How do banners affect PA sound

You can see quite often in big events that PA speakers are hidden behind large banners. Often times the event organizers want the speaker stacks covered with some client-related graphics. The question which comes to mind is how much does this kind of covering hurt the sound quality? What is the reality? Do Graphic Vinyl Banners Affect Sound Transmission? article at John Huntington’s Blog takes a practical approach to test what is the effect. The end result was that it never seemed that this vinyl affected the sound all that much. Normally the wind, humidity, and other factors outdoors have a far greater impact on the quality of the sound than these banners. The article has some measurement results (using SMAART) for those interested in them.