In addition to a constant background noise, sound recordings on magnetic tape may also suffer from modulation noise and 'drop-outs'. The level of the modulation noise is proportional to that of the signa!. Modulation noise is more readily audiblein recordings of solo instruments than in orchestral music; listening tests have shown that for the flute the audibility threshold lies at a signal-to-noise ratio as high as 40 dB. The frequency with which drop-outs occur is a matter of tape quality. Listening tests have been carried out to determine the annoyance caused by drop-outs as a function oftheir duration, the degree of attenuation and the frequency of occurrence. The results of these experiments have been applied in the DAMA annoyance meter, which can measure these quantities for an audio tape and assign a quality rating to it
