Fire Alarm Bell design question

[ElectDesigner]

Hypothetically say you have a 20,000sqft open space retail floor with no walls, and say you were to place 10" fire alarm bells (85dba at 10ft) throughout the area such that it would be loud enough to be heard anywhere on the floor.

My question is this:

1. How many fire alarm bells would you need?
2. How does one determine this from the fire alarm bell spec sheet ?

Basically what Im trying to get at is the effective radius of a fire alarm bell, and how it can be determined from a cut sheet. Also if the area had partition walls how does this affect the sound pressure level.

So what is the rule of thumb?

 

[nhfire77]

The rule of thumb is to not use bells inside. You would use horn strobes inside. (unless you have some weird spec that requires bells) They must be 15dB above ambient noise levels, you would do well by reading the annex of 72. There are 'average' ambient noise level by occupancy charts. You could attempt to extrapolate coverage distances from the devices spl, but it would be inaccurate due to acoustics in the space. It's one of those you have to have experience to know how many to use. With speakers you can make a closer guess, but that's a different issue.

You can't have too many horns, unless they cause the SPL to exceed 120dB. However, the larger the load the shorter your circuit will be and he less devices you will be able to fit on it. Additionally, batteries get bigger, which means you may need to then deversify the load. This would require more power supplies. So, again, you need experience to know what will work. Even then site conditions may require the addition of more horns after you complete the install. Thankfully, must horns now come with a low,medium, high field selectable output.

 

[gadfly56]

Your 20,000 square foot space would be a square with roughly 140' long walls. Dead center of the square would be 70' from any wall. You lose 6db for every doubling of the distance, so 10' to 20' to 40' and finally let's say 80' (to keep the math simple). Loose 18db and your 85db bell is giving you 53db in the center of the room. But wait! I have four (4) bells, one in the center of each wall. I should be good, no? Well, no. You convert the db level to power, multiply by 4, reconvert to db and now the center of the room is at a whopping........59db. Slightly above the level of your normal office and certainly NOT 15db above ambient. You can't do this with wall-mount bells.

The solution? Ceiling mounted horn/strobes, use the 110 cd setting on the strobe and buy lots of NAC boosters. I designed open area coverage for a 500,000 square foot facility. Nothing like seeing 370+ strobes going off at once!

 

[ElectDesigner]

The general trend I've seen here (in Canada) is a move away from bells and towards horn/strobes and speakers. However there are a lot of existing buildings will bells, and expansion to these buildings would require adding more bells.

I guess the rules are different in both countries.

I guess for low-medium ceiling heights (t-bar/grid clg) one would not have to worry but for open areas like say the receiving area of a store which is open, lets assume its a 20ft roof, in this case would the horn/strobe be attached directly to a joist or would have to be suspended to a lower height or rather mounted on the wall?

 

[nhfire77]

The general trend I've seen here (in Canada) is a move away from bells and towards horn/strobes and speakers. However there are a lot of existing buildings will bells, and expansion to these buildings would require adding more bells.

I guess the rules are different in both countries.

I guess for low-medium ceiling heights (t-bar/grid clg) one would not have to worry but for open areas like say the receiving area of a store which is open, lets assume its a 20ft roof, in this case would the horn/strobe be attached directly to a joist or would have to be suspended to a lower height or rather mounted on the wall?

Please get a copy of 72 and read it, the experience be very informative. No they would not be suspended lower. Please read this application guide to address the ceiling mounted considerations:

http://www.systemsensor.com/pdf/A05-0218.pdf

 

[ElectDesigner]

Thanks for the link!, was looking for something like this.