14 CFR § 121.318 - Public address system.
(b) Is approved in accordance with § 21.305 of this chapter;
(c) Is accessible for immediate use from each of two flight crewmember stations in the pilot compartment;
(d) For each required floor-level passenger emergency exit which has an adjacent flight attendant seat, has a microphone which is readily accessible to the seated flight attendant, except that one microphone may serve more than one exit, provided the proximity of the exits allows unassisted verbal communication between seated flight attendants;
(e) Is capable of operation within 10 seconds by a flight attendant at each of those stations in the passenger compartment from which its use is accessible;
(f) Is audible at all passenger seats, lavatories, and flight attendant seats and work stations; and