On many speaker data sheets, there are two speaker output ratings provided. One is referred to as reverberant and the other is anechoic. In the US, Underwriters Laboratories uses a reverberant chamber to test speakers. A reverberant chamber is a specially constructed room with walls that have almost no sound absorption. Since the walls are so highly reflective of sound waves, the sound energy distribution in the room is very uniform. When designing systems for UL compliance, the sound output measured using the reverberant method should be used.
Underwriters Laboratories of Canada takes just the opposite approach and uses an anechoic chamber. An anechoic chamber has almost no sound reflection. This chamber produces a different result in the speaker specification. When designing systems for ULC compliance, the sound output measured with the anechoic method should be used.