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Is it possible to convert from sound power to sound pressure level?Helpful Member! 

ColonelSanders83 (Mechanical) (OP)
23 Sep 10 14:03
Hello All,

Due to some ignorance on the part of our sales department we ended up with a contract which referanced a maximum Sound Power Level of XX DBa. The exuipment manufacturer is garunteeing Sound pressure level.

I have been tasked with converting between the two, but I have very little to go on and all my research on the topic has so far come up empty.

1) Is it possible to convert from sound power level to sound pressure level?

2) If possible how do I go about doing so? Is there a standard covering this? Perhaps a discription in a book?

Any help on this matter would be greatly appreciated.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

Helpful Member!  Hatch (Mechanical)
23 Sep 10 14:30
The short answer to your question is Yes.

You need to make/determine a few parameters.

1) Distance to the sound pressure level point, (usually 3 feet or 1 metre).

2) Is the measurement point indoors or outdoors.  Most contracts just assume outdoor conditions or "free field conditions".  i.e. Q=2.

If the source is indoors and your guaranteed pressure level is indoors, then you will need complete room acoustics information which is covered in a typical acoustics book.

3) Or you can do a very simple estimate/conversion by subtracting 12 dB from the sound power level to get a ballpark sound pressure level (free field assumed).

4) One last thing to note is that the conversion is frequency dependant so the best conversion is done with octave or third octave band levels.  This is not very significant if the distance is short such as 3 feet but can be very significant if the distance starts to approach two or three hundred feet or more.

I don't know the specifics of your contract but if your guarantee is 90 dBA power level and the equipment supplier has guaranteed 90 dBA pressure level, you have a much tighter limit to meet. (90 dBA power level in free field is about 80 dBA pressure level).
ColonelSanders83 (Mechanical) (OP)
23 Sep 10 15:29
Hello Hatch,

Thanks for your responce.

1) I believe distance is 3 ft

2) The unit is an outdoor unit installed as part of a package.

3) That simple conversion leads to the conclusion that were screwed..... oh well, cest la vie.

4) I will keep that in mind as this ugly topic is broached with the customer.

Is there a national standard or industry accepted method you could point me towards for performing a formal calculation myself. The old "some dude on the internet said so" responce won't be good enough on this one.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

Strong (Mechanical)
23 Sep 10 16:51
Here is one place to poke around for definitions:

One significant issue to calculate sound power form sound pressure or vice-versa is the size of the source. 3-ft distance from a gas turbine enclosure is quite different than 3-ft from a small residential AC condensing unit. The area factor must be addressed.

GregLocock (Automotive)
23 Sep 10 19:49
The short answer to your question is maybe. You may be able to get to an equivalence that satisfies your customer in this case, but generally there is a big problem. The reason is that a resonant sound field can display very high pressure levels while requiring little sound power to generate them.



Greg Locock

New here? Try reading these, they might help FAQ731-376: Forum Policies

Hatch (Mechanical)
24 Sep 10 13:22
The basic equation is in "Noise and Vibration Control Engineering" by Beranek and Ver.  A simplified form is below.

Lp = Lw - 20 log (r) + DI -11

Lp = sound pressure level.
Lw = sound power level
r  = distance from source to receiver
DI = directivity index.  Use 3 for free field (outdoors).

The above book is an excellent book.   
hydtools (Mechanical)
26 Sep 10 0:18
You may want to register at the B&K site for access to their literature:


jeyaselvan (Mechanical)
6 Oct 10 8:47

Look into page 12 of ISO 3744 wherein you can refer teh same to your customer


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