机械噪声控制技术.ppt

1、Chapter 7 Mechanical Noise Control Techniques 第7 机械噪声控制技术,7. 1 Noise Reduction by Sound Absorption 7. 2 Noise Insulation 7. 3 Silencers and Mufflers 7. 4 Vibration and Noise Reduction by Damping,Types of sound-absorbing materials Sound-absorbing materials are utilized in almost all areas of noise co

2、ntrol engineering. The porous sound-absorbing materials are available in the form of mats, boards, mineral fibers, open cell foams. They have open pores with typical dimensions below 1 mm that are very smaller than the wavelength of sound. Here, each can be treated as a lossy homogeneous medium.,Cha

3、pter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,Full reticulated plastic foam, Partially reticulated plastic foam, Glass fiber, Mineral wool.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound

4、 Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,How porous materials absorb sound Owing to the acting sound pressure, the air molecules oscillate in the interstices int:stis (空隙)of a porous material with the frequency of exciting sound wave. The oscillations result in frictional losses, a

5、nd they convert the sound energy into heat. Changes in flow direction and expansions and contractions of the flow through irregular pores result in loss of momentum in the direction of wave propagation.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Poro

6、us Sound-Absorbing Materials（多孔吸声材料）,Physical properties of porous sound-absorbing materials It is impossible to predict the behaviour of most sound-absorbing materials entirely on the basis of theoretical models, principally because of their geometric and structural complexities. Commercial softwar

7、e for modelling vibro-acoustic fields in poroelastic materials is now available, but it requires substantial inputs of empirical empirikl（实验） data.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,Many experimental

8、studies of the behaviour of common sound-absorbing material of which the structural skeletons are effectively rigid has shown that there are three gross parameters that principally control their sound absorption characteristics.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by S

9、ound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,Porosity （孔隙率） Porosity is defined as the ratio of the volume of voids to the total volume occupied by the porous structure: it is symbolized herein by h.,where Vg is the volume of gas phase, Vs is the volume of solid phase, Vm is the vo

10、lume of material,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,Porosity,s is density of the solid (frame) and m is bulk density(容积密度) of the porous material. Consider a fiberglass insulation product:,It is genera

11、lly in excess of 95% in mineral and glass wools and porous plastic foams.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,(2) Flow Resistivity (比流阻）,where p is the steady pressure differential across a homogeneous

12、layer of thickness x , is face velocity of the flow through the material (actually it is the average velocity within the material) , V is the volume velocity of air passing through the test sample during the time period t, and S is face area (one side) of the sample.,Flow resistivity (specific flow

13、resistance) is a most important physical characteristic of a porous material. It is defined as,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,(2) Flow Resistivity The flow resistance of a sheet of material of thic

14、kness x is given by x . The flow resistivity of common absorbing materials typically lies in the range 2 to 2 kg/(m3s). For a given material bulk density, flow resistivity increases strongly as fiber diameter is decreased.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound A

15、bsorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,(3) Structure Factor （结构因子）,The various influences of the geometric form of the skeleton on effective density and compressibility are lumped together into a structure factor symbolized by s.,The structure factor decreases with increasing frequ

16、ency and ranges from extreme high value of s=6 down to s=1 but generally falls in the range of s=1.3 . Most numerical calculations use s=1.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.1 Porous Sound-Absorbing Materials（多孔吸声材料）,(1) The modified plane wav

17、e equation,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.2 Plane Wave Sound Propagation in Porous Materials,where is the effective bulk modulus of the gas.,(2) Harmonic solution of the modified plane wave equation,Setting,Substituting it into the modified

18、 wave equation yields,or,is called complex wavenumber.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.2 Plane Wave Sound Propagation in Porous Materials,(2) Harmonic solution of the modified plane wave equation,For a harmonic progressive wave, . We write t

19、he complex wavenumber , in which is the attenuation constant and is the propagation constant.,Therefore, the general solution of the modified plane sound wave equation may be expressed as,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.2 Plane Wave Sound Pr

20、opagation in Porous Materials,(2) Harmonic solution of the modified plane wave equation,The spatial distribution of instantaneous pressure is illustrated by the followed Figure.,Exponential attenuation of a progressive harmonic wave: instantaneous pressure distribution,Chapter 7 Mechanical Noise Con

21、trol Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.2 Plane Wave Sound Propagation in Porous Materials,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,Sound energy absorption coefficient,where Ea and Ei are the absorbed and i

22、ncident energies, respectively, R is the reflection coefficient defined as the ratio of the reflected and incident sound pressure at the interface.,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,The sound pressures and particle veloc

23、ities in the air and porous materials are expressed as,At x=0, the particle velocity is zero, yields C=D,where,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,At x= -t , the continuities of sound pressure and particle velocity give,Co

24、mbining above expressions, one obtain the sound pressure reflection coefficient as,Sound absorption characteristics of finite thick layer of porous material Principle factors influencing sound absorption characteristics of finite thick layer of porous material are: Density of material Thickness of m

25、aterial Air gap behind the material Perforated facing Temperature and humidity,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,Sound absorption characteristics of finite thick layer of porous material,超细玻璃棉归一化吸声系数曲线,Chapter 7 Mechanic

26、al Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,下限频率,下半频带宽度,Sound absorption characteristics of finite thick layer of porous material,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,Prin

27、ciple factor influencing sound absorption characteristics of finite thick layer of porous material:,Density of material,Thickness of material,容重增加，低频吸声系数变大，但高频吸声系数降低；容重过大会使总的吸声效果明显降低。一般超细玻璃棉容重大约可取1525kg/m3，矿渣棉为120 130kg/m3。,如前页图，增加吸声材料厚度会使材料吸声系数曲线向低频方向移动。容重一定时，frD约等于材料中声速的四分之一。,Chapter 7 Mechanical

28、Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,Air gap behind the material,空气层的作用相当于增加材料厚度，改善低频吸声效果。 研究表明，当空气层厚度为入射声波1/4波长的奇数倍时，吸声材料处的声波质点振速最大，吸声效果最好。 当空气层厚度为入射声波1/2波长的倍数时，吸声材料处的声波质点振速最小，吸声效果最差。 一般取空气层厚度为715cm。,Chapter 7 Mechanical Noise Control Techniqu

29、es 7. 1 Noise Reduction by Sound Absorption 7.1.3 Large Plat Absorbers,Perforated facing,Temperature and humidity,护面层常用材料：玻璃丝布、塑料窗纱、金属丝网、穿孔板等。当穿孔率大于20%，可以忽略护面层对吸声效果的影响。,温度高、湿度大都对吸声效果有负面影响。前者使共振频率向高频移动，后者更是使吸声系数降低（材料空隙被水分占据）。,Structures of absorbers 吸声尖劈： 吸声尖劈的结构如图所示，属于阻抗渐变型结构。室内环境空间中的尖劈用多孔吸声材料做成，外包玻

30、璃纤维布或金属丝网。吸声尖劈具有优良的吸声性能，高于截止频率的频段的吸声系数均高于0.99。截止频率的大小由吸声材料、尖劈总长度及空气层厚,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,度决定。例如，采用玻璃棉、矿渣棉等优质吸声材料制作的尖劈总长度为lm，后留空气层厚度5一l0cm时，截止频率可达70Hz。因此吸声尖劈被广泛地用于消声室中。,b) 共振吸声结构 薄板共振结构： 薄板共振吸声结构的结构形式是在周边固定在

31、框架上金属板、胶合板等薄板后，设置一定深度空气层。由薄板的弹性和空气层的弹性与板的质量形成一个共振系统，在系统共振频率附近具有较大的吸声作用。,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,穿孔板吸声结构： 这种吸声结构是在钢板、胶合板等类薄板上穿孔，并在其后设置空气层，必要时在空腔中加衬多孔吸声材料。它可以看作是许多亥姆霍兹共振器的并联。密封的空腔通过板上的小孔与外界声场相通。孔颈处的空气柱有如质量，空腔内空气有如

32、弹簧，构成了弹性振动系统。当外来声波频率等于结构共振频率时，将引起孔颈中空气柱发生共振，此时空气柱的振动位移最大，振动速度最大，孔壁摩擦损耗也最大，对声能的消耗也最大,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,薄型塑料盒式吸声体 此类结构是用改性硬质PVC材料真空成形高频焊接加工而成的多层盒体结构，利用封闭盒体的谐振作用达到吸声目的。盒体厚度为50mm100mm，许多盒体连成0.5m0.5m的板。这种新型的吸声结

33、构吸声性能优良，物理性能稳定，重量轻，透光性好，易于施工，在工矿企业的噪声控制中得到广泛应用。,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,c) 微穿孔板吸声结构 微穿孔板吸声结构是在厚度小于1mm的薄板上每平方米钻上万个孔径小于1mm的微孔，穿孔率控制在1%-5，将这种板固定在刚性平面之上，并留有适当空腔。微穿孔板的声阻比穿孔板大得多，决定了共振吸声系数高，而声质量却小得多，声阻与声质量之比大为提高，加宽了吸声频

34、带。,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,resonance absorbers: a) Resonance frequency,式中P为穿孔率，D为穿孔板后空气层厚度l为穿孔的有效长

35、度。,对薄板共振结构，共振频率为,式中M为薄板密度，D为空气层厚度。 共振时的吸声系数约为0.20.5。,Main parameters evaluating the resonance absorbers,Chapter 7 Mechanical Noise Control Techniques 7. 1 Noise Reduction by Sound Absorption 7.1.4 Resonance Absorbers,b) Sound absorption coefficient at the resonance frequency,式中rA为声阻率R与空气特性阻抗0c0之比，称为

36、相对声阻率。,c) Effective bandwidth,微穿孔板吸声结构；串联式双层孔板结构,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.1 Definitions of Sound Insulation,The sound power transmission coefficient of a structure is defined as,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.1 D

37、efinitions of Sound Insulation,Transmission loss may be expressed as,where and are the incident and transmitted sound intensities, respectively.,General variation of the transmission loss with frequency for a homogeneous panel. 幻灯片 42,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulati

38、on 7.2.2 Sound Transmission through a Panel,(1) Region I: Stiffness-Controlled Region,At low frequencies, the panel (provide that the panel is very thin) vibrates as a whole, and sound transmission,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a

39、 Panel,through the panel is determined primarily by the stiffness of the panel.,At the surface of the panel (for a very thin panel), the particle velocities are both equal to the instantaneous velocity of the panel . We may write the following expressions from above Equations for :,If the panel has

40、a finite stiffness, the net force acting on the panel is equal to the “spring-force” of the panel. The specific mechanical compliance or mechanical compliance per unit area will be denoted by the symbol .,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission th

41、rough a Panel,The sound power transmission coefficient for normal incidence may be determined,The transmission loss for normal incidence may be written as follows:,where,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,For a rectangular pan

42、el, the expression for the specific mechanical compliance is given by :,The quantities a and b are the width and height of the panel; h is the thickness of the panel; and E and are the Youngs modulus and Poissons ratio for the panel material, respectively.,Chapter 7 Mechanical Noise Control Techniqu

43、es 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,For a circular panel with a diameter D and thickness h, the specific mechanical compliance is given by :,(2) Resonant Frequency,As the frequency of the incident wave is increased, the plate will resonate at a series of frequencies, ca

44、lled the resonant frequencies.幻灯片 38 The lowest resonant frequency marks the transition between Region I and Region II behavior. The resonant frequencies are a function of the plate dimensions. For a rectangular plate having dimensions ab h thick, the resonant frequencies are given by,The quantity c

45、L is the speed of longitudinal sound waves in the solid panel material,The quantity is the density of the panel material.,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,Usually, the lowest resonant frequency (the fundamental frequency) is

46、 the most predominant frequency. This frequency corresponds to m=n=1,The fundamental resonant frequency for circular plate is given by the following expressions. For a circular plate of diameter D and thickness h clamped(夹住) at the edge,For a circular plate with a simple supported edge, the fundamen

47、tal resonant frequency is given by,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,(3) Region II: Mass-controlled Region,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,At the fi

48、rst interface ( x=0) , the pressure in medium 1 and the pressure in medium 2 are equal, and the particle velocities in mediums 1 and 2 are also the same at the interface. Using these conditions, we find the following relations:,At the second interface (x=h) , the pressures and velocities are also eq

49、ual. Using this condition, we obtain a second set of relationships between the coefficients:,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,We may combine above Equations to obtain the ratio A1/A3,The magnitude of the ratio A1/A3 may be w

50、ritten as,or,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,If the materials are the same on both sides of the wall, i.e., Z1=Z3 the above equation reduces to,For the frequency range of interest in analysis of transmission of sound throug

51、h walls, the term k2h is small and Z2Z1.,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,The transmission loss for normal incidence is related to the sound power transmission coefficient for normal incidence:,If we introduce the quantity,

52、called the surface mass, the above equation may be written in the following form often called the mass law.,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,(4) Critical Frequency,As the frequency of the impinging sound wave increases in th

53、e mass-controlled region, the wavelength of the bending waves in the material, which are frequency-dependent approaches the wavelength of sound waves in the air. Coincidence (equality of the wavelengths) first occurs at grazing incidence（掠入射）, or for an angle ofincidence of . When this condition hap

54、pens, the incidence sound waves and bending waves in the panel reinforce each other. The resulting panel vibration causes a sharp decrease in the panel transmission loss. The point corresponds to the transition from Region behavior to Region behavior.,Chapter 7 Mechanical Noise Control Techniques 7.

55、 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,Condition for Coincidence:,or,are the wavelength and propagation speed of the bending wave in the panel.,/2,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,The critical frequency

56、 (or wave coincidence frequency) is given by,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,If we combine and the equation above,we find that the product (Msfc) is a function of the physical properties of the panel and sonic velocity (c)

57、in the air around the panel.,(5) Region : Damping-Controlled Region,For sound waves striking the panel at all angles (random incidence) at frequency greater than the critical frequency, the following empirical field-incidence expression applies for the transmission loss in the damping-controlled reg

58、ion,The quantity is the transmission loss for normal incidence at the critical frequency:,is the damping coefficient for the panel material.,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,Example,An oak door has dimension of 0.90m wide by

59、 1.80m high by 35mm thick. The air on both sides of the door has a temperature of 20, for which , , and . Determine the transmission loss for the following frequencies: (a) 63Hz, (b) 250Hz, and (c) 2000Hz.,Longitudinal wave speed Density Critical frequency product,Chapter 7 Mechanical Noise Control Techniques 7. 2 Noise Insulation 7.2.2 Sound Transmission through a Panel,Damping factor Youngs modulus Poissons ratio,Chapter 7 Mechanical Noise Con