HeadWize tells us that Sensitivity (loudness) is : "A measure of headphone efficiency in dBs SPL per milliwatt of input. A low number means that the headphones need more power to sound as loud as those which have a higher sensitivity.
Headphones for portables need to be fairly sensitive because of the lower power output of portable stereos. Modern dynamic headphones have sensitivity ratings of 90 dB or more. When shopping for portable headphones, look for a sensitivity rating of 100 dB or greater."
http://gilmore2.chem.northwestern.edu/articles/hguide_art.htm#sens
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DervishD 
you're going to hurt your ears and lose hearing, are you aware of it? ... be VERY careful with them and with the sound pressure levels or you will be deaf (or suffering with tinnitus) quite soon.
Look this guy was just asking about hardware, we can run out stuff as loud as we want, without your parental, big brother, unasked for, and much less needed concerns.
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..due to the source not being able to deliver enough current to the headphones, so the voltage drops and the sound is "clipped", so to say.
I do want to thank you for your often needed, but rarely expressed simple explanation of clipping. Clipping arises because, there will always be a limit above which an amplifier is unable to produce output currents. When we try to ‘turn up the volume’ too far, then we find that the loudest sections of the signal will be clipped.
Clipped music experiences amplitude compression, whereby all notes begin to sound equally loud because loud notes are being clipped to the same output level as softer notes.
