Sound pressure level for new ACCU

[PacificSteve]

I will sure appreciate any help that you expert can provide. Design challenge: new aircon system, using ACCU, to be lcoated about 4 feet from the property line. The adjacent building (2 feet away from equipment) will act as a reflecting wall.

I have Octave band and "sound level" information from the manufacturer. I have spent alot of time searching for acoustic calculations and came up blank.

So - the goal is to predict the decibel level at the property line. 70 dB is allowable.

The manufacturer tell me that their "sound rating" is really sound power as if a point source.

They state sound rating for ACCU as 86.2 dBa.
Octaves as follows
63 --
125 93
250 86
500 83
1000 80
2000 78
4000 73
8000 71

So-by making some broad generallizations about reflecting quality of the steel building wall, or by ignoring the presence of this wall altogether (i.e. if this cant meet 70 dB with the wall "not there" in calcs, it sure won't meet 70dB with the wall there), what would be the sound pressure level in dBa at the property line?

OK, if anyone is still with me....is there a calculation method for turning the octave band data into a point source sound power?

Many thanks for anyone who could contribute to this solution and method. Thanks!

PacificSteve

 

[okdb]

Hi PacificSteve,

How to calculate sound levels from sound powers:

Lp = Lw - 20log(r) - 11 (free field, 1/1 sphere)+ 3dB for each reflection plan (for ex. ground reflection, wall,...)
By the way, I'm not sure that formula works in US units, so use METRIC UNITS to be sure.

"r" is the distance form the middle of the sound source to the measurement point.

I guess that the manufacturer has calculated the sound power levels after the sound levels from the machine above a hard surface. That means that there's already included +3dB in the sound power levels form ground reflection, the case of an hemisphere. So calculate the sound pressure levels with Lw - 20log(r) - 11 and add 3dB for reflection from the wall. If there's any other reflecting surfaces, you have to add some dB, 1-3, depends on where and what the reflecting surface is.

Tips: be sure the manufacturer is REALLY giving you sound POWER levels. Ask for the test results and procedure (ASTM, ISO) such as you can verify.

I hope there's nobody living close this machine with 70 dBA as noise limit!!

OkdB