Originally Posted by stv014
Frequency response specs are only meaningful with dB tolerances (e.g. 20 Hz to 20 kHz +/- 3 dB). This is almost never the case with headphones, so the manufacturers can invent whatever numbers they want. You can ignore them, and it is best to check actual frequency response graphs instead, preferably from more than one source, and ones that do not include equalization or smoothing.
I'm reminded of a Sony headphone a few years ago, that they spec'd as being able to cover something like 10hz to 110khz (yes that's infrasonic all the way up to the upper-threshold of bat hearing). I'm sure it was at something like -70 dB or some equally nonsense reference though.
Shameless repost: http://www.rane.com/note145.html
I think the only manufacturer where I pay attention to their FR spec is Koss, not because they're accurately done, but because Koss will indirectly rank their products for you by FR (the wider the FR, the higher up they think the product should be positioned; for whatever reason they don't just say this, and because most of their products exist within a very narrow price band, it's sometimes hard to sort them out). This doesn't mean I believe their numbers, it's just kind of a "trend" I've noticed based on their marketing literature. I'm sure other manufacturers participate in a similar numbers game, but I haven't ever taken the time to look into it for say, Sennheiser.
Also note that with some IEMs you're dealing with a pro audio supplier, and you may very well be seeing real numbers. I would suspect this is the case with at least Etymotic (at least the ER-4) and JH Audio, but could be mistaken.