Generally, absolute numbers obtained from measurements have little significance in acoustics. Instead, measurements are almost always compared with some base or reference, and they are usually quoted as levels above or below that reference level. The levels are usually ratios of observed values to the reference level, such as 2:1, 1:2, 1:10, because rarely are simple, linear relationships found between stimuli and effects in humans.
Zero level of sound pressure is not a true physical zero; that is, the absence of any sound pressure at all. Rather, zero level is something of an average threshold of human hearing, of sound at about 1000 Hz. The physical pressure associated with this threshold level is very small, 0.00002 Pa (N/m2).
Changes in human response tend to occur according to a ratio of the intensity of the stimuli producing the response. In acoustics, the ratio of 10:1 is called a bel, and one-tenth of a bel, a decibel (dB). Thus, power and intensity levels, in dB, are computed from
Sound power level refers to the power of a sound source relative to a reference power of 1012 W. (Note: At one time, 10^(-13) W was used; thus, it is imperative that the reference level always be explicitly stated.)
The ear responds in a roughly logarithmic manner to changes in stimulus intensity, but approximately as shown in Table 11.25.
Another comparison, which gives more meaning to various levels, is shown in Table 11.26.
Measurement Scales. Most measurements or evaluations of sound intensity or level are made with an electronic instrument that measures the sound pressure. The instrument is calibrated to read pressure levels in decibels (rather than volts). It can measure the overall sound pressure level throughout a frequency range of about 20 to 20,000 Hz, or within narrow frequency bands (such as an octave, third-octave, or even narrower ranges). Usually, the sound-level meter contains filters and circuitry
to bias the readings so that the instrument responds more like the human ear˜deaf to low frequencies and most sensitive to the midfrequencies (from about 500 to 5000 Hz). Such readings are called A-scale readings. Most noise level readings (and, unless otherwise specifically stated, most sound pressure levels with no stated qualifications) are A-scale readings (often expressed as dBA). This means that actual sound pressure readings have been modified electrically within the instrument to give a readout corresponding somewhat to the ears response (Fig. 11.91).
For various measurements and evaluations of performance for materials, constructions, systems, and spaces, see Art. 11.81.