[warnerwh]

I'm confused here. I believe I read point source speakers lose 6db per meter. That would be 12db at two meters. What I'm confused about is this for one speaker playing or two?

Line source speakers lose apparently only 3db per meter if memory serves me. How come?

Thanks

[Gordy]

Here's an excellent paper by Jim Griffin on line array's... hope it helps! http://www.audiodiycentral.com/resource/pdf/nflawp.pdf

[JohninCR]

I'm confused here. I believe I read point source speakers lose 6db per meter. That would be 12db at two meters. What I'm confused about is this for one speaker playing or two?

Line source speakers lose apparently only 3db per meter if memory serves me. How come?

Thanks

Warner it's -6db per doubling of distance for a point source and -3db per doubling distance for a line array within the range of distance that it functions as an array.  The simple answer is that the sound from an array doesn't disperse vertically.  Since the sound energy is more focused, it travels farther.  That's why line arrays must be used a big concerts, otherwise the sound would be way too loud up front and too quiet in the back.

[Daryl]

Hi Warner,

SPL decreases as distance increases due to the wavefront (and it's energy) being distributed over a larger area.

The wavefront for a point source expands in three dimensions.

The surface area of a three dimensional shape (or the area of a point sources wavefront) is proportional to the square of it's radius.

Thus the relative SPL at a given distance from a point source will be 1/Distance^2.

So if 1m=0db (1) then 2m=-6db (1/4), 3m=-9.5db (1/9), 4m=-12db (1/16) and so on.

For line sources the wavefront expands in only two dimensions (an infinite column).

The perimeter of a two dimensional shape (or the relative surface area of our theoretical infinite column) is proportional to it's radius.

Thus the relative SPL at a given distance from a line source will be 1/Distance.

So if 1m=0db (1) then 2m=-3db (1/2), 3m=-5db (1/3), 4m=-6db (1/4) and so on.

Daryl

[Scotty]

Warner, you also need to bear in mind that the math involved assumes the point source and line arrays are in a free space condition with no boundary surfaces near the speakers with-in one wave length of the lowest frequency that the system operates down to. When speakers are operated in a reverberant environment like your listening room with dimensions smaller than the wave-length of the lowest frequency that system is designed for room gain will occur and energy emitted at shorter wave-lengths is not lost either so the difference between the two speaker designs relative efficiency is not as simple as it would first appear. Also a line source only behaves like it is infinitely long as a result of the floor and ceiling reflections of the energy emitted by the speaker column.
If the line source is in a free space condition it will show the predicted efficiency only when the measurement is taken in a near-field location. That being a distance that is no further away than the line is long. Once again
operation in a reverberant situation will tend to change the results somewhat.
The virtual length of the line source will tend to vary with frequency depending on
how reflective or absorbent the floor and ceiling are within operational bandwidth
of the line source.
Scotty

[JohninCR]

If the line source is in a free space condition it will show the predicted efficiency only when the measurement is taken in a near-field location. That being a distance that is no further away than the line is long.

Scotty,
While I agree with most of your post in principle, I disagree with the spirit of it.  Finite length arrays in our acoustically small rooms enjoy a quite significant SPL/distance advantage, with the biggest benefit being a much larger prime listening area for arrays.  You get a listening area vs a listening position.  A floor to ceiling array will behave like an infinite length array and won't be subject to a nearfield farfield transition in-room.  A shorter array isn't limited in distance to the length of the array.  The nearfield/farfield transition distance is both line length and frequency dependant.  Lower frequencies need longer line lengths, but even a 3ft line may be sufficient for the higher frequencies in most rooms.  A 6ft array works quite well in a typical room.   Dr. Griffin's white papers are a great read for anyone interested in domestic line arrays.