Member
Joined 2003
I once compared GP measurements between a sheet of smooth MDF vs a concrete slab...concrete dropped HF about 1db above 10k. Not much change, and I was specifically looking for the difference.
How many feet was the distance? It does depend on the distance as well as the specifics of the surface. The point is that the results depend on factors that are often not revealed. I once saw someone doing this test at 10 meters! That would be a lot of loss.
Member
Joined 2003
Shading of acoustically short line array
Over the past 5-6 weeks I've "shaded" the mid-bass portion of my two way DIY line arrays. I have 3 two-way line arrays. The mid-bass portion of the arrays (L/C/R) are used from about 100 Hz to 2000Hz. They are all located on a the front wall, more or less half-way between floor and ceiling. Each is about 42" from driver edge to driver edge. The listening distance is 12-13 ft.
I used a shading method based on path length difference from center of the array. I shaded symmetrically about the center with resistors. The outer drivers are attenuated by approximately 3 dB.
These are acoustically short line arrays which I have been very happy with for a number of years. I altered one speaker at a time and listened for a week or more.
I immediately observed an improvement in resolution, especially with voice. When listening to stereo program material with just the left side shaded the singer "moved" to the right a bit. This effect became more pronounced (and balanced) as I shaded the left and center speaker. The overall improvement in sound quality is rather dramatic. By far the best value in an upgrade that I have ever done ($10 worth of power resistors).
What I don't know is why. To my mind it can be two things: reduction in floor ceiling bounce (reduced vertical lobes in the midrange) or increased coherency of signal time arrival due to the reduction in amplitude of delayed "outer" driver signals. Of course it can be a combination.
It is not lost on me that this type of shading "shortens" the line. I am starting with a line that is being used probably 2 octaves lower than it should be, so I am only in the near field for 2.25 of the 4.25 octaves. This situation certainly as not gotten better with shading but there is no question in my mind of the improvement in "musicality" of the system with shading in place. it has become more difficult for me to believe that a floor to ceiling line-array will somehow need less or no shading.
Anyone else have a similar or contrary experience?
Frank
Over the past 5-6 weeks I've "shaded" the mid-bass portion of my two way DIY line arrays. I have 3 two-way line arrays. The mid-bass portion of the arrays (L/C/R) are used from about 100 Hz to 2000Hz. They are all located on a the front wall, more or less half-way between floor and ceiling. Each is about 42" from driver edge to driver edge. The listening distance is 12-13 ft.
I used a shading method based on path length difference from center of the array. I shaded symmetrically about the center with resistors. The outer drivers are attenuated by approximately 3 dB.
These are acoustically short line arrays which I have been very happy with for a number of years. I altered one speaker at a time and listened for a week or more.
I immediately observed an improvement in resolution, especially with voice. When listening to stereo program material with just the left side shaded the singer "moved" to the right a bit. This effect became more pronounced (and balanced) as I shaded the left and center speaker. The overall improvement in sound quality is rather dramatic. By far the best value in an upgrade that I have ever done ($10 worth of power resistors).
What I don't know is why. To my mind it can be two things: reduction in floor ceiling bounce (reduced vertical lobes in the midrange) or increased coherency of signal time arrival due to the reduction in amplitude of delayed "outer" driver signals. Of course it can be a combination.
It is not lost on me that this type of shading "shortens" the line. I am starting with a line that is being used probably 2 octaves lower than it should be, so I am only in the near field for 2.25 of the 4.25 octaves. This situation certainly as not gotten better with shading but there is no question in my mind of the improvement in "musicality" of the system with shading in place. it has become more difficult for me to believe that a floor to ceiling line-array will somehow need less or no shading.
Anyone else have a similar or contrary experience?
Frank
I am a believer in shading arrays and did so with a McIntosh 16 tweeter array as described in my paper. The primary goal of shading a short array is to smooth out the polar response and reduce the frequency response variation that is strongly seen with variations in listener height.
I found that between a seated height and standing height there was a dramatic variation in response that made it impossible to do sensible EQ. This is especially true of short arrays in the 3-4 ft length range.
As to your impressions of sound change, it doesn't sound as though you have done any re-EQ after the shading, so it is hard to predict what you started with or what you ended up with. The first order effect is always frequency response and your changed perception is likely due to the response changes that came along with any polar curve changes. The better design approach would be to experiment with shading until you got acceptably low response variation, then to crossover and equalize the array as it then is.
Note that 3 dB rolloff of the outer elements is very little shading and wouldn't make too much difference, in my experience. For the Mac 16 tweeter array (from memory) I broke it into 4 groups and gave them 0dB, -1dB, -3dB, -9dB. A lot of shading schemes can work and you just need to find a good compromise between response variation with height and array output (collapsing the array to a single central element is guaranteed to give the best polar response, but tends to be wasteful of the outer elements!)
If I get a chance next week I may put together a simulation of short arrays with plausible weighting schemes.
David
I found that between a seated height and standing height there was a dramatic variation in response that made it impossible to do sensible EQ. This is especially true of short arrays in the 3-4 ft length range.
As to your impressions of sound change, it doesn't sound as though you have done any re-EQ after the shading, so it is hard to predict what you started with or what you ended up with. The first order effect is always frequency response and your changed perception is likely due to the response changes that came along with any polar curve changes. The better design approach would be to experiment with shading until you got acceptably low response variation, then to crossover and equalize the array as it then is.
Note that 3 dB rolloff of the outer elements is very little shading and wouldn't make too much difference, in my experience. For the Mac 16 tweeter array (from memory) I broke it into 4 groups and gave them 0dB, -1dB, -3dB, -9dB. A lot of shading schemes can work and you just need to find a good compromise between response variation with height and array output (collapsing the array to a single central element is guaranteed to give the best polar response, but tends to be wasteful of the outer elements!)
If I get a chance next week I may put together a simulation of short arrays with plausible weighting schemes.
David
The Hann shading scheme used by Keele on strait arrays puts the driver(s) between the center and the end at -6db. the center always being 0db and the outside ends way down there if not truncated. As David says this can be quite wasteful of drivers vs. efficiency. Keele shows graphs of the shading weight vs. length for both types of shading he uses in his CBT papers.
flg,
Thanks for the heads up. I missed the weighting/height analysis in Keele's papers.
Point is well taken regarding efficiency. Obviously deeply attenuated additional end-drivers give diminishing returns.
Frank
Thanks for the heads up. I missed the weighting/height analysis in Keele's papers.
Point is well taken regarding efficiency. Obviously deeply attenuated additional end-drivers give diminishing returns.
Frank
For me that was a problem with Keeles arrays the 2 times I heard them. They didn't get loud enough without overheating the resistor array and pushing the drivers too hard. If all the drivers were full on, that wouldn't happen. But then the array wouldn't work right. It's a trade-off.
More robust or more efficient drivers in the CBT would be nice to hear. The concept works well.
More robust or more efficient drivers in the CBT would be nice to hear. The concept works well.
The tweeters are quite low in sensitivity with the CBT-36 but that keeps the cost down and Don Keele wanted minimal driver spacing; hence, the small tweeter diameter. With the drivers I've used the arrays will play very loud but the cost also increases due to the amount of neo material in the magnetics.
Having built both shaded and non-shaded arrays I definitely believe the small loss of output with shading is outweighed by the advantages.
One thing I would like to suggest is that the horizontal response of a CBT shouldn't be dismissed just because it doesn't conform to what is assumed to be the epitome of directivity.
The same reason you'd shade any array. To improve the off axis response, reduce comb filtering, and widen the vertical directivity.
You shade an array to make it sound better.
David Smith and Rick Craig are a couple of experts on array design, and both are using and have recommended shading in this thread.
You shade an array to make it sound better.
David Smith and Rick Craig are a couple of experts on array design, and both are using and have recommended shading in this thread.
If a floor to ceiling array is trying to approximate an infinite line source by using the floor and ceiling reflections, it's not any array, though...
If the array is floor to ceiling (assuming they are reflective), there is no off-axis - everything is on axis, there is no vertical "axis".
He (David Smith) did not always say that:
Source: http://www.diyaudio.com/forums/newreply.php?do=newreply&p=2647010
With "reflective" floor and ceiling in mind shading a floor to ceiling array makes no real sense to me.
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I would still agree with that. It is the short arrays that exhibit the greater variation with relatively small vertical position shift.
At Mac I did simulations of floor and ceiling extended arrays. Even with some gap and reduced reflection level (representing boundary absorption), the results weren't too different. Once an array reaches a sufficient length, even infinite extension has minimal effect.
David
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