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+Jason Hugo

#56 - Fire Alarm Bell Mounting Height

Posted by Jason Hugo & Cameron Sharp on 7/29/2016 to Fire Sprinkler System Components
The fire alarm bell has been around forever; it's been saving lives and irking building occupants with inconvenient drills since 1852. Today fire alarm systems are required in all public buildings, such as schools, offices, hotels, warehouses, libraries and courthouses, for their ability to inform quickly large amounts of people of imposing danger. 

Read enough, and want to start browsing? Simply jump to our selection of fire alarm bells!

The most common fire alarm system that fire bells connect to are sprinkler systems. You can imagine why; loud noises and water spraying everywhere is enough to get anyone out of a building! Alarm bells should comply with all local codes and/or NFPA 72: National Fire Alarm Code when installed on sprinkler systems. Alarm bells are most often located on the street address side of the building, generally directly above the fire department connection and the post indicator valve. When the fire alarm bell sounds, it quickly directs firefighters to the right location for connecting their hose to the sprinkler system. For residential sprinkler systems, an exterior bell is also recommended, and should be located so that it's easily heard from all sleeping areas. The recommended size for commercial building sprinkler systems is a 10-inch fire alarm bell, residential systems are compatible with all sizes - 6", 8", 10". 
6" Fire Alarm Bell

Regardless of the systems, they operate on and buildings they serve, fire alarm bells should be installed so that the sound level at any nearby location is higher than 65 dB, or higher than 6 dB plus the ambient sound level. You read that right, dBs and AMBIENT SOUND LEVEL. The former is used widely in electronics, signals, communications, and for measuring sound; they are abstract mathematical units used to describe quantities and ratios (power, sound pressure, voltage). The ambient sound level is a measurement used in studying the strength of intrusive sound sources. 

To achieve the 65 dB target at any nearby location from an alarm bell, NFPA 72 states: . . . wall-mounted appliances shall have their tops above the finished floors at heights of not less than 2,290 mm (90 in.) and below the finished ceilings at distances of not less than 150 mm (6 in.). Where low ceiling heights do not permit mounting at a minimum of 2,290 mm (90 in.), visible appliances shall be mounted within 150 mm (6 in.) of the ceiling.

Sound Output

NFPA 72 has varying requirements for the different functions and areas that fire alarm bells operate in: 

For alert and evacuation tones in a high ambient noise area, such as industrial factories, the maximum total sound pressure should be 110 dB(a). 

Public mode audibility, used for signaling occupants to take action on their own or relocate, is required to be at least 75 dB. 

Private mode audibility, used for signaling to facility staff or emergency personnel to take action, should be 70 dB. 

Lastly, audibility in sleeping areas should also be 75 dB. 

Sound levels are assessed directly in line with the middle of the alarm bell or other sound emitting device, also known as “on-axis." This is due to the effect that a change in axis or listening position has on the frequency response. On-axis is believed to be the loudest position, however, every time the distance from the fire alarm bell doubles, sound level reduces by 6dB. This is useful to keep in mind when assessing sound levels at any particular point in a room, which is done by measuring the distance from the alarm bell and the angle off-axis, for head height varies depending on the size of the person. For instance, audible notification appliances are rated at 10 feet from the appliance. So, if you had fire alarm bell rated at 95 dB(A) at 10 feet, the sound level could be as low as 89 dB(a) at 20 feet. 

Decibel loss can also occur through doorways. It is estimated that as much as 20 dB(A) is lost through a normal door and 30 dB(A) through a fire door. This form of decibel loss has influenced policies, such as those surrounding sleeping areas. Alarm bells used for bedrooms are required to produce 75 dB(A). If those devices are installed outside of the room, it is unlikely it could achieve that amount of sound because of decibel loss. Therefore, alarm bells must be installed inside sleeping areas. 

Components

Fire alarm bells are protected by heavy-duty, dome housing. Attached to the outer shell of this dome is the gong. This piece is made from selected, alloy steel that give it the loud resonant tones required for fire alarm systems. A 4-wire design makes installation quick and easy. Learn how to install fire alarm bells in 6 easy steps here! Planning on installing your alarm bell outside? If so, you're going to want to consider an air-tight, water-resistant Alarm Bell Back Box and Durable Alarm Bell Wire Guard. We also advise that you check out our blog - Can I mount a Fire Alarm Bell Outside?- for more information.
Fire Alarm Bell w/ Back Box

QRFS is your one-stop shop for quality, durable fire protection equipment. Fire alarm bells are certainly part of that selection, and we provide 6", 8", and 10" models, either 24 volts or 120 volts. Our bells come standard with corrosion resistant red enamel finish, giving them high visibility, and a heavy steel gong which produces sound levels up to 96 dB! Visit us today and get started on your complete fire protection program.
6", 8", 10" Fire Alarm Bells


This blog originally posted by Jason Hugo and Cameron Sharp at QRFS.com/blog on July 22, 2016. If you like what you've read, check us out at Facebook.com/QuickResponseFireSupply or at Twitter @QuickRsponseFS.

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