Beware of drumming of rectangular ducts, particularly the top surface. Round ducts are free from this trouble.

If rectangular ducts are used they must be very amply stiffened or “cross-folded”.

In public buildings the ducts were formerly made in builders’ works as they were less likely to cause trouble. Square corners were unavoidable to get into the space provided, but the addi­tion of vanes reduced the pressure loss.

No sharp edges of any form should be left on which air is blown. No splitters of light gauge which might vibrate. Edges turned over to air flow.

Beware of noise from room to room with short connections on a main duct. See Figure 3.78.

Batter: Acoustic treatment In branchas is desirable

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