Technical acoustics

Most disagreements on site come from a specification that asked for the wrong quantity. These are the terms that matter, and what each actually governs.

• Reverberation time — how long sound takes to decay in a room. It governs clarity and how loud a full room feels, and it varies with frequency, which is why a single number is rarely enough.
• Absorption coefficient — how much of the sound striking a material is absorbed rather than reflected, given per frequency band.
• Speech intelligibility — how much of what is said is understood. This is what an auditorium is ultimately judged on.
• Background noise level — the sound present when nothing is happening, usually set by ventilation and outside noise.

• Sound transmission class — a single-figure rating for how much airborne sound a construction blocks.
• Impact sound — noise transmitted by footfall and knocks, which behaves differently from airborne sound and needs different treatment.
• Flanking — sound that bypasses a good wall by another route, such as a ceiling void or a shared duct. Most isolation failures are flanking.

• Sound power and sound pressure — what a source emits, and what arrives at a listener. Equipment data gives the first; specifications usually want the second.
• Coverage — how evenly a system reaches the seats, normally shown as a plot across the seating.

Specify performance, not products. A clause that names a quantity and a target can be verified; one that names a brand cannot.