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Old 4th January 2004, 08:05 PM   #1
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Default line array open baffle

In reading the information on this site, and the linkwitz site, I am convinced that my next pair of speakers should be open baffle dipoles. However, I don't want to have to make an active eq for my speakers, and I want them to play down to 100Hz. In addition, I don't think I have enough room in my living room for 2 6x6 foot panels. If I were going to make panels that big, I may as well just make a new wall for an almost infinite baffle.

So, I have been trying to think of ways to extend the bass response in an open baffle without making the speakers too big or using active eq, and I have thought of two (one of which I think may work well).

1. The phoenix open baffle system has folded back pieces of wood on the sides. If I were to extend the folded back panel to about 2.5 feet, I could add extra bass response at about 100Hz. This would make a quarter wave chamber for extra bass at 100Hz, but it would make a half wave, 3/4 wave, etc. for higher frequencies. Of course, that would pretty much ruin the response at higher frequencies, so that's out.

2. I have also done some reading on line arrays. (This was a good paper http://www.audiodiycentral.com/resource/pdf/nflawp.pdf ) Line arrays basically focus sound pressure in the plane perpendicular to the line array. (I tried to make a picture of a vertical array as an example of how there is more sound pressure toward the center of the vertical line array, and less at the top and bottom). I was thinking that if I used a tall but narrow open baffle (6 feet by 2 or 3 feet) with a horizontal line array I could extend the base response, without having a very wide speaker. I think it would work because sound pressure in the horizontal plane would be focused toward the center of the speaker in the near field. In the vertical plane there would be plenty of height for 100Hz response. The problem with this set up would be that the speaker would become directional, (kinda like an electrostatic speaker). Speaking of an electrostatic speaker, this is probably why a 1 or 2 foot wide electrostatic speaker can play down to 100Hz or lower. Has anybody tried this? Does anyone have any thoughts? How wide would I need to make my line array? I think I am going to try it for my next set of speakers, but I don't really have any measurement equipment to show if it works.
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Old 5th January 2004, 11:48 AM   #2
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Speakerguy,

I have seen an OB line array on the net, it is quite interesting:

OB line array

Line array is not really about improving bass response. It has more to do with efficiency, output, minimising vertical reflections and projecting sound further. Seems to me like a very good combination with OB actually.

OB without active filters is a case for a more traditional approach, and you should search the forum for other discussions of this. With the Linkwitz method, you either need to follow his design or be ready for a lot of work. Or you can make a larger baffle and use drivers that are more suited to OB use.

I see no need to reinvent the wheel here.

Also you could consider a hybrid - sealed bass with OB above 100 Hz.

regards,
Paul
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Old 5th January 2004, 02:50 PM   #3
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I must not have got my point across. I have actually read that line array site before. It is a good site, but it is not the horizontal line array that I am speaking of. I have included another picture of what I am thinking about. The idea is that sound pressure will be focused along the vertical axis instead of the horizontal. The vertical axis has the longer baffle, so the bass response will be improved.

The basic problem with bass in an open baffle is that sound pressure cancels out at the edge of the baffle. The less distance between cancelation, the higher the natural high pass cross over frequency.
Look at page four of this pdf: http://www.audiodiycentral.com/resource/pdf/nflawp.pdf. Now imagine that instead of looking at the side of the speaker, imagine you are looking at the top of the speaker, and imagine the rear wave has to do the same thing. In this case, instead of sound traveling directly sideways as in the normal open baffle case, it first travels outward from the speaker. Thus the sound has to travel further before cancelation, resulting in a lower natural cutoff frequency. Does this make sense?

John

P.S. Not trying to reinvent the wheel, just trying to grease it bearings with teflon to make it more efficient.
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Old 6th January 2004, 02:59 AM   #4
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The image helps. Your point is now more clear.

I really doubt the improvement in bass response will be worth messing with the stereo image so much. I'm trying to get my head around what it would do to the stereo image, but I can't imagine it helping.

What I find impressive about the line array OB is that there is a synergy between the two speaker typologies. I think where bass leads the design, the stereo image and midrange will fall behind.

Still, if you have selected drivers, I can't see any harm in trying out ideas like that to see how you like them. MDF is cheap, and your ears should always be the final judge.

regards,
Paul
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Old 6th January 2004, 06:05 AM   #5
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Quote:

I really doubt the improvement in bass response will be worth messing with the stereo image so much. I'm trying to get my head around what it would do to the stereo image, but I can't imagine it helping.
Actually, I think, stero imaging will be improved because less sound pressure will be reflected off the side walls. The speakers should be more directional however. Thus, there will be less optimal listening area. I think if I don't make the array too long, then I will still have a resonable listening area. Again, this will be kind of like an electrostatic panel.

Quote:

What I find impressive about the line array OB is that there is a synergy between the two speaker typologies. I think where bass leads the design, the stereo image and midrange will fall behind.


Maybe the midrange won't be as good, but I haven't thought of a reason yet. I am only trying to get the main panels down to 100Hz without eq.

Quote:

Still, if you have selected drivers, I can't see any harm in trying out ideas like that to see how you like them. MDF is cheap, and your ears should always be the final judge.
I am going to be ordering my drivers soon. I think I am going to get the stryke sa071's, just because they are cheap, and they seem to be really flat from 100Hz to 1000Hz. I would like to go with a ribbon speaker for above 1000Hz, but I don't think I can afford ribbons that go that low. I may go with tang band W3-881S for mid range from 1k to 10k because they are cheap, but they don't have the flatest response, so I may try to find something else. I haven't thought about the tweets that much. I may just use the midrange. I know from electrical engineering lab expiriments that I can only hear up to 14k anyway.
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Old 6th January 2004, 08:42 AM   #6
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The drivers you mentioned seem to be good value. But still, given that you want to get around eq, why not use drivers with parameters suited for OB use?

TB have some good fullrange drivers that look very promising, many of them can do 100 Hz - 15k quite well, and they are cheap. An array of them could be a good idea.

I was just looking at some other threads with OB discusions, you should have a look at them if you haven't already. Here is one:

dipoles for me? help me decide

The right driver on a reasonably sized baffle can do quite well fullrange and can be very simple. Some even use glass to reduce visual impact, which can be quite an elegant solution.

I found one driver mentioned on the website of Thorsten Loesch that looks interesting. "Afterburner, and inexpsnsive DIY speaker plan that uses the Eminence high efficiency widerange driver fairly affordable, which has a high Qts and looks interesting for dipole use:

http://www.fortunecity.com/rivendell...terburner.html

This is another direction worth considering with drivers.

regards,
Paul
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