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17th November 2004, 04:36 PM  #1 
diyAudio Member

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 didn´t help me either. Anyone? Please :) Thanks in advance Regards HansHenning
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17th November 2004, 07:31 PM  #2 
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Join Date: Jun 2002
Location: USA, MN

What kind of source? Piston, point source, line source, simple or dipole, etc...?
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17th November 2004, 08:44 PM  #3 
diyAudio Member
Join Date: Sep 2002

Point source 6dB for every doubling of distance
Line source 3dB for every doubling of distance This is for ideal monopole sources. /Peter 
17th November 2004, 08:58 PM  #4 
diyAudio Member

Cased Solved
Excellent. Thank you.
Case closed Regs. HansHenning
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18th November 2004, 12:15 AM  #5 
Banned
Join Date: May 2004
Location: New Hampshire

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.

18th November 2004, 12:41 AM  #6 
diyAudio Member
Join Date: Sep 2002

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 it´s 700/f ? /Peter 
18th November 2004, 01:12 AM  #7  
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Join Date: Jun 2002
Location: USA, MN

Quote:
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18th November 2004, 04:22 AM  #8 
diyAudio Member

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.
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18th November 2004, 09:06 AM  #9 
diyAudio Member

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 you´re 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. HansHenning Ps: and sorry for my bad english.
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If you cannot measure it  you cannot improve it! 
18th November 2004, 02:00 PM  #10 
Banned
Join Date: May 2004
Location: New Hampshire

A 'true' line source with no farfield doesn't exist; can we agree to limit discussion to realworld 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 prosound 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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