1、8.1CHAPTER 8SOUND 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 Respo
2、nse 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 the threshold of hearing to the thresho
3、ld of pain covers a rangeof approximately 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 100 ft 200 140 Extreme dangerArtillery fire at 10 ft 63.2 130Passenger jet takeoff at 50 ft 20 120 Threshold of painLoud rock band 6.3 110 Threshold of discomfortAutomobile hor
8、n at 10 ft 2 100Unmuffled large diesel engine at 130 ft0.6 90 Very loudAccelerating diesel truck at 50 ft 0.2 80Freight train at 100 ft 0.06 70 LoudConversational speech at 3 ft 0.02 60Window air conditioner at 3 ft 0.006 50 ModerateQuiet residential area 0.002 40 QuietWhispered conversation at 6 ft
9、 0.0006 30Buzzing insect at 3 ft 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 HandbookFundamentalssquared to give quantities proportional to intensity or energy. Areference quantity i
10、s needed so 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
11、many different 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 acoustica
12、l quanti-ties, 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
13、pressure levels 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
14、is simple harmonic, 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 freque
15、ncy range for 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 t
16、hefluids density and bulk modulus of elasticity. In air, at roomtemperature, the speed of sound is about 1100 fps; in water, about5000 fps. 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 frequ
17、ency, and are 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 the
18、ir speed changes 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
19、disturbancepropagating in that medium at a single instant in time. Wavelength,speed, and frequency are related by = c/f (4)where = wavelength, ftc = speed of sound, fpsf =frequency, HzSound Power and Sound Power LevelThe sound power of a source is its rate of emission of acousticalenergy and is expr
20、essed in watts. 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-eren
21、ce is 1012W or 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
22、 sound. For example, 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
23、pressure to the referencepressure.Most mechanical equipment is rated in terms of sound power lev-els 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 equ
24、ipment. AMCA Publication 303-79 providesguidelines for using sound power level ratings. Also, AMCA Stan-dards 301-90 and 311-05 provide methods for developing fan soundratings from laboratory test data. Note, however, some HVACequipment has sound data available only in terms of sound pressurelevels;
25、 for example, AHRI Standard 575-2008 is used for water-cooled chiller sound rating for indoor applications. In such cases,special care must be taken in predicting the sound pressure level ina specific room (e.g., manufacturers sound pressure data may beobtained in large spaces nearly free of sound r
26、eflection, whereas anHVAC equipment room can often be small and very reverberant).Sound Intensity and Sound Intensity LevelThe sound intensity I at a point in a specified direction is the rateof flow of sound energy (i.e., power) through unit area at that point.The unit area is perpendicular to the
27、specified direction, and theunits of intensity are watts per square metre. (SI units are used hererather than I-P units because of international agreement on theTable 2 Examples of Sound Power Outputs and Sound Power LevelsSourceSound Power, WSound Power Level,dB re 1012WSpace shuttle launch 108200J
28、et aircraft at takeoff 104160Large pipe organ 10 130Small aircraft engine 1 120Large HVAC 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 electri
29、c clock 10840Voice, soft whisper 10930Rustling leaves 101020Human breath 101110Sound and Vibration 8.3definition.) Sound intensity level LIis expressed in dB with a ref-erence quantity of 1012W/m2, thusLI= 10 log(I/1012)(6)The instantaneous intensity I is the product of the pressure andvelocity of a
30、ir motion (e.g., particle velocity), as shown here.I = pv (7)Both pressure and particle velocity are oscillating, with 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
31、 the time average of Equation (7) over one period yieldsIave= Re pv(8)where Re is the real part of the complex (with amplitude 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(0.075 lb/ft3), and c is
32、the acoustic phase speed in air (1100 fps).Equation (9) implies that the relationship between sound intensityand sound pressure 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 nor
33、mallyexperienced in HVAC environments. Therefore, sound pressurelevel is a good measure of the intensity level at locations 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 soundpress
34、ure level is equal to sound intensity level.Combining Sound LevelsTo estimate the levels from multiple sources from the levels 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
35、level again, so the combination ofmultiple levels L1, L2, etc., produces a level Lsumgiven byLsum= 10 log (10)where for sound 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,
36、although not exact, results in errors of 1 dB or less.The process with a series of levels may be shortened by combiningthe 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
37、 the combined level. Theprocess may then be stopped.The procedures in Table 3 and Equation (10) are valid if the indi-vidual 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 p
38、ure tones at thesame frequency, the pressures (amplitude and phase) should beadded and the level (20 log) taken of the sum 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
39、the phase difference between the tones. Com-bined sound levels from two tones of equal amplitude and frequencycan range from 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
40、 infrequency but not exactly the same, the combined sound leveloscillates as the tones move in and out of phase. This effect 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 sourc
41、esare made in the presence of background noise (i.e., noise fromsources other than the ones of interest). Thus, the measurementincludes noise from the source and background noise. To removebackground noise, the levels are unlogged and the square of thebackground sound pressure subtracted from the sq
42、uare of thesound pressure for the combination of the source and backgroundnoise:Lp(source) = 10 log (10L(comb)/10 10L(bkgd)/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 o
43、n and off is greater than 10 dB,then background noise levels are low enough that the effect of back-ground noise on the levels measured with the source on can beignored.ResonancesAcoustic resonances occur in enclosures, such as a room or HVACplenum, and mechanical resonances occur in structures, suc
44、h as thenatural frequency of vibration of a duct wall. Resonances occur at dis-crete frequencies where system response to excitation 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 f
45、requency of tonal noise with any frequencies ofresonance of the space into which the noise is radiated.At resonance, multiple 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 (minimu
46、m acoustic pressure) and antinodes (maximumacoustic pressure) is one-quarter of an acoustic wavelength for thefrequency of 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 incidents
47、ound that is transmitted through the surface and either dissipated(as in acoustic tiles) or transmitted into the adjoining space (asthrough an intervening partition). The fraction of acoustic intensityincident on the surface that is absorbed is called the absorptioncoefficient , as defined by the fo
48、llowing equation: = Iabs/Iinc(12)where Iabsis the intensity of absorbed sound and Iincis the intensityof sound incident on 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
49、measured in largereverberant rooms. The difference in the rates at which sounddecays after the source is turned off is measured 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 HandbookF
