There are many types of perforation options for a projection screen. When the audience is seated closer to the screen it is best to choose a micro or mini perforation pattern to make sure the audience does not see the holes. As the audience is seated further away, larger hole patterns can be used without being visible.
A technique for enhancing the realism of video on screen is to place the speakers directly behind the screen so that on-screen voices and sound effects are heard to come directly from their sources on the screen. To accomplish this, screen perforation is necessary to allow the sound to pass through the viewing surface. Perforation refers to the small holes in the viewing surface, which allow sound from speakers placed behind the screen to move through the material to be heard by the audience.
Sound pressure that passes through the viewing surface, measured in decibels (dB), varies by frequency and is charted on sound transmission curves. The characteristics of sound movement are unique for each perforation pattern. Screen and sound designers evaluate these curves along with the visual requirements of the setting to plan the type of perforation chosen and the sound control systems needed to optimize the viewing experience.
Here is a fairly typical example of a sound transmission curve for a generic type of screen perforation. The horizontal axis shows frequency measured in Hertz (Hz). Most humans can detect frequencies between 20 Hz (low) and 20,000 Hz (high). Lower Hz sounds are the lower bass sounds, which have long wavelengths. High Hz sounds are the high pitched treble sounds and have short wavelengths. Sound in the higher Hz range is more impacted by passage through the screen surface. On this graph, the vertical axis is expressed in the decibels (dB), used to describe how loud a frequency is heard. The image below shows the decrease in dB relative to Hz as sound hits the screen. Sound engineers refer to this as a roll-off of the high frequency sounds and design the systems to equalize the sound pressure across the audible spectrum.