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Application GuideHVAC Acoustic FundamentalsDecibelSound 200.00010.000020BDBarrierBHS1R1Measurement PointPath Length DifferenceEngineered for flexibility and performance. AG 31-010
Table of ContentsTable of Contents . 2Introduction . 4Using This Manual . 4Sound Basics . 5General . 5Wavelength and Frequency. 6Decibels . 6Sound Pressure vs. Sound Power. 7Octave Bands. 7Human Response to Sound . 8Sound Pressure Evaluation Criteria . 9Acceptable Sound Levels . 14Sound Testing Methods . 15HVAC Equipment Acoustics. 16Calculating Sound Pressure from Sound Power . 16Outdoor Sound Analysis. 22General . 22Outdoor Sound Analysis Basics. 22Open Field Analysis. 23Sound Barriers. 24Reflecting Walls. 27Indoor Sound Analysis – Zoned Comfort Systems . 29General . 29Sound in a Room . 29Room Constant and Sound Absorption Coefficient . 29Thompson and Schultz Equations. 32Indoor Sound Analysis – Ducted Zoned Comfort Systems . 36Radiated and Discharge Sound Power. 36Multiple Path Concept . 36Duct Sound Path . 37Converting Ducted Sound Power to Sound Pressure. 41Duct Breakout Sound Path . 44Return Air Sound Path . 48Diffuser Sound Path. 50Radiated Sound Path. 52Evaluating All the Sound Paths. 53Indoor Sound Analysis – Central Systems . 54General . 54Multiple Paths. 54Central System Duct Sound Path. 55Transmitted Sound. 64Main Duct Breakout . 70Return Air Path . 72Evaluating Multiple Sound Paths and Locations . 75Regenerated Noise. 76General . 76Evaluating Regenerated Noise. 76Conclusions . 78Appendix 1 - References . 79Appendix 2 – HVAC Equipment Sound Measurement Stds. 80General . 80Appendix 3 – Various Acoustic Properties of Materials . 81General . 812Application Guide AG 31-010
The data and suggestions in this document are believed current and accurate at the time ofpublication, but they are not a substitute for trained, experienced professional service.Individual applications and site variations can significantly affect the results andeffectiveness of any information, the reader must satisfy him/herself regarding theapplicability of any article and seek professional evaluation of all materials. McQuaydisclaims any responsibility for actions based on this document.Application Guide AG 31-0103
IntroductionOccupant comfort is the goal of all HVAC designers. Sound (or noise) is a key parameter inmeasuring comfort, in addition to temperature, humidity and Indoor Air Quality (IAQ). Whileacoustics consultants are usually involved in critical applications (such as performing arts centers),the task of creating a comfortable acoustic environment in most other applications falls on the HVACengineer. This is because most background sound sources are generated by the HVAC equipment.The purpose of this manual is to familiarize the designer will the basics of acoustics, and to applythese basics to typical HVAC designs.Using This ManualThe manual can be used as an application guide or as a primer for using the McQuay AcousticAnalyzer software program. The equations and approach described herein were used in thecreation of the Acoustic Analyzer program. Small differences can exist between the software and themanual because the Acoustic Analyzer program uses equations to estimate values as opposed to thetables listed in the manual. However, the difference in values will be very small.Examples that show how to perform some analyses are included in double lined boxes. Helpful tipsare also provided. Tip; In this manual, Sound Pressure will be indicatedby Lp RE 10-12 Pa, while Sound Power will be indicatedby Lw RE 10-12 W.“Tips” box exampleDecibel ExampleCalculate the loudest possiblesound at standard atmosphericpressure (101.3 kPa)Lp 20Log(101,300/0.00002) 194 dB RE 20µ PaInformation in double linedboxes show calculationexamples4Note: 20µ Pa is another way towrite 0.00002 Pa.Application Guide AG 31-010
Sound BasicsGeneralSound is defined as a disturbance in an elastic medium that can be detected by the human ear. Themedium can be gas, liquid or solid. Noise is undesirable sound or sound without value. The pressurewaves (or sound) act on the inner ear, which is what we hear. The best sound is not necessarily nosound. In an open office concept, background sound offers privacy for conversations. The quality ofsound is also important. Tonal sounds are usually not desirable.The following list will help better understand sound.Figure 1- Typical Sound Pressure Levels The amplitude of the sound waverepresents the loudness and ismeasured in decibels (Pascals). Thelouder the sound, the larger theamplitude. The loudest atmosphericsound has zero atmospheric pressureat the low point and two timesatmospheric pressure at the highpoint. This is 194 dB.The frequency of sound representsthe pitch and is measured in Hertz.The higher the frequency, the higherthe sound. The human hearing rangeis from about 16 Hz to 16,000 Hz.Below 30 Hz, sound can be felt aswell as heard.DecibelSound 0010.00010.00002 The wavelengths for sound can vary from 70 feet (21.3 m) at 16 Hz to 0.07 feet (0.02 m) at16,000 Hz. This is important because sound absorbing materials tend to work well when theirdimensions are close to the wavelength. Therefore, a 1-inch (25 mm) ceiling tile is effective atabsorbing higher frequency sounds, but low frequency sounds are much more difficult toattenuate. The human ear can respond to very wide range of sound levels. At the low end, the ear issensitive to sound pressure waves as little as 0.00002 Pa. At the high end, the human ear can hearabout 20 Pa without pain, which is 1,000,000 times louder. This is a key reason why Decibellogarithmic scales are used. The speed of sound is dependent on the density of the medium it is travelling through. The lowerthe density, the slower the sound wave. At standard atmospheric conditions, the speed of sound(Mach 1) is 764 miles per hour (1120 feet per second, 341 m/s). Sound waves do not actually pass through walls or other solid objects. Instead, they impinge onthe exterior surface of the wall or object, causing it to vibrate. This, in turn, causes the airmolecules in the space to vibrate. What is actually happening is that the sound wave is makingthe wall or object move!Application Guide AG 31-0105
Wavelength and FrequencyThe wavelength of sound in air is given by;Eq. 1λ co /fWhereλ is the wavelength in feet (m).co is the speed of sound, which is 1120 feet per second(341 m/s) at sea level.f is the frequency in Hz.DecibelsThe very large range in sound pressure makes a logarithmicscale more convenient. Decibels (dB) are always referencedto base signal. Knowing the base reference is criticalbecause the term “decibels” in acoustics is used for soundpressure and sound power. In the case of sound pressure,the reference is 0.00002 Pascals, which is the threshold ofhearing.Decibel ExampleCalculate the loudest possiblesound at standard atmosphericpressure (101.3 kPa)Lp 20Log(101,300/0.00002) 194 dB RE 20µ PaNote: 20µ Pa is another way towrite 0.00002 Pa.Eq. 2Lp 20Log(P/0.00002)Decibel Addition ExampleAdd the follow values together;L1 80 dBL2 82 dBL3 84 dBL4 93 dBL5 72 dBWhereLp is the sound pressure in Decibels (dB)P is the sound pressure in Pascals.For sound power the reference is 10-12 Watts.80/10Eq. 3Lw 10Log(W/10-12 )WhereLw is the sound power in Decibels (dB)W is the sound power in WattsDecibel Addition and SubtractionSince Decibels are logarithmic, they cannot simply beadded. For instance, 40 dB 40 dB is not 80 dB, it is 43dB. Decibels can be added as follows:Eq. 4Ls 10Log(10L1/10 10L2/10 10L3/10 .)Decibels can be quickly added together with an accuracy ofabout 1 dB by using the relationship shown in Table 1Decibel Addition Chart.6LTotal 10Log(10 82/1084/1093/1072/1010 10 10 10) 94 dBUsing Table 1- DecibelAddition ChartBetween L1 and L2, there is adifference of 2 dB so add 2 dBto L2 for a total of 84 dB.Between 84 dB and L3, there isa difference of 0 dB so add 3dB to 84 dB for a total of 87dB.Between 87 dB and L4, there isa difference of 6 dB so add 1dB to L4 for a total of 94 dB.Between 94 dB and L5, there isa difference of 22 dB so add 0dB to 94 dB for a total of 94 dB.Application Guide AG 31-010
Table 1- Decibel Addition ChartWh
Application Guide AG 31-010 Engineered for flexibility and performance. HVAC Acoustic Fundamentals 0 20 40 60 80 100 120 140 0.00002 0.0001 0.001 0.01 0.1 1.0
