Chapter 6 Noise Analysis and Measurement 第6章 噪声分析与测量,6. 1 Combination of Sound Levels 6. 2 Spectral Analysis of Noise 6. 3 Weighted Sound Levels 6. 4 Noise Measurements 6. 5 Noise Source Identification,The root-mean square (rms) sound pressure,Combined sound pressure of p1 and p2,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,(1) 两列频率相同且以恒定相位差相交的单频率噪声声波的叠加,相干波：频率相同、相位差固定不变的波称为相干波。,if,if,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,这两种情况下的平均声能密度分别为,(2) 两列不相干噪声声波的叠加,or the frequency is same, but the phase angle variation is of random,Time is long enough,两列具有不同频率，或频率相同但相交时相位差无规律变化的噪声声波叠加后的合成声场，其平均声能量密度等于每列噪声声波平均能量密度之和，这两列噪声波称为不相干波，它们的合成声场将遵守“能量相加法则”。,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,Noise is emitted usually from more than one source or at different frequencies, and it is necessary to calculate the cumulative or overall sound levels. Obviously, since the sound level scales are logarithmic, they cannot be added algebraically. Combining of sound levels may be performed using the energy addition theorem and the overall level L(OA) is determined by,Here, each sound level to be combined might be the noise from n individual sources or the sound levels associated with n octave bands. They may be sound power levels or sound pressure levels.,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,Another easy and useful method for combining sound levels is to utilize the table as bellow. The combined effect of sounds depends on the difference in their decibel levels.,实际计算表明，两列声压级相同的噪声叠加后，总声压级仅增加3dB；两列噪声的声压级相差10dB以上，则声压级低的那列噪声对总声压级的贡献可以忽略不计，总声压级近似等于声压级高的那个值.,Chapter 6 Noise Analysis and Measurement 6. 1 Combination of Sound Levels,(1) Octave Band and 1/3 Octave Band Spectral Analysis,In noise control engineering, the bands are related by the following relationship,n=1, octave band n=1/3, one-third-octave band,The center frequency of any such band is defined as the geometric mean,The lower and upper limit frequencies may be determined from the center frequency as,Chapter 6 Noise Analysis and Measurement 6. 2 Spectral Analysis of Noise,Chapter 6 Noise Analysis and Measurement 6. 2 Spectral Analysis of Noise,Chapter 6 Noise Analysis and Measurement 6. 2 Spectral Analysis of Noise,(1) Octave Band and 1/3 Octave Band Spectral Analysis,Band Number 1 2 3 4 5 6 7 8 Center Freq. (Hz) 63 125 250 500 1000 2000 4000 8000 SPL at 3 ft (dB) 126 132 128 119 115 108 98 90,133,134.3,134.4,Overall SPL 134 dB,Chapter 6 Noise Analysis and Measurement 6. 2 Spectral Analysis of Noise,128.5,115.8,98.6,115.9,Chapter 6 Noise Analysis and Measurement 6. 2 Spectral Analysis of Noise,(2) Constant Bandwidth Narrow Band Spectral Analysis,在倍频程分析中，中心频率越高，对应的带宽越大，得出的数据越粗糙。在恒定窄带分析中，在高频域仍能保持同样的带宽，可达到很高的分析精度。但其代价是大大增加了分析的工作量。,If the noise has frequencies evenly distributed throughout the audible frequency range it is known as white noise.,Chapter 6 Noise Analysis and Measurement 6. 2 Spectral Analysis of Noise,A electrical weighting networks for sound level meter,Chapter 6 Noise Analysis and Measurement 6. 3 Weighted Sound Levels,Band Number 1 2 3 4 5 6 7 8 Center Freq. (Hz) 63 125 250 500 1000 2000 4000 8000 SPL at 3 ft (dB) 126 132 128 119 115 108 98 90 A-weighting -26.2 -16.1 -8.6 -3.2 +0 +1.2 +1.0 -1.1 A-weighted SPL 99.8 115.9 119.4 115.8 115 109.2 99 88.9 116 121 116 99.4 122.2 116.1 123.2 Overall A-weighted SPL 123 dB(A),Chapter 6 Noise Analysis and Measurement 6. 3 Weighted Sound Levels,Home Works,距离发动机3米处测得的1/3倍频程声压级列于表A，试画出倍频程频谱图，并计算总的A声压级。,表A. 1/3倍频程声压级的测量值,