The interaction between a low-level noise band and a finite-amplitude tone is studied. Experiments are performed in the air and in a plane-progressive tube. Three cases are considered: where the tone is located at the low end of a three-octave noise band, where the tone is at the upper end of a narrower noise band, and where the tone is over an octave above a one-octave noise band. It is found that the noise portion of the spectrum becomes broadband and flat at short distances. Both these characteristics become extensive before the tone reaches the shock-formation stage. The source of the high-frequency noise energy is identified as the tone and its harmonics rather than the original noise band. Good agreement is found between these results and calculations based on the Earnshaw solution. It is noted that the study is directly applicable to jet aircraft noise.