Sound Calcs 1

[RonShap]

I have a large room with an ambient sound of 87 dBA. I wish to sound an alarm that is heard at 15dBA above ambient (or 102dBA) throughout.
If I have a speaker that can output 104dBA at 10', how do I calculate the spacing of the speakers.

 

[Myndex]

I'm assuming by dBA you mean dB SPL A weighted.

Two non coherent sound sources of 104 dB SPL when mixed will produce 107 dB SPL (mixing two equal level sources that are not in phase results in an increase of 3 dB SPL).

If at 10' the signal is 104 dB SPL, at 20' it would be 98 dB SPL (doubling distance results in a 6 dB decrease from point sources).


If two of your speakers were 40' apart, and you stood equa-distance between them, the level would be 101 dB SPL, assuming there was no other noise in the equation.


Because you have a background noise of 87 dB, the overall level measured will be higher by a bit - that math is somewhat more complicated.


For a good reference, see "The New Audiocyclopedia".


Regards,



Andy

Andy Somers
Myndex Technologies Inc.

 

[paulfr]

This may sound daft,but have you considered a visual alarm, or a system with ducking for the main audio (or is it noise)
are you familiar with noise at work regs? - PPE, risk assessment...

 

[RonShap]

There are very specific requirements for fire alarm system audio devices. It requires 15 dBA above ambient with a maximum of 120dBA. There are already enough visual notification devices in the room to make it a disco dance hall, but the code requires audio devices in case the occupant is blind.
Thanks for the responses.

 

[melone]

Couldn't you always output 120dBA?

 

[RonShap]

There are no fire alarm speakers (listed as fie alarm device) that I've found that could output more than 104 dBA @ 10'. There are cluster type speakers, that are like octopus arms with speakers on the end, but there ratings are not much different.

 

[melone]

Then how will you ever meet the requirement if the ambient noise ever goes above 105 dBA?

 

[RonShap]

In my case ambient is 87dBA
If there were a situation where the ambient is over 105 (such as an active generator room), the fire inspector is more flexible.

 

[walker1]

Just for the fun of it.
Does it have to be a speaker?
Couldn't one or several of these mechanical sirenes do the job?

They are usually a lot more energy efficient too.

 

[RonShap]

Unfortunately, it had to be a speaker. After the alert tone, the message not only had to be audible, but intelligible (understandable).

 

[SolderingGunslinger]

I seem to remember that J.W.Davis used to have a page in their catalog for calculating speaker placement. It was meant for placement of 8" public address and background music ceiling speakers, however it might be an assistance in solving your problem.

As an undergrad (decades ago) I worked for a sound installation company. I used what I had learned in CALC 201 and came up with essentially the same answer the boss got from a cheap cardboard slipstick. IIRC, that little tool came from J.W.Davis as well...and is probably a collectable all these years later.

I remain,

The Old Soldering Gunslinger