CHAPTER
6 Sound Pressure, Power, and Loudness
In this chapter, we will discuss the quality of loudness and the physical parameters that determine it. The principal such parameter, we learned in Table 5.1, is sound pressure. Related to the sound pressure are the sound power emitted by the source and the sound intensity (the power carried across a unit area by the sound wave). The output signal of a microphone is generally proportional to the sound pressure, so sound pressure can be measured with a microphone and a voltmeter. In this chapter, you should learn: About sound pressure level, sound intensity level, and sound power level; About the decibel scale for comparing sound levels; How to combine sound levels from several sources; What determines the perceived loudness of a sound; How one sound can mask another.
6.1 DECIBELS Decibel scales are widely used to compare two quantities. We may express the power gain of an amplifier in decibels (abbreviated dB), or we may express the relative power of two sound sources. We could even compare our bank balance at the beginning with the balance at the end of the month. (“My bank account decreased 27 decibels last month.”) The decibel difference between two power levels, L, is defined in terms of their power ratio W2/W1: