The paper deals with part of a comprehensive investigation which is being carried out by a manufacturer of electrical plant on the noise emitted by machinery and other apparatus, a subject of great present and future importance. The need for treating the subject in a thorough and quantitative manner for engineering purposes is pointed out. The physical magnitudes involved in the measurement of the sound set up by a vibrating body are discussed generally, together with the characteristics of the ear. The characteristics that should be possessed by a sound-measuring apparatus intended for use in engineering problems are pointed out and a brief survey is made of existing types of apparatus. The principle, development and final form of the apparatus, which enables a complete analysis of a complex sound to be made, are described, together with the methods employed to verify the accuracy of the apparatus. The influence of the conditions under which sound measurements on engineering apparatus are carried out is discussed, and a special laboratory which has been set up for acoustical work is described. Some sound-test results on engineering apparatus are given as examples.