T effects of freestream disturbances on boundary-layer transition in flows with zero pressure gradient have been studied by several investigators. Some of the notable results have been published by Dryden, Hall and Hislop, and Schubauer and Skramstad. A study similar to that of Schubauer and Skramstad was performed and reported by Wells. In the two latter studies the transition Reynolds number was found to increase systematically with decreasing freestream disturbance intensity until a maximum was reached which was not affected by further reductions in disturbance intensity. The maximum transition Reynolds number found by Wells was 80% greater than that reported by Schubauer and Skramstad, being 5.0 X 10 and 2.8 X 10, respectively, for disturbance intensities less than 0.1% of the freestream velocity Examination of the energy spectra of the freestream disturbances indicated that most of the energy in Wells' experiments occurred at frequencies below 150 cps with acoustic content less than 10% of the total energy. The tests of Schubauer and Skramstad involved significant energy levels out to 400 cps, and in addition the spectrum exhibited large acoustic energy peaks at 60 and 95 cps which accounted for approximately 90% of the total disturbance energy that was measured for intensities less than about 0.05%. These results suggest an influence of both the spectral energy distribution and the nature of the disturbances in determining the transition Reynolds number. The effects of acoustical noise are particularly interesting in the light of recent studies by Pfenninger, Bacon, and Carlson" and Jackson and Heckl concerning the stability of laminar boundary layers in the presence of intense acoustical fields. This report describes a recent series of experiments designed to extend the study reported in Ref. 4. In particular, the effects of acoustic noise fields of discrete frequencies and the broad-band turbulence created by coarse-mesh grids placed in the freestream have been investigated. These experiments are meant to be only illustrative of the problem and do not attempt to explain completely the relation between transition and free-stream disturbance energy. The results do emphasize the importance of the frequency spectra and nature of the disturbance, as well as the rms intensity of disturbance energy in predicting transition.