ABC计权1.doc

噪声计中的频率计权网络有A、B、C三种标准计权网络。A网络是模拟人耳对等响曲线中40方纯音的响应，它的噪声计曲 线形状与40方的等响曲线相反，从而使电信号的中、低频段有较大的衰减。B网络是模拟人耳对70方纯音的响应，它使电信号的低频段有一定的衰减。C网络是模拟人耳对100方纯音的响应，在整个声频范围内有近乎平直的响应。声级计经过频率计权网络测得的声压级称为声级，根据所使用的计权网不同，分别称为A声级、B声级和C声级，单位记作dB(A)、dB(B)和dB(C)。为了模拟人耳听觉在不同频率有不同的灵敏性，在声级计内设有一种能够模拟人耳的听觉特性，把电信号修正为与听感近似值的网络，这种网络叫作计权网络。通过计权网络测得的声压级，已不再是客观物理量的声压级（叫线性声压级），而是经过听感修正的声压级，叫作计权声级或噪声级。 计权网络一般有A、B、C三种。A计权声级是模拟人耳对55dB以下低强度噪声的频率特性，B计权声级是模拟55dB到85dB的中等强度噪声的频率特性，C计权声级是模拟高强度噪声的频率特性。三者的主要差别是对噪声低频成分的衰减程度，A衰减最多，B次之，C最少。A计权声级由于其特性曲线接近于人耳的听感特性，因此是目前世界上噪声测量中应用最广泛的一种，B、C已逐渐不用。 从声级计上得出的噪声级读数，必须注明测量条件，如单位为dB，且使用的是A计权网络，则应记为dB（A）。什么是计权? 就是滤波曲线, A,B,C计权分别对应三种滤波曲线.滤波的原因是因为人的耳朵对不同频率的敏感度不同,人耳对高低频都不太敏感,所以这些滤波器就是为了模仿人的耳朵听到的声音强度,把高低两端作些衰减.区别嘛就是三种曲线衰减的程度不同.首先要了解等响曲线的概念,人耳对不同频率的声音,灵敏度是不同的,对2-4kHz的声音最敏感,所以如果相同的声压级,听2-4kHz时,会觉得它最响.去找找等响曲线图,就明白了.规定1kHz的声压级大小为方,比如1kHz,40分贝, 人耳感受到的响度就为40方,对其他频率的声压级感受到的响度,和1KHz的40dB的一样响,就画出了一条响度为40方的等响曲线.不同响度的等响曲线是不一样的.ISO226规定了等响曲线.人耳对低频,高频较不敏感, 在响度低时,低频衰减得比较厉害,所以等响曲线以1kHz为中心,两端往上翘, 当响度增加时,对低频的灵敏度会上升.这样等响曲线的翘度会渐渐变小, 大概到90dB左右,等响曲线才差不多成为一直线,就是说20Hz,90dB的信号,同1kHz,90dB的信号听起来一样响.因为不同响度(也就是声波的声压级不同时),人耳又不同的等响曲线,所以对不同级别大小的声音,规定了不同的计权,也就是A,B,C,D计权,我这个说得是最详细的,你在网上其他地方找不到.A计权是模仿60dB以下的声压,这是大多数的日常情况,也是最符合人耳的响应曲线了,它基本上是把40方的那条等响曲线,翻个个,就是A计权的滤波曲线了.B计权是模仿60-80dB的声压,滤波曲线两端没有A计权曲线下压得那么厉害C计权曲线同B计权很像,它是模仿80dB以上的声压D记权是高频段(1kHz以上)的处理曲线,它不但不滤波,反而对这个频段的信号加以提升,这个计权是测量100dB以上的声压时用,比如航空业的强噪声,引擎噪声什么的,一般这种都在耳朵的痛阈以上了,所以不但不能降还要给你提. 应用相当专业.一般A,C这两种计权用得最多, A计权是用在MIC测量时人耳听到的响度,比如测一个小区内的环境噪声,就用A计权, C计权是用来测量工业噪声,那种周期短的的脉冲强噪声,爆炸声等.实现方法就是在声级计里加上个滤波器,你选了那个A档以后,它给你减去一个与频率对应的dB值,得到的dB值就是计权声压级,就像小学生做减法那么简单.分别记作dB(A),或dB(B), dB(C), dB(D)这个链接有A计权比较详细的计算http:/www.diracdelta.co.uk/science/source/a/w/aweighting/source.html A WeightingThis is the most generally used filter when making overall noise measurements. The attenuation of the sound signal with an A-weighted filter corresponds to the fact that the human ear is not as sensitive to sound of the lower frequencies as it is at the higher frequencies.The A-Weighted Sound Level is a single number measure of the relative loudness of noise that is used extensively in outdoor environmental noise standards. The ratings correlate well with human judgments of relative loudness, but do not take into account the spectral balance or sound quality. Many different sounding spectra can result in the same numeric value, but have quite different subjective qualities. The A-Weighted Sound Level can be measured with simple sound level meters; the rating is expressed as a number followed by dB(A) or dBA. For example, 35dB(A).The A weighting curve approximately follows the equal loudness curve of 40 phons.The 40 phon curve shown in red and the inverted (40dB-A weighting) A-weighting curve shown in blue.The inverted A weighting curve was calculated based on the fact that the A-weighting is 0dB at 1000Hz and the 40 phon curve is 40dB at 1000Hz, therefore, these two points coincide.A-weighted levels are not a suitable descriptor for low frequency noise 20 to 200Hz. A-weighted sound pressure level has been found to be an unsatisfactory descriptor to predict effects caused by low frequency noise. According to previous studies, specific sound characteristics that are not fully assessed by an A-weighted sound pressure level could be of importance for adverse effects of low frequency noise.The A-weighting value in decibels as a funcion of frequency is given by:where WA = weighting to be applied, dB f = frequency, Hz Enter frequency and level in either dB or dB(A) to get conversion and weighting or frequency only to get weighting. 窗体顶端A WeightingFrequency Hz Level dB Level dB(A) A weighting dB 窗体底端The following are tabulated standard A weighting values for the 1/3 Octave centre frequencies.Frequency HzA weighting dBFrequency HzA weighting dB20-50.5800-0.825-44.71000031.5-39.412500.640-34.61600150-30.220001.263-26.225001.380-22.531501.2100-19.140001125-16.150000.5160-13.46300-0.1200-10.98000-1.1250-8.610000-2.5315-6.612500-4.3400-4.816000-6.6500-3.220000-9.3630-1.9The s-domain transfer function is: where s is the complex variable. VBA Function Function Aweighting(freq As Double) As Double If NumberArg 0 Then f2 = freq 2 f4 = freq 4 Return A-weighting for this frequency. Aweighting = 10 * Log(1.562339 * f4 / (f2 + 107.65265 2) _ * (f2 + 737.86223 2) / Log(10) _ + 10 * Log(2.242881E+16 * f4 / (f2 + 20.598997 2) 2 _ * (f2 + 12194.22 2) 2) / Log(10) Else Aweighting = -1E+32 End If End If End Function B WeightingThe B-weigthing curve approximately follows the 70 phon curve.The B-weighting curve follows the equation: where WB = weighting to be applied, dB f = frequency, Hz Enter frequency and level in either dB or dB(B) to get conversion and weighting or frequency only to get weighting. 窗体顶端B WeightingFrequency Hz Level dB Level dB(B) B weighting dB 窗体底端The following are tabulated standard B weighting values for the 1/3 Octave centre frequencies.Frequency HzB weighting dBFrequency HzB weighting dB20-24.28000.025-20.410000.031.5-17.112500.040-14.216000.050-11.62000-0.163-9.32500-0.280-7.43150-0.4100-5.64000-0.7125-4.25000-1.2160-3.06300-1.9200-2.08000-2.9250-1.310000-4.4315-0.812500-6.1400-0.516000-8.4500-0.320000-11.1630-0.1The s-domain transfer function is: where s is the complex variable. C WeightingThe C-weighting curve approximately follows the 100 phon curve.The C-weighting is described by the equation below: where WC = weighting to be applied, dB f = frequency, Hz Enter frequency and level in either dB or dB(B) to get conversion and weighting or frequency only to get weighting. The following are tabulated standard C weighting values for the 1/3 Octave centre frequencies.FrequencyC-wgtFrequencyC-wgt20-6.28000.025-4.410000.031.5-3.012500.040-2.01600-0.150-1.32000-0.263-0.82500-0.380-0.53150-0.5100-0.34000-0.8125-0.25000-1.3160-0.16300-2.02000.08000-3.02500.010000-4.33150.012500-6.24000.016000-8.55000.020000-11.26300.0The s-domain transfer function is: where s is the complex variable.