When working with an acoustician in the design or renovation of a hall it is helpful for all to have an understanding of the basic concepts in auditorium acoustic design. You can't really design a hall just by knowing the basics of auditorium design. The acoustician maintains an arsenal of trade secrets and insider techniques, reserved to managing sound once it's been launched from the loudspeaker. But by understanding the basics, you can at least keep track of what and why various things are being done. You can explain things to fellow parishioners. Also, long before you even start, you have to find someone to help with the design work. Understanding auditorium design helps you interview candidates and recognize who is not able to answer straightforward questions. Finally understanding auditorium acoustics will help give you second thoughts when you find there is a problem with understanding sound from a loudspeaker. Your speaker was probably installed by perfectly competent sound contractor and is still working just fine. If it's loud enough to hear and you still can't understand it, then your problem isn't loud speakers, its acoustics. Besides keeping the rain out, the next most important thing an auditorium must do is to provide a place where speech can be clearly understood. This means a good auditorium will have a good intelligibility rating. The set of minimum acoustic requirements that are met by a working auditorium starts with the direct sound from the speaker being loud enough, that means it replicates conversational sound levels. The background noise in the hall has to be fairly quiet. The hall acoustics should be fairly free from echoes and other types of late reflections. And finally, the hall acoustic is not very reverberant at all. Here we take a look at each of these factors as they apply to the three basic types of auditoriums that have evolved over the last century. Reviewing the Basics of Auditorium Design
Auditorium design begins with the loudspeaker and how it plays sound into the hall. It ends with how the hall returns reflections of the sound back to the audience. The speakers should produce a sound level at about 65 dB,A everywhere in the seating area. It should have at least 20 dB of "head room" so that short lived bursts of sound up to 85 dB,A can be replicated without any hint of speaker or electronic distortion. Electronic distortion must be avoided. Distortion of the signal is one of the fastest ways to cause people to lose their understanding of the sound. In addition, the loudspeaker system should sound similar no matter where a person is seated. This is achieved when the speaker system is tested and confirmed to provide a fairly flat frequency response curve for every seat in the house. The natural noise floor of the hall itself should be at least 20 to 30 dB quieter than the speaker level heard at the seats throughout the entire frequency range for speech. Generally the ambient noise levels for an empty good auditorium would be about 25 to 30 dB,A. This noise is the sound of the hall when everything is turned on, lights, air conditioning and even the sound system. The only thing that isn't happening is that someone isn't talking into the mic. Only a fairly good sound meter can measure sound levels this low. Then we open the doors and the audience moves into the hall. The background noise levels rise up to about 35 dB,A because of the breathing and other rustling that people naturally do. It is a well-established fact of human behavior that in a group, we collectively manage to make just a little more noise than the background noise level. That's why we act quietly in a library and noisily in a packed diner. (See Figure D)
Fig-D Noise levels, sound levels and head room
Early reflections are very helpful but not necessary for achieving good intelligibility in a quiet hall. But many halls are quite perfect and that's when early reflections become a good little...