Acoustical Evaluation of Terminal Units

With the ever increasing competitive environment for office space, the focus on indoor air quality, and the demand for productive working environments, acoustics plays an important role in variable air volume system design and product selection. One of the most significant challenges in acoustic design is determining the allowable sound power level generated by the air terminal.

Considerations

To selector evaluate the performance of different terminal units, the designer must first determine two variables the sound level required for the space in terms of NC (Noise Criteria) or RC (Room Criteria) level; and the acoustic attenuation transfer function of the application. The latter requires the designer to predict system attenuation elements on sound generated by the terminal under design conditions for both casing and discharge sound paths. This factor alone most influences terminal unit selection. If the acoustic transfer function is underestimated, the resulting NC level in the space will exceed design. If overestimated, the first cost of the terminal units may be greater than necessary due to unit oversizing and acoustic treatments. And, in cases where the NC level is too low, noise must actually be added back into the system to mask noise created in the space by the occupants.

Acoustical Evaluation

Before starting any comparison, it is important to know how the product sound power performance data was obtained and to have confidence in its representation. ARI Standard 880 provides industry agreed upon methods for determining the sound power ratings of air terminals and air distribution devices. The ARI certification program polices participating manufacturers in the industry, assuring designers that actual equipment performance matches cataloged data. A note to designers: Not all terminal unit manufacturers participate in the ARI Certification Program.

In most cases, the radiated sound path through the casing is the most critical path. Several variables to determine when considering casing radiated sound are: system operating condition, plenum size, sound path, ductwork design, ceiling type, room size and construction (hard or soft), and distance of receiver from sound source.

For greater accuracy in calculating expected unit performance in the occupied space, the actual installation should be modeled.

Evaluation Tools

Until recently, formal tools available to aid system design and equipment selections based on acoustics were limited to the manufacturer’s catalog data, acoustical mock-ups, computer modeling and ASHRAE standards. A new method utilizing ARI Standard 885-90 is now a feasible alternative.

Manufacturers Catalog Data

Most manufacturers rate acoustical performance in terms of sound power. To assist the designer, assumptions for plenum, ceiling, room attenuation, etc., are also made to offer a catalog NC level at a set of specific operating conditions. Using this NC data is an acceptable means of comparing different products from the same manufacturer. However, this method can be very unreliable when evaluating products of different manufacturers. Typically, their application assumptions are not the same since no guidelines existed (until recently) to enable standardization. Further, assumptions about application and operating conditions rarely resemble the actual installation.

Acoustical Mock-Up

The most accurate, yet most costly, method of calculating expected unit performance is to construct a full scale mock-up of the specific installation that incorporates the selected terminal unit(s). The terminal unit is then operated at actual design conditions, and sound pressure measurements are taken to determine the NC level in the space.

Computer Modeling

A more affordable alternative is an acoustical modeling computer program. These programs calculate acoustical performance in the space based on a system model input by the designer. A trade-off certainly exists between modeling accuracy and simplicity of user input with this method. Depending on the complexity of their designs and input systems, acoustics programs can be relatively easy or difficult to use and provide varying degrees of accuracy.

ASHRAE Standards ASHRAE has published general standards on acoustics and HVAC equipment application. These standards are essentially guidelines that can be used in conjunction with other evaluation techniques, but are incomplete when used alone as an evaluation tool. ASHRAE does not offer a procedure for calculating system attenuation.

ARI 885-90

ARI Standard 885-90 is a procedure for estimating occupied space sound levels in the application of air terminals and air outlets. This standard assists the designer in manually calculating appropriate plenum, ceiling and room attenuation values by octave band for an application. These values can then be applied to the manufacturer’s sound power catalog data to generate estimated sound pressure in the space which can, in turn, be converted to an NC level.

Trane’s NC values recorded in the catalog data section are based upon ARI Standard 885. the designer may also choose to develop his own acoustical attenuation transfer function from this standard to compare various manufacturers.

Acoustical Conclusions

When comparing different manufacturers’ products using NC values, it is imperative that the same assumptions of application and operating condition are used to make a good comparison. Many evaluation tools exist to help the designer model specific applications when they do not match the manufacturer’s cataloged NC data.

Bypass System Application

Traditional bypass system applications involve bypassing air to the return plenum at the terminal unit level. Rather than provide bypass at the unit level, Trane offers the standard VCCE cooling only unit with a static pressure controller as a system bypass unit. One box may be used to dump the required air to the return plenum which makes this a lower cost alternative than conventional bypass boxes.

The VCCE unit used in bypass applications is available only with an electronic static pressure controller and electric actuator. This unit is installed off of the main trunk and set to control at the static pressure setpoint at that location, If there are multiple trunk ducts, a VCCE unit would be placed off of each trunk and each would control to its own unique static pressure setpoint.