One last hanging chad from my three part Acoustics set is the concept of the sound being the same in every seat. This is only partially an acoustical issue, so it gets its own post.
I hope it’s obvious why this is a goal of sound in theater. While I understand that there will always be some seats that are better than others, what we do not want is for some seats to be noticeably worse in ways that the audience member does not really understand. If they’re in the back and can’t see very well, they will understand that they had a lousy seat. If they’re in the middle somewhere and can’t understand the words, the show’s sound was bad (in their mind). If someone gripes to a friend that they had a lousy seat, that is very different than griping that the sound was bad. One turns into an expression of sympathy, the other turns into bad press.
All of the components of acoustics discussed so far decidedly contribute to this goal. Reverberant fields by nature leave hot spots and dead spots scattered around the room. In an extreme case, a resonance can leave individual seats completely unable to hear a single note that one seat in any direction would find uncomfortably loud.
But here is where we add in the sound system. The system can’t fix problems with the acoustics, but it can certainly be good or bad on its own.
A properly designed sound system for theater will produce the same sound in every seat and will produce the same volume at every frequency… to within a small tolerance. It is common to see a sound system specification that says: “The system shall reproduce from 50Hz-18KHz +- 3dB in every seat in the room.” The numbers may change a bit from system to system, but that’s the gist of it. A normal electric bass guitar goes down to 42Hz, bass drums might be a little lower than that, and the human voice (bass, obviously) commonly gets down to around 65Hz, so we need to cover more or less down to those options. Going up, the range in the mid-teen KHz is important for hi-hat, triangle, and for intelligibility of speech. Nominally we can hear from 20-20KHz, but as a general rule the real extremes aren’t as useful. The “+- 3dB” part indicates that the volume will not differ terribly much from one location to another.
The more stringent the specification is, the more difficult it will be to create a system that can meet it. Even the example I use above would require computer modeling of the room and the sound system using tools such as EASE that can account for the acoustics as well as the speakers being installed and their installation locations.
Clearly, if you’re trying to do this correctly, simply knowing something about sound isn’t nearly enough.