Sound Waves and Vibrations
In 2013, NASA sensors detected infrasound waves from the Chelyabinsk meteor explosion at just 0.01 Hz — 2,000 times below what human ears can detect.
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Q1 · Why can you feel the bass from a loudspeaker but not the high notes, even when both are equally loud?
Q2 · If an alarm clock rings inside a glass jar and all the air is slowly removed, what do you predict will happen to the sound?
● Know
- Sound is produced by vibrations.
- Sound travels as a longitudinal wave through a medium.
● Understand
- Sound requires a medium because it relies on particle collisions to transfer energy.
● Can do
- Explain why sound cannot travel through a vacuum and identify the medium in different sound examples.
Strike a tuning fork and hold it an centimetre from your ear: you hear a clear tone, yet nothing visible is moving from the fork to your ear. What is actually happening is that the vibrating prongs push air molecules forward then pull them back 440 times per second, creating a series of high-pressure bunches (compressions) and low-pressure gaps (rarefactions) that travel outward at about 340 m/s. That travelling pattern of pressure changes is sound — a longitudinal pressure wave.
Because sound is a mechanical wave, it needs a medium. In a vacuum, there are no particles to compress and rarefy, so sound cannot travel at all. This is why space is silent despite the violent processes occurring in stars and galaxies. It is also why an alarm clock in a sealed vacuum chamber becomes inaudible even though the bell is still ringing.
The frequency of the sound wave determines the pitch you hear. The amplitude determines the loudness. A high-frequency sound wave has compressions and rarefactions close together; a low-frequency wave has them far apart. Your ear detects these pressure variations and your brain interprets them as sound.
When a bass guitar plays a low E note at about 40 Hz, the speaker cone moves back and forth 40 times per second. Each forward push creates a compression; each backward pull creates a rarefaction. These pressure waves travel through the air, reach your eardrum, and cause it to vibrate at 40 Hz. Your brain interprets this as a deep, low note.
Australian recording studios: Studios in Melbourne and Sydney use soundproof rooms with thick walls and air gaps to isolate performers. Because sound needs a medium, removing air or using dense barriers reduces transmission. Understanding sound as a pressure wave helps engineers design spaces where artists can record cleanly.
Sound can travel through space if it is loud enough. No amount of loudness can make sound travel through a vacuum. Loudness is amplitude, and amplitude still requires particles to push. Even the loudest explosion in space is completely silent because there is no medium to carry the pressure wave.
Sound cannot travel through a vacuum. If you rang a bell inside a sealed glass jar and slowly removed all the air, what would happen to the sound?
How close was your prediction?
Nice calibration — your intuition is good for this kind of problem.
Good — being surprised is the point. This answer is worth remembering.
All sounds begin with vibration. When a guitar string is plucked, it vibrates back and forth. When a drum is struck, the skin vibrates. When you speak, your vocal cords vibrate. These vibrations disturb the surrounding medium and create sound waves.
The vibrating object pushes nearby air molecules together, creating a compression. As the object moves back, it leaves a region of lower pressure, a rarefaction. This push-and-pull pattern propagates outward in all directions. When it reaches your ear, the pressure variations push and pull on your eardrum, which vibrates in sympathy with the original source.
The properties of the sound wave mirror the properties of the vibration. A fast vibration produces high frequency (high pitch). A large vibration produces high amplitude (loud sound). A complex vibration, like that of a violin string, produces a complex wave with many frequencies combined, giving the instrument its characteristic timbre.
Place a vibrating tuning fork near a small suspended ball of paper. The ball will be pushed away repeatedly by the compressions from the fork. This demonstrates that the fork is doing real mechanical work on the air. The sound wave is not abstract; it is a physical force that can move objects.
Didgeridoo acoustics: The didgeridoo, played by Indigenous Australians for thousands of years, produces sound through lip vibration creating standing waves inside a hollow tube. The length of the tube determines the fundamental frequency. Traditional makers understood wave physics intuitively long before it was formalised in equations.
Sound is just a sensation in your head, not a real physical wave. Sound is very real. It is a pressure wave that can shatter glass, trigger avalanches, and cause permanent hearing damage. The fact that your brain interprets it subjectively does not make it any less physical than light or heat.
Pitch is the perceived highness or lowness of a sound, and it is determined entirely by frequency. A high-frequency sound wave has many compressions passing your ear each second, and your brain interprets this as a high-pitched note. A low-frequency wave has few compressions per second, and you hear it as a low-pitched note.
The human hearing range is approximately 20 Hz to 20,000 Hz. Below 20 Hz is infrasound, which you cannot hear but can sometimes feel as vibrations. Above 20,000 Hz is ultrasound, which is also inaudible but is used in medical imaging and animal communication.
It is important to distinguish pitch from loudness. Pitch is frequency; loudness is amplitude. A whispered high note has high frequency but low amplitude. A shouted low note has low frequency but high amplitude. These two properties are completely independent.
A piano keyboard spans about 27 Hz (lowest A) to 4186 Hz (highest C). When you press a key on the right side, a small hammer strikes a short, thin string that vibrates thousands of times per second. When you press a key on the left, a hammer strikes a long, thick string that vibrates slowly. The physical difference in the strings creates the frequency difference you hear as pitch.
Australian wildlife communication: Many Australian birds, such as lyrebirds, produce an extraordinary range of frequencies in their calls. Some calls exceed 10 kHz, well within human hearing. Scientists analyse these frequencies to identify species, track populations and understand how birds communicate across noisy forest environments.
Fast vibrations always mean loud sounds. Speed of vibration is frequency, which controls pitch. Loudness is controlled by how far the object vibrates, which is amplitude. A mosquito wing beats at hundreds of hertz (high pitch) but moves only a tiny distance (quiet). A bass drum vibrates slowly (low pitch) but moves a large distance (loud).
frequency sounds are heard as notes and can be felt as vibrations. frequency sounds are heard as notes and carry more detail.
Beyond the basic properties of sound, several specialised terms help us describe specific phenomena. Infrasound refers to sound waves with frequencies below the human hearing threshold of about 20 Hz. You cannot hear infrasound, but you can feel it as pressure or vibration. Elephants and some whales use infrasound to communicate over distances of many kilometres.
Ultrasound is the opposite: sound with frequencies above about 20,000 Hz, also beyond human hearing. Bats and dolphins use ultrasound for echolocation. Medical ultrasound sends high-frequency pulses into the body and analyses the reflections to create images of organs and unborn babies.
Decibels (dB) measure sound intensity on a logarithmic scale. Every 10 dB increase represents a tenfold increase in intensity. Resonance occurs when an object naturally vibrates at the same frequency as an incoming sound, causing dramatic amplification. Opera singers can shatter wine glasses by singing at the glass resonant frequency.
A dog whistle produces ultrasound at about 25,000 Hz. Humans hear nothing, but dogs prick up their ears because their hearing extends to about 45,000 Hz. The same principle applies to pest deterrents that emit high-frequency sounds unpleasant to rodents but inaudible to humans.
Australian ultrasound technology: Australian companies develop portable ultrasound devices used in remote communities and emergency services. These devices send ultrasound pulses into the body and process the returning echoes into images. The technology relies entirely on the longitudinal wave properties of sound and its reflection at tissue boundaries.
Ultrasound is a completely different kind of wave from normal sound. Ultrasound is just sound with a frequency above human hearing. It is still a longitudinal mechanical wave. It still needs a medium. It still obeys v = fλ. The only difference is that human ears cannot detect it.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
The human ear is a remarkable detector, but it has limits. We can generally hear sounds with frequencies between about 20 Hz and 20,000 Hz. This range varies between individuals and shrinks with age, particularly at the high-frequency end. Sounds outside this range are not silent; they are simply inaudible to us.
Many animals exceed human capabilities. Dogs hear up to about 45,000 Hz. Bats and dolphins extend well above 100,000 Hz. Elephants and whales communicate using infrasound below 20 Hz. These different ranges reflect different evolutionary needs: predators need precise localisation; prey need early warning; social animals need long-distance communication.
Sound intensity, measured in decibels, determines whether a sound is audible even when it is within the frequency range. A 30 dB whisper at 1000 Hz is easily heard; a 30 dB tone at 50 Hz may be inaudible because our ears are less sensitive at very low frequencies. Both frequency and intensity matter for perception.
As people age, they typically lose sensitivity to frequencies above about 15,000 Hz first. This is called presbycusis. A teenager might hear a 17,000 Hz mosquito tone that their parents cannot. This is not because the sound does not exist, but because the adult cochlea has lost the hair cells that detect those high frequencies.
Australian hearing conservation: Australian workplaces are required to protect employees from noise-induced hearing loss. Safe Work Australia mandates hearing tests for workers exposed to noise above 85 dB. Understanding the relationship between frequency, intensity and hearing damage helps design effective ear protection for construction workers, musicians and factory workers.
If I cannot hear a sound, it does not exist. Many sounds exist that humans cannot hear. Your dog responds to whistles you cannot hear. Your phone charger emits high-frequency electronic noise you cannot detect. Infrasound from wind turbines is measurable with instruments even when residents claim they cannot hear it. Existence and audibility are not the same thing.
- Infrasound
- Ultrasound
- Pitch
- Loudness
- Compression
- Rarefaction
- Below 20 Hz
- Spread-out region in sound wave
- Above 20 kHz
- Squeezed region in sound wave
- Perceived amplitude
- Perceived frequency
The reason you can feel bass but not treble lies in the physics of low-frequency sound waves. Bass frequencies (roughly 20-200 Hz) have long wavelengths — from about 17 metres down to 1.7 metres in air. These long wavelengths mean each compression involves a large mass of air moving together, transferring significant mechanical energy to surfaces, walls, floors and your body.
High frequencies (2000-20,000 Hz) have short wavelengths — from 17 cm down to 1.7 cm. Each compression involves only a small mass of air. While your sensitive eardrum can detect these tiny pressure variations, your body as a whole does not respond to them. Short wavelengths are also more directional, beaming forward like light rather than spreading and flooding a room.
This is why subwoofers are large and often placed on the floor: they need to move lots of air to create those long wavelengths, and they use the floor and walls to couple that energy into the room. Tweeters are small and placed at ear level: they need only tiny movements to create short wavelengths, and they need to aim those wavelengths precisely at the listener.
At a live music concert, you might feel your chest vibrate during the bass solo even if you are 50 metres from the stage. The long wavelengths from the subwoofers wrap around obstacles and travel through the air with little absorption. By contrast, the high-hat cymbal, playing at 10,000 Hz, seems to come from a specific point on stage. Its short wavelengths beam forward and are absorbed by the crowd.
Australian live music venues: Venues like the Forum in Melbourne or the Enmore in Sydney employ acoustic engineers to balance bass and treble. Too much bass creates uncomfortable vibrations; too little leaves the music sounding thin. Engineers calculate wavelengths and room resonance to ensure every frequency range is heard and felt appropriately.
Bass is felt because it is louder than treble. Bass is not necessarily louder; it is lower in frequency. A 100 dB treble note at 10,000 Hz is painfully loud but not physically felt. A 100 dB bass note at 50 Hz shakes the room. The difference is wavelength and the amount of air moved, not just the decibel level.
You learned that sound is produced by vibrations and travels as a longitudinal wave through a medium.
If you were on the Moon and your friend tapped a metal pole with a hammer, could you hear it by touching the pole with your hand? Explain why or why not.
The hook invited you to imagine placing your hand on a speaker and feeling the air vibrate — and pointed out that the same thing can't happen in space because there's nothing to vibrate.
Now that you understand sound as a pressure wave that compresses and rarefies a medium, how would you explain to someone why space is truly silent? How has your thinking shifted from that first feeling of vibration?
1. What type of wave is a sound wave?
2. Why can sound not travel through a vacuum?
3. In which material does sound travel fastest?
4. What determines the pitch of a sound?
5. What do we call regions where air particles are pushed close together in a sound wave?
Explain how a tuning fork produces sound and how that sound reaches your ear. Use the terms vibration, compression, and rarefaction. (3 marks)
Hint: Describe the sequence from the tuning fork to the air to your eardrum.
Describe an experiment you could do to show that sound needs a medium to travel. (3 marks)
Hint: Think about using a bell jar and a vacuum pump, or another practical setup.
Explain why you can hear someone knocking on a door from the other side, even though the door is solid and the air gap is small. (3 marks)
Hint: Consider how sound travels through different media and which medium transmits it best.