Mechanical Noise Sources as Calibrated Noise Standards

Abstract

In a large number of industrial applications the most convenient method of obtaining sound power ratings is by comparison with an acoustically calibrated mechanical noise source. The sound power outputs are compared in a random incidence (reverberant) sound field with standardized and relatively inexpensive electronic equipment which does not need to be calibrated. The acoustical absorption also does not need to be measured, provided it does not change appreciably during a test. This paper will describe two types of mechanical noise sources which are very stable. One of these operates from a compressed air supply. A group of these units have been calibrated by the Geiger-Hamme Laboratories and have become the standard sources for the trade association called the Compressed Air and Gas Institute. Another aerodynamic source operated by an electric motor gives a very uniform broad-band spectrum over eight octaves. This latter source has been calibrated by the Armour Research Foundation and is being used by many laboratories in the air motion industry. Both sources will be demonstrated. Some of the technical requirements and precautions for using this equipment for secondary sound power calibrations will be described.