right, is the big belt on the left side of the engine for the alternator/power steering????

the tensioner thats in the middle/ish of this belt is wobbling slightly and making a whiring sort of sound at the same time, and also in key with that woble and whiring ive noticed that the lights are flashing in time with it??? that make sense????? lol

done the multimeter reading an with only the engine running it was just over 14, sometimes tipped 15 but never settled, with engine off it slowly went down to just under 13. any ideas?