It's a good question votard, and one that I've thought about for many years - I've never found any studies into the matter but my 2c worth would be:
- ultra high frequencies would be unlikely to damage your hearing as your hearing is "generated" by the vibration of the hairs within your ears - the shorter the hair the higher the frequency it vibrates at - thus, (seeing we can't hear above e.g. 20KHz) there are probably no hairs short enough to vibrate at that frequency. Now whether the lower hamonics are affected is another question altogether......
- ultra low frequencies - probably the same answer, but when a frequency gets down near the resonant frequency of your body (a few Hz) it can make you feel very sick........