physics of line arrays

[Craig]

Hey all. I'm thinking about making my next project a line array set up, but even a cursory look at the physics of line arrays suggest that they may significantly blur the sound because of interference. A quick calculation suggests that even with mid-bass woofers crossed at ~3000 Hz, there significant phase cancellation due to path difference of the waves with as little as a 70 cm array line length. With the tweeters, this problem is even more marked. So my question is how does this affect the sound produced? Does the large number of drivers provide an averaging effect? I'm confused.

[Pete Mazz]

Have you read Jim Griffin's White Paper on NF LAs?

Pete

[Craig]

No. Been looking around for it though - heard there was something like that floating around, but could'n't locate it, so cheers!

[SY]

You have to be a little careful about modeling, especially if your model thinks of the drivers as point sources- which they decidedly are not at wavelengths on the order of the driver size. If you space the drivers tightly and the array is a true line array (i.e., roughly floor to ceiling), you'll find that the vertical polar response can be made quite smooth up to the frequency corresponding to the gap between diaphragms. And even then, the polar pattern will smooth out as you get some distance between the listener and the array.

[GRollins]

You need to differentiate between a line source and a line array. A true line source such as the Bohlender-Graebner RD-75 is a single diaphram roughly six feet long. A line array is a collection of smaller (usually point source, but sometimes short line drivers) drivers stacked on top of one another. The two are not equivalent.
A true line source will give you lobing in the horizontal plane when used with other drivers, but that's all. A line array will give you a three dimensional waffle; difficult to work with.
Be alert to driver height from the floor vs. your listening height. You'll get the best response--at least easiest to work with--if there's a driver pretty much in line with your ear.

Grey

[Jim Griffin]

Originally posted by GRollins
You need to differentiate between a line source and a line array. A true line source such as the Bohlender-Graebner RD-75 is a single diaphram roughly six feet long. A line array is a collection of smaller (usually point source, but sometimes short line drivers) drivers stacked on top of one another. The two are not equivalent.
A true line source will give you lobing in the horizontal plane when used with other drivers, but that's all. A line array will give you a three dimensional waffle; difficult to work with.
Be alert to driver height from the floor vs. your listening height. You'll get the best response--at least easiest to work with--if there's a driver pretty much in line with your ear.

Grey

Grey,

Those interested can take a look at my white paper and understand better the conditions (driver spacing) necessary for a line array to emulate a line source. The white paper is located at:

http://www.audiodiycentral.com/resource/pdf/nflawp.pdf

The major rub that users have reported with the long B-G ribbon (B-G 75) is its roll-off in the upper octave (~17,000 Hz and beyond). A couple more shortcomings are its low sensitivity and high cost.

Jim

[ronc]

Seems to me, course i am just a dumb ole texan,that if you curved the array you would get a multi element focused array,problem solved.
ron

[Jim Griffin]

Ron said:

"Seems to me, course i am just a dumb ole texan,that if you curved the array you would get a multi element focused array,problem solved."


My reply is that if you do so, you will have a very limited listening distance as the sound is focused at one specific point away from the source. A better solution is to implement a near field line array which will cover the entire listening room with great sound.

Jim

[ronc]

I was just speaking from my experience using ultrasonic focused arrays in non destructive testing of materials Jim.
Yes true the focus point(sweet spot) is small but have you ever listened to a front firing front loaded horn? I would imagine the results would be simular.
ron

[Leonidas]

Did anyone ever try an open baffle "Bessel"array?
looking for some experiences.

Leon