Case in point:
We had a big youth conference on at church a year or two back, and part of the stage decoration was some big dummy speaker stacks. They weren't plugged in, they weren't even loaded IIRC. The stage design was left in for the Sunday morning service, and the PA guys got countless complaints about it being 'too loud because of the new speakers.' I was in the room, it was no louder than any other Sunday.
To the untrained ear/eye, something that looks like it should affect volume, does. Essentially, people hear what they want to hear. So regardless of how much effect a perspex screen really has one way or the other on volume, because people can see that there's some 'soundproofing' between them and the kit, they percieve it as quieter.