Some sound defects on film tracks are:
A ‘ragged’ edge to loud sounds, but acceptable quality at low levels. This may be caused by an incorrectly-positioned scanner slit in the projector, or overloading during the sound transfer.
Fine particles of dirt, scratches or cinches in the negative, all result in showers of white spots on the print. These are heard as a background of clicks and crackles. Since white spots affect only the dark areas of the track, this noise is most noticeable during quiet passages where the bias line is narrowest. Noise during loud passages originates in the clear areas of the print. Loose dirt or scratches on the print, or an excessive and smeary application of lubricating wax to the print, are likely causes.
Some high-frequency loss is inevitable in a photographic system, but is normally diminished by film loss equalization (electronically boosting the high frequencies of a recording during transfer). A frequent cause of excessive high-frequency loss (producing a woolly, muffled sound) is poor printer contact or slippage resulting in a visibly-blurred track.
Hiss results from a random, unwanted signal being reproduced. In an optical sound track this may come from a grainy or slightly fogged track area, or from scanning beyond the edge of the soundtrack.
This may arise from a colour print that has not been redeveloped, or from an excessively light print (of the sound). Moderate variations in print density, however, have little effect on sound level, and volume errors are most likely to be the result of an incorrect recording level during mixing or transfer.
Low-frequency pops or thuds are caused by relatively large holes or spots in the positive soundtrack. Often these occur when bubbles form in the redeveloper bead during application, leaving spots in the soundtrack without silver, and light blue in appearance.
Incorrect negative or positive densities result in cross modulation distortion, heard as spitting ‘s’ and ‘muddy’ orchestral sounds. (NB. Relates to variable area sound tracks only)
Wow and flutter
A gradual (wow) or rapid (flutter) variation in pitch is caused by an uneven playing speed at some stage of sound reproduction. This cannot be caused during printing, however, since negative and raw stock must inevitably run through the printer at the same speed. (Case, 1985)
Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston