This is the fourth in a series of five papers that describe Lockheed's work on tone-excited jets and appear in this issue of the journal. This paper describes the acoustic results from a well-controlled experiment designed to obtain an understanding of the noise generated by a tone-excited jet, with particular emphasis on the mechanism of broadband jet-noise amplification. Results for a 5·05-cm-diameter nozzle are presented for both unheated and heated jets, with and without the effect of forward flight simulation for a range of excitation frequencies and levels, and for two mode orders: the zero-order and the first-order spinning mode. Broadband jet-noise amplifications of up to 5 dB have been observed in the present study; these levels would have been much higher if the excitation levels had been higher than those used. It is also shown that the relative velocity effects are the same for both the unexcited and the excited jets. Finally, it is concluded that it is the small-scale turbulence that generates the additional broadband noise upon excitation of the jet by an upstreaminjected discrete tone acoustic wave.