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18th November 2004, 01:30 PM  #11 
diyAudio Member
Join Date: Sep 2002

But sure prosound use with (normally) no ceiling, and home use where the line can be a "true" linesource is two different situations, no?
I have not done my homework on linesource theory, though I understand the basics so so. /Peter 
18th November 2004, 02:02 PM  #12 
Banned
Join Date: May 2004
Location: New Hampshire

There are similarities. Consider a live sound venue where the required throw is 100 meters. To reach that at 80 Hz in the nearfield would require an array height of about 30 meters. That's not practical in most cases.
In a home with a throw of five meters getting to 80Hz in the nearfield requires an array almost 7 meters high, also not a viable option. But realistically it doesn't matter, as once the frequency is low enough that room reflections and modes are a significant percentage of response the nearfield/farfield transition frequency and distance doesn't matter than much anyway. 
18th November 2004, 02:33 PM  #13 
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Join Date: Sep 2002

But in a home, with a line from floor to ceiling, there are no "limits", the line is infinite.
So in a room, using a ribbon from floor to ceiling, where the floor and ceiling acts as mirrors extending the line there must be other rules than for free field use.´ I´m aware of room contribution for sound sources in all registers. Guess I´ll have to go read Jim Griffins paper to get the final understanding of linesources. /Peter 
18th November 2004, 04:25 PM  #14 
diyAudio Member

"A 'true' line source with no farfield doesn't exist; can we agree to limit discussion to realworld circumstance?"
Actually no. Not in this case since i sought an answer for a theoretical question, an answer to be used in a theoretical paper that is to be judged by theoretical lectors at an university. If we were to limit the discussion to real world cases  the formula of pointsource is far more complex than i´m currently, and probably ever will be, cabable of using or get anything usefull out of. I´ve read the fundamentals of the pointsource in "Loudspeaker and Headphone Handbook" and being a 3rd. semester student (7 more to go) I didn´t understand much of it. And since this project has focus in a completely other and very different subject  this part of the project is a minor detail  and there for the theoretically simplest formula is more than adequate. So  as i said  cased closed. However i´m happy to reveal that there are wellinformed peoble in this forum. I do though suggest that posts that goes far beyond the question in a thread should go in threads were they do belong :) So thanks everyone :) HansHenning / Denmark
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18th November 2004, 04:58 PM  #15 
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Freedom,
I really think you need to get your arms around that point source formula if you're are going to be dealing with people at that kind of theoretical level, because I believe that formula applies to a lot of things other than just sound wave dispersion.
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18th November 2004, 07:54 PM  #16 
diyAudio Member

I´ll do so  if needed. So far it has not been the case  but  as i wrote  there are another 3½ years to come.
So thanks for the advice. Regs. HansHenning PS I may need an armextender in order to get hold of all of it  anywhere you know where i can get this :) ?
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18th November 2004, 08:10 PM  #17 
diyAudio Member

Sorry I can't help you. I don't need to know it, so the only thing in my memory warehouse is the 6db rolloff for point source speakers and 3db for line arrays in the nearfield.
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18th November 2004, 08:49 PM  #18 
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Join Date: Jun 2002
Location: USA, MN

There is a fairly easy way to get an intuitive feel for the 6dB point source freefield rule. For each doubliing of distance the sound intensity in watts per square meter is reduced by a factor of four.
10 * log (0.25) = 6 (6.0206...) The actual formula for sound level is: dB = 10 * log(Intensity/1e12) such that 1 watt per square meter is 120dB If you wish to use pressure rather than intensity you may use: dB = 20* log (P/0.00002), where P is sound pressure in Pascals. 0.00002 is the sound pressure in pascals defined to be the 0dB level. Bonus question: What happens to SPL when the distance goes to zero, assuming dB = 90 at 1 meter
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