Table 169 Required Composite and Component FSTC for Various Voice Levels Chanaud 1983

Background Noise = 44 dBA Minimum Composite FSTC for

Voice Normal Confidential

Level Privacy Privacy

Normal 26 32

Raised 36 42

Shouting 46 52

Recommended Component FSTC for

Voice Normal Confidential

Level Privacy Privacy

Normal 32 38

Raised 42 48

Shouting 52 58

sound pressure level due to a normal voice in one room would generate about 18 dBA sound pressure level in the adjacent room. If the masking level in the receiving space were 35 dBA, we could achieve Confidential Privacy since the signal-to-noise ratio is less than -12 dB. This describes a typical private office with a background level of about NC 30 although the NC spectrum is not identical with the masking spectrum.

Plenum Flanking

The choice of an appropriate wall type is not the end of the design exercise, since there are many other routes that the sound can take from one room to another. If the wall does not extend from slab to slab, then the sound can travel over the top of the wall, passing through the plenum above the T-bar ceiling on either side. Since the mass of acoustical tile is quite low, it does little to attenuate noise from passing through it. Blazier (1981) has published measured values of the noise reduction of an acoustical tile ceiling, which are shown in Fig. 13.15. Even if these are doubled and increased by 6 dB, following Eq. 9.50 they still represent a significant degradation in transmission loss relative to the wall performance. Grille openings for the return air plenum can further degrade these ratings.

Figure 16.22 gives several examples of FSTC values for partial-height walls flanked by plenums. In this figure there are substantial differences in FSTC values depending on the ceiling material. Manufacturers of acoustical ceilings have sought to develop so-called high transmission loss tiles, which combine both absorption and transmission loss into a single product. These tiles have a modest absorption coefficient, between 0.5 to 0.7, and a relatively low effective STC rating (as measured with a wall in the STC 30 to 35 range). Even though they are better than lighter acoustical tiles, they can be compromised by openings for return air grilles.

Figure 16.21 Full Height Separation Wall Configurations

Full Height Wall Full Height Wall One Layer

FSTO 50 FSTO 4=1

Figure 16.21 Full Height Separation Wall Configurations

Full Height WaI FSTO 3®t

Full Height Wai FSTO 34

Full Height Wall One Side FSTO 3fe
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