Noise level calculation

[remp]

I have the sound power level form the casing of an AHU at different freqyuencies. How do I calculate the noise level at 5m 10 m, 20 m etc from the unit. I need a formula that give the noise level at different distances from the source.
Any good web sites?


Remp

 

[BradPainting]

Refer to:

http://hep.physics.indiana.edu/~rickv/Sound_intensity.html

"Sound power level" is the raw power, determined only by the mechanical vibrations and given by the formula:

--> sound power level = LW = 10 log( W / W0)

W0 is the "reference sound power", equal to 10^(-12) Watts. W0 is basically represents silence.

If the value you have is in DB, you have the LW value. Use the above equation to solve for W, in Watts.

What you need is "sound intensity level" (sticking with the website's terminology), given by:

--> sound intensity level = LI = 10 log ( I / I0 )

Sound intensity level depends on the radial distance from the source, and is given in units of W/m^2. I0 is analogous to W0 and equals 10^(-12) W/m^2. "I" is the distance dependent variable, given by:

--> sound intensity = sound power / (4 pi R2)

I hope I didn't over-complicate that.

 

[KiwiMace]

Use the well known CADET rule for acoustics:

Can't Add, Don't Even Try

Adding dB
Lpt = 10 log10 [log-1(dB1/10) + log-1(dB2/10) + log-1(dB3/10)]

eg. For the addition of 3 sound pressure levels : 90dB, 95dB and 88dB

= 10 log10 (109 + 109.5 + 108.8)

 

[nightowl5473]

This is actually a relatively easy calculation to do. You aren't trying to calculate intensity, you are trying to calculate the sound pressure level at a distance based on a known sound power level.

The equation you use depends on your environment. You would have to use the appropriate equation based on whether you are outdoors in a "free field" or indoors. If you are indoors, you need to account for both the "direct" and "indirect" sound fields. The sound absorption in the room you are dealing with controls the "indirect" aka reverberant sound field.

Here is a good page to do the calculation correctly:

http://www.engineeringtoolbox.com/sound-propagation-indoor-d_72.html

Of course, you would have to do the calculation for each frequency band based on the sound power level AND the average absorption coefficient for each frequency band. Sound absorption is a function of frequency, so you really shouldn't use a single value there.

Hope this helps.

 

[BravoCompany]

All Standards on ARIs Website are free and can be downloaded to pdf.

I believe this one should help you out:

http://www.ahrinet.org/ARI/util/showdoc.aspx?doc=953

If this isn' what your looking for look around in the standards section.

I hope this is helpful.

Good Luck!

I hope this is helpful.