The decibel (dB) is a logarithmic unit..
If you check out: http://en.wikipedia.org/wiki/Decibel it may make more sense.
Everytime you step up 10dB you are effectively multiplying the power ratio by 10.
To find the amplitude ratio, take the square root of the power ratio.
To demonstrate the comparitive power of your headphones have a look at this chart:
At 90dB your headphones should max out at the equivalent of a train whistle at 500' or truck traffic.
Ipods are generally quite quiet.. You need to put them through a proper amp to test them out (turn it down first of course!!).
My old Ipod sounded so weak in comparison I could hear people on the back row of a bus chatting. (Kinda what I was trying to block out).
If they were cheap, then they won't sound amazing or put down a lot of power before they blow out.. But then I have had a go on headphones that cost quite a bit (rhymes with 'Leaks Lordio') and they are a complete con.. No detail, power, nothing..
Last pair of cans I bought cost less than £20 and sounded better than a pair I bought for £50!
If they turn out to be ok on an amp, best bet is to buy a portable headphone amp or even build one..