If it's not too rude, could I point you to
the wiki article on spectralism, and recommend a detour through
this article about a Xenakis piece and maybe a quick look at the article on
IRCAM? I'll happily try and fill in the gaps from there.
I think it's really unfair on all parties to say it 'sounds like Sonic Youth' though. SY have only a mildly unusual approach to composition (as such) - tonally speaking, they're very tame.