Handbook for Sound Engineers

70 Chapter 4

recording noise analyzers available today. There are a
number of fine units that are capable of producing reli-
able and very useful site surveys. Fig. 4-5 is an example
of a 24 hour site survey made with the Gold Line TEF
25 running Noise Level Analysis™ software. One can
also use a handheld sound level meter (SLM) if outfitted
with the appropriate options. Some real-time frequency
analyzers such as the Brüel and Kjaer 2143 are also
appropriate. It is also acceptable to use a dosimeter such
as the Quest Technologies model 300, Fig. 4-6.

No matter which analyzer is used, the system must
be calibrated using a microphone calibrator. The
weather conditions, especially temperature and relative
humidity, should be noted at the time of calibration. The
measuring microphone may be mounted in a weather-
proof housing at the desired location with the micro-
phone cable running to the equipment indoors. There
are a number of terms which will appear on any display
of a noise survey. There will be a series of Ln levels
indicated, see Table 4-3. These are called exceedance or
percentile levels. L 10 refers to the noise level exceeded
10% of the time, L 50 the level exceeded 50% of the time,
l 90 the level exceeded 90% of the time and so forth. In
the United States L 10 is considered to indicate the
average maximum level and L 90 the average minimum
or background level.^6 Since many noise levels vary
dramatically over time, it is useful to have a number
which represents the equivalent constant decibel level.
The Leq. This is the steady continuous level that would
yield the same energy over a given period of time as the

measured levels. Ldn indicates a 24 hour Leq with 10 dB

added to the levels accumulated between 2200 and

0700 h to account for the increased annoyance potential

during the nighttime hours. The Community Noise

Equivalent Level (CNEL) also is used to document

noise levels over a 24 hour period. It differs from the

Ldn as weighting factors for the evening period between

1900 to 2200 h are included. The Leq for evening hours

is increased by 5 dB while the Leq for the nighttime

hours is increased by 10 dB.

The lmean is the arithmetic mean of the measured

levels. Lmin and Lmax refer to the lowest and highest

measured instantaneous levels, respectively.

Ideally, the site survey should take place over a

minimum of 24 hours. A 24 hour observation captures

Figure 4-5. A 24 hour NLA site survey made with the Gold
Line TEF 25.

70

64

58

52

46

40

Leq

122.9 seconds

07:49 Time (HH:MM) 3 hr/div

duration 

24:00

Lmin = 44.4 dB Lmax = 90.8 dB Leq = 54.3 dB

L 10 = 57.1 dB L 50 = 51.5 dB L 90 = 47.8 dB Lmean = 52.1 dB

07:49

Channel A Preamp

Gain: 48 dB, Weight: F1t, Output: Off

Figure 4-6. Quest Technologies model 300 dosimeter.

Courtesy Quest Technologies.

Table 4-3. Common Level Designations in Noise

Surveys

L 10 Noise level exceeded 10% of the time

L 50 Noise level exceeded 50% of the time

L 90 Noise level exceeded 90% of the time

Ldn 24 hour Leq

lmean Arithmetic mean of measured levels