At school today, one of my classmates was giving a demo of the ECS (AC and pressurization). While a group of us was in the aft cargo bin of a 727-100 going over the pressure relief valves. A question came up when he gave the pressure ranges for the cabin. He said, and these are rough numbers, the overflow valve allows for a maximum positive pressure of 8 or 9'ish PSID (with the cabin able to stand up to 12 PSID). He also said that the maximum negative pressure allowed was 1 PSID.
My question to him and the instructor that remains unanswered is:
How do you develop negative pressure differential?
This may seem dumb to some people here but, I could not see how it would be done.
Thanks in advance