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  • High School line array project

    Since retiring from teaching and moving to Washington State, I have remained active with the high school science club students in my new home area. Last year I helped them design and construct basic 2-way loudspeakers. This year we are designing and building a pair of inexpensive vertical line arrays which the school can use for dances, assemblies, even small outdoor events. The drivers chosen are the closeout Fountek FE83 midwoofers and Dayton Audio PTMini-6 tweeters. We first constructed a test baffle on which were mounted 12 each of the drivers (picture attached). This baffle enabled the students to experiment with various combinations of the number of drivers and their series/parallel wiring. Our goal, in order to maximize output efficiency, is to eliminate any unnecessary resistors from a series path to the drivers within the crossover while maintaining an acceptable amplifier impedance load. In other words, we wanted to balance the drivers in the crossover frequency range (4k Hz) solely by choice of the number and wiring of the drivers. The students used my Clio system in order to assess the driver outputs in the range from 2k Hz to 8k Hz, all gated measurements occurring at a distance of 2m from the baffle. This is still a work in progress, so further posts will occur over the next week or so. Additional construction pics are attached. The box is trapezoidal to reduce standing waves and will be ported in the rear. We will probably employ 4th order electrical crossovers in order to minimize driver interference (4k Hz is somewhat high for a line array crossover frequency) and to protect the tweeters. In any case, they decided the best driver match involves 24 woofers (4 series groups of 6 in parallel) and 18 tweeters (3 parallel groups of 6 in series). The nominal woofer impedance is 5.33 ohms and the tweeter impedance is 12 ohms. Tomorrow's science club session involves assembling and testing the crossover, which will be mounted in the bottom (separate) box chamber.
    Attached Files

  • #2
    Re: High School line array project

    Wish I would have had a teacher like you when I was in high school 30 years ago, this looks like a fun project. Imagine how this may impact these young peoples lives and possibly start a life-long love for the hobby. Looking forward to following this thread. Good Luck with the project!
    My "No-Name" CC Speaker
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    • #3
      Re: High School line array project

      Originally posted by Kevin K. View Post
      Wish I would have had a teacher like you when I was in high school 30 years ago, this looks like a fun project. Imagine how this may impact these young peoples lives and possibly start a life-long love for the hobby. Looking forward to following this thread. Good Luck with the project!
      +1 I would have paid a lot more attention in school :D

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      • #4
        Re: High School line array project

        Originally posted by ratch View Post
        ...The students used my Clio system in order to assess the driver outputs in the range from 2k Hz to 8k Hz, all gated measurements occurring at a distance of 2m from the baffle. This is still a work in progress, so further posts will occur over the next week or so. ...
        What is the likelihood it could be taken outdoors and measured at varying distances?
        It would provide an excellent demonstration of the field transitions and the aspects of propagation such as directivity and how it varies with frequency and distance.
        "Not a Speaker Designer - Not even on the Internet"
        “Pride is your greatest enemy, humility is your greatest friend.”
        "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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        • #5
          Re: High School line array project

          Originally posted by dynamo View Post
          +1 I would have paid a lot more attention in school :D
          I went to a different kind of school. As long as I did what needed to be done they let me use the computer lab as much as I wanted, but I would have been in every class with this teacher.

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          • #6
            Re: High School line array project

            Originally posted by ratch View Post
            ...all gated measurements occurring at a distance of 2m from the baffle.
            Two meters is awful close to be measuring such a long array when the intended listening area will be much further.

            Originally posted by ratch View Post
            In any case, they decided the best driver match involves 24 woofers (4 series groups of 6 in parallel) and 18 tweeters (3 parallel groups of 6 in series).
            Especially since you mentioned the possibility of these being used in dances, I would highly recommend re-wiring the woofers to 6 parallel groups of 4 series drivers. It will give you the same system impedance and the same output, but better protection for the transducers. With 24 transducers in the array, all it takes is one weak or damaged transducer to blow 5 more and end up with a non-functioning array. In your proposed wiring scheme, one failed transducer will end up with greater power applied to the remaining 5 in that parallel group. The overall SPL output will not drop enough to even know that you have lost one transducer. If you continue to drive them at high power and one of those remaining 5 fails, it will result in even more power to the last 4, and so on, until all 6 of that group have failed, and then the entire array will not function since the remaining 3 groups are wired in series with this section that is now open circuit. If you wire them as 6 parallel groups of 4 series transducers, then a single transducer failure results in 4 transducers not playing (the other 3 are still OK), but the remaining 20 transducers will not see any difference in power and should continue to operate normally.

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            • #7
              Re: High School line array project

              Originally posted by Sydney View Post
              What is the likelihood it could be taken outdoors and measured at varying distances?
              It would provide an excellent demonstration of the field transitions and the aspects of propagation such as directivity and how it varies with frequency and distance.
              I agree. One of the coolest parts about measuring a line array is that you can take a ground plane measurement, which is actually the same as taking a free-field measurement of an array twice its length. This could even be accomplished in a gymnasium where you would not have to worry about wind and ambient outdoor noise affecting measurements.

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              • #8
                Re: High School line array project

                Hey, Ratch! Nice to see you here.
                Been awhile...
                Wolf
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
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                • #9
                  Re: High School line array project

                  Originally posted by 1100xxben View Post
                  Two meters is awful close to be measuring such a long array when the intended listening area will be much further.



                  Especially since you mentioned the possibility of these being used in dances, I would highly recommend re-wiring the woofers to 6 parallel groups of 4 series drivers. It will give you the same system impedance and the same output, but better protection for the transducers. With 24 transducers in the array, all it takes is one weak or damaged transducer to blow 5 more and end up with a non-functioning array. In your proposed wiring scheme, one failed transducer will end up with greater power applied to the remaining 5 in that parallel group. The overall SPL output will not drop enough to even know that you have lost one transducer. If you continue to drive them at high power and one of those remaining 5 fails, it will result in even more power to the last 4, and so on, until all 6 of that group have failed, and then the entire array will not function since the remaining 3 groups are wired in series with this section that is now open circuit. If you wire them as 6 parallel groups of 4 series transducers, then a single transducer failure results in 4 transducers not playing (the other 3 are still OK), but the remaining 20 transducers will not see any difference in power and should continue to operate normally.
                  Thanks for pointing out the risks of wiring as originally proposed. Since the woofers aren't even wired yet (happens first thing in the morning), it is a simple alteration. I merely need to drill an additional hole in 2 of the chamber dividers to allow series connections across them. This is so because each chamber isolates 6 woofers, but the new wiring plan will connect groups of 4 in series. Therefore, chamber 1 holds a group of 4 and half of the next group and so on. There are 3 dividers to isolate 4 chambers. The center divider will not split a series grouping, so no additional connection needs to pass through it.

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                  • #10
                    Re: High School line array project

                    Originally posted by 1100xxben View Post
                    Two meters is awful close to be measuring such a long array when the intended listening area will be much further....
                    Precisely. The traditional guideline for un-tapered arrays is much further than 2x the array length. Far enough that the driver spatial offset effects are greatly reduced
                    ( see Ureda's papers )
                    "Not a Speaker Designer - Not even on the Internet"
                    “Pride is your greatest enemy, humility is your greatest friend.”
                    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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                    • #11
                      Re: High School line array project

                      Hi Ratch, how are you? Long time no see. This thread reminds me of Farm's line array, the ones I listened to at your house.
                      Alan

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                      • #12
                        Re: High School line array project

                        Hi Ratch, how are you? Long time no see. This thread reminds me of Farm's line array, the ones I listened to at your house.
                        Alan
                        Hi, Alan. I'm doing well in retirement. Play with the grandkids every chance I have. How have you been?

                        The students I am working with here are very different from those in Sacramento. Here, the level of science understanding and knowledge is much higher. However, exactly the same lack of hands-on projects describes their experience. Wood and metal working shops are nonexistent along with electronics. The robotics club is even more popular than the science club because the students clamor for practical applications and projects. So, as far as I can see, it is the afterschool clubs that excite the students. I incorporated the projects directly into the science curriculum. The trend now is increasingly "to teach to the test". Standardized testing is shortchanging students because of a lock-step curriculum which is paper based and largely irrelevant or uninspiring to the students.

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                        • #13
                          Re: High School line array project

                          Originally posted by ratch View Post
                          Hi, Alan. I'm doing well in retirement. Play with the grandkids every chance I have. How have you been?

                          The students I am working with here are very different from those in Sacramento. Here, the level of science understanding and knowledge is much higher. However, exactly the same lack of hands-on projects describes their experience. Wood and metal working shops are nonexistent along with electronics. The robotics club is even more popular than the science club because the students clamor for practical applications and projects. So, as far as I can see, it is the afterschool clubs that excite the students. I incorporated the projects directly into the science curriculum. The trend now is increasingly "to teach to the test". Standardized testing is shortchanging students because of a lock-step curriculum which is paper based and largely irrelevant or uninspiring to the students.

                          Good to see you around again! Love this approach. I was fortunate to be able to attend specific let's-learn-about-speakers "classes" in the early 70s while I was still in high school, the ones hosted by Pat Snyder and Dave Graebener at their modest Speakerlab store in Seattle. I learned so much and the environment was informal and encouraged asking questions. It sparked a lifetime of learning and building. I made my first pair of speaker cabinets from the plans that came with the floorstanding 2-way Speakerlab S2 model. I bought the parts kit and followed the measured drawing. Pat and Dave stressed how important a solidly-built cabinet is, so I added cross-bracing and mitered joints. I still have those enclosures to this day. Comparisons were inevitable since my friends thought I was a fool for not buying a name-brand. The S2s became counterparts for numerous listening sessions next to my friends speakers and I ended up making many cabinets for other Speakerlab models that replaced their OEM brands. It definitely helped refine my cabinet-building skills with all the usual mistakes along the way and learning from them. We can all benefit from young people learning more about this hobby specifically, and being interested in science also.

                          I loved the hands-on I got in wood and metal shop. I'd get my friends together after school at my place and we'd work on and ride our small dirt bikes on the property...my dad made us a track on the back acre. Dad was an operating engineer and grandpa was a consummate motorcycle rider and mechanic, so we got to learn a lot about keeping our bikes maintained and how to work on them. My high school in Tacoma started offering an auto mechanics elective after I graduated in '74, calling it Power Mechanics.



                          John A.
                          "Children play with b-a-l-l-s and sticks, men race, and real men race motorcycles"-John Surtees
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                          • #14
                            Re: High School line array project

                            Originally posted by ratch View Post
                            Hi, Alan. I'm doing well in retirement. Play with the grandkids every chance I have. How have you been?
                            Hi, thank you for your reply. I wish I could say that I was doing well, but it has been a real struggle for me several ways these last couple of years. However, I do have plenty to be thankful for, and I keep my chin up.
                            My two grandsons just turned 5, I see them regularly.

                            As far as loudspeaker building is concerned, that is pretty much on hold for now, currently no place to work on anything, my friend lost his cabinet shop business. My latest hobby seems to be photography, I've been upgrading my camera and lens collection more than anything else.

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                            • #15
                              Originally Posted by ratch
                              The students here are quite different and the level of science and knowledge understanding is higher. The electronics and robotics club is more popular tand the students want to work on the new applications too . So far, we have done not much but the club excites the students to write my essay fast. I combined several projects in the science curriculum and my classroom. They are concentrated to be taught to the test. Standardized tests ae paper based and students need to get prepared.
                              Hello, I'm a high school teacher from Corpus Christi. We're planning to create a powered Line Array for our auditorium. Could you please share the details of your project? My students are really enthusiastic about this and I'm trying to collect all possible materials to help them understanf everything clearly. Thank you.
                              Terry

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