SEATTLE — While duct silencers help quiet the rumble of air conditioning systems, choosing the wrong ones can create another source of unwanted noise.

In some cases, system designers fail to consider a couple of key factors when including silencers in duct designs, engineers said at the ASHRAE 1999 Annual Meeting, held here.

“It’s an issue of manufacturers’ data versus actual field performance,” Douglas Reynolds of DDR, Inc., said during a session on factors affecting the efficiency of duct silencers.

“It’s also a matter of using manufacturers’ data correctly.”

Information gap

Contributing to the problem is the fact that guidelines for choosing duct silencers have not kept pace with changing technology. Issues concerning breakout noise, duct rumble, and low-pressure systems have not been properly addressed, Reynolds said.

System engineers must consider three types of data when selecting which duct silencers to use.

1. How much the noise level will be cut is the main one.

Silencer manufacturers provide noise-reduction estimates, but specific systems may not achieve those levels for a variety of reasons. Engineers also must consider how much noise the silencers themselves will generate as air passes around them.

“In the middle and high frequencies, the silencers become a primary noise source themselves,” Reynolds said.

2. Pressure loss is another key issue.

“If you plan for a half-inch of pressure loss and put in a system that creates 2% or 3% pressure loss, that could have a drastic effect,” Reynolds said. “The fan is not going to be operating as efficiently. It’s not going to move enough air and it’s going to make more noise.”

3. Finally, the type of silencer used also can affect overall noise reduction and system performance.

Resistance silencers, which include baffles filled with fiberglass, are the most common and they work well, Reynolds said.

While built much the same, reactive silencers lack the fiberglass inside the baffles. Designed for medical buildings and similar facilities where mold, mildew, and other growths are a concern, reactive silencers are less efficient.

A little more high-tech, noise-cancellation silencers cut sound levels by producing a signal 180 degrees out of phase with the noise generated inside air ducts. After reading the frequency level inside a duct system, a noise-cancellation silencer produces the out-of-phase signal and projects it through a speaker inside the duct, quieting the system, Reynolds said.

By considering added noise and pressure issues, engineers and system designers can put together silencer systems that significantly reduce the roar of air-handling systems.