Three components are needed for sound to be heard:
- A source – where the sound is made.
- A medium – something for the sound to travel through.
- A receiver – something to detect the sound.
Source
Sound is made up of pressure waves1 that are generated when something vibrates, causing particles to move backwards and forwards and bump into each other.
When we talk, our vocal cords vibrate, making air particles2 vibrate in the oesophagus3 (throat), and a sound wave is produced. A fast vibration produces waves that are close together and have a higher pitch4.
Trained opera singers can make their vocal cords vibrate at close to 10,000Hz – in musical terms, a C6 or higher. The vocal range of a dolphin really puts humans to shame. They have been recorded producing sounds up to 150,000Hz – 15 times higher than the best human soprano.
Many things can act as the source. Under water, snapping shrimps make sound by very quickly snapping together a large claw. The resulting water movement that produces the sound is so forceful that it can stun nearby animals.
Find out more about how sound travels under water in this animated video.
Medium
Sound travels at different speeds through different substances. Scientists often refer to air as a standard measure – the speed of sound in air is 343m/s at room temperature5 but it travels much, much faster under water (1,482m/s).
Sound waves are transmitted by the medium’s particles bumping into each other. The closer the particles are, the faster the transfer of vibration from one particle6 to the next and the faster the speed of sound.
Sound cannot travel in a vacuum7, unlike light, because there are no particles to do the bumping. This is why we can see stars in the sky but cannot hear their nuclear reactions. Scientists do sometimes refer to listening to stars or the sky but they are referring to the detection of electromagnetic radiation8, not sound waves.
Receiver
For sound to be detected, there has to be a receiver. Often we refer back to human terms of reference – our own sense of hearing. Humans can detect noises ranging from around 20Hz to 20,000Hz depending on age. As you get older, it becomes progressively more difficult for our sensory systems to detect high-frequency (high-pitched) noises, and some people also begin to lose the perception of low-frequency (low-pitched) sounds.
Sound at frequencies below the human range of hearing is called infrasound9. Occasionally, if the sound is powerful enough, we can detect these sounds, but it is often a physical response rather than true hearing – think subwoofers and explosions.
Sound at frequencies above the human hearing range – more than 20,000Hz (20kHz) is called ultrasound10. Humans have developed many uses for sounds in this range, including diagnostic ultrasound that is used during pregnancy. Medical ultrasound machines use frequencies in the range 7–18MHz (7,000,000–18,000,000Hz).
In the natural world, many animals can detect sound in the ultrasound ranges and can use this to communicate or navigate. A dolphin whistle can include sounds that reach 170kHz, while fish such as herrings have been shown to detect sound at frequencies up to 180kHz.
While we normally think about these concepts from a human point of view, it is important to realise that other animals can often produce and detect sound much better than humans can. If we think our world is noisy, think about what it might be like for some of the species11 we live with.
Discover more about human hearing, focusing on the function of the ear structure.
Related content
The Connected article Can you hear that? provides an overview of sound, briefly addressing: characteristics of sound waves, how the human ear works, hearing loss in humans, how animal ears work, echolocation12 and sonar13.
This article explains some technologies helping people with hearing loss.
Use the recorded PLD webinar Sounds of Aotearoa to explore fun ways you can learn and teach about sound.
Activity ideas
You might like to try one of these activities:
- Sound detectives – students take part in a class14 experiment to locate sounds when blindfolded.
- Modelling waves with slinkies – students model how sound travels by sending waves along two stretched plastic15 slinkies tied together.
- Make and use a hydrophone – students make a hydrophone16 and use it to listen to underwater sounds.
- Sounds in a pool – students listen to sounds made in a swimming pool while being under water themselves.
- Measuring the speed of sound – use a timing app to measure the speed of sound.
- longitudinal wave: Also known as a compression wave, where the material moves in the same direction as the wave moves, for example, sound waves.
- air particles: The structural components of air. It includes gas molecules such as oxygen and nitrogen as well as larger dust, pollen and ash particles.
- oesophagus: A thin muscular-walled tube that runs from mouth to stomach. Also known as the gullet, it allows food to be transported to the stomach by peristaltic muscular movement.
- pitch: 1. (Flight) The movement of the nose (front) of an airplane or spacecraft up or down about a transverse axis. 2. (Sound) A human perception of how high or low the frequency of the sound being heard appears to be.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- particle: A tiny piece of matter. A particle may refer to an atom, part of an atom, a molecule or an ion.
- vacuum: An absence of matter. In practice, a space that contains a very low density of matter (very low pressure) is often referred to as a vacuum.
- radiation: Energy that is transmitted (radiates) from a source in the form of rays or waves or particles.
- infrasound: Any sound whose frequency is below the range of normal human hearing (less than approximately 20 Hz).
- ultrasound: Any sound whose frequency is above the range of normal human hearing (greater than approximately 20 kHz). Used in medical fields as an imaging technique and also used by animals in nature for navigation.
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- echolocation: The location of objects by reflected sound, used by animals such as dolphins and bats.
- sonar: A method of detecting, locating, and determining the speed of objects through the use of reflected sound waves. A sound signal is produced, and the time it takes for the signal to reach an object and for its echo to return is used to calculate the object's distance.
- class: A classification grouping that ranks above order and below phylum (kingdom > phylum > class > order > family > genus > species).
- plastic: A synthetic material made from a wide range of organic polymers (such as polyethylene, PVC and nylon) that can be moulded into shape while soft and then set into a rigid or slightly elastic form.
- hydrophone: An underwater microphone used to listen for and record sound under water.
