Test and Measurement 1609measured at a single point (the microphone position).
Sound power measurements must consider all of the
radiated sound from a device and sound intensity
measurements must consider the sound power flowing
through an area. Sound power and sound intensity
measurements are usually performed by acoustical labo-
ratories rather than in the field, so neither is considered
in this chapter. All measurements described in this
chapter will be measurements of sound pressures
expressed as levels in dB ref. 0.00002 Pa.
Sound level measurements must usually be
processed for the data to correlate with human percep-
tion. Humans do not hear all frequencies with equal
sensitivity, and to complicate things further our
response is dependent on the level that we are hearing.
The well-known Fletcher-Munson curves describe the
frequency/level characteristics for an average listener,
see Chapter 2. Sound level measurements are passed
through weighting filters that make the meter “hear”
with a response similar to a human. Each scale corre-
lates with human hearing sensitivity at a different range
of levels. For a sound level measurement to be mean-
ingful, the weighting scale that was used must be indi-
cated, in addition to the response time of the meter.
Here are some examples of meaningful (if not univer-
sally accepted) expressions of sound level:- The system produced an LP= 100 dBA (slow re-
sponse) at mix position. - The peak sound level was LA= 115 dB at my seat.
- The average sound pressure level was 100 dBC at
30 ft. - The loudspeaker produced a continuous LP of 100 dB
at one meter (assumes no weighting used). - The equivalent sound level LEQ was 90 dBA at the
farthest seat.
Level specifications should be stated clearly enough
to allow someone to repeat the test from the description
given. Because of the large differences between the
weighting scales, it is meaningless to specify a sound
level without indicating the scale that was used. An
event that produces an LP= 115 dB using a C scale may
only measure as an LP= 95 dB using the A scale.
The measurement distance should also be specified
(but rarely is). Probably all sound reinforcement
systems produce an LP= 100 dB at some distance, but
not all do so at the back row of the audience!
Lpk is the level of the highest instantaneous peak in
the measured time interval. Peaks are of interest
because our sound system components must be able to
pass them without clipping them. A peak that is clipped
produces high levels of harmonic distortion that degrade
sound quality. Also, clipping reduces the crest factor of
the waveform, causing more heat to be generated in the
loudspeaker causing premature failure. Humans are not
extremely sensitive to the loudness of peaks because our
auditory system integrates energy over time with regard
to loudness. We are, unfortunately, susceptible to
damage from peaks, so they should not be ignored.
Research suggests that it takes the brain about 35 ms to
process sound information (frequency-dependent),
which means that sound events closer together than this
are blended together with regard to loudness. This is
why your voice sounds louder in a small, hard room. It
is also why the loudness of the vacuum cleaner varies
from room to room. Short interval reflections are inte-
grated with the direct sound by the ear/brain system.
Most sound level meters have slow and fast settings that
change the response time of the meter. The slow setting
of most meters indicates the approximate
root-mean-square sound level. This is the effective level
of the signal, and should correlate well with its
perceived loudness.
A survey of audio practitioners on the Syn-Aud-Con
e-mail discussion group revealed that most accept an
LP= 95 dBA (slow response) as the maximum accept-
able sound level of a performance at any listener seat
for a broad age group audience. The A weighting is
used because it considers the sound level in the portion
of the spectrum where humans are most easily annoyed
and damaged. The slow response time allows the
measurement to ignore short duration peaks in the
program. A measurement of this type will not indicate
true levels for low-frequency information, but it is
normally the mid-frequency levels that are of interest.
There exist a number of ways to quantify sound
levels that are measured over time. They include:- LPK—the maximum instantaneous peak recorded
during the span. - LEQ—the equivalent level (the integrated energy over
a specified time interval). - LN—where L is the level exceeded N percent of the
time. - LDEN—a special scale that weights the gathered
sound levels based on the time of day. DEN stands
for day-evening-night. - DOSE—a measure of the total sound exposure.
A variety of instruments are available to measure
sound pressure levels, ranging from the simple sound
level meter (SLM) to sophisticated data-logging equip-
ment. SLMs are useful for making quick checks of