Applied Acoustics Dan Int-Hout, Chief Engineer, Krueger ASHRAE Fellow & Life Member

Objectives Basic Acoustical Concepts Understanding Sound Power & Sound Pressure The Goals The Tools Setting Meaningful Specifications

Setting Meaningful Specifications Getting the Best Overall Value! Why do we care? Setting Meaningful Specifications Effective Problem Solving Getting the Best Overall Value!

Basic Concepts

Single Number Ratings NC, NR, & RC dBA (A, B & C Scale) Sones & Bells STC & NRC

Evaluating Sound Data Few products have octave band sound power data. Octave band data is difficult to gather and use. A single number value cannot be used in an acoustical acceptability analysis.

Equipment Return Inlet Sound Tests AHRI 880 AHRI 260 ASHRAE 70 Discharge Sound Radiated Equipment Return Inlet

ASHRAE Standard 70 Method of test for diffusers, grilles and registers Sound measurement requires the equivalent of 10 diameters of straight duct to eliminate and inlet effects! Test method says to subtract 10dB for room effect, in all octave bands to estimate NC When using ANY manufacturer’s data, one should probably add 5dB/NC for top inlet ceiling diffusers with flex connections and more typical room absorption. Add 2dB/NC for side inlet plenums and ducted grilles. Krueger is the only manufacturer to publish octave band sound data for All air outlets. AHRI 885 spreadsheet has a way to include outlet generated sound.

End Reflection Low frequency sound traveling down a duct will partially reflect back when encountering a rapid change in area. The smaller the duct, the greater the effect. There is an end reflection reduction in the reverb room in AHRI 260, but not in 880 until recently. 880 has ignored reverb room end reflection for 40 years, but no longer!

End Reflection

End Reflection The result of applying the end reflection correction to reverb room data is as much as a 10dB increase at 125 Hz. Reported NC can go up as much as 10NC. (the smaller the discharge duct, the greater the correction) All manufacturers were required to include this correction in 2012. Not everyone has! Engineers should modify their sound requirements!

NC Curves

Typical NC Plot NC Rating is 45 since the highest tangent NC line is the one at 45. Approximate threshold of human hearing.

Diffuser NC Calculation

Diffuser & Box NC Calculation

NC & Speech Privacy The Blue Area is the preferred background levels that was determined by test conducted in the 70’s. The speech region is from 500 to 2000. As you notice, for an NC of 35, There is not much speech privacy.

Making Accurate Predictions

Sound Paths Sound Pressure Lp D 2 3 4 5 6 1 C O 1 = Casing Radiated & Inlet 2 = Duct Breakout 3 = Distribution Duct Breakout 4 = Flex Duct Breakout 5 = Discharge 6 = Outlet Generated Sound

AHRI 885 Application Standard AHRI Standard 885 formalized the process of estimating room sound pressure from known sound power. It includes ASHRAE tables for duct loss, breakout, and other factors. Designed for use with 880 data, it applies to 260 as well The most recent version is AHRI 885-2013 (www.ahri.org).

AHRI 885 Contains flexible duct insertion loss and breakout tables. Contains a unique table for the ceiling plenum ‘transfer function’. Defines the space effect.

Space Effect Diffuser NC typically assumes a 10 dB room effect. Actual room absorption is independent of room ‘harness’ and varies by band. Absorption is dependent on room volume and distance from the sound source.

Space Effect

Radiated Sound 885 Radiated sound is estimated by subtracting a combined ceiling/space effect (independent of room parameters) and the environmental effect from the unit’s radiated Sound Power.

Discharge Sound Factors Factors that effect discharge sound transmission include End Reflection, Duct Insertion Loss, Flexible Duct Loss, and Elbows. Power is reduced in all bands as the ducts branch out. The Space Effect (and Environmental Effect) must be included. End Reflection sound reduction should be included, even though sound power ratings were adjusted for it.

Discharge Sound Factors Mid Frequency, Hz 125 250 500 1000 2000 4000 Input Pwl 65 63 66 64 Lining Reduction 2 3 9 18 17 12 Elbow Reduction Tee Reduction Power Splits 5 End Reflection 4 1 Flex Duct Reduction 6 10 20 21 Other (Custom)   Environmental Adjustment Factor Room Attenuation 7 8 Total Attenuation 28 30 40 51 38 Output Spl 37 33 26 15 Output NC Output RC 19

Discharge Sound Factors In the past, it was difficult to compare different manufacturer’s units. In 1999, AHRI 885, Appendix E was established with standard deducts as a function of flow rate. Manufacturers are now required to use Appendix E to compute NC in catalogs.

Discharge Sound, Appendix E

Radiated Sound, Appendix E In appendix E, only one ceiling type is used.

Good Sound Specs Should… Be based on clearly stated assumptions. Reflect real project needs. Specify octave band sound power. Specify max allowed sound power. Require “end reflection adjusted” data And Should Not… Over silence. Hinder IAQ performance.

Calculation Spreadsheet

Wrapping Up

Summary Acoustical response is greatly effected by installation. Effects can be predicted. Avoid problems through careful design and installation. After construction, all problems are difficult. It’s easier to design right, than to correct!

Summary NC levels are well suited for product ratings. RC levels are a better indication of quality. Over-silencing may cause poor indoor air quality. Tools are available to assist in the analysis. Download AHRI 885 from www.ahri.org.

ANY QUESTIONS?

Contact Information Dan Int-Hout dint-hout@krueger-hvac.com Read my Blog on the Krueger website! www.krueger-hvac.com