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freedom 17th November 2004 03:36 PM

Loudspeaker formula
Dear All.

I need a formula telling me what the sound pressure level is at a certian distance from a source given that the spl of the source is known and that the distance is known. I know it is a "universal" formula - but are looking for the simplest form, where walls or other reflecting source are not taken into account.

I tried a google search, but no luck so far.

A search here in the forum didnīt help me either.

Anyone? Please :-)

Thanks in advance



Ron E 17th November 2004 06:31 PM

What kind of source? Piston, point source, line source, simple or dipole, etc...?

Pan 17th November 2004 07:44 PM

Point source -6dB for every doubling of distance
Line source -3dB for every doubling of distance

This is for ideal monopole sources.


freedom 17th November 2004 07:58 PM

Cased Solved
Excellent. Thank you.

Case closed

Regs. Hans-Henning

BillFitzmaurice 17th November 2004 11:15 PM

Not so fast. Line sources are -3dB per distance doubling only within the nearfield, which is calculated (in meters) by the formula R=L x L x f/700 where R is the distance to the nearfield/ farfield transition, L is the array height and f is the frequency. Once in the farfield the 6dB rule applies.

Pan 17th November 2004 11:41 PM

Bill, are you really sure that formula is correct? Feet or meters?

Say 100Hz/700 = 0.14

Line height 2.4 meter

0.14 x 2.4 x 2.4 = 0.8 meter. That would mean that farfield for 100Hz begins at 0.8 meter.

20.000Hz/700 = 28.57

28.57 x 2.4 x 2.4 = 164 meter transition for 20k.

Maybe itīs 700/f ?


Ron E 18th November 2004 12:12 AM


Originally posted by BillFitzmaurice
Not so fast. Line sources are -3dB per distance doubling only within the nearfield...
A "true" line source is infinite, there is no farfield. I asked what type of source before answering for a reason.

johninCR 18th November 2004 03:22 AM

There is no such thing as a true line source, so why even bring it up unless you are writing a text book on theory?

Also, the line array effect doesn't change from the 3db decrease to 6db at a finite point, it is a gradual change. Therefore what is needed are both forumlas, where the transition point starts and where it ends, in order to get a good picture of how the array will behave in the real world.

freedom 18th November 2004 08:06 AM

ok Ok OK!
No worries!

Concept of theory and pracsis are not the issue :-)

you guys are discussing weather infinite does exist or not. The answer depends on what glasses youīre wearing - eg. a mathematicians or a phycisists... Or more "earthnear" weather the universe has a boundary or not! - Try cracking that one :-)

In fact - a sound from any source will never never ever die completely - but it will decrease (in audiolevel), and during the decrease an energy conversion will take place...

I got the answer i needed - point of source is the issue in this case, so the -6dB is the case. Thanks again :-)

Regs. Hans-Henning

Ps: and sorry for my bad english.

BillFitzmaurice 18th November 2004 01:00 PM

A 'true' line source with no farfield doesn't exist; can we agree to limit discussion to real-world circumstance?

Yes, a 2.4 meter array will go from near to farfield .8 meters from the radiating plane. The same array goes to farfield at 82 meters at 10kHz. In pro-sound where line arrays today dominate it's generally accepted that trying to go lower than 80 Hz in the nearfield is not a worthwhile endeavor. The change from nearfield to far field is actually a fairly abrupt transition.

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