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RCF LF21N451 reflex enclosure for live sound

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  • RCF LF21N451 reflex enclosure for live sound

    Hi,

    I wanted to post this project. I built these about 2 years ago now. The design was done by a very knowledgeable friend of mine. These drivers were $300 cheaper when I bought them 2 yrs ago so it was a higher bang for the buck quotient back then. The neo motors are getting more expensive I guess. Here is the PE page for the driver http://www.parts-express.com/pe/show...number=294-895 It is a 4.5" voice coil neodymium motor 21" subwoofer with 13.2mm xmax and a 58mm xmech. Thermal power handling is 1500w RMS, 3000w program, 3000w max. They will take all of that if asked to.
    The design is a standard bass reflex enclosure with an optimized vent design. It is also designed to be augmented with DSP around Fb to extend F3 somewhat. The enclosure external dimensions are 836mm high by 756 cm deep by 594 cm wide. The design includes wheels on the back corner for easy tilt and role portability. The design of the vent is the strength of this sub. It is a slot port flared at both ends, but it is HUGE. This sort of vent size is needed to avoid port compression with a modern high excursion high power driver for sound reinforcement use. The port area at its smallest cross section is 728 sqcm which is 42% of the Sd of the driver and the port area at either end of the flare is 1084 sqcm. Fb is 32 hz and F3 before DSP is 37 hz but is about 35 hz with the recommended DSP applied. Pictures are below. If anyone is interested in building this design please email me and I will clear it with my friend who did the design work. I have an excel spreadsheet with all of the panel dimensions and recommended DSP as well as a google sketchup drawing.

    I built them out of 18mm baltic birch and this is the only material that should be used for such a build. It is a pleasure to work with. MDF need not apply for prosound cabinets. I used pockethole screws during the assembly which takes a little extra effort but avoids the need for tons of clamps and adds the mechanical fastener's strength to the glue joint. I finished them with DuraTex using the foam roller they sell with the product http://www.parts-express.com/pe/show...number=294-895 This is a nice product if you don't want to spend the money on bedliner at your local truck accessory shop. It isn't as durable but looks good and is much more durable than standard paints. I sourced the grill material from PE and then used some 1/2" flat steel stock around the edges to hide the cut edges of the PE grill. Please make sure that you allow plenty of room for driver excursion between the surround and the grill. These will slap the grill if you don't allow enough setback between the baffle and the grill. I found that out the hard way and had to modify my cabinets.

    Well, how do they work. These things are really really really impressive. They are used at our church. I initially built them to use in our portable system for the youth group but they were rapidly adopted into the main sanctuary. Our room can seat 1150 according to the fire marshal. It is a "gymatorium" about 100 feet square with 28 foot ceilings and a fair sized wing on each side with additional seating. The mix position is about 90 feet back from the stage. We play modern worship music with a full band of drums, bass, electric guitars, keyboards etc. I built two of these and the pair easily provide more low end than we ever need on a sunday morning. We usually run 90 dba at the mix position and 98-100 dbc. We have significant headroom left at these levels. The subs are each powered with a bridged Crown MA2402 which delivers around 1500 watts into the nominal 8 ohm load (around 115v). During testing with a shaped burst tone devised by Keele http://www.synaudcon.com/site/articl...st-test-track/ I have driven the amps into limit far enough to have them temporarily shut down and never heard a complaint from the drivers nor a chuff from the port. A significantly more powerful amp would be very reasonable if you wanted to get every last db out of these cabinets. With the MA2402 bridged per cabinet you can never hurt them unless you removed the LPF and started playing 15hz sine waves at full tilt, then perhaps, but in normal use they are bulletproof with this level of power. They sound full and deep for a PA application and will really thump you in the gut if you want them too. For reference the PA they are used with is EAW KF650Z main speakers ( a total of 7 of them in an L-C-R configuration). These RCF 21" cabinets replaced three EAW double 15" subs and there is no comparison. The 2 RCF cabinets stomp all over the 3 EAW dual 15" subs (while using only 2 amps compared to three of the same crown amps on the EAW subs. They are highly recommended.

    Here are some pics.

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    Loren Jones

    http://techtalk.parts-express.com/sh...-sound-drivers

    http://techtalk.parts-express.com/sh...for-live-sound

  • #2
    Re: RCF LF21N451 reflex enclosure for live sound

    Click image for larger version

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    Loren Jones

    http://techtalk.parts-express.com/sh...-sound-drivers

    http://techtalk.parts-express.com/sh...for-live-sound

    Comment


    • #3
      Re: RCF LF21N451 reflex enclosure for live sound

      Excellent work! I like the optimized elliptical-profile port. There was an AES paper by JBL some time ago that compared port geometries, and found that the elliptical-flare port sustained the highest velocity of air with the least vent noise.
      Best Regards,

      Rory Buszka
      Product Manager, Dayton Audio

      The best way to predict the future is to create it.

      Comment


      • #4
        Re: RCF LF21N451 reflex enclosure for live sound

        Originally posted by [email protected] View Post
        Excellent work! I like the optimized elliptical-profile port. There was an AES paper by JBL some time ago that compared port geometries, and found that the elliptical-flare port sustained the highest velocity of air with the least vent noise.
        Rory,

        Thanks. While I'm 100% sure that my friend who designed the enclosure was fully aware of that paper when he created this design, I must confess that absent his mentioning that paper to me I had no knowledge of it and thus can take no credit. He is quite adamant that with the increased capabilities of today's prosound LF drivers, compared to even those of 10 years ago, that it is becoming increasingly difficult to design a ported prosound subwoofer cabinet that doesn't end up having port compression be the limiting factor in the performance of the cabinet at maximum output levels.

        I must credit him though that while I haven't driven these with a big enough amp (not that the bridged MA2402 is a lightweight) to take full advantage of them I have never heard a hint of noise from the port. Now port noise and port compression of output aren't necessarily going to go hand in hand, but this port does seem to perform very well.

        Loren
        Loren Jones

        http://techtalk.parts-express.com/sh...-sound-drivers

        http://techtalk.parts-express.com/sh...for-live-sound

        Comment


        • #5
          Re: RCF LF21N451 reflex enclosure for live sound

          Is there any bracing on the side panels apart from the vent itself? Seems like those are pretty big unbraced spans.

          Other than that, looks good!

          Comment


          • #6
            Re: RCF LF21N451 reflex enclosure for live sound

            Originally posted by badman View Post
            Is there any bracing on the side panels apart from the vent itself? Seems like those are pretty big unbraced spans.

            Other than that, looks good!
            The bracing in the sketch up rendering is just a quick thing he threw in. The real bracing is more extensive.
            Loren Jones

            http://techtalk.parts-express.com/sh...-sound-drivers

            http://techtalk.parts-express.com/sh...for-live-sound

            Comment

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