Minimizing Excessive Sound in Ventilation System Design

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1 Minimizing Excessive Sound in Ventilation System Design s

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3 Minimizing Excessive Sound in Ventilation System Design Application Guide Rev. 4, June, 2009 Siemens Building Technologies, Inc. ii

4 Rev.4, June, 2009 NOTICE Document information is subject to change without notice by Siemens Building Technologies, Inc. Companies, names, and various data used in examples are fictitious unless otherwise noted. No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Siemens Building Technologies, Inc. All software described in this document is furnished under a license agreement and may be used or copied only in accordance with license terms. For further information, contact your nearest Siemens Building Technologies, Inc. representative. Copyright 2004 by Siemens Building Technologies, Inc. TO THE READER Your feedback is important to us. If you have comments about this manual, please submit them to: SBT_technical.editor@siemens.com CREDITS APOGEE is a trademark of Siemens Building Technologies, Inc. Other product or company names mentioned herein may be the trademarks of their respective owners. Printed in U.S.A. Siemens Building Technologies, Inc. ii

5 Table of Contents About this Application Guide Purpose of this Guide How this Guide is Organized Suggested Reference Materials Symbols Getting Help Where to Send Comments I I I II III III III Chapter 1 Introduction 1 Scope of This Guide 1 HVAC Sound Transmission 2 Background Sound 2 Laboratory Applicability 2 Computer Program Sound Analysis 2 Chapter 2 Physics of Sound 5 Sound Wave Propagation 5 Sound Wave Parameters 6 Sound Measurement Parameters 8 Sound Power Level 8 Decibels 9 Sound Pressure Level 11 Octave Bands 13 A-Weighted Sound Level 16 NC Curves 17 RC Curves 18 Determining an RC Rating 20 Step 1. Measure Existing Sound Pressure 20 Step 2. Mark Average Sound Pressure 20 Step 3. Plot Curve of Octave Band 21 Example RC Analysis 21 Chapter 3 HVAC Sound Sources 23 Sources of Sound in HAVC Systems 23 Fan Sound Components 24 Fan Aerodynamic Sound 24 Siemens Building Technologies, Inc. i

6 Purpose of this Guide Blade Frequency Increment 24 Fan Efficiency 25 Fan Sound Power Level Data 25 Fan Sound Power Level Calculation 26 Step 1. Actual Operating Conditions Increase 26 Step 2. Blade Frequency Increment (BFI) 26 Step 3. Efficiency Correction 27 Example Fan Sound Power Level Calculation 28 Step 1. Actual Operating Conditions Increase 28 Step 2. Blade Frequency Increment (BFI) 29 Step 3. Efficiency Correction 29 Damper Airflow Noise 30 U (Velocity Factor) 30 Calculate Pressure Loss Coefficient C 30 Calculate Damper Blockage Factor BF 31 Calculate the Velocity Factor U 31 K Factor 32 Example Damper Sound Power Level Calculation 33 Elbow Airflow Noise 34 K Factor 35 Example Elbow Sound Power Level Calculation 36 Junction and Takeoff Airflow Noise 38 K Factor 38 JC Factor 39 Example Duct Takeoff Sound Power Level Calculation 40 Air Delivery Device Sound 43 Flexible Duct Connection to Diffusers 44 Discharge Sound and Radiated Sound 44 Sound Breakout and Break-in 45 Laboratory Elements 45 Chapter 4 HVAC Sound Attenuation 47 Introduction to HVAC Sound Attenuation 47 Plenums 48 Example Plenum Attenuation Calculation 49 Duct Attenuation 51 Rectangular Unlined Sheet Metal Ducts 51 Example Rectangular Duct Attenuation Calculation 51 Rectangular Unlined, Externally Insulated, Sheet Metal Ducts 53 Rectangular Acoustically Lined Sheet Metal Ducts 54 Round Unlined Sheet Metal Ducts 57 ii Siemens Building Technologies, Inc.

7 About this Application Guide Round Acoustically Lined Sheet Metal Ducts 57 Duct Elbows 58 Example Rectangular Duct Elbow Attenuation Calculation 60 Duct Takeoffs and Divisions 60 Duct Silencers 62 End Reflection 62 Environment Adjustment Factor 63 Space Effect 63 Radiated Sound Attenuation 64 Chapter 5 HVAC System Sound Analysis 67 Introduction to HVAC System Sound Analysis 67 Example HVAC System Sound Analysis 67 Step 1. Actual Operating Conditions Increase 68 Step 2. Blade Frequency Increment (BFI) 69 Step 3. Efficiency Correction 69 Duct Section A 70 Duct Elbow B 70 Duct Section C 72 Junction D 73 Duct Section E 75 Junction F 76 Duct Section G 76 Duct Takeoff/Junction H 77 Duct Section I 80 Duct Elbow J 80 Reheat Terminal 82 Duct Sections L 82 Perforated Diffuser 83 End Reflection 83 Space Effect 83 Commentary on HVAC System Sound 88 Laboratory Room Sound Analysis 88 Laboratory Room Ambient Sound 89 Fume Hood Sound 89 Terminal Radiated Sound -Example Analysis 90 Radiated Sound 91 Discharge Sound 93 Terminal Radiated Sound -Example Analysis 2 94 Siemens Building Technologies, Inc. iii

8 Purpose of this Guide Chapter 6 Minimizing HVAC Sound 95 Introduction to Minimizing HVAC Sound 95 Basic System Design Criteria 95 Fans 96 Duct Configurations 97 Terminal Equipment 97 Sound Attenuation Devices 100 Passive Sound Attenuation Devices 100 Linings 100 Duct Silencers and Attenuators 100 Ceiling and Wall Absorbers 101 Enclosures 101 Active Sound Attenuation Devices 101 Sound Measurement Instrumentation 103 Sound Measurement Procedure 103 Appendix 105 NC and RC Curves, Tabular Listing 105 NC Curve 106 RC Curve 107 Sound Analysis Worksheet 108 Sound Measurement Worksheet 109 Glossary 111 Index 115 iv Siemens Building Technologies, Inc.

9 About this Application Guide This section discusses the following topics: Purpose of this guide How this guide is organized Suggested reference materials Conventions and symbols used It also provides information on how to access help and where to direct comments about this guide. Purpose of this Guide This application guide explains the nature of sound generation and attenuation within air movement components of HVAC systems, and is intended to help the reader understand how to achieve a ventilation system design that does not generate excessive or objectionable sound. It is the intent of this guide to provide a working level of HVAC sound dynamics knowledge for the benefit of those who may not have yet acquired a sufficient technical background in the subject of HVAC sound analysis. How this Guide is Organized This application guide contains the following chapters: Chapter 1, Introduction, discusses laboratory control and safety solutions. It includes a scope of this guide and discusses HVAC sound transmission, background sound, laboratory applicability, and computer program sound analysis. Chapter 2, Physics of Sound, discusses the properties of sound and how sound is measured. It includes sound wave propagation and parameters; measurement parameters; NC and RC curves; and how to determine an RC rating. Chapter 3, HVAC Sound Sources, discusses sources of sound associated with HVAC systems. It includes fan sound components and power level calculation; damper and elbow airflow noise; junction and takeoff airflow noise; and air delivery device noise. Chapter 4, Ventilation Systems Classification, discusses the attenuating effect of common HVAC system elements (also referred to as transmission loss or insertion loss). Chapter 5, HVAC System Sound Analysis, provides examples of how to analyze the components of a specific HVAC system. Siemens Building Technologies, Inc. I

10 About this Application Guide Chapter 6, Minimizing HVAC Sound, offers general guidance on minimizing excessive or objectionable HVAC sound. The Appendix contains blank copies of certain graphs and forms that appear in this document. They are intended to be copied and used for sound measurement and analysis. The Glossary describes the terms and acronyms used in this manual. The Index helps you locate information presented in this application guide. Suggested Reference Materials In addition to this application guide, the following publications are recommended sources of detailed technical information associated with minimizing sound in ventilation systems: American Society Of Heating, Refrigeration, & Air Conditioning Engineers, Inc.: A Practical Guide To Noise and Vibration Control HVAC Applications, 1991 (Chapter 42 - Sound and Vibration Control) Fundamentals, 1993 (Chapter 7 Sound and Vibration) Sheet Metal and Air Conditioning Contractors National Association Inc. (SMACNA): HVAC Systems Duct Design Air Movement and Control Association, Inc.: Laboratory Method of Testing in-duct Sound Power Measurement Procedure for Fans ANSI/AMCA Methods for Calculating Fan Sound Ratings from Laboratory Test Data AMCA Reverberant Room Method for Sound Testing of Fans AMCA Application of Sound Power level Ratings for Fans AMCA Air Conditioning & Refrigeration Institute: Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets (ARI ) Standard for Air Terminals (ARI ) American Society of Mechanical Engineers United Engineering Center: Measurement of Industrial Sound ANSI/ASME PTC II Siemens Building Technologies, Inc.

11 Symbols Symbols Department of Labor, Occupational Safety & Health Administration, Superintendent of Documents, U.S. GPO: Occupational Exposure to Hazardous Chemicals in Laboratories; Final Rule, 29 CFR Part 1910, 1990 The following table lists the symbols used in this guide to draw your attention to important information. Notation Symbol Meaning WARNING: Indicates that personal injury or loss of life may occur to the user if a procedure is not performed as specified. CAUTION: Indicates that equipment damage, or loss of data may occur if the user does not follow a procedure as specified. Note Tip Provides additional information or helpful hints that need to be brought to the reader's attention. Suggests alternative methods or shortcuts that may not be obvious, but can help the user better understand the capabilities of the product. Getting Help For more information about minimizing sound in ventilation systems, contact your local Siemens representative. Where to Send Comments Your feedback is important to us. If you have comments about this guide, please submit them to: SBT_technical.editor@siemens.com Siemens Building Technologies, Inc. III

12 About this Application Guide IV Siemens Building Technologies, Inc.

13 Chapter 1 Introduction Chapter 1 introduces laboratory control and safety solutions. It includes the following topics: Scope of this guide HVAC sound transmission Background sound Laboratory applicability Computer program sound analysis Scope of This Guide This application guide focuses on HVAC air movement and distribution system generated sound. It does not specifically address sound or vibration problems of other related mechanical system components such as boilers, chillers, cooling towers, pumping, and piping systems. It is the intent of this document to provide sufficient background information in the basics of sound and its application to air systems to enable the reader to properly use equipment manufacturer s sound rating data in the design of a ventilation system. Sound and vibration are a science in themselves and an all-inclusive study is beyond the scope of this guide. Additionally, it is believed that the reader need not delve too deep into the theory to achieve a practical working knowledge of the subject. For these reasons, this guide will limit its approach to only the essential elements of acoustics theory and will attempt to emphasize practicality rather than theory whenever possible. For those readers who want more background on the subject or need additional information, the Suggested Reference Materials section in the About this Application Guide lists a number of books, and other sources of more detailed and specialized technical information on the subject of HVAC sound and vibration. The information in this guide should assist in handling typical ventilation system design applications for offices, laboratories, classrooms, and the like. However, the reader is cautioned that more specific and detailed knowledge is warranted if the system design is intended for applications where sound is a much more critical issue. This includes acoustical laboratories, recording studios, and any location where maintaining a very low or specific sound level is crucial. If any of these types of applications are a part of an HVAC design project, it is recommended that the designer consult an appropriate acoustical or sound specialist for guidance. Siemens Building Technologies, Inc. 1

14 Chapter 1 Introduction HVAC Sound Transmission Ventilation system ductwork conducts or transmits sound in the same way that any conduit can convey sound. We re all familiar with how effectively a hose or pipe can conduct the sound waves of the human voice. In the same way, ductwork conducts fan noise, and other component sounds to the areas served. If the sound level is excessive or the sound pattern is annoying, it can cause dissatisfaction with an HVAC system that otherwise does an excellent job of maintaining comfort and providing the proper level of ventilation. Background Sound It is important to understand that the typical goal of a properly designed ventilation system is not to obtain the least possible amount of sound, but to achieve a specific sound level and profile. In most applications, a specific background sound level and sound profile are desirable since it helps cover or mask other objectionable sounds. In the workplace, a good ventilation system provides just enough background sound to prevent other sounds (telephone conversations, keypad clicking, copy machines, etc.) from being excessively annoying. This desirable background sound level and profile is sometimes termed white noise and is usually very noticeable when not present. (Recall how much louder common office sounds seem to be if the ventilation system is shut down and the white noise is not present.) However, there is a sound level threshold that is dependent upon the room and its activity, and when exceeded, results in excessive and objectionable sound. Laboratory Applicability Of the many HVAC applications where ambient sound level is an important design component, laboratory applications are a particular challenge to the HVAC designer, due to the necessity for providing high room ventilation rates to ensure the health and safety of the occupants. Computer Program Sound Analysis Computer programs that assist the HVAC designer in producing optimum duct design layouts often include a sound analysis capability that saves having to manually perform sound analysis calculations. As with all such design aids, it is important that the user be knowledgeable about the fundamentals of the subject in order to properly use the program. In addition, having this background knowledge enables the user to recognize whether a specific program incorporates an acceptable analytical approach. 2 Siemens Building Technologies, Inc.

15 Computer Program Sound Analysis Siemens Building Technologies, Inc. 3

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17 Chapter 2 Physics of Sound Chapter 2 discusses the properties of sound and how sound is measured. It includes the following topics: Sound wave propagation Sound wave parameters Sound measurement parameters NC Curves RC Curves Determining an RC rating Sound Wave Propagation The human ear hears or senses sound when oscillations or vibrations occur within its hearing mechanism. Under normal circumstances, these oscillations are transmitted to our ear as sound waves that are really air pressure waves. These air pressure waves impact upon the ear s sensing or hearing mechanism and cause it to oscillate or vibrate. As sound waves travel to the ear, they may travel not only through air but also use different mediums as well. Recall that a basic physics classroom experiment on sound consists of putting a sound generating device (sometimes an alarm clock) under a large glass container (typically a bell jar). The vibrations of the sound generating device cause sound waves in the air within the bell jar that travel outward until they reach the glass wall of the bell jar. There, they cause the wall of the bell jar to vibrate that in turn causes sound waves to be generated in the air outside of the bell jar. These sound waves then continue and eventually reach the ears of those in the classroom. As long as the bell jar contains room air at normal atmospheric pressure and density, the above scenario takes place and the sound is easily heard. However, after a vacuum pump removes most of the air from inside the bell jar, the sound made by the sound generating device is dramatically reduced since the density of the air within the bell jar has been substantially reduced and thus has a much more limited impact on the wall of the bell jar. This experiment shows that sound waves are highly dependent upon having an adequate medium for their transmission. Siemens Building Technologies, Inc. 5

18 Chapter 2 Physics of Sound This experiment also shows another important element of sound wave transmission; that sound waves traveling through air are not dependent upon movement of the air itself. Although the air in the bell jar could not leave the jar, the sound traveled outward from the jar without involving any physical movement of the air out from the inside of the bell jar. Likewise, sound movement in a ventilation system is not dependent upon the movement or direction of the airflow. Not only will sound generated in the supply side of a ventilation system travel in the direction that the air happens to be moving to the areas served, but also sound generated in an exhaust system will travel opposite the direction of airflow and also be heard in the areas served by the exhaust system. Sound Wave Parameters Any analysis or study of sound (acoustics) is especially concerned with the generation and reception of sound waves. It is necessary to first understand the fundamental concepts of sound wave generation and how this relates to the overall science of acoustics. Once these fundamentals are understood, actual quantifiers or measurement parameters can be applied and used in actual acoustical design practice. Figure 1 shows a diagram of the major parameters that apply to the analysis of sound. At the left side of the diagram is the sound source shown as a solid dot. Anytime sound is produced, there must be a sound source. When we speak, our vocal chords create sound and are the sound source. As a sound source creates sound in the air, it radiates energy outward in the form of compressed air waves or sound waves. Figure 1. Sound Wave Parameters. 6 Siemens Building Technologies, Inc.

19 Sound Wave Parameters Unless there is a barrier, the sound waves continue to travel outward in all directions in a spherical manner, until they either are absorbed by an object or their energy level is dissipated by the surrounding air. With regard to understanding the science of sound and its effects, it is necessary to have an understanding of two fundamental terms: sound power and sound pressure. These terms are not interchangeable and it is important to have a clear understanding of each term. The intensity of the sound at the source is expressed in terms of sound power and establishes the energy level of the sound. Sound power is the parameter that indicates the total energy or power output of the sound. It is universally expressed in terms of watts. The sound power spectrum that we are familiar with ranges from a high point of 10,000 (10 4 ) watts of sound power for a jetliner takeoff or gun fire, to a low of (10-9 ) watts for a soft whisper. Ultimately, sound waves, which in our context, are really compressed air waves, will ultimately impinge on a receiver and at that point their effect is expressed in terms of the sound pressure. For our purposes, the most common receiver will normally be the eardrum of a person who hears the sound. Another common example of a sound receiver is a microphone that is part of a sound amplification system. Sound power itself, however, does not really establish whether a sound will be interpreted as loud or soft by the receiver. That is entirely dependent upon the amount of energy loss or attenuation of the sound waves that occurs prior to impinging upon the receiver as sound pressure. Attenuation, which is simply a decrease in the sound power before it gets to the listener, occurs primarily due to two factors: distance and physical barriers. When sound is generated in an open or unconfined space, as in Figure 1, the primary attenuation factor is the distance between the sound source and the receiver. When a sound source generates sound, the sound power energy is radiated outward in all directions, as shown in Figure 1, and the sound power energy is dissipated over a rapidly increasing area. This can be likened to an ever expanding sphere surrounding the source of the sound. An analogy of the effect of sound power radiation would be like having a fixed quantity of paint and the task of achieving a uniform thickness paint coating on the surface of the sphere. In this analogy the paint quantity represents the available sound power level, while the resulting thickness of the paint coating on the surface of the sphere represents the sound pressure level. As the radius of a sphere increases, the surface area also increases and the thickness of the surface coating must be decreased. If a sphere having a radius of 1 foot (surface area square feet) is expanded until the radius is doubled (becomes 2 feet), the surface area would have increased to square feet or four times the original surface area. This of course means that the paint coating on the surface could then only be 1/4 of the previous thickness. Likewise, each time the distance between a sound source and the receiver is doubled, the effect at the receiver that is the sound pressure level is reduced by a factor of 4. Siemens Building Technologies, Inc. 7

20 Chapter 2 Physics of Sound The sound pressure level is the most widely used parameter in the field of acoustical engineering since it is the closest thing to what we experience in terms of loudness or softness of a sound. In the previous analogy, the thickness of the paint coating on the spherical surface represented the effect that a certain quantity of paint could have on the surface of the sphere. Therefore,sound pressure expresses the effect that the sound power energy has when impinging upon a unit area of the receiver. Sound Measurement Parameters To use sound generation and attenuation data in the design of HVAC systems, it is necessary to understand the measurement parameters with which sound power, sound pressure, and other factors involved are quantified. These key factors are each listed below with an explanation of their units of rating or measurement. Sound Power Level As discussed above, sound power expresses the overall sound energy of the sound source and sound power level is represented in terms of watts. However, as was previously indicated, the range between the highest and lowest sound power levels is too large to conveniently use actual watt values. In addition, the actual watt values that apply to the typical sound power levels encountered in HVAC are so small (that is, 10-3 to watts) that too many zeros would be needed after the decimal point to express specific watt values. A more convenient scale is to represent sound power level so the decibel () unit is used. With decibels, sound power low level begins at 0 decibels, which is just enough power for the human ear to begin hearing something that is right next to the ear. At the upper end of the scale is the sound power level of a jetliner taking off, which could be 160. (An explanation of decibels and how specific physical measurements are expressed in terms of decibels follows.) Be sure not to confuse sound power level with sound pressure level (even though both are expressed in terms of decibels). Remember: Sound power level expresses the power or energy of the sound source. Sound pressure level expresses the loudness or effect of the sound at the receiver. When we hear a soft whisper, we are experiencing a sound power level of approximately (10-9 ) watts. When we converse in a normal voice, we are experiencing a sound power at about (10-5 ) watts. If we had the unfortunate experience to be just below a jetliner taking off, we could later tell everyone that we experienced a sound power level of around 10,000 (10 4 ) watts. (However, it may take us a few days before we could again hear their reply.) As previously discussed, wattage based direct sound measurements are non-linear and vary over a very large range, thus it becomes clumsy to stay with watts as the basic unit of measurement. 8 Siemens Building Technologies, Inc.

21 Sound Measurement Parameters We could improve the situation by using a direct comparison or ratio between the two different sound power levels. For example, when comparing a normal voice to a whisper, we could divide 10-5 by 10-9 that would yield 10,000. In other words, a normal voice has about 10,000 times more sound power than a whisper. This same approach would also tell us that the sound power from a jetliner takeoff is 1,000,000,000 times more sound power than a normal voice. Unfortunately, these numbers are still awkward to work with because of the large number of zeros. We can remove these zeros by using logarithms. Recall that a logarithm to the base 10 (or log for short), means that the power of 10 would be raised to become the number that we re concerned with. In other words, if we re working with the number 10,000, the log is simply 4, since 10 4 = 10,000. If we re working with 1,000,000,000 the log would be 9 since 10 9 = 1,000,000,000. Decibels Using a comparison or ratio approach for large numbers tends to make it easier to relate to the data. In addition, converting large numbers to logarithms further reduces the amount of digits (and possible errors) when handling numbers comprised of many digits. This is where the use of decibels offers a practical approach to quantifying sound parameters since a decibel is based upon both a ratio and numbers converted into logarithms. The first thing to note with regard to a decibel is that it expresses a ratio or makes a comparison between two values; it is not a specific unit of measurement such as a watt, pound, or even a foot of length. (Since decibels are based upon a ratio, they can be applied to many other different scientific parameters besides sound.) Therefore, with regard to applying decibels (), a reference point must be established as one component of the ratio. With regard to sound power and sound pressure values, a bel is simply the logarithm of the ratio of two different sound power or sound pressure levels. A decibel is 10 bels. (The reason for using decibels instead of just staying with bels is that we do ourselves a favor by getting rid of any decimals in the final values.) This may sound complicated and possibly somewhat confusing, but you ll understand it better after going through the process of establishing decibels () for the previous examples of a whisper, a normal voice, and the jetliner takeoff. Then (hopefully) you ll see the advantage of using instead of the large decimal numbers that are required to express values in wattage. Since we re really only concerned with the sound that humans hear, we ll use the threshold of hearing as the common point of the comparison ratio. The sound power at the threshold of hearing is generally accepted as watts (10-12 watts), so this will always be the reference point or one of the two parts to each sound power ratio. Siemens Building Technologies, Inc. 9

22 Chapter 2 Physics of Sound The following formula will yield the decibels for any absolute value of sound power that we compare to the threshold of hearing: Where: Lw = 10 x Log (W Wref) Lw = the sound power level in. W = the power of the specific sound in watts. Wref = the reference point and is always watts. Using this formula, let s determine the sound power level of a whisper, which produces a very tiny amount of sound power around watts (10-9 watts). Using the above formula this becomes: Lw = 10 x Log ( ) = 10 x Log (10 3 ) = 10 x 3 = 30 Therefore, the sound power level of a whisper is approximately 30. Using the formula again, let s determine the sound power level of a normal conversational voice that is around watts (10-5 watts). Lw = 10 x Log10 ( ) = 10 x Log10 (10 7 ) = 10 x 7 = 70 And, using this same formula for the jetliner takeoff sound power level of 1,000.0 watts (10 4 watts) becomes: Lw = 10 x Log10 ( ) = 10 x Log10 (1016) = 10 x 16 = 160 It s much easier having the numbers determined above (30, 70, and 160 ) instead of having to refer to the actual wattage values when comparing sound power levels. Note that when we say that a whisper is 30, a regular voice is 70 and a jetliner takeoff is 160, we are really comparing these individual levels with respect to the threshold of hearing that is Siemens Building Technologies, Inc.

23 Sound Measurement Parameters Again, there are no units associated with decibels since they are a comparison between two values, (or more scientifically, a ratio between different magnitudes). Also, decibels are used for different parameters besides sound power level. Decibels are also used to express sound pressure level, which is discussed below, and as we are keenly aware, is a different sound parameter than the sound power level. Sound Pressure Level As previously stated, sound pressure is concerned with the effect that a specific sound power level has on a receiver that is usually some distance away from the sound source. Recall in our analogy about covering the surface of an expanding sphere with a fixed quantity of paint, a receiver is like a limited area of the sphere, it will receive only a small portion of the paint (sound power). Therefore, a receiver is only exposed to a portion of the total sound power. In other words, the effect of the sound power becomes less and less (is attenuated more and more) on the receiver. Therefore, the sound power level and sound pressure level are different parameters and cannot be used interchangeably. However, decibels also are used to express the ratios of the relative sound intensity or loudness at the receiver. The basic unit of acoustic pressure is the Pascal (Pa). (One PSI is equivalent to 6,895 Pascals.) Even though a Pascal is a very small unit of pressure measurement, the specific values of Pascals that are encountered with sound pressure are so small and vary over such a wide range that the decibel approach is applied to express sound pressure levels in a more practical manner. The basic formula to determine a specific sound pressure level in decibels is: Where: Lp = 10 x Log (P Pref) 2 Lp = the sound pressure level in. P = the pressure of a specific sound at the receiver in Pascals. Pref = the reference pressure and is always. 2 x 10-5 Pascals is approximately the sound pressure on an eardrum at the hearing threshold. A person speaking in a normal voice, about three feet away from a listener, will produce a sound pressure of around 0.02 Pa. Using this formula, let s determine the sound pressure level in that the listener would experience. Lp = 10 x Log (2 x x 10-5 )2 = 10 x Log (10 3 ) 2 = 10 x Log (10 6 ) Siemens Building Technologies, Inc. 11

24 Chapter 2 Physics of Sound = 60 Therefore, the sound pressure level of normal conversation for the listener is approximately 60. To give a feel for the common range of values for both sound power and sound pressure, Table 1 gives some typical values for the common levels in our environment. Table 1. Common Sound Power & Sound Pressure Levels. Source Sound Power Level Sound Pressure Level Jetliner takeoff * (at 100 ft away) Very loud sound (race car engine, gun fire) * Very noisy room (loud machinery) * Somewhat noisy room (computer printout room) Normal voice conversation (at 3 feet) General office Soft whisper (at 5 feet) Leaf rustling Hearing threshold 0 0 * Hearing protection should be used in sound pressure levels of 90 or more since permanent hearing loss will occur after exposure of 8 or more hours at 90. OSHA limits the permissible exposure time for unprotected workers in a sound pressure level that averages 90 or more. Remember, even though the decibel values in Table 1 are almost the same for the sound power level and the sound pressure level, they represent different physical parameters. In HVAC design, we are mostly concerned with the sound pressure level experienced by an occupant in an area served by an HVAC system. Even though you will find that the sound power level of a sound source such as an HVAC system supply fan may be considerably high (such as, 90 or more ), the objective is to design and configure the HVAC system so the sound pressure level will be attenuated down to an acceptable level, perhaps 35, when heard by an occupant of the area served by the HVAC system. Table 2 lists a few rules that generally predict how a person perceives loudness of sound as changes occur in the sound pressure level. Table 2. Effects of Sound Pressure Level Changes. Change Effect 0 to 2 None 3 Just noticeable 8 to 10 Increase Twice as loud 8 to 10 Decrease Half as loud 12 Siemens Building Technologies, Inc.

25 Sound Measurement Parameters Figure 2 illustrates another important rule regarding sound pressure levels and distance. Whenever the distance between a sound source and a receiver is doubled, the sound pressure level at the receiver is reduced by 6 from its previous value. This is a very important relationship. For instance, if a sound source produces a sound pressure level of 40 at a receiver 15 feet away, the sound pressure level would be reduced to 34 if the distance away were doubled to 30 feet. (Note that this is a non-linear relationship and the results cannot be interpolated. Therefore, in this example, it would be incorrect to assume that the level drops at 2 for every 5 foot increase in distance. If the distance were again doubled from 30 feet to 60 feet, the sound pressure level at 60 feet would be 28. This relationship continues each time the previous distance is doubled. If the sound pressure level was 60 at a 12 foot distance between the sound source and the receiver, what would the sound pressure level be at a distance of 100 feet? Using the distance doubling rule, the sound pressure levels at various distances would be: 60 at 12 ft 54 at 24 ft 48 at 48 ft 42 at 96 ft 36 at 192 ft Since 100 feet is slightly more than 96 feet, the level would be perhaps just a bit less than 42. Figure 2. Sound Pressure Level Decrease Due to Distance. Octave Bands Sound can vary in pitch or frequency from a very low base sound to a very high pitch sound such as a squeak. In terms of actual frequency, human hearing ranges from about 20 cycles per second (Hz) at the low end to around 20,000 Hz at the high end. The actual frequency span of hearing varies from person to person and tends to decline somewhat as we age with the upper frequency end of our hearing being the portion mostly affected by age. Siemens Building Technologies, Inc. 13

26 Chapter 2 Physics of Sound Previously, we discussed the terms sound power level and sound pressure level and arrived at how their intensity was expressed in decibels. If you recall how the screen of an oscilloscope looks when it s monitoring the audio output of a speaker, you can visualize that sounds are usually composed of a multitude of tones at different frequencies. To scientifically describe a particular sound accurately, a curve should be plotted showing the sound power level or sound pressure level in decibels with reference to the frequency. Since the normal audible spectrum covers the frequency range of 20 Hz to 20,000 Hz, it would be totally impractical to deal with each individual frequency. For this reason, it has become customary in sound analysis to divide the overall audible spectrum into 8 frequency bands called octave bands. (These are often referred to as 1/1 Octave Bands.) In each band the highest frequency is twice the lowest frequency, and the mid frequency of each band is used for identifying the octave band and as the specific frequency for expressing the sound power level or sound pressure level in decibels. Figure 3 illustrates how sound curves can be shown on a graph that plots the sound pressure level at each of the standard octave band mid frequencies. The resulting curves establish what s referred to as a sound criterion curve for the particular sound. Figure 3. Sound Pressure Level vs. Octave Band - Sound Criterion Curves. With reference to Figure 3, the scale ascends from 0 to 90 along the vertical axis and the center frequencies of 10 bands are along the horizontal axis. Note that the frequency scale is not linear but increases rapidly in moving from left to right. 14 Siemens Building Technologies, Inc.

27 Sound Measurement Parameters Note that a solid line curve in the lower left portion of the graph is labeled as the approximate threshold of a hearing curve. This represents the sound pressure level that must be present in a person s eardrum in order for the person to hear a particular sound frequency. Recall that in Table 1, the threshold for hearing is listed as 0 sound pressure level. With reference to Figure 3, this really applies to sound frequencies above 4,000 Hz that are in the area of the high pitched beep of a computer speaker. At the lower frequencies, the sound pressure level must be considerable higher to be audible. The sound pressure level of a particular sound such as a fan running, a transformer hum, or car horn can be measured with a sound level meter at a specific distance from the sound. The sound pressure in at each frequency band can be plotted on the graph and the resulting curve will show the profile of the sound similar to the dotted line and dashed line curves shown in Figure 3. The dotted line curve is a predominantly lower frequency curve since it has a high level in the lower frequency bands and a lower level in the higher frequency bands. This sound is characterized as rumbly or somewhat like a drumming sound. The dashed line curve is just the opposite and is characterized as a hissy type sound or somewhat like an air leak. In order for a sound to be acceptable for sound masking (white noise) or as an acceptable background, it must be fairly well balanced across the audible sound spectrum. Since neither of these two sound curves are well balanced, they would not be acceptable for sound masking and instead would probably be very annoying. Table 3. Adding Sound Pressure or Sound Power Levels. Difference between the highest and lowest of multiple sounds at a specific octave band center frequency Add this to the highest of the sounds to obtain the resultant at the octave band s center frequency For example, in Figure 3 at the 500 Hz frequency, one sound pressure level is at 24 and another at 50. The difference between them is 26. With reference to Table 3, this is well beyond the 16 difference. As a result, 0 is added to the higher one (50 ) that results in no change to the total sound pressure level. Siemens Building Technologies, Inc. 15

28 Chapter 2 Physics of Sound Figure 4 shows the resulting sound pressure level when combining the two curves of Figure 3 using Table 3. Note that where the individual curves of Figure 1 are more than 16 apart, the resultant always equals the higher value of the individual curves. Incidentally, the resulting sound produced by the combined sound curve of Figure 4 would be a combination of a rumble and hiss and would still be objectionable as an ambient sound. Figure 4. Two Sound Pressure Levels Combined. A-Weighted Sound Level In an effort to come up with a simpler method to address sound ratings for equipment, A-weighted sound levels that also use decibels, are sometimes used particularly when compliance with OSHA noise limits is the issue. However, the A-weighted criterion is limited to only being a reference of the overall loudness and does not represent the full frequency distribution characteristics of a sound. In particular, it does not specifically indicate the presence of the low frequency level sound component, which is the most important area of sound analysis. 16 Siemens Building Technologies, Inc.

29 NC Curves NC Curves Avoid using A-Weighted sound criterion when designing HVAC systems or conducting a detailed analysis of the sound pressure level in a room with the intent of improving the room ambient sound profile. Using A-Weighted values should be limited only to general noise level comparison measurements or when involved in ensuring against exceeding permissible occupational sound levels. In an effort to come up with ambient sound pressure level curves that provide a good balance between the sound frequency spectrum and the acceptable loudness for various room applications, standard Noise Criterion (NC) curves have been developed. Figure 5 shows the family of NC curves. Until now, this document has avoided using the term noise since there is no scientific way to define it. It is merely a term that each individual subjectively applies to a sound profile that for them ranges from unwelcome or bothersome to very annoying. With regard to the Noise Criterion curves, they establish balanced sound (noise) levels that are generally acceptable for specific room applications. In other words, the sound produced by an HVAC system serving a specific application would be acceptable by the vast majority of occupants if the sound pressure level that it produces does not exceed the level of the appropriate NC curve at any point, and it also has the same general shape as the referenced NC curve. When analyzing a given room sound profile, it is also acceptable to visually interpolate between the NC curves. For instance, if the highest penetration of a listed curve (NC 45 in this case) is 52 at 500 Hz, then the measured sound can be stated as having an NC 48 rating. The NC curves were developed in 1957 and are still widely used today. However, note that they do not include any values for frequencies below 63 Hz. In general, the most objectionable HVAC noise is the low frequency rumble that is produced by HVAC fans. The bulk of this sound occurs below the 63 Hz octave band. Siemens Building Technologies, Inc. 17

30 Chapter 2 Physics of Sound (See the Appendix for a copy of this graph that is suitable for reproduction.) Figure 5. Noise Criterion Curves. RC Curves The Room Criterion (RC) rating is a more recent development for analyzing and rating the sound present in a room. The RC rating should be used, whenever possible, in specific design applications since it is superior to the NC curves for the following reasons: 1. The RC curves extend down to 16 HZ that covers the low frequency sound spectrum more completely than the NC curves. 2. Establishing the applicable RC curve that applies to an actual sound profile is dependent on the overall shape or profile of the actual sound curve, rather than merely the highest penetration of the sound into the NC family of curves. 3. Since a given sound curve is likely to have a unique curvature or profile, each RC sound curve is further annotated as to its actual characteristics: A curve with a rumbly (low frequency) component is also given an R suffix. A curve with a hissy (high frequency) component is also given an H suffix. A more neutral curve without a rumbly or hissy component is given an N suffix. 18 Siemens Building Technologies, Inc.

31 RC Curves If an identifiable predominant tone exists in the sound (such as, clicking, whining, whistle, etc.), a T is also added to the above suffix. If excessive vibration is present, a V suffix is also added to the above. Figure 6 shows the standard family of Room Criterion curves. Table 4 lists specific applications and the maximum acceptable sound criterion that apply when referencing these curves. Note that the Criterion level is always the level of the particular RC curve as it passes through 1,000 Hz. Utilizing the NC curves for design purposes typically results in background sound characteristics having a noticeable rumble or hiss. Although the level may be acceptable for speaking, the overall sound profile is less likely to be as acceptable as a design that is based upon the RC Criterion. Table 4. Applicable NC and RC Sound Criterion Curves for Various Applications. Application Criterion Level General Office Private Office Conference Room Corridors Hospital Room Surgical Room Classroom Cafeteria Library Lobby Auditorium Washroom Research Laboratory Educational Laboratory Sound Studio Siemens Building Technologies, Inc. 19

32 Chapter 2 Physics of Sound (See the Appendix for a copy of this graph that is suitable for reproduction.) Figure 6. Room Criterion Curves. Determining an RC Rating To determine what RC rating should be applied to an existing room, follow the steps listed below. Step 1. Measure Existing Sound Pressure Measure the existing sound pressure level in the room in decibels at all of the octave band center frequencies. Calculate an average value from the room values obtained at 500, 1,000, and 2,000 Hz. Step 2. Mark Average Sound Pressure Mark the average obtained in Step 1 on an RC Criterion graph on the 1,000 Hz vertical scale. Create an RC reference curve by drawing a line through this point that parallels the standard RC curves. (Note that the standard slope of an RC curve is a loss of 5 per frequency band as it goes from left to right.) 20 Siemens Building Technologies, Inc.

33 Determining an RC Rating Step 3. Plot Curve of Octave Band Plot an actual curve of all of the octave band frequencies obtained in Step 2 on the graph, and compare this curve with the reference curve drawn in Step 2. If the actual curve does not depart from the reference curve throughout all octave bands by more than 5, the actual curve is considered to be neutral. The suffix N is added to the value obtained in Step 1. If the actual curve is above the reference curve by more than 5 at any octave frequency less than 500 Hz, the actual sound is considered to be rumbly. The suffix R is added to the value obtained in Step 1. If the actual curve is above the reference curve by more than 3 at any octave frequency greater than 500 Hz, the actual sound is considered to be hissy. The suffix H is added to the value obtained in Step 1. If the actual sound has an identifiable predominant tone such as a clicking, whining, whistle etc., the actual sound is considered to have a tonal character. The T suffix is also added to the N, R, and H suffixes. Example of RC Analysis If an existing room has an actual measured sound profile as listed in the following chart, what RC Criterion would apply? The average at 500, 1,000, and 2,000 Hz is calculated as: ( ) / 3 = 36. With respect to Figure 7, the RC reference curve is plotted as the dashed line and the actual sound curve is plotted as a solid line. Note that the actual curve does not exceed the reference curve by more than 5 below 500 Hz, nor more than 3 above 500 Hz. Thus, the sound RC criterion for this particular room sound would be classified as neutral and is summarized as: 36 (N). Although this particular sound has a slight rumble as indicated by the rise above the reference curve in the lower frequencies, it would still be very acceptable as an overall sound level for applications requiring an RC 35 level. Siemens Building Technologies, Inc. 21

34 Chapter 2 Physics of Sound Figure 7. Actual Room Sound Profile Curve vs. RC Reference Curve. 22 Siemens Building Technologies, Inc.

35 Chapter 3 HVAC Sound Sources Chapter 3 discusses sources of sound associated with HVAC systems. It includes the following topics: Sources of sound in HVAC systems Fan sound components Fan sound power level calculation Damper airflow noise Elbow airflow noise Junction and takeoff airflow noise Air delivery device noise Sources of Sound in HAVC Systems Sound associated with HVAC systems and equipment is generated from multiple sources. All operating equipment generates sound by the inherent vibration of its components. This includes HVAC fan systems, pumps, and the primary mechanical equipment (boilers, chillers, air compressors, etc.). All of these units contain mechanically rotating components that generate operational sound. This sound travels both as sound waves through the air and by transmission of vibrations through adjoining elements of the building structure including walls, floors, pipes etc. In addition to the sound generated by the mechanical components of rotating equipment, the air movement produced by a fan generates aerodynamic sounds due to interaction with the distribution system components including dampers, duct fittings, junctions, terminal units, air diffusers and inlet grilles. Rotational equipment sound is primarily attenuated by isolating the equipment from occupied areas of a building, incorporating physical barriers to sound waves and utilizing vibration isolation to prevent vibrations from being transmitted through the building structure. (Information on attenuating equipment operational sound is given in a later section. Siemens Building Technologies, Inc. 23

36 Chapter 3 HVAC Sound Sources HVAC aerodynamic sound is somewhat harder to attenuate since the ductwork provides a direct conduit for its transmission to the conditioned spaces. In addition, some aerodynamic sound is generated locally by HVAC system supply and exhaust components associated with the room served by the system. On the supply side, this includes VAV box dampers, reheat or cooling coils, air diffusers, and associated duct fittings. On the exhaust side, this primarily involves the room exhaust terminals, laboratory fume hoods and other specialized room exhaust units. Other sources of locally generated sound associated with HVAC systems includes water flow through reheat coil valves, fan powered terminal units, and sometimes even sound caused by bleeding or exhausting compressed air from the HVAC control system. Fan Sound Components Fans are the predominant source of HVAC system sound. The fan sound power level must be known to determine its contribution to the sound pressure level in a given space served by the fan system. Fan sound is made up of several components. However, before we discuss how to determine the overall fan sound power level, it will help to understand the nature of each individual component affecting fan sound. Fan Aerodynamic Sound Aerodynamic sound is generated by air in motion. As you blow out a candle, an aerodynamic sound is produced by the air rapidly passing through your lips. Since a fan imparts a high level of motion to the air, it also results in significant aerodynamic sound. Fans are tested for the sound power level produced by the manufacturers according to standard tests covered by ASHRAE Standard , and also by AMCA Standard Virtually all fan manufacturers also send their fans to the AMCA laboratory for certification of their test data. For greater accuracy of data, these tests cover the sound levels produced in 1/3 octave bands. (Each of the eight octave bands is further divided into three bands thus making 24 bands in all for the test. Three sound power level values are thus obtained for each of the eight octave band. This data is then converted into the sound power level for each of the eight octave bands and becomes the published data.) Although fan manufacturers provide sound power level data for each of their different sizes and types of fans, the data cannot cover each possible combination of operating conditions (airflow, static pressure, etc.) in which a given fan may be applied. Therefore, fan sound power level data is typically given at one set of standard operating conditions that also is a common denominator for all fans. This consists of an airflow of 1 cfm and a static pressure of 1.00 in. WC. With this data, the fan sound power level at other operating conditions can be determined through a calculation process that includes additional fan sound components. Blade Frequency Increment Before the arrival of electronic sound producing equipment, emergency warning sounds were commonly produced by a mechanical device such as the unmistakable wailing sound of a fire truck siren. The mechanical siren was very similar in design to a fan in that it had a rotor with blades or slots that produced vibrations as it rotated. 24 Siemens Building Technologies, Inc.