Handbook for Sound Engineers

Acoustical Noise Control 67

The field of room acoustics divides easily into two
broad categories; noise control and subjective acoustics.
These two branches of acoustics actually have rather
little in common. Noise control, to a great extent, must
be designed into a project. It is very difficult, if not
impossible to retroactively improve the isolation of a
room. However, it is often possible to change the way a
room sounds subjectively by simply modifying the wall
treatment. It is important to keep in mind that noise is a
subjective categorization. Sound pressure, sound inten-
sity, and sound transmission can all be measured. Noise
is unwanted sound. It is much more difficult to measure
and quantify the extent to which any given sound
annoys any given individual. To the Harley rider
driving his Fat Boy™ past your studio, the exhaust is
music to his ears but noise to you. Your music is noise
to the therapist trying to conduct a session of a very
different sort next door. The music in studio A is noise
to the band trying to record in studio B down the hall.
Throughout this chapter, the term sound room will be
used to designate any room that requires some measure
of quiet in order for the room to serve its purpose.

4.1 Noise Criteria
When specifying permissible noise levels, it is
customary to use some form of the noise criteria (NC).
The beauty of the NC contours is that a spectrum speci-
fication is inherent in a single NC number. The NC
contours of Fig. 4-1 are helpful in setting a background
noise goal for a sound room.^1 Other families of NC
contours have been suggested such as the PNC,^2 Fig.
4-2 which adds an additional octave to the low end of
the scale, and NR (noise rating), Fig. 4-3, used in
Europe. In 1989 Beranek proposed the NCB or
Balanced Noise Criteria.^3 The NCB adds the 16 Hz
octave band and the slopes of the curves are somewhat
modified relative to the NC or PNC curves, Fig. 4-4.

Beranek also proposed NCB limits for various applica-

tions as shown in Table 4-2.

Considering the spectrum of noise is far superior to

using a single, wideband noise level. However, if

desired, each NC contour can be expressed as an overall

decibel level by adding the sound power in each octave

band as in Table 4-1. These overall levels are conve-

nient for rough appraisal of noise levels from a single

sound level meter (SLM) reading. For example, if the

SLM reads 29 dB on the A-weighting scale for the

background noise of a studio, it could be estimated that

the NC of that room is close to NC-15 on the assump-

tion that the noise spectrum of that room matched the

corresponding NC contour, and that there are no domi-

nant pure tone components.

It is helpful to see recommended NC ranges for

recording studios and other rooms compared to criteria

applicable to spaces used for other purposes, Table 4-2.

The NC goals for concert halls and halls for opera,

Table 4-1. Noise Criteria (NC) Overall Levels*

NC Contour Equivalent Wideband Level (A-weighted)

15 28

20 33

25 38

30 42

35 46

40 50

45 55

50 60

55 65

60 70

65 75

*Source: Rettinger^5

Figure 4-1. Noise criteria (NC) curves. As displayed on Gold Line TEF 20.

32 64 125 250 500 1000 2000 4000 8000

Frequency—Hz

90

80

70

60

50

40

30

20

10

Re = 2 × 10

5

Pascals^7065

60

55

50

45

40

35

30

25

20

15