In order to determine noise specifications in absolute units for a certain type of large reduction gear measurements were made of the sounds produced by ten machines in operation at a large central generating station. An average weighted total intensity, and a frequency analysis were made of the noise of each machine. It appears that these data are suitable and sufficient for absolute noise specifications on this type of machine.It was found that 1 db. represents an important difference in loudness in these machines which places quite rigorous requirements on the apparatus and the methods of measurement in order to eliminate errors due to wave patterns, location, load, speed, etc. Data on these points are given.A very close check was obtained between the observed frequencies of the various notes, and the movements of certain parts of the machine. By a comparison of the frequency analyses of the different machines it was possible to locate which part of the machine was responsible for the noise, and something of the nature of the defect producing the noise.