AHRI 885 Calculator

Three Calculations in One Simple File

Discharge Sound

The program’s discharge tab defaults to the Appendix E attenuation values for a small (< 300 unit), the most conservative of the three airflow settings, but can easily be modified to any of the available attenuation values. The input sound power data is assumed to have been generated using either AHRI 880 (or AHRI 260). Please note that sound power data for VAV terminals from all manufacturers has recently (as of January 2012) been adjusted for end reflection in the reverb room. This does not mean that the end reflection in the calculations should not be used, as it will likely be quite different than the one calculated for the reverb room. Most installations have at least one end reflection in the path between the sound source and the room. Please note that a field is provided for a user input sound attenuation value. If a duct silencer is provided between the source and the room, input the values here, being sure that any self- generated sound created by input air velocities has been accounted for. A graphical representation of the input and output sound values is provided.

Radiated Sound

The radiated calculations assume a standard mineral tile ceiling (per appendix E) but can also be modified. The input sound power data is assumed to have been generated using either AHRI 880 (or AHRI 260). Please note that the ceiling/plenum/room attenuation is a combined number. This is from ASHRAE Research project 755, which determined that a noise source in a ceiling plenum acts as an area, not a point, source. If there is no ceiling, however, one should apply the radiated sound power as a discharge sound source, being sure to include the whole room volume in the room attenuation field. Please note that a field is provided for a user input sound attenuation value, such as a silencer on the induced air inlet. A graphical representation of the input and output sound values is provided.

Room Sound

The room sound tab combines the radiated and discharge room sound pressure values with a calculated air outlet sound pressure value. This requires that the outlet’s reverb room sound pressure be input, and carried forward the room volume and distance values from the discharge tab. It also allows for input of the number of outlets, assuming all are equa-distant from the observer. For ceiling diffusers, which typically have less than ideal inlet conditions, Krueger recommends adding 5dB to the device’s octave band sound power ratings. This calculation will be more realistic than the 10dB/band reduction provided in most manufacturer’s catalogs. Our Product Catalog and K-Select provides octave band sound power data for all air outlets. Output data includes NC, RC and dBA values. A graphical representation of the input and output sound values is provided.

Additional Information

Additional tabs include the calculations per AHRI 885, an NC table, and the dBA calculator. LEED V4 (due out in October 2013) is expected to reference AHRI 885 as a means of determining estimated HVAC sound in both LEED for schools (a prerequisite of 40 dBA) as well as the other LEED programs. At present, the ASHRAE Handbook, which is also referenced by LEED, does not include the ceiling/plenum table from 885; therefore, it cannot realistically be used in a room sound calculation for a plenum sound source.