The measurement of noise by the use of sound level meters has been accepted by the aeronautic and automotive industries. Such measurements may be of comparative value for reasonably continuous composite noise. This is particularly true when the types of noise produced by the objects under comparison are similar in nature. Under properly controlled conditions, such measurements are of value in determining the relative success of various attempts or methods of noise reduction. In attacking noise reduction from an engineering standpoint, it is generally desirable to consider the individual physical characteristics of noise. Some of these characteristics and their effect on methods of instrumentation are discussed.