Collinear interaction of noise with a finite-amplitude tone

The interaction between a low-level band of noise and a finite-amplitude tone has been measured in air in a plane progressive wave tube. Noise bands generally 1–3 octaves wide with center frequencies in the range 0.7–1.2 kHz were used. At the source the tone SPL was in the range 140–151 dB (re 0.0002 μbar); the frequency was either within the noise band or somewhat above. When the tone was absent, the noise propagated as a small-signal wave. When the tone was turned on, however, a strong modulation took place. After a propagation distance of only a few meters, the noise spectrum extended at least 4–5 octaves above the original noise band. Moreover, the spectrum was quite flat. The energy required to extend the noise apparently came from the tone and its harmonics, not the original noise band. [Work supported by NASA, AFOSR, and ONR.]