Hearing sound

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 waves 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 particles vibrate in the oesophagus (throat), and a sound wave is produced. A fast vibration produces waves that are close together and have a higher pitch.

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 temperature 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 particle to the next and the faster the speed of sound.

Sound cannot travel in a vacuum, 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 radiation, 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 infrasound. 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 ultrasound. 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 species we live with.

Discover more about human hearing, focusing on the function of the ear structure.