How to implement effective acoustic design with terminal units
As anyone wearing headphones at the office to keep coworkers from bothering them can tell you, noise is an issue in public spaces. While the primary focus of HVAC systems is temperature and humidity control, experienced designers know that controlling sound is vital to a comfortable and productive environment.
Total peace and quiet may sound relaxing, but actually isn’t ideal. Acoustic privacy is a state where someone at their desk can hear nearby conversations, but not enough to be distracted by them. Good acoustical design is not about trying to eliminate sound, but rather manage it appropriately. Too quiet and you destroy acoustic privacy. Too loud and you interfere with speech or even cause hearing damage. You also don’t want any annoying rumbles, hisses, identifiable machinery sounds, or undue modulation of the system cycling off and on over time.
Good design practice requires mechanical system designers to establish the acceptable noise for an occupied space and then determine the selection criteria for all system components. AHRI Standard 885, Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminal and Air Outlets, provides a consistent method and the most current application factors. This article won’t cover the calculations involved, but rather focuses on underlying factors and actionable recommendations.
The key approach to noise control breaks each issue into three overall components: the source that creates the noise, the paths that transmit the noise, and the receiver who hears the noise. Air terminals are often a primary source of sound in a mechanical system. The path for sound emanating from an air terminal is either through the plenum, down the duct into the conditioned space where it reaches the occupant, or some combination of both.
In any application, both radiated and discharge sound needs to be considered. Radiated sound escapes through the terminal casing and/or induction port and travels through the plenum and ceiling to enter the occupied space. Discharge sound travels out the terminal through the ductwork and air outlet. Radiated sound is the primary issue when using fan-powered terminals, while discharge sound is common with non-fan terminals. Unlined ductwork can result in higher discharge sound levels for all terminal types.
Once the radiated and discharge sound paths are determined, the resulting room sound level can be evaluated. Several factors outside the mechanical system play a part, including the dimensions of the room, carpet and furniture, just to name a few. An accurate analysis is a complex process, and as a reminder, AHRI Standard 885 provides a consistent method for accomplishing it.
Engineers can minimize the sound contribution of air terminals to an occupied space with the following good design practices:
- Whenever possible, terminals should be located away from occupied areas and placed over areas less sensitive to noise such as corridors or storage areas.
- Use lined ductwork or attenuators downstream of air terminals to help attenuate higher frequency discharge sound. Used in moderation, flexible duct is also an excellent attenuation element. However, remember to keep flexible duct bends as gentle as possible.
- Sharp edges and transitions in the duct design should be minimized to reduce turbulent airflow and the resulting sound contribution.
- Rather than using mechanical devices to restrict airflow, use fan speed controllers to reduce fan rpm. This form of motor control also often has the added benefit of being more energy efficient, especially when applied to ECMs.
- Separate the air terminal and return air grille as far as possible, in order to allow duct attenuation to reduce discharge sound levels.
- Design systems to operate at low supply air static pressure. Not only will this reduce the generated sound level, it also provides more energy efficient operation and allows the central fan to be downsized.
While it is crucial for designers to understand acoustical ratings in order to write good specifications, the careful selection and proper installation of every element of the entire system are required to ensure an optimal environment. Using the best products for the job will save you time, money and headaches in the long run.