Abstract: There is described a system for producing noise particularly for sonic testing of an article, such as a satellite, in a reverberation chamber. In one of its aspects the system comprises a low frequency high-level broad-band noise source; a tube means including a section formed as an acoustic horn connected at its small end to the low frequency source and at its large end to the chamber. A Hartmann-type air acoustic high level noise generator, or generators are located inside the tube at a predetermined position. In operation the generator is excited by the generated low frequency noise to provide an output of non-linearly modulated noise. Also described is a Hartmann-type noise generator which comprises a nozzle and an aligned acoustic tube spaced therefrom by an air gap. A reverberation cup is formed in the mouth of the tube facing the nozzle and a bridge member extends between the nozzle and tube and spans the air gap.