Acoustic transparent projector screens are those which allow you to place the speakers behind the projector screen without losing audio quality. This can be beneficial in helping you make room for your home theatre system, while also giving the impression that when people are speaking on screen their voice is actually coming directly from them.
Another advantage of acoustic transparent projector screens is that, unlike solid screens, they don't act as another wall to reflect sound off.
Types of audio transparent screens
There are two main types of acoustic transparent screens; perforated screens and woven screens.
Perforated screens are made up of a series of holes with about a 0.3mm diameter. These allow the sound to penetrate through the screen without filtering light through, eliminating the possibility of the picture also reflecting from the wall behind. Perforated screens can sometimes give an uneven sound that cannot be fixed by simple changes in equalisation though. Likewise, they're also more susceptible to the moiré effect.
Woven fabrics, on the other hand, are good to deliver sound through the screen, while also helping to minimise the effect of moiré. These may also lose small amounts of sound at certain frequencies as it is absorbed into the screen, but this can normally be fixed with tone controls and basic equalisation.