The art of noise

Engineered Systems, Feb, 2005 by Joanna R. Turpin

When it comes to managing noise levels in VAV systems, there's more there than meets the ear. How much is too much? That's a good place to start. After that, silencers, duct liners, and vinyl flexible duct are but three tools at your disposal. Elsewhere, designers may face a less predictable predicament: What is too little noise? Consider these angles to master the sound of success.

VAV systems are commonly used in a wide variety of commercial and institutional buildings, including hotels, hospitals, office buildings, and retail applications. These systems are usually seen as being beneficial to building owners and occupants, in that they both save energy and can provide a high level of comfort.

VAV systems are different from constant volume systems in that they adjust the airflow to the occupied space based on the building's heating and cooling requirements. When less heating or cooling is needed, the fan system reduces the amount of air that is supplied to the space.

VAV systems rely on control dampers and often include distributed fan boxes for the various environmental control zones. The dampers can create airflow-generated noise, which can travel into the occupied space. The terminal units used to control the airflow into the zone are often located above the occupied space, and their casing-radiated and discharge sound levels can affect the amount of noise in the occupied space.

There will almost always be noise in the space when the terminal unit is sitting above the ceiling, but how much noise is too much? That depends on numerous factors, including the application involved, ceiling height, room sound levels, occupants, and time of day. Since it is very difficult to make an existing VAV system quiet, the best advice is to make sure noise levels will be appropriate for the application during the design phase.

MEASURING NOISE LEVELS

Noise in any HVAC system can be measured as a function of frequency in decibels, typically divided into eight octave bands. That result is then compared against a criteria curve, such as the noise criterion (NC) family of curves. Karl L. Peterman, P.E., acoustical engineer for Vibro-Acoustics, notes that ASHRAE Handbook--Applications 2003 gives some guidelines for various spaces based on their use, but not based on the system type.

"Private offices are typically designed for NC 30 to 35, while open offices are designed for NC 40 to 45. Experience has shown that sound levels in offices above NC 45 can start to create complaints," noted Peterman.

Dan Int-Hout, chief engineer at Krueger, believes that measuring average spectra, such as dBA, typically doesn't tell the whole story. "That data is meaningless to anyone in trying to solve the problem. If you want to know what's going on, you have to take sound level as a function of frequency or octave band, and that takes a spectrum analyzer. Then you can do diagnostics."

Having said that, Int-Hout also acknowledges that classroom specifications are written in dBA, because the research on which they were based was measured in dBA. The research, and the A-scale in particular, typically focuses on speech intelligibility. Speech happens predominantly in the mid-frequencies, so that's all a specification may be concerned with. "But if you take readings in dBA there may be a huge low frequency component happening that you don't know about, because dBA is a weighted average," he added.

Indeed, some VAV systems have other problems that cannot be addressed through comparing the level against the NC curves. These include rumbles, whines, and tonal noise problems, which are typically the result of some improper design, noted Peterman.

"If the box above the ceiling has a bad motor, you're going to get a motor hum. It's something that's going to be more of a tonal type of noise, so it'll be more annoying," said David G. Paoli, acoustical engineer with Shiner + Associates, Inc., Chicago. "Sometimes the system might hiss, which also causes complaints. These types of noises usually cause more complaints than general broadband sound."

HOW MUCH IS TOO MUCH?

Noise is definitely subjective: What's objectionable to one person may be just fine to another. Occupants may be more aware of the noise in a VAV system simply because it causes changes in the airflow, and people notice that. Constant volume systems can be just as noisy, though their sound levels are constant.

One way around this problem is to use a series fan box. These VAV boxes have a fan on the discharge unit that runs at a constant speed. "These boxes have been used for 20 years, but they're expensive and they may use more energy, so they're typically found in high-end applications. But they keep the noise constant," noted Int-Hout.

Designing a system correctly is really the only way to make sure a VAV system isn't too noisy. Proper location of VAV devices and proper selection of noise control elements will help the design to meet standard background sound level criteria. When noise control or proper selection has been neglected, then there will be a greater probability for noise problems.