Previous to the development of amplifiers most of the instruments used in acoustical research depended for their operation upon purely mechanical principles. This paper includes a brief survey of such of these instruments as are still of interest in connection with the investigation of technical or research problems in acoustics, but it deals primarily with the more recent electrical devices used in the study of air-borne sound waves. The limitations and fields of application of various electrical instruments, including microphones, particularly adapted to definite types of acoustic measurements, are discussed.