Guide to Audible Signals
Our suppliers, including E2S Warning Signals, design and manufacture a comprehensive range of industrial alarm horns and speakers for harsh industrial and hazardous marine applications. Audible signaling effectiveness depends on the size and geometry of the area to be covered, ambient background noise levels, and the sounder’s sound pressure level (SPL), frequency, and tone pattern.


How to calculate alarm horn coverage and effective distance

Sound pressure level (SPL) decreases with increasing distance from the source due to sound divergence. As acoustic energy spreads over a larger area, sound intensity decreases according to the inverse square law, resulting in an approximate reduction of 6 dB(A) for each doubling of distance.

For example, an alarm horn rated at 106 dB(A) @ 1m (3.3 ft) will project sound approximately twice as far as a sounder rated at 100 dB(A) @ 1m. A 100 dB(A) sounder will produce approximately 94 dB(A) at 2 m, 88 dB(A) at 4 m, and so on.

Distance attenuation (Inverse square rule)

Impact of ambient background noise

The effective distance of an alarm sounder is defined as the distance at which the calculated SPL remains at least 5 dB(A) above ambient background noise.

Example:
For a 100 dB(A) sounder operating in a 65 dB(A) environment, the effective distance is where the sound level drops to 70 dB(A). This corresponds to approximately 105ft.

By comparison, a 120 dB(A) sounder reaches the same threshold at approximately 980 ft, providing ten times the distance and one hundred times the coverage area.

Installation and environmental considerations

• In open areas, sound propagates in all directions; enclosed spaces may increase effective levels.

• Ceiling-mounted sounders benefit from reflected sound energy.

• Wall-mounted sounders are typically more effective than pillar-mounted units.

• Recommended mounting height is 6.5-8 ft above floor level.

• Synchronizing multiple sounders improves audibility.

• Personnel wearing hearing protection must be considered.

Frequency, tone pattern, and sound attenuation
Sound propagation is frequency-dependent. Lower-frequency signals travel further and experience less attenuation.

Frequency adjustment guidance

Audibility depends on tone pattern as well as sound level. Multi-frequency, pulsed, alternating, and dynamic tones are more effective, especially in high-noise environments.

Determining the number of alarm horns required

Simply adding more sounders does not guarantee better coverage and may result in:

Uneven sound distribution

Excessively loud local sound pressure levels

Minimal improvement in overall audibility

Correct output selection and placement are often more effective than increasing device count.


Key principle: match alarm horn output to ambient noise

• Minimum alarm level: 65–70 dB(A)

• At least 5 dB(A) above ambient noise

• Recommended maximum: 10–15 dB(A) above ambient

• Fire doors reduce sound by ~30 dB; standard doors by ~20 Db

Design example: sounder output vs quantity

Scenario

• Room size: 30 m × 20 m

• Ambient noise: 65 dB(A)

• Sounder: 100 dB(A) @ 1 m

• Effective coverage radius: ≈ 30 m

This space is adequately covered by one sounder under low-noise conditions.

Increased noise condition
New ambient noise: 85 dB(A)  (heavy machinery)

Question
How many sounders are required?

Correct answer

One sounder - if output is increased to 120 dB(A). A 20 dB increase provides 10×distance and 100× coverage area.

Disaster warning / wide-area signaling considerations

• Ground effects, buildings, and barriers

• Atmospheric refraction and temperature gradients

• Wind direction and speed

• Building construction
• Human perception of sound

Disaster warning sounders should be mounted horizontally at 10–15 m above ground, positioned above obstructions, and installed with clear line of sight.

Because these variables interact in complex and often unpredictable ways, sound coverage predictions can only ever be estimates.

For optimal performance, disaster warning sounders should generally be:

• Mounted horizontally, at a height of 10–15 meters (33–49 ft) above ground level

• Installed at the highest practical point within the coverage area, but not so high that sound overshoots the target zone

• Positioned at least 2 meters (≈6.5 ft) above any obstruction within a 50 m (164 ft) radius

• Installed with a clear line of sight between the sound source and the intended coverage area whenever possible

Explore The Signal Source range

• Alarm Horns

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