Sound and the Environment: What Can You Hear?
Extension topic – this chapter belongs to the Extension Part of the 2025 junior secondary science curriculum.
Hong Kong is a busy, noisy city. In this chapter you will learn which frequencies humans and other animals can hear, how loudness is measured in decibels (dB), how noise pollution affects health and the environment, how acoustic protection works, and how noise from ships and construction affects dolphins and whales.
Quick reminder (from Properties of sound): frequency is the number of vibrations per second, in hertz (Hz). Higher frequency means higher pitch.
1. The Range of Sounds We Can Hear
The human audible range
The range of frequencies that we can hear is called the audible frequency range. For a healthy young person it is about 20 Hz to 20 000 Hz:
\(20\ \text{Hz} \le f \le 20\,000\ \text{Hz}\)
20 000 Hz is also written as 20 kHz, because \(1\ \text{kHz} = 1000\ \text{Hz}\).
Infrasound and ultrasound
Infrasound – sound below 20 Hz, too low for us to hear. Earthquakes, thunder, large machines, elephants and some whales produce it.
Ultrasound – sound above 20 000 Hz, too high for us to hear. Bats and dolphins use it, and doctors use it to scan babies before birth.
Memory aid: infra means "below" and ultra means "beyond". Both are still real sound waves – we just cannot hear them.
Your hearing range changes with age
As people grow older, the sensory cells in the inner ear slowly wear out. The upper limit falls first, so many adults cannot hear very high-pitched sounds that teenagers hear clearly. Loud noise also damages these cells and shrinks the range at any age (see the chapter Hearing for the parts of the ear).
Did you know? A tone of about 17 000 Hz may be clear to students but silent to many adults!
Try it: find your class's hearing range
Connect a signal generator to a loudspeaker. Start at about 20 Hz and slowly raise the frequency; students raise a hand while they can hear the sound. Record each student's lowest and highest audible frequency and compare.
Variables: independent variable – frequency; dependent variable – whether the sound is heard; control variables – volume setting, loudspeaker and distance from it.
Safety: keep the volume low. Ordinary loudspeakers do not play very low or high frequencies well, so results are approximate.
Key takeaway: Humans hear about 20 Hz to 20 000 Hz. Below is infrasound; above is ultrasound. The upper limit falls with age and with noise damage.
2. Animals Hear Different Ranges
The audible range of humans is different from that of other animals.
Hearing ranges of some animals
Approximate values:
Dogs – up to about 45 000 Hz (ultrasound)
Bats – up to about 100 000 Hz or more (ultrasound)
Dolphins – up to about 150 000 Hz (ultrasound)
Elephants – can hear some infrasound, but their upper limit is lower than ours
Large whales – make very low calls, some of them infrasound
Why different ranges help animals
Bats and dolphins send out ultrasound and listen for echoes to find food in the dark or in muddy water – echolocation (see Properties of sound).
Elephants make deep rumbles, partly infrasound, that reach other elephants several kilometres away. Large whales make low calls that travel long distances through the ocean.
A dog whistle makes sound mostly above 20 000 Hz. Dogs hear it clearly; we hardly hear it at all!
Key takeaway: Dogs, bats and dolphins hear ultrasound; elephants and large whales use very low sounds, including infrasound.
3. Noise and the Decibel Scale
What is noise?
Noise is unwanted or unpleasant sound. When it is so loud or lasts so long that it harms people or other living things, it is noise pollution. In Hong Kong, common sources include road traffic, construction and renovation, aircraft and ships.
Measuring loudness in decibels (dB)
Loudness is measured with a sound level meter in decibels (dB). 0 dB is about the quietest sound a healthy young ear can hear.
Typical levels (approximate):
About 30 dB – a whisper
About 60 dB – normal conversation
About 70–85 dB – busy road traffic
About 100 dB – a road drill close by
About 120 dB or more – a jet taking off nearby; this can be painful
An increase of 10 dB sounds roughly twice as loud. Experts generally advise that long exposure to sounds above about 85 dB can damage hearing – the louder the sound, the shorter the safe time.
Common mistake: "Ultrasound means very loud sound." Wrong! Infrasound and ultrasound describe frequency (pitch), in Hz. Loudness is measured in dB. A sound can be high-pitched but quiet.
Key takeaway: Noise is unwanted sound. Loudness is measured in dB; +10 dB sounds about twice as loud, and long exposure above about 85 dB can damage hearing.
4. Effects of Noise on Health and the Environment
Effects on our health
1. Hearing loss – loud sound damages the sensory cells in the cochlea. They do not grow back, so the loss is permanent. It often happens slowly and painlessly.
2. Tinnitus – ringing or buzzing in the ears after loud noise.
3. Stress – noise makes people annoyed and tense; long-term noise is linked to high blood pressure.
4. Sleep problems – night-time traffic or construction noise disturbs sleep, leaving people tired.
5. Poor concentration – noise makes it harder to study, work and talk.
Effects on the environment and wildlife
Masking – noise covers up the sounds animals use to find mates, food or warn of danger.
Leaving their habitat – animals move away from noisy areas and lose places to feed and breed.
Changing behaviour – some city birds sing louder or higher to be heard over traffic.
Quiet places such as Hong Kong's Country Parks are valuable for people and wildlife.
Common mistake: "If a noise does not hurt, it cannot damage my hearing." Wrong! Damage from long exposure usually causes no pain, builds up slowly and cannot be reversed.
Key takeaway: Noise pollution can cause permanent hearing loss, tinnitus, stress, sleep problems and poor concentration, and it disturbs wildlife.
5. Acoustic Protection: Fighting Back Against Noise
Acoustic protection means reducing noise or protecting people from it. It matters because damaged hearing cannot be repaired.
Memory aid: stop noise at the Source, along the Path or at the Receiver – S-P-R.
At the source: make less noise
Use quieter equipment and silencers on engines; keep machines well maintained; on building sites, press or drill piles in instead of hammering them; use road surfaces that reduce tyre noise.
Time limits on construction noise
Under Hong Kong's noise laws, noisy construction work with powered machines at night (about 7 p.m. to 7 a.m.) or on Sundays and public holidays needs a permit, and hammering piles is not allowed at those times. These time limits let residents rest.
Along the path: block the noise
Noise barriers – tall panels along Hong Kong's highways reflect and absorb sound before it reaches nearby flats; some roads have noise enclosures like tunnels.
Distance and design – placing buildings further from roads, with bedrooms facing away.
At the receiver: protect the listener
Ear plugs – fit inside the ear canal.
Ear defenders (ear muffs) – cover the whole ear; worn by construction and airport workers.
Double-glazed windows – two layers of glass with a gap between; little sound gets through.
Soft materials – carpets and curtains reduce echoes indoors.
Everyday tip: the "60/60 rule" – earphones at no more than 60% volume for no more than 60 minutes at a time.
Key takeaway: Reduce noise at the source (quieter equipment, time limits), along the path (noise barriers) and at the receiver (ear protection, double glazing).
6. Industrial Noise and Marine Animals
The maritime industry (shipping) and the construction industry produce noise that can affect the habitats of dolphins and whales.
Why sound matters so much under the sea
Light does not travel far in seawater, but sound travels faster and much further than in air. So marine mammals depend on sound to find food, find their way and keep in touch with their group.
Noise from the maritime industry
Large ships' engines and propellers make loud, continuous, low-frequency noise. High-speed ferries are also very noisy underwater, including high-frequency sounds that overlap with those dolphins use.
Noise from the construction industry
Building bridges, piers and reclaimed land may involve percussive piling (hammering piles into the seabed) and dredging. Each blow sends a very loud pulse of sound through the water.
How noise harms dolphins and whales
Masking – noise drowns out echolocation clicks and calls, so hunting and staying together is harder.
Stress – animals may stop feeding or resting.
Leaving their habitat – they may avoid noisy areas, even good feeding grounds.
Hearing damage – very loud sounds nearby can damage their hearing.
Case study: Chinese White Dolphins in Hong Kong
Hong Kong's Chinese White Dolphins belong to the Pearl River Estuary population and live mainly in the western waters around Lantau Island. Adults often look pink! Their busy home is shared with ferries, cargo ships and marine works, whose noise can disturb their feeding and communication.
How people help:
Marine parks protect important dolphin habitats.
Bubble curtains – walls of air bubbles around underwater piling – reduce the noise spreading out.
Quieter construction methods are chosen where possible.
Some high-speed ferries must slow down or follow set routes in important dolphin areas.
Whales only occasionally visit Hong Kong, but worldwide, ship noise is a serious problem for them as it overlaps with their low calls.
Did you know? The Chinese White Dolphin was the official mascot of the 1997 Hong Kong handover ceremony.
Key takeaway: Ships, ferries and marine piling make underwater noise that masks dolphin and whale sounds, causes stress, drives them away and can damage hearing.
Quick Review
1. Humans hear about 20 Hz to 20 000 Hz; the upper limit falls with age.
2. Infrasound is below 20 Hz; ultrasound is above 20 000 Hz.
3. Dogs, bats and dolphins hear ultrasound; elephants and large whales use infrasound.
4. Loudness is measured in decibels (dB); noise pollution harms health and wildlife.
5. Acoustic protection works at the source, along the path and at the receiver.
6. Shipping and construction noise harms dolphins and whales, including Hong Kong's Chinese White Dolphins.
Well done – keep your ears safe!