From our discussion of sound-related terminology, we can infer that specifying quality sound for an application requires us to:

• Determine the desired acoustical character, and ...
• Choose an appropriate single-number descriptor, keeping in mind the limitations inherent in each numbering scheme.

For example, suppose an air-cooled chiller will be placed adjacent to a building where a local ordinance limits sound to 50 dBA. Such a requirement might be stated in the specification as:

"The A-weighted sound pressure level shall not exceed 50 dB re 20 micropascals, measured on the slow response scale, anywhere along the property line. The period of observation shall be at least 60 seconds at each measurement location."

Similarly, the specification for an air handling unit to be situated indoors might state:

"The sound pressure measured re 20 micropascals shall not exceed NC 40 anywhere in the occupied space. Measurements shall be taken on the slow setting, and the period of observation shall be at least 60 seconds at each measurement location."

Analysis Is Key

Both examples highlight an important point: including a single-number descriptor in the specification means that someone must make an acoustical analysis to determine if the proposed HVAC equipment will satisfy acoustical requirements. To make such a prediction, the analysis must convert equipment sound power ratings to sound pressure and assess the effect of environmental factors.

Unless the application is extraordinarily simple, sound that reaches the occupied space will be altered by ductwork, room furnishings and the like. The validity of an acoustical analysis, therefore, depends on the analyst's familiarity with construction details.

The source-path-receiver model provides a systematic approach to acoustical analysis. As its name suggests, this modeling method traces sound from its origin (e.g., at a fan or compressor) to the site at which it's heard (e.g., around a conference table). Everything that sound encounters as it travels between these two points constitutes the "path."

Sound emanating from a source will likely follow more than one path, so the sound level at the receiver will be the collective sum of the paths' analyses. Figure 5 shows the typical sound paths associated with an air handler installed in a mechanical equipment room next to an occupied space.

Acoustical Alchemy

Defining the model's endpoints is straightforward. Manufacturers provide sound power data for source equipment and owners set sound pressure targets for the receiver rooms. The work, and art, of acoustical analysis lies in identifying and quantifying the path elements that attenuate or amplify sound. Theoretical equations aid the analysis of some path elements, but prediction equations based on test data and experience prevail.

ASHRAE collected and developed numerous logarithmic prediction equations for path components in HVAC systems, and subsequently published them in their Algorithms for HVAC Acoustics handbook. (Fortunately, software tools are available to spare analysts from solving these iterative, calculation-intensive equations manually.)

An acoustical analysis based on the source-path-receiver model can help the system designer write a specification that's more likely to satisfy the acoustical target and provide "quality sound." From such an analysis, the designer knows the path attenuation provided and can directly specify the maximum allowable equipment sound power. For example, a typical sound power specification for an air handler might read:

"Sound power levels for the unit shall be determined in accordance with AMCA 300-95, and shall not exceed the values in the following table at design conditions ... "

Octave
Band,           Sound Power Level 
Hz      (Unweighted), dB re 1 pW         
        Discharge       Inlet + Casing  
63        102             100   
125       100              99   
250       101              99   
500        98              97   
1000       95              95   
2000       92              90   
4000       90              87   
8000       90              85   

Continue on to Putting It Together.