Acoustic-noise measurements on nominally identical small electrical machines

A considerable variation in noise data between members of a group of nominally identical small electric machines has been observed. Data for such members are here analysed by the analysis of variance to assess the relative importance of the possible sources of variability. The presentation of the noise produced by the mass-produced machines is made with the aid of statistical techniques. The errors involved in using different numbers of motors and different numbers of sound-pressure sampling points around each motor and of using approximate calculating procedures are also studied. The paper should be of particular interest to industry for assessing, with a desired degree of accuracy, the noise of mass-produced small machines, so that the noise may be compared with that which may be quoted in forthcoming international and British standards in which noise limits for electric machines will be specified. With the aid of the techniques put forward, reliable judgments may be made of the effects of new design features on the noise output of small machines. Numerical examples for data obtained using the A weight ing network are given. The method can be applied in the same manner to analysis of noise data in octave bands, one-third-octave bands and narrowbands.