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
Hans-Henning
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
Hans-Henning
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.
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
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
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.
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.
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. Hans-Henning
Ps: and sorry for my bad english.
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. Hans-Henning
Ps: and sorry for my bad english.
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.
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.
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.
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.
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
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
"A 'true' line source with no farfield doesn't exist; can we agree to limit discussion to real-world 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 point-source 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 point-source 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 well-informed 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
Hans-Henning / Denmark
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 point-source 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 point-source 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 well-informed 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
Hans-Henning / Denmark
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/1e-12) 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
10 * log (0.25) = -6 (-6.0206...)
The actual formula for sound level is:
dB = 10 * log(Intensity/1e-12) 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
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.