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Thread: Modifying the Altec 825 bottom compartment area into a rear loaded horn

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    Modifying the Altec 825 bottom compartment area into a rear loaded horn

    Just picked up a rough pair of Altec 825 3/4" particle board boxes for nixs. The bottom rear panel had some water damaged so I've removed it and the front vented panels areas as well.

    Just wondering if anyone has tried to rear horn load the bottom compartment like the attached photo. I wouldn't do this to a pristine plywood pair of 825s but as they are a little rough I thought I might try some experimenting. Is this a dumb idea or is there merit to my thoughts? Thanks


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    Quote Originally Posted by altecman View Post
    Just picked up a rough pair of Altec 825 3/4" particle board boxes for nixs. The bottom rear panel had some water damaged so I've removed it and the front vented panels areas as well.

    Just wondering if anyone has tried to rear horn load the bottom compartment like the attached photo. I wouldn't do this to a pristine plywood pair of 825s but as they are a little rough I thought I might try some experimenting. Is this a dumb idea or is there merit to my thoughts? Thanks


    My first question to you, is what brand and model of driver do you plan to use? That is what will determine if your concept is viable. If you plan to use any ALTEC driver in a rear loaded format I can save you the trouble by telling you that you would not be happy with the results. What you are suggesting is called a compound horn. RCA had a model back in the 1930's.

    I will explain when you reveal the make and model. There are many calculations and considerations involved in designing a rear loaded horn. More than a straight axis.

    BTW - Someone already beat you to it. I have a rendering I found on the internet several years ago done by some Japanese fellow. I do not know what make or model driver that was used. I also couldn't say how reliable the math was behind his design.

    H.F.

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    Haven't really thought about the driver yet, just want to know if anyone has tried this approach.

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    Quote Originally Posted by altecman View Post
    Haven't really thought about the driver yet, just want to know if anyone has tried this approach.
    Okay. If you chose to proceed I recommend that you find a pair of 2220 or 130A. Many on this forum feel the E145 is a good horn loading candidate, but I do not share that opinion. I have been searching manufacturers for several years to find a decent 15" horn loader in current production. There are none.

    The first parameter I consider for choosing a cone driver for horn loading is the Efficiency Bandwidth Product. The lower the number, the larger the chamber is required. The higher the number, the smaller the chamber required. This is not my opinion. My assertion is based on using equations found in papers written by Paul Klipsch, Richard Small, and Don Keele. The ALTEC 515G for example requires a chamber volume if approximately 5 cubic feet. The dimensions of an A7 cabinet precludes any idea of putting a rear loaded horn behind a 5 cubic foot chamber. Doing so would result not in a horn, but a discontinuity due to it's short length. If that is the case, then designing a horn loaded vent would be in order. The calculations for that method are considerably more simple.

    The chamber volume required for a 2220 / 130A is approximately 2.2 cubic feet. The E130 is a good candidate, if you can find a pair that hasn't been beat to death. The D/K/E 140 is a no go, along with the 2226. The 2227 looks like a promising candidate, but finding even one on Ebay would be a miracle. Loading a horn such as the Paragon with an LE 15 has to be JBL's mother of bonehead moves.



    The throat area and chamber volume are inter-related, as the combination of the two serve as an acoustical low-pass filter. The length of the horn is determined by the throat / chamber parameter combination that determines the filter frequency. The EBP for the E145 is 134.6. The EBP for the 2220, depending on the suffix model is approximately 200.

    H.F.

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    Quote Originally Posted by Horn Fanatic View Post
    Okay. If you chose to proceed I recommend designing a horn loaded vent would be in order. The calculations for that method are considerably more simple.

    The chamber volume required for a 2220 / 130A is approximately 2.2 cubic feet. The E130 is a good candidate, if you can find a pair that hasn't been beat to death. The D/K/E 140 is a no go, along with the 2226. The 2227 looks like a promising candidate, but finding even one on Ebay would be a miracle. Loading a horn such as the Paragon with an LE 15 has to be JBL's mother of bonehead moves.



    The throat area and chamber volume are inter-related, as the combination of the two serve as an acoustical low-pass filter. The length of the horn is determined by the throat / chamber parameter combination that determines the filter frequency. The EBP for the E145 is 134.6. The EBP for the 2220, depending on the suffix model is approximately 200.

    H.F.
    Would you be able to sketch what you mean so I can get an idea on how it would look. Thanks

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    Quote Originally Posted by altecman View Post
    Would you be able to sketch what you mean so I can get an idea on how it would look. Thanks
    If you mean a sketch of a typical compound horn, you already posted it. What your image is lacking is the suggestion of a rear horn throat.

    If you mean a horn loaded port, it's more of a calculation and deciding the geometry of the enclosure. Treat that design as a conical taper with parallel sides. The vent geometry will depend on the driver free air resonance, the chamber volume, and the tuning frequency. Maintaining the proper cross-section around a bend if one is required, is much easier to accomplish than that of a horn flare.

    Should you convert the 828 to a straight axis horn / horn loaded port arrangement, you have much more flexibility with the chamber volume. It's a safe bet you will end up with a stronger low-frequency than a stock 828 will provide. Let's face it, the bass response an A7 / 828 cabinet provides leaves much do be desired considering it's cavernous volume. The horn loaded port approach also paves the way for a broader selection of drivers.

    I found the drawing of a modified A7 cabinet. Obviously someone's pipe dream. As I stated in a previous post, the chap who came up with this plan does not indicate which driver is to be used. By the looks of the rear chamber volume, any ALTEC model would be out of the question. I also have no idea of the math he used to design the horn, or to determine it's length. Or if he used any calculations at all. An educated guess tells me that by the area of the bass horn it is totally incorrect. It looks like he tried to match the areas of both horns. For that bass horn area a properly designed horn would never fit in that box. Typically, the treble horn in a compound horn arrangement is always smaller than the bass horn. I can state with certainty the guy had no idea what he was doing.

    http://photo.qip.ru/users/tda-audio/...#mainImageLink


    You can join this website for free. If you do, search for U.S. patent 2,694,462. Ignore the image of the box in the patent. The patent explains how to design a horn loaded port.

    http://www.freepatentsonline.com/



    H.F.

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    Do you reckon this could work? Probably replace the reflectors with smaller ones. If I could get the response down to about 50hz would be fine as it will hopefully keep up with the mid horn.

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    Quote Originally Posted by altecman View Post
    Do you reckon this could work? Probably replace the reflectors with smaller ones. If I could get the response down to about 50hz would be fine as it will hopefully keep up with the mid horn.

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    It is my opinion that attempting to turn an 828 into a compound horn is a waste of time and material. Why would you settle for 50 Hz when you could take advantage of the volume and re-tune the enclosure to resonate at a lower frequency? It is not certain that the drawings and sketches that guy provided are complete. He only shows the panels on one plane. There may be numerous divergent panels that are not illustrated. On the right side of the image you can see that it has been cut off. Where is that rest of the drawing? There are many such drawings on that webpage that are missing pages or views.

    My advise, is if you really want a compound horn, design one from scratch. Jack Dinsdale has plans for a compound corner horn that utilizes a 6 inch full range driver. It could easily be modified to accept a specific model 8 inch Audio Nirvana speaker, and the cabinet would be considerably less intrusive in a domestic environment.

    http://diyaudioprojects.com/Technica...ign-Part-1.pdf

    http://diyaudioprojects.com/Technica...ign-Part-2.pdf

    http://diyaudioprojects.com/Technica...ign-Part-3.pdf

    John Crabbe also wrote a very informative 8 part article on designing a compound horn.

    His articles can be found on this link;

    http://www.saturn-sound.com/history/...20projects.htm

    Here is an image of the RCA compound horn I mentioned in a previous post;

    http://www.medienstimmen.de/wp-conte...z21936_101.jpg


    Perhaps before you fire up the table saw, you should hit the books. Horn design is not rocket science, but to design a horn one must first know the rules.

    H.F.

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    Quote Originally Posted by Horn Fanatic View Post
    It is my opinion that attempting to turn an 828 into a compound horn is a waste of time and material. Why would you settle for 50 Hz when you could take advantage of the volume and re-tune the enclosure to resonate at a lower frequency? It is not certain that the drawings and sketches that guy provided are complete. He only shows the panels on one plane. There may be numerous divergent panels that are not illustrated. On the right side of the image you can see that it has been cut off. Where is that rest of the drawing? There are many such drawings on that webpage that are missing pages or views.

    H.F.
    Not too hard to work out what parts are missing especially since its been hand sketched onto a 828 drawing.
    Why would I bother with the Dindsale project when I want to use the 825 as a try and see project, no point really in playing with toy 6" or 8" speakers, been there, done that, not my cup of tea.
    I look at it this way, if it doesn't work I'll convert the horn line into a lossy damped 7 ft labyrinth tuned to about 38Hz.

    Here's the drawing with some dimensions I calculated by scaling. I'll try and make the line a bit smoother, might be good for mid 40's. Don't really want to go vented, lets try something different.
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