What happens if i use a cable with less voltage than the appliance?

I have an ipod dock but the cable is missing. I have one that will fit but its only 12v and it says you need 18v on the appliance. What will happen if i use the 12v? Anything? PLEASE HELP!|||the cable will eather get hot or not work at all !|||need to make sure the positive is to the tip on the 12 volt transformer dc out lead, most are positive to the tip of the plug and most if not all have a diagram to tell you this on the the power adapter with symbols that represent pos to tip or neg to tip , if your 12v power transformer is negative to tip you will cook your safety circuits in the docker|||Well it won't explode, that's for sure, but it won't operate normally i guess.