i've just been reading a few old threads that seem to have gone dead a good while ago about how headphones can cause damage to hearing. Excessive noise levels over a particular period of time may cause damage to hearing; According to UK legislation, an LAeq (dB) of 80 can be listened to for a maximum duration of 8 hours. In an environment where exposure to noise levels is over 80db employees of a recording studio must use hearing protection. more info can be found at http://www.hse.gov.uk/noise/regulations.htm ( quoting one of my own reports) So with the aforementioned information in mind and using the free app for the iPhone dB volume and sticking the microphone next to one diaphragm, ( i know not the most accurate test but a good rough guide ) i was surprised at how low the volume was to stay safely under a peak level of 80dB. I have been listening to headphones far too loudly and at the age of 27 no wonder i have an occasional ringing in my ears, tho my last hearing test showed that thankfully i haven't suffered any significant hearing loss of any of the frequencies yet. For the test i was using an iPhone 4 with the dB volume app, while running KRK KNS 8400 headphones out of an apogee one, using several different music genres to test. A recommended max volume on my setup turned out to be 8 bars where the highest peak i found was 79 dB. (Source of information) Noise levels are not difficult to measure, just use a meter and take a reading. But expressing exposure to noise, or any high sound level, over a period of time is more tricky. Real-world noise fluctuates and long periods of low ambient sound levels may be punctuated by brief bursts of high intensity noise. One way of expressing noise levels over a period of time is the Leq. This is the sound level which, integrated - 'evened out' - over a period of time, is equivalent to a steady sound pressure level. In fact, the expression should ideally be Leq,T - where T is the time over which the exposure is integrated. Sometimes also you will see LAeq,T - this signifies that an 'A weighting' filter has been used to mimic the frequency response of the human ear. Legislation in the UK, which is similar in intent and spirit to legislation elsewhere, specifies that the daily duration of exposure to noise should be no longer than a stated maximum over a range of sound levels. LAeq Permissible duration of exposure 80 8 hours 83 4 hours 86 2 hours 89 1 hour 92 30 minutes 95 15 minutes 98 8 minutes 101 4 minutes This makes interesting reading. Music that we would consider to be loud is regularly over the 100 dB SPL level. The point however is that the level is integrated over a period of time. The fact is that a typical recording session involves a lot of discussion in and around takes and playbacks. So if levels peaked at 111 dB SPL on one single occasion that lasted two seconds, it may not make all that much of a difference. Legally, whoever is in charge of a session is responsible for controlling levels. The studio that you hired does not have that responsibility (Standard legal disclaimer - get your own legal advice on these matters!). So if you wanted to ensure that you don't get a lawsuit from a session musician in twenty years time when he finds his hearing is failing, consult your local legislation and get a Leq meter. Use it on all sessions and keep a log of the readings. Factory managers take noise levels very seriously. In music this is something that is often ignored. An awareness of the potential for hearing damage wouldn't go amiss though.