In my July 2025 Q&A article, I tackled questions often raised by those in the field trying to understand the requirements that affect their fire alarm system installations. The same format is below.

Note: An asterisk* after a code section number indicates additional information regarding the requirement can be found in Annex A of NFPA 72-2025.

I am currently working on designing a fire alarm system for a facility that, under normal conditions, has very high ambient sound pressure levels. In fact, the employees that work throughout this facility must wear ear protection.

How should I design the fire alarm system notification appliances to meet this unusual situation? 

NFPA 72-2025, National Fire Alarm and Signaling Code, addresses this very issue as follows:

18.4.1.1* “An average ambient sound level greater than 105 dBA shall require the use of a visible notification appliance(s) in accordance with Section 18.5 where the application is public mode or Section 18.6 where the application is private mode.”

The Annex A material states in A.18.4.1.1: “The Code does not require that all audible notification appliances within a building be of the same type. However, a mixture of different types of audible notification appliances within a space is not the desired method. Audible notification appliances that convey a similar audible signal are preferred. For example, a space that uses mechanical horns and bells might not be desirable. A space that is provided with mechanical horns and electronic horns with similar audible signal output is preferred. However, the cost of replacing all existing appliances to match new appliances can impose substantial economic impact where other methods can be used to avoid occupant confusion of signals and signal content. Examples of other methods used to avoid confusion include, but are not limited to, training of occupants, signage, consistent use of temporal code signal pattern, and fire drills. Hearing protection can attenuate both the ambient noise level and the audible fire alarm signal. Specifications from hearing protection manufacturers might allow the effect of hearing protection devices to be evaluated. In spaces where hearing protection is worn due to high ambient noise conditions, visible signal appliances should be considered. In addition, where hearing protection is worn due to high ambient noise conditions, the audible fire alarm signal and ambient noise measurements can be analyzed, and the audible fire alarm signal can be adjusted to account for attenuation caused by the hearing protection devices.”

In this instance you must provide audible notification appliances for those periods when the facility may not be operating. And you must provide visible notification appliances to alert occupants when the facility operations produce excessive sound pressure levels. In some facilities, you can design the interface of the fire alarm system operating into a fire alarm condition to shut down the equipment that is producing high ambient noise levels. For example, this feature may be employed at casinos where the fire alarm system interrupts power to the gaming machines to allow the occupants to hear the fire alarm system audible notification appliances.

You may also wish to investigate the use of special audible notification appliances that provide a carefully engineered sound that occupants can perceive even with the high ambient sound pressure levels.

18.4.7* Narrow Band Tone Signaling for Exceeding Masked Thresholds.

“A.18.4.7 This subsection permits a more rigorous analysis and design for audible signaling. Acoustic design practice and psychoacoustic research have long recognized that for a signal to be audible, it need only penetrate the background noise in a one-third or a one octave band. The averaging resulting from A-weighted analysis and design is a simplification that often results in systems being overdesigned. This overdesign is not dangerous but can be costly and is certainly not needed for effective system performance.

“18.4.7.1 Masked Threshold Allowance. Audible tone signaling shall be permitted to comply with the masked threshold requirements in this subsection in lieu of the A-weighted signaling requirements in 18.4.3 and 18.4.4.

“18.4.7.2* Calculation Method. The effective masked threshold shall be calculated in accordance with ISO 7731, Danger signals for workplaces - Auditory danger signals.

“A.18.4.7.2 Noise at a lower frequency can mask a signal at an adjacent higher frequency. Thus, it is necessary to calculate the effective masked level of the noise in accordance with established procedures. Figure A.18.4.7.2 shows an example of an octave band analysis of noise along with the calculated effective masked threshold and the proposed fire alarm signal.

“18.4.7.3 Noise Data. Noise data for calculating the effective masked threshold shall be the peak value of noise lasting 60 seconds or more for each octave or one-third octave band.

“18.4.7.4 Documentation. Analysis and design documentation shall be submitted to the authority having jurisdiction and shall contain the following information:

(1) Frequency data for the ambient noise, including the date, time, and location where measurements were taken for existing environments, or projected data for environments not yet constructed

(2) Frequency data of the audible notification appliance

(3) Calculations of the effective masked threshold for each set of noise data

(4) A statement of the sound pressure level that would be required by 18.4.4 or 18.4.5 if masked threshold signaling had not been done

“18.4.7.5 Sound Pressure Level. For masked threshold signaling, the audible signal tone shall meet the requirements of either 18.4.7.5.1 or 18.4.7.5.2 but not for the reproduction of prerecorded, synthesized, or live messages.

“18.4.6.5.1 The sound pressure level of the audible tone signal shall exceed the masked threshold in one or more octave bands by at least 10 dB in the octave band under consideration.

“18.4.6.5.2 The sound pressure level of the audible tone signal shall exceed the masked threshold in one or more one-third octave bands by at least 13 dB in the one-third octave band under consideration.”

These alternatives all offer viable ways of overcoming the problems presented by providing fire alarm system notification in occupancies with very high ambient noise levels. In my experience, the use of visible notification appliances will provide the most useful and most cost-effective option.