4. Hearing sound

Our ears have two functions, hearing and balance, but what do they hear and how? We can experience a whole world of sound due to the precise coordination of highly evolved anatomical, electro-chemical, and neurological processing systems. ‘Hearing sound’ describes the anatomy of the outer, middle, and inner ear along with the nerve signals sent to the brain. What happens in the brain’s hearing and language centres is not entirely clear, but the first stage in processing is to extract salient features from the stream of input data from the auditory nerve. Problems with hearing, including deafness and tinnitus, are explained along with how we make different sounds.

8. Sound out of place

Noise can be two very different things: for a scientist, noise is extraneous acoustic or electromagnetic energy, but for most of us ‘noise’ is any sound that is unwanted by the person exposed to it. ‘Sounds out of place’ considers the main sources of noise pollution, noise-induced hearing loss, and what can be done to eliminate or reduce noise: active noise cancellation, zone separation, and tackling reverberation in public spaces. Although our hearing systems are important, their performance is not usually a matter of life and death. This is not the case for some marine creatures, such as whales and dolphins, which can be devastated by the effects of noise underwater.

1. Past sounds

‘Past sounds’ provides a history of sound from the origin of sound waves 300,000 years after the Big Bang to the modern day of ultrasound and electroacoustic technology. Primordial sound was of a very low frequency, but powerful and omnipresent, and the environment in which the first living things evolved was an acoustically rich one, profoundly affecting the forms, habits, and destinies of those creatures. Hearing evolution is described along with the human development of music and musical instruments. The Greeks built amphitheatres that dealt with the practicalities of sound and Pythagoras studied harmony on a monochord. The World Wars of the twentieth century accelerated electronics development and inspired underwater acoustic research and sonar systems.

6. Ultrasound and infrasound

‘Ultrasound and infrasound’ explains that humans can hear sounds with frequencies in the range of 20 Hz to around 17 kHz, but there is a wide ultrasound soundscape beyond this range. Bats, for example, are able to navigate and hunt using echolocation, generating ultrasounds and timing the delays until they hear the echoes, which inform them of the distances to nearby objects. Ultrasound can also be used medically to help repair damaged muscles, scan foetuses, or treat inoperable tumours. At extremely high frequencies, sounds behave like particles called phonons. Infrasound occurs at lower frequencies and can travel far further than audible sound, through sea, ground, or air.

2. The nature of sound

Sound is a physical phenomenon as well as a sensual one. The relationships between the physical and sensual aspects of sound are complex in that many of the impressions sound makes on us are related to its physical parameters but not reducible to them. ‘The nature of sound’ considers the physical aspects of sound, which are far better understood than the emotional ones. It discusses pressure waves; how sound is carried; the velocity, refraction, frequency, and diffraction of sound; the power of sound, including loudness and the decibel measurement system; standing waves and resonances; charting sound; sound filters; and sound synthesis.

7. Sound underwater and underground

‘Sound underwater and underground’ focuses on the areas of sound that we can’t hear. It was not until World War II that the richness of the underwater soundscape became clear. Listening with hydrophones revealed a cacophony of sounds extending well above and below the human frequency range. From the 1910s, primitive hydrophones were gradually replaced by echo sounding, from which modern sonar systems developed. Sound waves that travel through the earth can be detected with geophones, which are highly directional and are usually deployed to respond to sounds from directly below. Modern microelectromechanical systems are relatively insensitive and are used mainly for the monitoring of actively seismic regions.

3. Sounds in harmony

The words ‘tone’ and ‘note’ reflect the subjective/objective nature of sound: a tone is a sound wave with a particular frequency, a note is its subjective impact, with a particular pitch. ‘Sounds in harmony’ looks at what makes a note. In addition to pitch, a note also has duration, loudness, and timbre. Unlike pitch, duration, and loudness, timbre is a dynamic quality that can change over time and it is the only thing unique to a particular instrument or person. Why do we like what we do? Auditory pleasantness—consonance—and harmony are explained along with octaves, the pentatonic scale, major and minor scales, melodies, tempo, and metre.

5. Electronic sound

‘Electronic sound’ explains the conversion of sound to electricity through the development of microphones, amplifiers, and loudspeakers. There are several types of microphones, but only three types in common use: the dynamic or moving coil microphone, the condenser microphone, and the piezoelectric microphone. Directionality is also important; an omnidirectional microphone is equally sensitive to sound from any direction, and is required to capture a full soundscape, whereas a unidirectional microphone picks up sound from one direction only—ideal for picking up speech or song in noisy environments. The development of sound storage from stereophonic phonographs, to analogue cassette tapes, compact discs, and now MP3 audio files is also described.