The paper deals with a detailed investigation of noise measurement for the purpose of establishing methods generally applicable to the sustained noises encountered in engineering. The work follows on that described in a previous paper. The necessity for redetermining the properties of the hearing system of the average individual in terms of free-space conditions of listening is explained, and the results obtained on 60 persons are given. The inadequacy of the decibel scale for indicating subjective loudness is discussed and two alternatives are considered. The second of these is suggested as being the more helpful in conveying numerically the desired impression of loudness.Desirable improvements in the noise-analysing apparatus described in the previous paper are indicated, and a new equipment embodying these features is briefly described.The various methods of assessing the loudness of a complex noise are considered in relation to engineering requirements, and the reasons are given for the choice of the aural balance method using a pure reference tone with a frequency of 800 cycles per sec. Experience obtained with this method in assessing typical noises is described. Finally, consideration is given to the problem of specifying the noise which a piece of apparatus may make when installed in a particular location.