14 Chapter 1
the sound source. The larger the source, the longer the pressure–equalization time. Only
after this does the frequency analysis mechanism tell anything about the pitch and timbre
of the sound.
The aforementioned results suggest that anything in a sound reproduction system that
impairs the reproduction of a transient pressure change will damage localization and
the assessment of the pressure–equalization time. Clearly, in an audio system that
claims to offer any degree of precision, every component must be able to reproduce
transients accurately and must have at least a minimum phase characteristic if it cannot
be phase linear. In this respect, digital audio represents a distinct technical performance
advantage, although much of this is later lost in poor transducer design, especially in
loudspeakers.
1.7 Level and Loudness .................................................................................................
At its best, the ear can detect a sound pressure variation of only 2 1 0 ^5 Pascals root
mean square (rms) and so this fi gure is used as the reference against which the sound
pressure level (SPL) is measured. The sensation of loudness is a logarithmic function of
SPL; consequently, a logarithmic unit, the decibel, was adopted for audio measurement.
The decibel is explained in detail in Section 1.12.
The dynamic range of the ear exceeds 130 dB, but at the extremes of this range, the ear
either is straining to hear or is in pain. The frequency response of the ear is not at all
uniform and it also changes with SPL. The subjective response to level is called loudness
and is measured inphons. The phon scale is defi ned to coincide with the SPL scale at
1 kHz, but at other frequencies the phon scale deviates because it displays the actual SPLs
judged by a human subject to be equally loud as a given level at 1 kHz. Figure 1.9 shows
the so-called equal loudness contours, which were originally measured by Fletcher and
Munson and subsequently by Robinson and Dadson. Note the irregularities caused by
resonances in the meatus at about 4 and 13 kHz.
Usually, people’s ears are at their most sensitive between about 2 and 5 kHz; although
some people can detect 20 kHz at high level, there is much evidence to suggest that most
listeners cannot tell if the upper frequency limit of sound is 20 or 16 kHz. 2,3 For a long
time it was thought that frequencies below about 40 Hz were unimportant, but it is now
clear that the reproduction of frequencies down to 20 Hz improves reality and ambience.^4