Handbook for Sound Engineers

1610 Chapter 46

sound levels. Most have at least an A- and C-weighting
scale, and some have octave band filters that allow
band-limited measurements. A useful feature on an SLM
is an output jack that allows access to the measured data
in the form of an ac voltage. Software applications are
available that can log the meter’s response versus time
and display the results in various ways. A plot of sound
level versus time is the most complete way to record the
level of an event. Fig. 46-2 is such a measurement. Note
that a start time and stop time are specified. Such
measurements usually provide statistical summaries for
the recorded data. An increasing number of venues
monitor the levels of performing acts in this manner due
to growing concerns over litigation about hearing
damage to patrons. SLMs vary dramatically in price,
depending on quality and accuracy.
All sound level meters provide accurate indications
for relative levels. For absolute level measurements a
calibrator must be used to calibrate the measurement
system. Many PC-based measurement systems have
routines that automate the calibration process. The cali-
brator is placed on the microphone, Fig. 46-3, and the
calibrator level (usually 94 or 114 dB ref. 20μPa) is
entered into a data field. The measurement tool now has
a true level to use as a reference for displaying
measured data.
Noise criteria ratings provide a one-number specifi-
cation for allowable levels of ambient noise. Sound
level measurements are performed in octave bands, and
the results are plotted on the chart shown in Fig. 46-4.
The NC rating is read on the right vertical axis. Note
that the NC curve is frequency-weighted. It permits an
increased level of low-frequency noise, but becomes
more stringent at higher frequencies. A sound system
specification should include an NC rating for the space,
since excessive ambient noise will reduce system clarity
and require additional acoustic gain. This must be
considered when designing the sound system. Instru-
mentation is available to automate noise criteria
measurements.

46.3.1.1 Conclusion

Stated sound level measurements are often so ambigu-
ous as to become meaningless. When stating a sound
level, it is important to indicate:

1. The sound pressure level.


2. Any weighting scale used.


3. Meter response time (fast, slow or other).


4. The distance or location at which the measurement
   was made.
   5. The type of program measured (i.e., music, speech,
   ambient noise).

Some correct examples:

• “The house system produced 90 dBA-Slow in section
  C for broadband program.”

Figure 46-3. A calibrator must be fitted with a disc to

provide a snug fit to the microphone. Most microphone

manufacturers can provide the disc.

Figure 46-4. A noise criteria specification should accom-

pany a sound system specification.

70

65

60

55

50

45

40

35

30

25

20

15

90

80

70

60

50

40

30

20

(^10) 0 125 250 500 1000 2000 4000 8000
Octave band sound pressure level–dB
Frequency–Hz
NC - 40
TEF 20