Chapter 2 Sound, decibels and hearing Important aspects of hearing sensitivity and frequency range. An introduction to the decibel in its various applications. The speed of sound and the concept of wavelength. Relation between absorbers and wavelength. Sound power, sound pressure, sound intensity. Double-distance rule. The dBA and dBC concepts. Sound insulation and noise perception. Aspects of hearing and the concept of psychoacoustics. The sensitivity of the ear and the differences of perception from one person to another. The effect on the perception of loudspeakers vis-à-vis live music. 2.1 Perception of sound That our perception of sound via our hearing systems is logarithmic becomes an obvious necessity when one considers that the difference in sound power between the smallest perceivable sound in a quiet room and a loud rock band in a concert is about 10 12 ­ one to one-million-million times. A rocket launch at close distance can increase that by a further one million times. The ear actually responds to the sound pressure though, which is related to the square root of the sound power, so the pressure difference between the quietest sound and a loud rock band is 10 6 ­ one to one-million. When a pure tone of mid frequency is increased in power by ten times, the tone will subjectively approximately double in loudness. This ten times power increase is represented by a unit called a bel. One tenth of that power increase is represented by a decibel (dB) and it just so happens that one- decibel represents the smallest mean detectable change in level that can be heard on a pure tone at mid frequencies. Ten decibels (one bel) represents a doubling or halving of loudness. However, the terms `pure tone' and `mid frequencies' are all-important here. Figure 2.1 shows two representations of equal loudness contours for human hearing. Each higher line represents a doubling of subjective loudness. It can be seen from the plots that at the frequency extremes the lines converge showing that, especially at low fre- quencies, smaller changes than 10 dB can be perceived to double or halve the loudness. This is an important fact that will enter the discussions many times during the course of this book. It is the concept of the doubling of loudness for every 10 dB increase in sound pressure level that fits so well with our logarithmic hearing. A street,