Abstract: An amplifier system comprises a plurality of modules located within a cabinet in a manner providing minimum signal paths. The signal to be amplified is divided into frequency ranges for main amplification, and each main amplifier has an independent power supply. The modules are constructed from extrusions shaped and assembled to facilitate servicing and to function as heat sinks or as a cooling system as well as to provide mechanical protection and electrostatic shielding.

Abstract: A microphone unit comprises a pair of like electroacoustic transducers each having sound-responsive elements in the form of identical diaphragms which are parallel to each other and mounted in a common housing that is open at at least one end for the reception of desired sounds in a direction perpendicular to the diaphragms. The diaphragms are spaced apart a distance no greater than one-quarter of the shortest wavelength of the range of frequencies of the noises to be cancelled; and the housing has further apertures that open into the housing in directions parallel to the planes of the diaphragms, these further apertures being disposed at least one on each side of each diaphragm and the interior ones of said further apertures being spaced apart from their associated diaphragm a distance no greater than one-eighth of the shortest wavelength of the range of frequencies of the noises to be cancelled. The noise sources and channels that the microphone may be exposed to are1. air-borne ambient noises;2.