Last revised: 23 September 2024
Reviewed by: Scott Orlosky, consulting engineer

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:

• • • 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
    • Security

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
• Environmental resistance (IP) 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

Audible Alarms FAQs

What are the power consumption requirements for audible alarms?

Audible alarms typically require anywhere from tens of milliamps (mA) to over 80 mA to operate. Designs using inductors or transformers generally require more current (40+ mA), while those using discrete components or ICs use less than 20 mA.

What materials are used in the construction of audible alarms?

Piezoelectric transducers in audible alarms are typically made of a piezoceramic material that is bonded to a metal horn, usually of brass or stainless steel. Brass is cheaper and easier to process, making it more reliable for most applications. Stainless steel is used for harsh saltwater environments.

What is the importance of environmental testing for audible alarms?

Environmental testing for audible alarms is crucial to ensure their reliability and effectiveness in various conditions. Here are some key points highlighting the importance of environmental testing.

Ensuring Reliability

Audible alarms play a critical role in safety and emergency systems. Environmental testing ensures that these alarms will function reliably under different conditions, such as extreme temperatures, humidity, and exposure to chemicals or salt spray. This reliability is essential for the alarms to perform their intended function when needed most.

Testing Methods

Environmental and electrical testing methods for audible alarms should include:

• Reverse Voltage: Ensures the alarm can handle incorrect voltage polarity without damage.
• Surge Voltage: Tests the alarm's ability to withstand voltage spikes.
• Vibration and Shock: Ensures the alarm can operate correctly in environments with mechanical vibrations or impacts.
• Salt Spray: Tests the alarm's resistance to corrosion, especially important for marine or coastal applications.
• Thermal Shock: Ensures the alarm can handle rapid temperature changes.
• Hot and Cold Life and Storage Tests: Verifies the alarm's performance over its expected lifespan in extreme temperatures.
• Intermittent Voltage Life Tests: Ensures the alarm can handle fluctuating power conditions.

Sealed vs Unsealed Units

Manufacturers offer both sealed and unsealed units. Sealed units, often back-filled with epoxy, provide additional protection against environmental factors such as moisture and dust. This sealing can be crucial for applications in harsh environments, ensuring the alarm remains functional and reliable 

Application-Specific Requirements

Different applications may have unique environmental requirements. For example, alarms used in marine environments need to be resistant to saltwater corrosion, while those in industrial settings may need to withstand high levels of dust and vibration. Environmental testing helps tailor the alarm's design to meet these specific needs.

Quality Assurance

Environmental testing is part of the overall quality assurance process. It helps identify potential weaknesses in the alarm's design and construction, allowing manufacturers to make necessary improvements before the product reaches the market. This process ensures that only high-quality, reliable alarms are deployed in critical applications.

Environmental testing is essential for verifying the durability and reliability of audible alarms in various conditions. It ensures that these alarms can perform their critical safety functions effectively, regardless of the environmental challenges they may face.

What are some market trends in the audible alarm industry?

Recent trends in the audible alarm industry include increased demand for smaller and louder PCB mount units, the development of surface mount audible alarms and the need for lower frequency alarms due to aging populations.

Another trend is the use of specialty sounds like chimes, chirps, warbles, and sirens.

What are the advantages of using a transducer type audible alarm?

Transducer type audible alarms, which are piezoelectric audible alarms without a driving circuit, have lower part costs but require significant circuit design and testing to ensure reliability. They may be justified in high-volume applications where design and testing resources are available.

What are the advantages of using an indicator model audible alarm?

Indicator models — which are piezoelectric audible alarms with a driving circuit —although more expensive, add value by simplifying the design process and ensuring reliability across various conditions.

What are some considerations for the construction quality of audible alarms?

When evaluating an audible alarm, users must consider factors such as housing strength, cleanliness of the circuit board, quality of solder joint, layout of the circuit board, and adhesion strength of silicone adhesive used for mounting the transducer.

Audible Alarms Media Gallery

References

Electronics360—Warning Indicators Emit Light and Sound for Industrial, Retail or Mobile Applications

GlobalSpec—IEC 60601-1-8 AMD2:2020, medical alarms and FAQs

GlobalSpec—Finding Safe Harbor for Electronic Sounding Devices

Image credits:

Federal Signal Corporation/Industrial Systems|Schneider Electric