Lining the inside surfaces of ducts, pipe chases, or electrical channels with sound-absorbing materials can effectively reduce the noise transmitted through such passageways. In typical duct installations, noise reductions of 10 dB/m for an acoustical lining 2.5 cm thick are well within reason for high-frequency noise. A comparable degree of noise reduction for the lower frequency sounds is more difficult to achieve because it usually requires at least a doubling of the thickness and length of the acoustical treatment. Figure 6.7.1 shows various types of duct lining baffles and silencers.
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