects as echoes, and how pitch and loudness are linked to vibrations. You will also read waveforms and investigate loudness with a phone app. How your ear picks up sound is covered in the chapter Hearing.
1. Sound Is Made by Vibrations
A vibration is a quick back-and-forth movement. All sounds are produced by vibrating objects. When the vibration stops, the sound stops.
Feel it for yourself
1. Your voice – Put your fingers on your throat and hum. You can feel your vocal cords vibrating.
2. A ruler on a desk – Hold one end on the desk and flick the other. It vibrates and goes "twang".
3. A tuning fork – Strike it and dip the prongs into water. The water splashes because the prongs vibrate fast.
How does the vibration travel?
A vibrating object pushes and pulls the air particles next to it, and they push the next ones. This makes regions where particles are squashed together (compressions) and spread apart (rarefactions).
The particles only move back and forth. It is the energy that travels as a sound wave, like a Mexican wave in a stadium: each fan only stands up and sits down, but the wave moves round.
Key takeaway: Sound is produced by vibrations and travels as a wave that passes energy from particle to particle.
2. Sound Needs a Medium
A medium is the material that sound travels through. It can be a solid, a liquid or a gas. Without particles to pass on the vibration, sound cannot travel.
The bell-in-a-jar experiment
Set-up: An electric bell rings inside a sealed glass bell jar. A vacuum pump slowly pumps the air out of the jar.
Observation: The ringing becomes fainter and fainter. You can still see the hammer hitting the bell, but you can hardly hear it. When air is let back in, the ringing becomes loud again.
Conclusion: Sound cannot travel through a vacuum (a space with no particles). Light can, so you still see the bell. A faint sound may remain, as some air is left and some vibration passes through the supports.
Silence in space
Outer space is almost a vacuum, so astronauts on a spacewalk talk by radio, as radio waves can cross a vacuum. Inside a space station there is air, so in the Tiangong Classroom lessons from China's space station, the astronauts could talk normally.
Did you know? The loud explosions in space battle movies are not realistic. A real explosion in space would be silent to a nearby spaceship!
Key takeaway: Sound needs a medium (solid, liquid or gas). It cannot travel through a vacuum, but light can.
3. How Fast Does Sound Travel?
The speed of sound depends on the medium. Sound travels fastest in solids, slower in liquids and slowest in gases. Particles in solids are close together and strongly linked, so they pass on vibrations quickly.
Approximate speeds of sound:
Air: about 340 m/s
Water: about 1500 m/s
Steel: about 5000 m/s
Memory aid: "Solid, Liquid, Gas – fast, medium, slow." The closer the particles, the faster the sound. Try it: put your ear on a desk while a friend taps the far end. The tap sounds clearer through the desk.
Sound is much slower than light
Light travels nearly a million times faster than sound in air. Lightning and thunder are produced at the same moment, but you see the flash first and hear the thunder later. We can estimate how far away the storm is:
\(\text{distance} = \text{speed} \times \text{time}\)
Example: You hear thunder 3 s after the flash. \(\text{distance} = 340 \times 3 = 1020\ \text{m}\), about 1 km away.
Key takeaway: Sound travels at different speeds in different media: fastest in solids, slowest in gases (about 340 m/s in air).
4. Sound Can Be Reflected: Echoes
Like light, sound bounces off surfaces. This is reflection of sound. A reflected sound heard separately from the original sound is an echo.
Hard, smooth, large surfaces such as cliffs, buildings and tunnel walls reflect sound well. Soft materials such as curtains and carpets absorb sound. That is why an empty flat sounds "echoey" until the furniture and curtains move in.
A clear echo is heard only if the surface is far enough away (roughly 17 m or more), e.g. a hillside in a Country Park. If it is too close, the echo mixes with the original sound.
Measuring distance with echoes
An echo travels to the surface and back, so the sound covers twice the distance:
\(\text{distance to surface} = \dfrac{\text{speed} \times \text{time}}{2}\)
Example: You clap and hear the echo from a cliff 0.5 s later. \(\text{distance} = \dfrac{340 \times 0.5}{2} = 85\ \text{m}\)
Animals that use echoes
Bats – Many bats hunt insects at night, in the dark. A bat sends out very high-pitched calls (mostly too high for us to hear) and listens to the echoes to tell where an insect or obstacle is and how far away. This is echolocation.
Dolphins – Dolphins send out rapid clicks underwater and listen to the echoes to find fish and find their way, even in murky water such as where Hong Kong's Chinese White Dolphins live.
Echolocation helps these animals navigate and adapt to places where their eyes are not much help.
Key takeaway: Sound reflects off hard surfaces to form echoes. Bats and dolphins use echoes to find food and their way around.
5. Pitch and Loudness of a Note
A note is a musical sound with a steady pitch, such as one piano key. Every note has a pitch and a loudness.
Pitch depends on frequency
Pitch is how high or low a sound is. It depends on the frequency of the vibration: the number of complete vibrations per second. The unit is the hertz (Hz), and \(1\ \text{Hz} = 1\) vibration per second. A tuning fork marked 256 Hz vibrates 256 times every second.
Higher frequency → higher pitch (e.g. a whistle, a bird singing).
Lower frequency → lower pitch (e.g. a big drum, a ship's horn).
Loudness depends on amplitude
Loudness is how loud or soft a sound is. It depends on the amplitude of the vibration: the largest distance the object moves from its rest position.
Larger amplitude → louder sound, because the vibration carries more energy. Hitting a drum harder gives a bigger amplitude and a louder sound, but the pitch stays the same.
Memory aid: Fast vibrations → high pitch (Frequency). Ample (big) vibrations → loud sound (Amplitude).
Musical instruments in action
Guitar strings – A shorter, tighter or thinner string vibrates faster and gives a higher pitch. Plucking harder gives a louder note.
Ruler on a desk – A shorter overhanging part vibrates faster, so the pitch is higher. Pushing the end down further gives a larger amplitude, so the sound is louder.
Drums – A tighter skin or a smaller drum gives a higher pitch. Hitting harder gives a louder sound.
Key takeaway: Pitch depends on frequency; loudness depends on amplitude. Changing one does not have to change the other.
6. Seeing Sound: Waveforms
A microphone changes sound into an electrical signal. An oscilloscope, or a sound app on a phone, draws it as a wavy line called a waveform. The horizontal axis shows time.
How to read a waveform
Count the waves: more complete waves in the same time → higher frequency → higher pitch.
Look at the height: taller waves → larger amplitude → louder sound.
Comparing four notes
Imagine four waveforms drawn on the same scales:
Note A – 4 waves, height 2 squares (our reference).
Note B – 8 waves, height 2 squares → higher pitch, same loudness as A.
Note C – 4 waves, height 4 squares → same pitch as A, but louder.
Note D – 2 waves, height 1 square → lower pitch and softer than A.
Only compare waveforms drawn with the same screen settings, like control variables in a fair test!
Key takeaway: On a waveform, more waves in the same time means a higher pitch, and taller waves mean a louder sound.
7. Investigation: How Does Loudness Change with Distance?
Aim: to find out how the loudness of a note changes with distance from the source, using mobile devices.
Apparatus: a phone playing a steady note (tone generator app), a second phone with a sound meter app, and a measuring tape. The app shows loudness in decibels (dB) (more in Sound and the environment).
Variables
Independent variable: distance from the source.
Dependent variable: loudness reading on the app.
Control variables: the same note and volume setting, the same phones and app, the same height and direction, and a quiet room.
Method
1. Measure the background noise with the source off.
2. Play the note and record the reading at 0.5 m from the source.
3. Repeat at 1.0 m, 1.5 m, 2.0 m, 2.5 m and 3.0 m.
4. Take three readings at each distance and find the average.
5. Plot a graph of loudness against distance.
Results and conclusion
The loudness reading decreases as the distance increases. It drops quickly near the source, then more slowly. This is because the sound energy spreads out over a larger and larger area as it travels, and some is absorbed by the air.
Accuracy and precision: Repeated readings that are close together are precise. Phone apps are not always calibrated, so readings may not be accurate, but they still show the trend. Work in a quiet place with few hard walls nearby to reduce echoes.
Key takeaway: The loudness of a note decreases as the distance from the source increases.
Common Mistakes
Mistake 1: "Louder sounds have a higher pitch." Wrong! Loudness depends on amplitude; pitch depends on frequency. A note can be loud and low, or soft and high.
Mistake 2: "Sound travels fastest in air." Wrong! It is slowest in gases and fastest in solids.
Mistake 3: Forgetting to divide by 2 in echo calculations. The sound goes to the surface and back.
Quick Review
1. Sound is produced by vibrations.
2. Sound needs a medium and cannot travel through a vacuum.
3. Speed of sound: solids > liquids > gases; about 340 m/s in air.
4. Reflected sound is an echo; bats and dolphins use echolocation.
5. Higher frequency (Hz) → higher pitch; larger amplitude → louder.
6. Loudness decreases with distance from the source.
Great work, sound scientist! Now you know what happens when you hear an echo.