Horn / Siren Symbol
Definition: The Horn / Siren symbol represents an electromechanical or electronic audible-signalling device — a horn, siren, or annunciator — that produces a loud audible alert when energised, shown in electrical schematics and wiring diagrams as a two-terminal device with a positive (+) and negative (−) terminal, referenced under IEC 60617 (signalling devices) and ANSI/IEEE 315-1975 with the designator HA (horn/audible annunciator).
Also known as: siren, alarm horn, audible annunciator, buzzer horn, electric horn, klaxon, warning siren, evacuation horn.
What the Horn / Siren symbol means
The Horn / Siren symbol denotes any audible-signalling device that converts electrical energy into sound to alert personnel to a specific condition — fire, intrusion, process alarm, shift start, or emergency evacuation. In a wiring diagram, the symbol appears as a two-terminal load connected between a supply positive (+) and negative (−) or between a switched live and neutral, typically energised by a relay contact, alarm panel output, or PLC output module.
Horns and sirens range from small 6 V DC electronic buzzers to 120 dB+ industrial pneumatic horns operating at 24 V DC or 120 V AC. The symbol's two terminals correspond to the device's electrical supply connections; no polarity is implied for AC-operated devices, though DC devices have a defined + and − to ensure correct operation of internal electronics.
How to identify the Horn / Siren symbol
The horn/siren symbol is drawn as a small funnel or megaphone shape pointing outward (indicating sound projection), or in simplified industrial schematics as a rectangle labelled 'HA' or a circle with sound-wave lines emanating from it. In the wiring diagram representation used here, the symbol appears as a compact block with two terminals: + (positive) exiting from the top and − (negative) exiting from the bottom, representing the DC supply polarity.
Function in a circuit
A horn or siren receives electrical energy at its two terminals and converts it to audible sound through an electromagnetic solenoid driving a diaphragm (electromagnetic horn), a rotating impeller driven by a motor (motor siren), or a piezoelectric transducer driven by an oscillator circuit (electronic horn/siren). The sound output — measured in decibels (dB) at 1 metre — typically ranges from 80 dB (small electronic horn) to 120 dB (industrial or emergency siren). Some models produce a continuous tone; others produce a pulsed, warble, or whoop pattern depending on internal electronics.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 represents audible signalling devices (horns, bells, buzzers) under signalling device symbols. IEC 60849 governs sound systems for emergency purposes. The IEC symbol for a general audible annunciator is a rectangle or funnel with a sound-wave annotation, labelled with the device function. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 designates audible signalling devices as HA (horn/audible device) or BZ (buzzer). NFPA 72 (National Fire Alarm and Signalling Code) covers fire alarm horn/strobe notification appliances. The ANSI symbol is a stylised megaphone or bell shape. |
| Key difference | IEC and ANSI/IEEE symbols for horns are visually similar — both use a funnel/megaphone shape or a labelled rectangle. ANSI/IEEE 315 uses the designator HA; IEC uses the designator HA or a locally defined designation. NFPA 72 fire alarm horns follow a separate set of appliance specifications and wiring requirements. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Supply voltage: 6–24 V DC (electronic horn), 24 V DC (fire alarm/industrial), 120 V AC or 240 V AC (large industrial siren). Sound output: 80–120 dB at 1 m. Current draw: 50 mA (small electronic) to 1 A+ (large motor siren). Enclosure rating: IP54–IP66 for outdoor/industrial models.
Where the Horn / Siren symbol is used
- Fire alarm systems: horn/strobe notification appliances on fire alarm circuits (24 V DC, NFPA 72) mounted in corridors, stairwells, and open areas to alert building occupants to evacuate
- Industrial process alarm annunciation: 120 dB horn on a process control panel energised by a relay output when a PLC detects a high-temperature, overpressure, or equipment fault alarm
- Intrusion alarm systems: siren horn on a burglar alarm panel activated when a sensor detects an unauthorised entry, producing a loud continuous or pulsing tone
- Shift and production signals: electric horn in a factory energised by a time clock relay to signal shift start, break time, or end of shift
- Emergency vehicle warning: automotive horn or siren connected to the vehicle's 12 V DC electrical system and activated by the driver for traffic clearance
- Hazardous gas detection: audible siren paired with a gas detector output to warn personnel of flammable or toxic gas accumulation in a confined space or process area
Example
In a fire alarm riser wiring diagram, the Horn / Siren symbol appears on each floor's notification appliance circuit (NAC) powered by the fire alarm control panel at 24 V DC. The + terminal of each horn connects to the positive NAC circuit wire, and the − terminal connects to the return wire. The fire alarm panel supervises the NAC for open-circuit and short-circuit faults.
Key facts
- The Horn / Siren symbol represents an audible-signalling device with two terminals: + (positive supply) and − (negative/return), energised by a relay contact, alarm panel output, or PLC digital output.
- In ANSI/IEEE 315-1975, audible-signalling devices including horns and sirens are designated HA; buzzers use the designator BZ.
- Fire alarm horn/strobe appliances operate at 24 V DC per NFPA 72 (National Fire Alarm and Signalling Code) and are connected in notification appliance circuits (NAC) supervised for wiring faults.
- Sound output of horns ranges from 80 dB (small electronic annunciator) to 120 dB (large industrial siren), measured at 1 metre distance in free-field conditions.
- IEC 60849 governs sound systems for emergency purposes including evacuation horns, requiring defined sound pressure levels (≥65 dB(A) at listening positions) in public buildings.
- DC-operated electronic horns have a defined polarity (+ and −); AC-operated horns (120 V or 240 V) have no polarity and can be connected either way.
- The horn/siren symbol in wiring diagrams always includes a control element — a relay contact, alarm panel output, or manual switch — in series with the supply terminals.
- Industrial outdoor horns carry IP enclosure ratings of IP54–IP66 to resist dust and water ingress in manufacturing, petrochemical, and outdoor applications.
Diagrams that use this symbol
- horn relay diagram
- relay diagram horn
- horn relay wiring diagram
- 4 pin horn relay wiring diagram
- horn wiring diagram
- horn diagram
- car horn wiring diagram
- dual horn diagram
Frequently asked questions
What does the horn/siren symbol mean in a wiring diagram?
The horn/siren symbol represents an audible-signalling device that produces a loud sound when electrically energised. It is shown as a two-terminal device (+ and −) connected to a supply through a relay contact or alarm panel output, and is used in fire alarm, process alarm, intrusion, and industrial signalling circuits.
What does the horn symbol look like in a schematic?
The horn symbol is commonly drawn as a funnel or megaphone shape pointing outward to indicate sound projection, or as a labelled rectangle in industrial wiring diagrams. In the two-terminal representation used here, a compact block has a + terminal at the top and a − terminal at the bottom, with the device body between them.
What voltage do industrial horns operate at?
Industrial and fire alarm horns most commonly operate at 24 V DC, which is the standard notification appliance circuit voltage per NFPA 72 and is compatible with PLC output modules and relay-controlled alarm panels. Larger industrial sirens may operate at 120 V AC or 240 V AC for higher sound output.
What designator is used for a horn in a schematic?
ANSI/IEEE 315-1975 designates horns and audible-signalling devices as HA (horn/audible annunciator). Buzzers specifically use the designator BZ. IEC 60617 uses HA or a locally defined tag for audible devices. In fire alarm drawings, horns are tagged per NFPA 72 notification appliance numbering conventions.
What is the difference between a horn and a siren?
A horn typically produces a single continuous or intermittent tone and is used for process alarms, shift signals, and general notifications. A siren produces a rising-and-falling or warbling tone and is associated with emergency evacuation, intrusion alarms, or emergency vehicle warning. Both are represented by the same horn/siren symbol in electrical wiring diagrams.
What standard covers fire alarm horns?
NFPA 72 (National Fire Alarm and Signalling Code) governs fire alarm notification appliances including horns and horn/strobe combination devices in North America. IEC 60849 covers sound systems for emergency purposes internationally. Both standards specify minimum sound pressure levels, circuit supervision requirements, and installation guidelines.
Does the horn/siren symbol have polarity?
DC-operated electronic horns have a defined polarity (+ and −) because their internal oscillator or amplifier circuit requires correct supply polarity. AC-operated horns (120 V or 240 V) have no polarity, and the two terminals can be connected either way. In wiring diagrams, the + and − labels on the symbol indicate the expected DC polarity for correct operation.
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