Handbook for Sound Engineers

Acoustical Noise Control 89

50 dB wall TL gives the overall TL with a window of
45 dB. (Calculated from Eq. 4-4 gives 45.7 dB.)

It is usually easier and more economical to get high
TL in wall construction than in window construction.
The possibility arises of compensating for a deficient
window by overdesigning the wall. For example, recog-
nizing that an STC-70 masonry wall is possible, how far
will it lift an STC-45 window? Using Eq. 4-4 again, we
find the overall STC to be 52.2 dB, an increase of over
7 dB over the STC-45 window. Actually, using Eq. 4-4
with STC values is a gross oversimplification
embracing all the inaccuracies of fitting measured TL
values with a single-number STC rating. Making the
calculations from measured values of TL at each
frequency point is much preferred. Of course, all this
assumes an airtight seal has been achieved.

Everything that bridges the isolation system is a
potential short circuit for noise. Such bridges include,

HVAC ducts, electrical conduit, sprinkler systems,

plumbing, raceways, and the like.

Now we have the formula for empirically looking at

the effect of a crack in the wall (Fig. 4-17 was plotted

using Eq. 4-4). Let us assume that an observation

window and wall combination have a calculated

composite TL of 50 dB. The window, installed with less

than ideal craftsmanship, developed a^1 einch (0.125 in)

crack around the window frame as the mortar dried and

pulled from the frame. Since this is the window of Fig.

4-41, the length of the crack is 21.6 ft, giving a crack

area of 0.225 ft^2. What effect will this crack have on the

otherwise 50 dB wall? Substituting into Eq. 4-4, we find

the new TL of the wall with the crack to be 28 dB. This

is similar to leaving off an entire pane of glass or a layer

of gypsum board. If the crack were only^1 einch wide,

the TL of the wall would be reduced from 50 dB to

31.2 dB. A crack only 0.001 inch wide would reduce

the TL of 50 dB to 40.3 dB. Let the builder beware!

4.3.12 Isolation Systems Summary

Noise migrates from one area to another in two ways. It

travels through the air and it travels through the struc-

ture. To reduce or eliminate airborne noise, one must

eliminate all air paths between the spaces. To reduce

structure-borne noise one must create isolation systems

that eliminate mechanical connections between spaces.

It is a rather simple matter to make theses statements.

Implementing the solutions is obviously much more

difficult. The following points should be kept in mind:

• Make seams airtight.


• Analyze all possible flanking paths that noise will
  take and realize that all must be controlled if
  significant isolation is desired.


• A room built entirely on a floating slab with the
  ceiling supported entirely by the walls will always be
  superior to any other method.

4.4 Heating, Ventilating, and Air Conditioning

(HVAC) Systems for Low Noise

So far in this chapter we have considered systems that

keep unwanted sound out. When we consider HVAC

systems we are dealing with systems that (a) breach the

acoustical shell designed to keep noise out, (b) intro-

duce considerable noise of their own, and (c) provide a

pathway for sound (noise) to easily migrate from one

space to another. HVAC systems can sometimes under-

mine all the efforts of isolation. Often the cheapest solu-

tion to providing HVAC to sound sensitive spaces is to

Figure 4-41. Typical observation window in a wall between

control room and studio.

Figure 4-42. Graphical determination of the effect on the
overall transmission loss (TL) of a wall by an observation
window.

15'

6.4'

Window area

28.2 ft^2

18.8%

Net wall area = 150 28.2 = 121.8 ft^2

81.2%

4.4' 10'

window area

wall area

28.2

121.8

= 0.23

=

40

30

20

10

(^0) 0.01 0.02 0.040.06 0.1 0.2 0.4 0.6 1.0
Glass area / Total wall area
Reduction in wall TL–dB
30 dB
20 dB
10 dB
Wall TL
window TL = 40 dB