Acoustics for dummies

[PacificSteve]

Hello all,

I am looking for some help with some rule of thumb calc methods for a rough estimate of dBa at a distance from a source dBa. i.e. for lighting design, footcandles diminish with the distance squared.

Issue I am working on, cooling tower producing 70dBa. What will the dBa be 200' away, across level ground, half grass, half asphalt, no obstructions.

I appreciate any and all feedback related to more advanced considerations for better calculations, but my prime focus is keeping it simple and getting an answer to the above example.

Thank you!

PacificSteve

 

[kstifle]

PacificSteve,

You might want to check out the booklet "Environmental Noise" from B&K. You can download a copy from their web site (

http://www.bksv.com).

Kirk Stifle, P.E.

 

[Rob45]

The accepted rule of thumb for this kind of thing is a 6dB drop for every doubling of the distance.
The limitations on this "rule" are that it is for point sources, and for hemispherical radiators. So, when you say the cooling tower is "70 dBA", is that source power, or is that 70 dBA measured at some distance?
If it is 70 dBA measured at a distance, and the distance is sufficient to consider the cooling tower to be a point source, then you can estimate the sound level at a variety of distances using the 6 dB rule.
Needless to say, having very absorptive ground cover will prvide greater attenuation with distance than the 6dB rule would indicate.

 

[PacificSteve]

kstifle: thanks for the link, there are some some "primers" on this site.

Rob45: Thanks for the info. I can clarify my question somewhat. Manufacturer states 70dBA at 5 feet. The sound source is a cooling tower that is roughly 14 feet square, mounted 4' off the ground, and the unit is 8' tall (total height 12' from ground to top of tower).

Seems like it would be stretching the rule of thumb a little to call this a point source, especially in relation to the 5 foot sound data. What do you think?

Can I throw one more twist in the mix? On one side of the tower is a 30' tall steel sided building. This will tend to reflect the sound in exactly the direction I don't want it go (neghbors 20' away). The building is about 6' away from the closest side of the tower.

Thanks for the help!

PacificSteve

 

[GregLocock]

This all very rule of thumb anyway. The reflective wall could double the intensity on the neighbour's side, which is 6 dB.

So roughly you get 70 dBA at 5 feet, less 6 dB per doubling of distance, ie 30 dB, plus 6 dB for the reflection, to give an estimate of 46 dBA. Note that the less point-like the source, the less attenuation you get at distance.

This could be in error by +- 10 dB, and rather more than that if your installation has a bad resonance.

46 dBA is the sound level in a quiet office - so if your neighbour was sat in their garden they would be able to notice your plant. perhaps you could measur ethe current ambient noise at the neighbour's property to get an idea of the likely problem.

It would also be a good idea to find one of these things installed, and measure the nosie at 200 feet.

Cheers

Greg Locock