During a recent open week at the National Gas Turbine Establishment at Pyestock, an experiment involving just over 1,000 subjects was made to establish some measure of the disturbance value of such rhythmic or amplitude modulated noises as are generated by helicopter rotors. The results were (i) that, as may be implied from Niese's results with modulated tones, the subjective assessment of such a noise depends upon its peak sound pressure level (i.e. 82·5 db. in Fig. 3) rather than upon its r.m.s. value, (ii) that the effect of the modulation frequency is negligible in the range from 4 to 12 cycles/sec. and (iii) that these results are invariant with peak sound pressure level in the range from 85 to 95 db. re 2 x 10–4 dynes/cm2.