Recently Morfey and Howell (AIAA Paper 79-0595) have shown that the acoustic power emitted by jets does not appear to be in agreement with Lighthill's U8 scaling law. In the present investigation it is argued that source convection is the principal mechanism leading to a departure from the U8 law (unheated jets only). This claim is tested by computation of “effective sound power spectra” from published data. The jet noise data are corrected for Doppler shift as well as “convective amplification.” The results suggest that the excess noise of clean jets is only apparent and can be accounted for by a “source-oriented” sound power balance. The method is also applied to heated jets. In this case a unique interpretation is not possible owing to the possibility of coherence between the various source mechanisms. [Research supported by Natural Science & Engineering Research Council, Canada and the University of Toronto.]