You're on the right track! Let's break down how to calculate the required number of fire alarm horns, focusing on NFPA 72, ambient sound levels, and horn dBA ratings.

Key Principles NFPA 72 is the Governing Standard

Everything you do must comply with the latest edition of NFPA 72, National Fire Alarm and Signaling Code.

Audibility is Paramount

The core goal is to ensure the fire alarm signal is clearly heard above the ambient noise.

Location-Specific Design

The sound level required varies depending on the type of space and potential ambient noise.

Steps to Calculate the Number of Horns1. Determine Ambient Sound Levels



Measure or Estimate

This is the most critical step.

Measurement

Use a calibrated sound level meter to measure the average ambient sound level during the loudest foreseeable conditions in each area. This needs to be done for various times of the day and during typical activities. (e.g., during peak class changes in a school, during manufacturing operations in a factory).

Estimation

Only when measurement is impractical, you can use NFPA 72's prescriptive ambient sound levels.

However, this is generally NOT recommended unless you have really low noise enviroment such as office space. ALWAYS TRY TO MEASURE AMBIENT SOUND LEVELS. If the building has a different location such as sleeping area, or general space, you would need to determine the ambient sound level for all locations to estimate the worst case sound level. NFPA 72 Prescriptive Ambient Levels (Table 18.4.3)

Sleeping Areas: Assumed to be 35 dBA. Public Areas: Assumed to be 70 dBA. Other Areas: Measure or use reliable data.

Consider Occupancy

Occupancies with higher potential noise (factories, restaurants, workshops) will require higher alarm sound levels.2.

Determine Required Alarm Sound Level (dBA)



NFPA 72, Section 18.4 (Audibility)

This section outlines the required alarm sound levels. The basic rule is:

At least 15 dBA above the average ambient sound level 5 dBA above the maximum sound level that lasts at least 60 seconds Maximum 120 dBA Sleeping Areas

Specific requirements apply:

Minimum 75 dBA at pillow level. This takes precedence over the 15 dBA above ambient. Use low-frequency signals (520 Hz) to wake people more effectively. Consider Intelligibility

In areas with voice evacuation systems, ensure the voice message is intelligible. This often means even higher sound levels.3.

Select a Horn



Review Product Data Sheets

Your Mircom example datasheet is helpful. Here's what to look for:

UL 464 Compliance

Make sure the horn is UL 464 listed for fire alarm use.

Sound Output (dBA)

Pay close attention to the sound output at a specified distance (e.g., 85 dBA at 10 feet).

Sound Pattern (Temporal 3)

Fire alarm horns must produce the Temporal 3 pattern (three pulses followed by a pause). This is a specific coded signal.

Voltage

Ensure the horn is compatible with your fire alarm control panel's (FACP) voltage (usually 24VDC).

Current Draw

Calculate the total current draw of all horns on a circuit to ensure it's within the FACP's capacity.

dBA vs. Distance

Horn sound output decreases with distance. The datasheets should give you the dBA at a given distance. You need to use the dBA and distance information to calculate the actual dBA at various points in the room.

Inverse Square Law

The sound level decreases by 6 dB for every doubling of distance (in a free field). In a room, reflections complicate this, but it's a good starting point.

Example

If a horn is rated 85 dBA at 10 feet, it will be approximately 79 dBA at 20 feet (85 - 6 = 79).

Adjustable dBA

Some horns have adjustable dBA output. This is very useful for fine-tuning the design.4.

Placement and Calculation



Divide the Area

Divide the building into areas with similar ambient noise levels.

Initial Placement

Strategically place horns based on the geometry of the space. Consider: Room Size Obstructions (walls, partitions, equipment) Ceiling Height

Sound Level Calculations

For each horn, calculate the sound level at various points within the area. Use the inverse square law as a starting point, but be aware that this is an estimate.

Worst-Case Scenario

Identify the location within the area that will have the lowest sound level. This is your "worst-case" point.

Iterate

Adjust horn placement and number until the worst-case sound level meets the minimum required dBA (15 dBA above ambient).

Documentation

Document all calculations, assumptions, and horn locations. This is essential for plan review and AHJ approval.

Important Considerations AHJ (Authority Having Jurisdiction)

Always check with your local fire marshal or building department for any specific requirements or interpretations of NFPA 72. They have the final say.

Room Acoustics

Room acoustics significantly affect sound levels. Hard, reflective surfaces increase sound levels, while soft, absorbent surfaces decrease them.

Reverberation

High reverberation can reduce speech intelligibility.

Testing

After installation, conduct sound level testing to verify that the system meets the design requirements. Use a calibrated sound level meter. Document the results.

Accessibility

Consider accessibility requirements (ADA) for people with hearing impairments. This may involve visual notification appliances (strobes) in addition to horns.

Voice Evacuation Systems

Voice evacuation systems are becoming increasingly common. They require careful design to ensure intelligibility, which may involve higher sound levels and careful speaker placement.

Software

Fire alarm design software can help with calculations and placement, but it's crucial to understand the underlying principles.

Example Using Mircom FH-400 Datasheet

Let's say you have a general office area where you've measured the ambient sound level to be 65 dBA. You need an alarm level of at least 80 dBA (65 + 15 = 80).1. Look at the Datasheet

The Mircom FH-400 datasheet shows various sound output levels. For example, it might say "UL464 Temporal 3, High Volume: 90 dBA at 10 ft."2.

Distance Attenuation

At 20 ft it will be approximately 84dBA, and at 30 ft approximately 81dBA.3.

Placement

You would place the horns so that the farthest point in the room is within this distance range from a horn, and at all location it can achieve at least 80dBA to meet the requirement.

In summary, calculating the number of fire alarm horns is an iterative process that requires accurate ambient sound level measurements, careful horn selection, strategic placement, and thorough calculations to ensure that the alarm signal is audible throughout the building. Always adhere to NFPA 72 and consult with the AHJ. Flag for review