Two vocabularies meet in this field: the mosque audio world and the commercial paging world. They often mean the same thing by different words. These are the terms that appear across this site, defined plainly.
The Islamic call to prayer, recited five times a day to announce that the time for a prayer has begun. Also written azan, athan or ezan. In an automated system, the adhan is an audio recording played at a calculated time.
The second, shorter call announcing that the congregational prayer is about to start. The gap between the adhan and the iqamah gives people time to arrive, and is typically a fixed number of minutes set by the mosque rather than calculated astronomically.
The person who recites the adhan. A "virtual muezzin" is the common informal name for a system that plays a recorded adhan on schedule in place of a live reciter.
The congregation, or congregational prayer. Often used interchangeably with iqamah when talking about timings — the "jamaah time" is when the congregation prays.
The dawn prayer, and the one that most exposes a bad prayer-time system: its time moves substantially through the year and it happens when nobody is available to press a button.
The astronomical convention used to derive prayer times for a location, differing mainly in the sun-angle values used for Fajr and Isha. Different mosques legitimately follow different conventions, which is why a fixed timetable and a calculated one can disagree by several minutes and both be correct.
The Friday congregational prayer, held around midday and usually the busiest gathering of the week. It is scheduled separately from the daily prayers because most masjids fix it at an agreed clock time rather than deriving it from a calculation.
Which of two juristic opinions is used to determine the start of Asr. The Hanafi position places it noticeably later than the Shafi'i, Maliki and Hanbali position — commonly around an hour — so two correct timetables for the same city can differ substantially on that one prayer. Any system calculating prayer times must let the committee choose.
The moment of sunrise, which ends the window for Fajr. It is not a prayer time in itself, but display boards usually show it because it is the practical deadline for praying Fajr.
The date in the Islamic lunar calendar, shown alongside the Gregorian date on most prayer-time displays. Because the lunar year is shorter, the two drift against each other continuously.
The number of minutes between the adhan and the congregational prayer, set per prayer. Expressing iqamah as an offset rather than a fixed clock time means it follows the season automatically; a fixed time is easier to remember but needs revising through the year.
A screen or dedicated clock in the prayer hall showing today's adhan and iqamah times, usually with a countdown to the next one.
Any system that distributes announcements to a space through amplified loudspeakers. The term covers everything from a single speaker to a campus-wide installation.
A loudspeaker system certified for emergency use, designed and installed to a standard such as ISO 7240-19, with monitored circuits and battery backup. A voice alarm is regulated life-safety equipment. Voice alerting over an ordinary PA is not the same thing and is not a substitute for it, however clear the message.
Moving people into a building or holding them inside it, rather than evacuating — the correct response to an external hazard. Schools increasingly specify it alongside evacuation and lockdown, and it needs its own message because the instruction is the opposite of the one people were drilled on.
Securing people in place behind locked doors in response to a threat inside or near the site. As an audio requirement it is distinctive: the message must be unambiguous, must reach every space, and must not be interrupted by anything scheduled.
A distribution scheme where amplifiers drive speakers at a high constant voltage — 100 V in the UK and much of Europe, 70 V elsewhere — with a transformer at each speaker. It allows long cable runs and many speakers on one pair, and it is what most existing building PA already uses.
How much louder a system can go before it distorts. It matters for bells and alerts because a message set to a comfortable level in a quiet corridor may be inaudible once machinery, a crowd or a fire alarm is also sounding.
An independently controllable area of a PA system. Announcing to one zone means the others do not hear it. Zoning granularity is set by how the building is wired: with relay-based muting, a zone is an amplifier circuit; with addressable IP speakers, a zone can be a single speaker.
Making a live or recorded announcement over a PA system, usually to a subset of zones. "Paging" and "announcement" are used interchangeably; paging tends to imply the live case.
Holding a control to stream a microphone live to the speakers, releasing to stop. The defining property is that it pre-empts whatever else is playing — the person holding the microphone owns the speakers.
Automatically lowering background audio while an announcement plays, then restoring it. Distinct from stopping: ducked audio continues underneath and returns to level afterwards.
Overlapping the end of one track with the start of the next so there is no silence between them. Used in background and radio-style playback rather than for bells.
Playing the same audio out of several sound outputs simultaneously, from one decoded stream, so multiple amplifiers stay in sync. Without it, a multi-rack site hears a slap-back echo.
The rule deciding what happens when two sounds want the speakers at once. A typical scheme: emergencies and live announcements interrupt immediately, secondary content waits for a quiet moment, and queued content plays after the current item finishes.
A time-based rule that overrides the master volume during a window — for example, dropping levels overnight without changing anything else.
A short sound used as a signal rather than as content: a bell, a chime, a buzzer, a siren. Distinct tones for distinct meanings is the cheapest usability improvement available to a bell system.
The cabinet holding the amplifiers driving a building's speakers. Most retrofit installations connect to a line input on an existing rack rather than replacing it.
A pair of terminals that a device shorts together to signal something — the simplest possible interface between two systems. Fire panels, nurse-call systems and emergency buttons commonly offer one, which is why a scheduling system able to read a contact closure can integrate with equipment far older than itself.
A loudspeaker with a network port, powered and fed audio over the same Ethernet cable (Power over Ethernet). Each speaker is individually addressable, which allows very fine zoning, at the cost of replacing every speaker.
A board of electrically controlled switches, driven over USB or serial, used to open and close circuits — muting an amplifier, ringing a physical bell, or lighting an indicator. The cheap way to add zoning to a conventional PA.
General-purpose input/output — the pin header on a Raspberry Pi and similar boards, used here to read a physical emergency button.
The wiring driving a traditional electric bell or siren, independent of any audio system. A relay can pulse it on a schedule, so a site can keep the bell it already trusts.
An audio input expecting a standard signal level, as opposed to a microphone input. The 3.5 mm jack or phono pair on an amplifier rack is a line input, and is normally where a scheduling PC connects.
The component deciding what plays when. In this context it holds the timetable, the recurrence rules, and the date-scoped exceptions.
How long a machine takes to synthesise speech relative to the length of the speech produced. An RTF of 2 means two seconds of computation per second of audio. It matters only for text typed on the spot: scheduled announcements are synthesised in advance, so their RTF is invisible. Measured figures are published on the measured data page.
How long after a request the first sound is heard. It differs from real-time factor because speech can be produced clause by clause, so playback starts while the rest is still being synthesised. It is the number a listener actually experiences.
A bell marking the boundary between lessons in a school, as distinct from break, lunch and dismissal bells. Systems worth buying let each of these be a different sound, because staff and pupils hear them several times a day for a year.
The industrial equivalent of a period bell, marking the start and end of a shift or a break on a factory floor. Its scheduling problem is harder than a school's, because shift patterns often repeat on a cycle that does not divide into a week.
An exception layered on top of a base schedule, matching on date, weekday, month or slot, and changing one aspect — the sound, the time, or the spoken text — without editing the underlying timetable.
Swapping an entire configuration file on a date rule, used when a day is fundamentally different (a holiday, study leave) rather than an exception to a normal day.
An HTTP request the system sends outward when something happens, so another system can react. Used here to report bells fired and emergencies triggered or reset.
A lightweight publish/subscribe messaging protocol common in industrial and IoT installations. A site already running an MQTT broker can trigger announcements through it without new infrastructure.
An open XML standard for exchanging public emergency alerts between systems, used by national warning services. A system that consumes CAP can react automatically to an official alert.
An inbound interface letting another system — a nurse-call panel, timetable software, a PLC gateway — cause an announcement, authenticated per integration.
A method of controlling a device over the internet without opening any inbound ports: the device polls a server for commands. Used where port forwarding is not permitted, which is most institutional networks.
A record of what was done, by whom and when. For a PA system the useful contents are what played, what triggered it, and who reset an emergency.
Running without a monitor, keyboard or mouse — the normal state for a scheduling box in a cupboard, controlled entirely over the network.
Synthesising spoken audio from written text, so an announcement can be typed rather than recorded. On-device synthesis means it works with no internet connection.
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