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Handbook of air conditioning and refrigeration / Shan K

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19.26 CHAPTER NINETEEN<br />

Environmental Adjustment Factor<br />

Plenum Ceiling Effect<br />

FIGURE 19.7 Sound path <strong>of</strong> a typical terminal unit installed in the ceiling plenum.<br />

According to Industry St<strong>and</strong>ard (IS) 880, sound power levels <strong>of</strong> the terminal unit as listed in the<br />

manufacturers’ catalogs are based on free field (outdoor) calibration <strong>of</strong> the reference sound source.<br />

At low frequencies, actual rooms are highly reverberant. The adjustment factor that takes into account<br />

the difference in sound power levels <strong>of</strong> a commonly used reference sound source measured in<br />

a free field <strong>and</strong> a reverberant field is called the environmental adjustment factor E f, in dB. IS 885<br />

recommends the following E f values:<br />

B<strong>and</strong> frequency 63 125 250 500 1000 2000 4000 8000<br />

E f, dB, re 10 �12 W � 7 � 3 � 2 � 1 � 1 � 1 � 1 � 1<br />

Sound attenuation <strong>of</strong> the plenum ceiling combination not only includes the transmission loss <strong>of</strong> the<br />

ceiling, but also considers the effect <strong>of</strong> the plenum. For a -in.- (16-mm-) thick mineral fiber<br />

acoustic tile ceiling with a density <strong>of</strong> 35 lb/ft 3 (560 kg/m 3 5�8 ), IS 885 recommends the following values,<br />

which are the same as those in Table 19.14:<br />

Octave b<strong>and</strong> center frequency, Hz 125 250 500 1000 2000 4000<br />

Plenum ceiling effect, dB � 5 � 9 � 10 � 12 � 14 � 15

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