The Bogen SPT15A is the standard in loud horn speaker paging. Designed for indoor or outdoor paging or sound systems. The SPT15A is made of rugged all metal construction and is ideal for use in industrial plants, warehouses, schools, construction sites, transportation terminals, and recreational areas. TheSPT15A has a built-in rotary impedance selector switch for matching the speaker power requirements to a 25V or 70V constant-voltage line. How many horns do you need for your facility? Rule of thumb is 6,000 sq ft per horn for large open spaces. Very loud noisy facilities need special attention and do not follow this rule.