That sound you hear is Apple’s (AAPL) iPad cannibalizing sales of personal computers. Fortune reports that Needham & Company analyst Charlie Wolf says that there is now “definitive evidence” that the iPad is gobbling up customers who in years past would have purchased PCs. Wolf’s evidence is that Mac shipments to schools in Q3 2012 fell by 0.1% while overall PC sales to schools dropped nearly 13.9%, even as Apple sold more than 1 million iPads to schools over the same period. To make things worse for PC OEMs, Wolf says that “the twin halo effects emanating from the iPhone and iPad” will drive even more converts into Apple’s arms the next time they look to buy a new desktop or laptop.