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Author Topic: Flair Equation for BT7?  (Read 3881 times)

Antone Atmarama Bajor

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Flair Equation for BT7?
« on: May 07, 2005, 05:06:56 am »

     Sorry I keep bugging ya' all about this but I was wondering if
anyone has the numbers I need to graph the flair.  I'm trying to go by this guys equations

http://www.diy-systems.com/en-us/pg_18.html html I need Fs of the BT7 module etc.

    I imagine it should be easy enough for me to design a flair extender if I had all of the right info.

    Or is there an easier way to generate a graph of the flair.  I could probably work with 1 curve of the hyperbola since they are totally symmetrical.  I know the length of the BT7 is 10.6Ft T=.55
90" mouth for 2 cabs.  And low cut off of about 27Hz.

    Any help much appreciated.
Thanks

Sum
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Elliot Thompson

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Re: Flair Equation for BT7?
« Reply #1 on: May 07, 2005, 09:07:15 am »

Why don't you look at the Lab Sub. The characteristics
between the two are closer than you may think.

Best Regards,
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Elliot

Mark Seaton

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Re: Flair Equation for BT7?
« Reply #2 on: May 07, 2005, 12:17:40 pm »

sumsound wrote on Sat, 07 May 2005 04:06

     Sorry I keep bugging ya' all about this but I was wondering if
anyone has the numbers I need to graph the flair.  I'm trying to go by this guys equations

http://www.diy-systems.com/en-us/pg_18.html html I need Fs of the BT7 module etc.

    I imagine it should be easy enough for me to design a flair extender if I had all of the right info.

    Or is there an easier way to generate a graph of the flair.  I could probably work with 1 curve of the hyperbola since they are totally symmetrical.  I know the length of the BT7 is 10.6Ft T=.55
90" mouth for 2 cabs.  And low cut off of about 27Hz.

    Any help much appreciated.
Thanks

Sum


I would point out that you are probably making it more complicated than is justified.  Everything in the real world has tolerances.  The ultimate question is how great a variation can you have before there are detrimental or noticable changes.  In the case of the BT7, Tom Danley already did the hard part.  As he explained many times in the early stages of the LAB sub design, it is the small end of the horn where variations are most significant.  The further you move from the throat, and the larger the cross sectional area of the horn, the less significant variations are.

For the large end of a horn like a BT7, and similarly the LAB sub, you are plenty safe in following the simple rule-of-thumb which Tom directed you to previously, where the area should roughly double in the next 24" of length.  

Short answer:  Make teh length longer by 22.5-24" and make the area 45" x 45" for each BT7.  Grab your ruler and connect the dots.  Curvature at this point in the horn won't matter.  In fact, the dimensions are not terribly critical within a few inches.

Tom Danley and I had worked out a few creative loadings of single and multiple BT7s for permanent installs under stages and in corners (all of which Tom or I have described in concept on this forum).  If someone needs detailed drawings or help with a specific install, our input is certainly available with appropriate compensation for time.

Best Regards,
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Mark Seaton
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"Make no little plans; they have no magic to stir men's blood..." - Daniel H. Burnham

Antone Atmarama Bajor

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Re: Flair Equation for BT7?
« Reply #3 on: May 07, 2005, 02:03:17 pm »

     Ok just 22.5-24"  make one mouth about 90" so a pair would be about 180" (for Asymmetric).

    I may go for a lab sub set up one of these days but since I have the BT7's I want to play around with them.  I would probably use the equation to tweak with scalability of the design.  Though I'm unclear on what sort of vertical expansion the horn has/requires.

    I was wondering if these folded horn subs can be thought of as being horizontally (orientation dependant) polarized acoustic radiators.  I believe the horn mouth is like a cross section of a Hyperbolic Funnel if you will.

    If this is the case what are the effects of having the bass energy propagating horizontally Vs. vertically.

Anyhow thanks as always.
Sum
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Don Snyder

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LABhorn spreadsheet
« Reply #4 on: May 07, 2005, 05:54:11 pm »

I did a page on horn extensions including a spreadsheet for the LABhorn. The data for the BT-7 is almost identical, and you can alter the cells at the top of the spreadsheet to make it identical. The hard part is making it stiff enough that it doesn't resonate. Concrete block works well !!!

It's all posted on Pete Sylvester's website ...

The Page:
http://home.comcast.net/~labhorn/extension

The Spreadsheet:
http://home.comcast.net/~labhorn/extension/labhorn2.xls

~Don Snyder
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Antone Atmarama Bajor

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Re: LABhorn spreadsheet
« Reply #5 on: May 09, 2005, 12:52:29 am »

Cool Thanks for those plans and Numbers.

    I have a question though.  If the horns "width" (I guess it would be width if standing the horns vertically) only expands for the first 10" of horn length then remains constant at 21" wide. Why does it need more width expansion for the flair extender?  Is it because when they are used in clusters side by side that the width of the mouth is now the sum of the cabinets widths?  And for a single cab it needs the width expansion as well as height to increase in efficiency?

     That leads me to ask the question.  If I made a flair extender for a 2X2 monoblock of BT7's would do I need to increase the height still since close coupling of the 4 Cabs in a monobloc effectively doubles the horn mouth height (since 4 BT7's are typically stacked laying horizontally I would be more comfortable saying height, but that is configuration dependant).  

    So if I wanted a flair extender for the 2X2 monoblock could I build a flair that Just expands the Flairs Width and length but not height?     It seems If I built a single flair extenders for single or paired cabs I would have to increase the Height, length, and width for significant increase in efficiency?

    Also if a single cab is layed horizontally on the ground can't you ditch the downward expansion of the mouth since you are getting coupling with the ground plane, which effectively doubles the mouth area due to ground plane reflection?  I imagine if a single cab is stacked vertically it would be better to have the full flair expansion?  But that would give a very wide vertical coverage and a very narrow Horizontal coverage for frequencies with directivity?

Anyhow Thank you.
Sum



     
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Don Snyder

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Lots O' theory
« Reply #6 on: May 09, 2005, 02:34:01 pm »

From the old Physics book, a perfect horn is one-half wavelength long and has a mouth with a circumference of one wavelength(see fig a.). Since one wavelength at 26.4 Hz is about 42 feet, this is clearly impossible for us to do, but we have found several ways to cheat.

First, if we use half of the horn (as shown in fig b.) the air sees its reflection on the ground and doesn't know it's not in a perfect horn. Next, the air doesn't notice if the horn is only 1/4 wavelength long. When the wave leaves the horn, it takes almost 1/10 of a wavelength before it realizes that the horn is no longer there.

Since it's hard to make curved horns out of 3/4" ply, we have found if you make a horn with a rectangular cross section, but keep the volume the same, the wave doesn't seem to mind too much (see fig c.). If you can't make the mouth large enough, you can build multiple horns, and they will share their mouth areas. For best results with a ply horn, the mouth area of the stack and it's reflection together should make a square.

What about this directionality thing? Until a wave nears the point where the flare is one-half wavelength in width or height, it's  omnidirectional. A 45 high by 90" wide stack(either 2x2 or 4 standing in a row) is not directional below 200 Hz. The same four boxes, laid down in a line (21" high by 180" wide) will still work OK for 40 Hz, but 80 Hz will be a firehose, with sound beaming down the center and nothing in the wings.

As far as your problem with how to build extensions for the stack of 4 BT-7's, I don't think it matters if your extension makes the stack wider, taller or both, but 27" long by 120" wide by 60" tall woud give you all the mouth you could ever need without getting too directional. I'm afraid that you will be dissapointed, because you are already at 45% effeciency and the extension will only boost that to 55%.

index.php/fa/1632/0/
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Antone Atmarama Bajor

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Re: Lots O' theory
« Reply #7 on: May 10, 2005, 01:08:07 am »

Hmm interesting stuff.  10% efficiency gain isn't mind blowing but its still significant.  Has anyone figured the maximum obtainable efficiency's from scaling and stacking the horns (before diminishing returns)?

     From What I've learned about wave propagation, 1/4 and 1/2 Wavelength are the magic numbers (I've seen 3/4 wave and 3 1/4 wave antennas for RF never Full wave).  Although full wave sounds nice in theory it isn't as efficient as a 1/4, 1/2 radiator especially with a "wave guide" like a horn.  The mouth of a pair of BT7's is 90" about 1/2wave of 74Hz and 1/4 37Hz.  With extenders making the mouth 180" wide a 1/2wave of 37Hz.  Though the Horn Length won't be much more than 1/4 wave of  around 26Hz.

    Can making the BT7 mouth 180" wide give Horizontal directionality down to 37.5Hz!!!?  Or is the horn length not developed enough for the directionality to happen?


    I guess the BT7 and the Lab subs are optimized for flair width in pairs, from what I've guessed and what you've said (to make a large symmetric horn) but you need to stack them to increase Height expansion to an optimal dimension?  

    I thought square wave guides/horns caused moding problems and where inefficient compared to Rectangular, elliptical, and circular.
 I'm intrigued by the picture you posted.

     As a side question the dimensions of the horn throat are what limit the horns High frequency response as the wavelength approaches 1/2 wavelength of the throat (a mechanical lowpass filter).

    Maybe I'm just mixing up RF with mechanical waves but aren't the mouths of coupled Bass horns like BT7 and Lab subs Exponential Dipoles?  Or better yet exponential continuous 1/4wave tuning stubs for mechanical energy?

    Sorry guys I like to Blah Blah Blah.
I need to take some more extensive physics classes.

Have a good night.

Sum.        
       
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Michael_EllistonĀ¶

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Re: Lots O' theory
« Reply #8 on: June 02, 2005, 05:17:42 pm »

sumsound wrote on Tue, 10 May 2005 17:08

Hmm interesting stuff.  10% efficiency gain isn't mind blowing but its still significant.  Has anyone figured the maximum obtainable efficiency's from scaling and stacking the horns (before diminishing returns)?

Its possible. Depends what you mean by scaling? The BT7/Labhorn are pretty much as close to ideal,in a stack of correct number as you can get.
 


Quote:

     I guess the BT7 and the Lab subs are optimized for flair width in pairs, from what I've guessed and what you've said (to make a large symmetric horn) but you need to stack them to increase Height expansion to an optimal dimension?  

    I thought square wave guides/horns caused moding problems and where inefficient compared to Rectangular, elliptical, and circular.
 I'm intrigued by the picture you posted.


Its all about the dimensions being similar in acoustic size.
26hz waves,being 13metres long,dont particularly care about 1metre dimensions.The panel modes will be out of the passband unless you build a massively big flare extension.



Quote:

      As a side question the dimensions of the horn throat are what limit the horns High frequency response as the wavelength approaches 1/2 wavelength of the throat (a mechanical lowpass filter).

    Maybe I'm just mixing up RF with mechanical waves but aren't the mouths of coupled Bass horns like BT7 and Lab subs Exponential Dipoles?  Or better yet exponential continuous 1/4wave tuning stubs for mechanical energy?

    Sorry guys I like to Blah Blah Blah.
I need to take some more extensive physics classes.

Have a good night.

Sum.        
       


This is only a problem in HF horns,where phase plugs are used.Normally 26hz basshorns arent used much past 120hz,so no problems here. Vocals sound funny through folded horns anyway.

They are 1/4wave damped resonates.Note 'damped'.

These horns arent Dipoles,their mostly omnidirectional With some on axis beaming in the bass when many units are used due to enough mouth area controlling those long bass wavelengths.

A dipole is a speaker in free space- two out of phase sources

http://www.kettering.edu/~drussell/Demos/rad2/dipole.gif

http://www.kettering.edu/~drussell/Demos/rad2/mdq.html
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Antone Atmarama Bajor

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Re: Lots O' theory
« Reply #9 on: June 04, 2005, 06:15:30 pm »

Cool Thanks for that link.  Dipole speaker has a Bi-Radial Dispersion pattern ehh.

    I was wondering about scalability especially for The BT7 because I've heard It has an Fs of 8Hz, so theoretically you can make a Horn for it that is tuned Much lower than the BT7, though probably impractically massive?


Sum-
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