Image Credits: Federal Signal Corporation/Industrial Systems|Schneider Electric

Audible alarms provide an audible indication of a hazardous situation. Audible alarms may also be available as audio / visual alarm combinations.Applications for audio alarms include:

• Security
• Burglary or theft
• Fire alarms or fire sirens
• Light sirens
• Machine failure or malfunction
• Medical
• Process monitoring
• Power distribution
• Confined space or personal safety
• Smoke
• Structural failure
• Vehicle safety
• Warning sirens

Types of Audible Alarms

Audible alarms are available in several possible types.

• Air horns (or airhorns) are operated by compressed air.
• Sirens, often electrically operated, produce a penetrating warning sound.
• Bells are hollow metal devices that give off a reverberating sound when struck.
• Buzzers are electronic signaling devices that make a buzzing sound (a low continuous humming like that of a bee).
• Speakers are designed to reproduce attention penetrating tones and voice commands for emergency signaling evacuation system. These sounders may also be referred to as hooters.

Specifications and Features

One of the most important specifications to consider when selecting an audible alarm is sound level. Sound level is often measured as sound intensity, which is defined as the sound power per unit area. The most common approach to sound intensity measurement is to use the decibel scale.

Decibels measure the ratio of a given intensity, I, to the threshold of hearing intensity, I₀, so that this threshold takes the value 0 decibels (0 dB. Decibel output is commonly rated at 1). A-weighting is often used to more closely match the measurement to the sound heard by the human ear. Some other specialized weighting modes are B, C, D, and G. The alarm may provide a continuous tone or a pulsed (intermittent) tone.

Some features that may be available with audible alarms are:

• Acknowledge or silence
• Explosion-proof housings
• Intrinsically safe operation
• Voice evacuation
• Outdoor ratings
• Submersible housings
• Volume control options

An additional feature of some audible alarms is synchronization.In most cases this refers to audible alarms with a visual alarm component. NFPA 72 requires visual appliances to flash in synchronization in rooms or areas where there are two or more visual devices that are not a minimum of 55 feet apart. NFPA 72 requires that fire alarm signals be distinctive in sound from other signals and used for that purpose only. This is known as the three-pulse temporal pattern.

Standards

MIL-A-15303 - AUDIBLE SIGNALS: ALARMS, BELLS, BUZZERS, HORNS

ULC CAN-S525 - AUDIBLE SIGNAL DEVICES FOR FIRE ALARM SYSTEMS, INCLUDING ACCESSORIES

ULC CAN-S531 - STANDARD FOR SMOKE ALARMS

Image credits:

Federal Signal Corporation/Industrial Systems|Schneider Electric