User Guide

Sound System Design Reference Manual
Sound Pressure and Loudness Contours
We will see the term dB-SPL time and again in
professional sound work. It refers to sound pressure
levels in dB above the reference of 20 x 10
-6
N/m
2
.
We commonly use a sound level meter (SLM) to
measure SPL. Loudness and sound pressure
obviously bear a relation to each other, but they are
not the same thing. Loudness is a subjective
sensation which differs from the measured level in
certain important aspects. To specify loudness in
scientific terms, a different unit is used, the phon .
Phons and decibels share the same numerical value
only at 1000 Hz. At other frequencies, the phon scale
deviates more or less from the sound level scale,
depending on the particular frequency and the
sound pressures; Figure 2-2 shows the relationship
between phons and decibels, and illustrates the
well-known Robinson-Dadson equal loudness
contours. These show that, in general, the ear
becomes less sensitive to sounds at low frequencies
as the level is reduced.
When measuring sound pressure levels,
weighted response may be employed to more closely
approximate the response of the ear. Working with
sound systems, the most useful scales on the sound
level meter will be the A-weighting scale and the
linear scale, shown in Figure 2-3. Inexpensive sound
level meters, which cannot provide linear response
over the full range of human hearing, often have no
linear scale but offer a C-weighting scale instead. As
can be seen from the illustration, the C-scale rolls off
somewhat at the frequency extremes. Precision
sound level meters normally offer A, B, and C scales
in addition to linear response. Measurements made
with a sound level meter are normally identified by
noting the weighting factor, such as: dB(A) or dB(lin).
Typical levels of familiar sounds, as shown in
Figure 2-4, help us to estimate dB(A) ratings when a
sound level meter is not available. For example,
normal conversational level in quiet surrounds is
about 60 dB(A). Most people find levels higher than
100 dB(A) uncomfortable, depending on the length of
exposure. Levels much above 120 dB(A) are
definitely dangerous to hearing and are perceived as
painful by all except dedicated rock music fans.
Figure 2-2. Free-field equal loudness contours
2-4