Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 84

The range of sound pressure magnitudes and sound powers of sources experienced in practice is very large. Thus, logarithmic rather than linear measures are often used for sound pressure and sound power. The most common measure of sound is the decibel. Decibels are also used to measure vibration, which can have a similar large range of magnitudes. The decibel represents a relative measurement or ratio. Each quantity in decibels is expressed as a ratio relative to a reference sound pressure, sound power, or sound intensity, or in the case of vibration relative to a reference displacement, velocity, or acceleration. Whenever a quantity is expressed in decibels, the result is known as a level.

The decibel (dB) is the ratio R₁ given by

(3.21)

Thus, R₁ = 10^0.1 = 1.26. The decibel is seen to represent the ratio 1.26. A larger ratio, the bel, is sometimes used. The bel is the ratio R₂ given by log₁₀ R₂ = 1. Thus, R₂ = 10¹ = 10. The bel represents the ratio 10 and is thus much larger than a decibel. For simplicity, in the following sections of this book we denote log( ) = log₁₀( ).