The art of noise

Engineered Systems, Feb, 2005 by Joanna R. Turpin

He added that prefabricated duct silencers are effective elements that can substantially reduce the noise levels across the audible spectrum--especially in the lower frequency region. "Silencers can be supplied in lieu of duct liner at a cost savings and to meet noise criteria that may otherwise be unachievable," noted Peterman.

This may also be a good option when certain applications, such as hospitals, do not allow the use of duct liner due to potential microbial growth. As Paoli noted, "There are such products as hospital-grade duct silencers, which are basically standard silencers with a Tedlar liner to keep the fiberglass filling from entering the air stream. Packless silencers, which have no fiberglass fill, are also a possibility. They're designed with perforations in the silencer baffles to attenuate the sound."

HOW MUCH IS TOO LITTLE?

While noisy VAV systems are a common problem, another issue is when they are too quiet. This is especially true in open-plan offices, where people don't want co-workers to hear everything they're saying. Speech privacy, which is a condition in which you don't understand enough of what else is going on to be distracted by the conversation, is a desirable condition. To achieve speech privacy, a system should be at around NC 40, but in a VAV system, that noise isn't constant.

Over the years, designers have used white noise sound-masking speaker systems in the ceiling in order to keep noise levels constant. With the advent of desktop computers, those speakers were less necessary, because the fans in the computers make constant (desirable) white noise. Today, however, more people are turning to laptops, which don't make as much noise. This may result in the need for noise-generating equipment again.

VAV systems may also be too quiet if the designer-specified ductwork, diffusers, and terminal boxes that are just too big. This results in higher thermal losses, as well, because the diffusers can't properly distribute air in the space. The only way to fix this speech privacy problem may be to install a sound-masking system.

Designers definitely need to pay attention to the noise levels written into a specification. "During the design phase of open-plan offices, we won't design to NC 30 or 35 because of the speech privacy issues. If we know the client wants background sound masking, we won't design a VAV system to meet an NC 35, because there's no need for it. The sound masking system will increase that level up to NC 40 anyway," said Paoli.

Consulting the proper standard--or hiring an acoustical engineer--is the best insurance a designer can have in making sure VAV systems make just the right amount of noise.

Turpin is ES' contributing editor.