Both of these fittings can increase duct noise.
The ¿noises¿ heard in a room by an occupant can be measured by a meter that detects sound pressure. Because sound pressure is affected by the characteristics of the room ¿ large, small, hard or absorbent surfaces, etc. ¿ a/c equipment is often rated in terms of sound power which is independent of a particular room.

Some manufacturers of registers and grilles provide information on the sound characteristics of their devices. Others suggest minimum velocities to avoid noise problems.

Field installations substantially differ from the conditions of the sound tests, and can seriously alter the actual sound levels generated. Sound measurements in the laboratory are made with a uniform air velocity profile through the outlet. Thus, in illustration ¿A¿ the nonuniform flow through the outlet could increase the noise generated. Often, equalizing grids are specified to control noise as well as mixing performance. Severely offsetting the outlet from the duct connection, as in illustration ¿B¿ can also increase noise above ratings. Increases as much as 10 to 20 decibels are possible depending upon the severity of the offset.

Perhaps the most common and potentially greatest source of noise coming from a register is caused by the balancing dampers immediately behind the grille. Depending upon how severely the dampers must be closed to balance the system, noise levels can increase from 5 to 20 db above ratings. In critical applications, it is preferable to use dampers placed well upstream in the supply duct to balance the air flow.

While velocity is perhaps the most important criterion governing register noise, size is also a factor. Given two identical diffuser models, experiencing the same air velocity through the outlet, the larger outlet will generate more noise. This is also true for return air intakes. Thus, the general rule is to size larger returns based on a lower face velocity than smaller intakes.

Adding insulation helps reduce high frequency sounds.

Insulating elbows helps

Elbows, take off fittings, transitions, etc. help reduce (attenuate) low frequency sounds through the duct system ¿ especially in the 500 Hz range as generated by fans. These duct fittings tend to reflect sounds back upstream. Adding sound insulation helps reduce high frequency sounds and placing the insulation in elbows improves performance.

Lining a duct cannot solve a noise problem if the register is the actual source of the noise ¿ say if fins/bars are vibrating, damper is rattling or air is whistling through the openings. Generally, if the noise is a rumble, it is the result of a fan or blower. A whistle on the other hand is a result of high frequency sound. If the noise disappears when the outlet or inlet is removed, obviously the register is to blame.

Furnace closet applications are probably the most critical duct system application in homes, offices, apartments, etc. in terms of noise abatement. Usually, the closet is adjacent to the occupants and the slightest noise can be disturbing or at least distracting.

This type of installation often features very short return connections to the equipment ¿ and sometimes only one. Next to undersizing the return intake, the short return connection is itself the major cause of noise transmission. Even though the air is moving inward, the sound can radiate outward with the short duct acting like a cheerleader¿s megaphone. The intake grille as a cover is of modest help in reducing equipment noise transmission.

The best ¿ and really only recommended solution ¿ is to avoid short returns completely. As illustrated in B, the return intake should be based on a very low face velocity, return duct lengthened and lined, plus the air should be made to turn at least twice in its path toward the equipment. The fan should, of course, be adequately sized and rotating at modest speeds to minimize the sound power level at its origin.

(Editor¿s note: the above is an excerpt from the Northamerican Heating & Airconditioning Wholesalers Association Home Study Institute booklet, ¿Selection of Supply Outlets & Return Inlets.¿ Contact them at 1389 Dublin Road, P.O. Box 16790, Columbus, Ohio 43216-6790; 614-488-1835; fax 614-488-0482;