1、8.1CHAPTER 8 SOUND AND VIBRATIONAcoustical Design Objective . 8.1Characteristics of Sound . 8.1Measuring Sound 8.4Determining Sound Power 8.7Converting from Sound Power to Sound Pressure 8.8Sound Transmission Paths 8.9Typical Sources of Sound 8.10Controlling Sound . 8.11System Effects 8.13Human Resp
2、onse to Sound 8.14Sound Rating Systems and Acoustical Design Goals 8.15Fundamentals of Vibration 8.17Vibration Measurement Basics 8.18Symbols . 8.19F FUNDAMENTAL principles of sound and vibration controlI are applied in the design, installation, and use of HVAC thethreshold of hearing to the thresho
3、ld of pain covers a range ofapproximately 1014:1. Table 1 gives approximate values of soundpressure by various sources at specified distances from the source.The range of sound pressure in Table 1 is so large that it is moreconvenient to use a scale proportional to the logarithm of this quan-tity. T
4、herefore, the decibel (dB) scale is the preferred method ofpresenting quantities in acoustics, not only because it collapses alarge range of pressures to a more manageable range, but alsobecause its levels correlate better with human responses to the mag-nitude of sound than do sound pressures. Equa
5、tion (1) describes lev-els of power, intensity, and energy, which are proportional to thesquare of other physical properties, such as sound pressure andvibration acceleration. Thus, the sound pressure level Lpcorre-sponding to a sound pressure is given byLp= 10 log = 20 log (2)where p is the root me
6、an square (RMS) value of acoustic pressurein pascals. The root mean square is the square root of the time aver-age of the square of the acoustic pressure ratio. The ratio p/prefisThe preparation of this chapter is assigned to TC 2.6, Sound and VibrationControl.AAref-Table 1 Typical Sound Pressures a
7、nd Sound Pressure LevelsSourceSound Pressure, PaSound Pressure Level, dB re 20 PaSubjective ReactionMilitary jet takeoff at 30 m 200 140 Extreme dangerArtillery fire at 3 m 63.2 130Passenger jet takeoff at 15 m 20 120 Threshold of painLoud rock band 6.3 110 Threshold of discomfortAutomobile horn at
8、3 m 2 100Unmuffled large diesel engine at 40 m0.6 90 Very loudAccelerating diesel truck at 15 m 0.2 80Freight train at 30 m 0.06 70 LoudConversational speech at 1 m 0.02 60Window air conditioner at 3 m 0.006 50 ModerateQuiet residential area 0.002 40 QuietWhispered conversation at 2 m 0.0006 30Buzzi
9、ng insect at 1 m 0.0002 20 PerceptibleThreshold of good hearing 0.00006 10 FaintThreshold of excellent youthful hearing0.00002 0 Threshold of hearingppref-2ppref -8.2 2013 ASHRAE HandbookFundamentals (SI)squared to give quantities proportional to intensity or energy. Areference quantity is needed so
10、 the term in parentheses is nondi-mensional. For sound pressure levels in air, the reference pressureprefis 20 Pa, which corresponds to the approximate threshold ofhearing for a young person with good hearing exposed to a puretone with a frequency of 1000 Hz.The decibel scale is used for many differ
11、ent descriptors relatingto sound: source strength, sound level at a specified location, andattenuation along propagation paths; each has a different referencequantity. For this reason, it is important to be aware of the context inwhich the term decibel or level is used. For most acoustical quanti-ti
12、es, there is an internationally accepted reference value. A refer-ence quantity is always implied even if it does not appear.Sound pressure level is relatively easy to measure and thus isused by most noise codes and criteria. (The human ear and micro-phones are pressure sensitive.) Sound pressure le
13、vels for the corre-sponding sound pressures are also given in Table 1.FrequencyFrequency is the number of oscillations (or cycles) completedper second by a vibrating object. The international unit for fre-quency is hertz (Hz) with dimension s1. When the motion of vibrat-ing air particles is simple h
14、armonic, the sound is said to be a puretone and the sound pressure p as a function of time and frequencycan be described byp(t, f ) = p0sin (2ft)(3)where f is frequency in hertz, p0is the maximum amplitude of oscil-lating (or acoustic) pressure, and t is time in seconds.The audible frequency range f
15、or humans with unimpaired hear-ing extends from about 20 Hz to 20 kHz. In some cases, infrasound(20 kHz) are important, but methods andinstrumentation for these frequency regions are specialized and arenot considered here.SpeedThe speed of a longitudinal wave in a fluid is a function of thefluids de
16、nsity and bulk modulus of elasticity. In air, at roomtemperature, the speed of sound is about 340 m/s; in water, about1500 m/s. In solids, there are several different types of waves, eachwith a different speed. The speeds of compressional, torsional,and shear waves do not vary with frequency, and ar
17、e often greaterthan the speed of sound in air. However, these types of waves arenot the primary source of radiated noise because resultant dis-placements at the surface are small compared to the internal dis-placements. Bending waves, however, are significant sources ofradiation, and their speed cha
18、nges with frequency. At lower fre-quencies, bending waves are slower than sound in air, but canexceed this value at higher frequencies (e.g., above approximately1000 Hz).WavelengthThe wavelength of sound in a medium is the distance betweensuccessive maxima or minima of a simple harmonic disturbancep
19、ropagating in that medium at a single instant in time. Wavelength,speed, and frequency are related by = c/f (4)where = wavelength, mc = speed of sound, m/sf =frequency, HzSound Power and Sound Power LevelThe sound power of a source is its rate of emission of acousticalenergy and is expressed in watt
20、s. Sound power depends on operat-ing conditions but not distance of observation location from thesource or surrounding environment. Approximate sound poweroutputs for common sources are shown in Table 2 with correspond-ing sound power levels. For sound power level Lw, the power ref-erence is 1012W o
21、r 1 picowatt. The definition of sound powerlevel is thereforeLw= 10 log(w/1012)(5)where w is the sound power emitted by the source in watts. (Soundpower emitted by a source is not the same as the power consumed bythe source. Only a small fraction of the consumed power is con-verted into sound. For e
22、xample, a loudspeaker rated at 100 W maybe only 1 to 5% efficient, generating only 1 to 5 watts of soundpower.) Note that the sound power level is 10 times the logarithm ofthe ratio of the power to the reference power, and the sound pressureis 20 times the logarithm of the ratio of the pressure to t
23、he referencepressure.Most mechanical equipment is rated in terms of sound powerlevels so that comparisons can be made using a common referenceindependent of distance and acoustical conditions in the room.AHRI Standard 370-2011 is a common source for rating large air-cooled outdoor equipment. AMCA Pu
24、blication 303-79 providesguidelines for using sound power level ratings. Also, AMCA Stan-dards 301-90 and 311-05 provide methods for developing fansound ratings from laboratory test data. Note, however, someHVAC equipment has sound data available only in terms of soundpressure levels; for example, A
25、HRI Standard 575-2008 is used forwater-cooled chiller sound rating for indoor applications. In suchcases, special care must be taken in predicting the sound pressurelevel in a specific room (e.g., manufacturers sound pressure datamay be obtained in large spaces nearly free of sound reflection,wherea
26、s an HVAC equipment room can often be small and veryreverberant).Sound Intensity and Sound Intensity LevelThe sound intensity I at a point in a specified direction is therate of flow of sound energy (i.e., power) through unit area at thatpoint. The unit area is perpendicular to the specified directi
27、on, andthe units of intensity are watts per square metre. Sound intensityTable 2 Examples of Sound Power Outputs and Sound Power LevelsSourceSound Power, WSound Power Level,dB re 1012WSpace shuttle launch 108200Jet aircraft at takeoff 104160Large pipe organ 10 130Small aircraft engine 1 120Large HVA
28、C fan 0.1 110Heavy truck at highway speed 0.01 100Voice, shouting 0.001 90Garbage disposal unit 10480Voice, conversation level 10570Electronic equipment ventilation fan 10660Office air diffuser 10750Small electric clock 10840Voice, soft whisper 10930Rustling leaves 101020Human breath 101110Sound and
29、 Vibration 8.3level LIis expressed in dB with a reference quantity of 1012W/m2,thusLI= 10 log(I/1012)(6)The instantaneous intensity I is the product of the pressure andvelocity of air motion (e.g., particle velocity), as shown here.I = pv (7)Both pressure and particle velocity are oscillating, with
30、a magni-tude and time variation. Usually, the time-averaged intensity Iave(i.e., the net power flow through a surface area, often simply called“the intensity”) is of interest.Taking the time average of Equation (7) over one period yieldsIave= Re pv(8)where Re is the real part of the complex (with am
31、plitude and phase)quantity. At locations far from the source and reflecting surfaces,Iave p2/0c (9)where p is the RMS sound pressure, 0is the density of air(1.2 kg/m3), and c is the acoustic phase speed in air (335 m/s).Equation (9) implies that the relationship between sound intensityand sound pres
32、sure varies with air temperature and density. Conve-niently, the sound intensity level differs from the sound pressurelevel by less than 0.5 dB for temperature and densities normallyexperienced in HVAC environments. Therefore, sound pressurelevel is a good measure of the intensity level at locations
33、 far fromsources and reflecting surfaces.Note that all equations in this chapter that relate sound powerlevel to sound pressure level are based on the assumption that soundpressure level is equal to sound intensity level.Combining Sound LevelsTo estimate the levels from multiple sources from the lev
34、els fromeach source, the intensities (not the levels) must be added. Thus, thelevels must first be converted to find intensities, the intensitiessummed, and then converted to a level again, so the combination ofmultiple levels L1, L2, etc., produces a level Lsumgiven byLsum= 10 log (10)where for sou
35、nd pressure level (Lp), 10Li/10is p2i/p2ref, and Liis thesound pressure level for the ith source.A simpler and slightly less accurate method is outlined in Table3. This method, although not exact, results in errors of 1 dB or less.The process with a series of levels may be shortened by combiningthe
36、largest with the next largest, then combining this sum with thethird largest, then the fourth largest, and so on until the combinationof the remaining levels is 10 dB lower than the combined level. Theprocess may then be stopped.The procedures in Table 3 and Equation (10) are valid if the indi-vidua
37、l sound levels are not highly correlated, which is true for mostsounds encountered in HVAC systems. One notable exception is thepure tone. If two or more sound signals contain pure tones at thesame frequency, the pressures (amplitude and phase) should beadded and the level (20 log) taken of the sum
38、to find the sound pres-sure level of the two combined tones. The combined sound level isa function of not only the level of each tone (i.e., amplitude of thepressure), but also the phase difference between the tones. Com-bined sound levels from two tones of equal amplitude and frequencycan range fro
39、m zero (if the tones are 180 out of phase) up to 6 dBgreater than the level of either tone (if the tones are exactly inphase). When two tones of similar amplitude are very close infrequency but not exactly the same, the combined sound leveloscillates as the tones move in and out of phase. This effec
40、t createsan audible “beating” with a period equal to the inverse of the differ-ence in frequency between the two tones.Measurements of sound levels generated by individual sourcesare made in the presence of background noise (i.e., noise fromsources other than the ones of interest). Thus, the measure
41、mentincludes noise from the source and background noise. To removebackground noise, the levels are unlogged and the square of thebackground sound pressure subtracted from the square of thesound pressure for the combination of the source and backgroundnoise:Lp(source) = 10 log (10L(comb)/10 10L(bkgd)
42、/10) (11)where L(bkgd) is the sound pressure level of the background noise,measured with the source of interest turned off. If the differencebetween the levels with the source on and off is greater than 10 dB,then background noise levels are low enough that the effect of back-ground noise on the lev
43、els measured with the source on can beignored.ResonancesAcoustic resonances occur in enclosures, such as a room or HVACplenum, and mechanical resonances occur in structures, such as thenatural frequency of vibration of a duct wall. Resonances occur at dis-crete frequencies where system response to e
44、xcitation is high. To pre-vent this, the frequencies at which resonances occur must be knownand avoided, particularly by sources of discrete-frequency tones.Avoid aligning the frequency of tonal noise with any frequencies ofresonance of the space into which the noise is radiated.At resonance, multip
45、le reflections inside the space form a stand-ing wave pattern (called a mode shape) with nodes at minimumpressure and antinodes at maximum pressure. Spacing betweennodes (minimum acoustic pressure) and antinodes (maximumacoustic pressure) is one-quarter of an acoustic wavelength for thefrequency of
46、resonance.Absorption and Reflection of SoundSound incident on a surface, such as a ceiling, is either absorbed,reflected, or transmitted. Absorbed sound is the part of incidentsound that is transmitted through the surface and either dissipated(as in acoustic tiles) or transmitted into the adjoining
47、space (asthrough an intervening partition). The fraction of acoustic intensityincident on the surface that is absorbed is called the absorptioncoefficient , as defined by the following equation: = Iabs/Iinc(12)where Iabsis the intensity of absorbed sound and Iincis the intensityof sound incident on
48、the surface.The absorption coefficient depends on the frequency andangle of incident sound. In frequency bands, the absorption coef-ficient of nearly randomly incident sound is measured in largereverberant rooms. The difference in the rates at which sounddecays after the source is turned off is meas
49、ured before and afterthe sample is placed in the reverberant room. The rate at whichsound decays is related to the total absorption in the room via theSabine equation:10Li10iTable 3 Combining Two Sound LevelsDifference between levels to be combined, dB 0 to 1 2 to 4 5 to 910 and MoreNumber of decibels to add to highest level to obtain combined level 3 2 1 08.4 2013 ASHRAE HandbookFundamentals (SI)T60= 0.161(V/A) (13)whereT60= reverberation time (time required for average sound pressure level in room to
