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The ELF

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  • The ELF

    I know I've talked about this before, but haven't done anything about it. Now I have a bit more motivation, so I've decided to give it a go.

    For those who don't know, ELF stands for extreme low frequency. What you do is take a sealed speaker cabinet with a resonant frequency at the highest frequency you want it to reproduce, then make a low pass filter with the corner frequency set at the lowest frequency you want to reproduce. What this does is tilt the response such that the output is flat from the corner frequency up to the resonant frequency, above which it falls off at 12dB/octave. Theoretically, you can make the cabinet flat down to any frequency above DC. In practice, of course, you'[re limited by the power handling and excursion of the driver used in the speaker.

    First step was proof of concept. I have an old homemade bass cabinet with a resonant frequency of about 80Hz. I figured I'd make a passive low pass filter with a corner frequency of 20Hz, giving me a two octave pass band for the system. A passive second order filter is problematic for a few reasons, but I figured it was worth a try. I put it together on a breadboard, and kluged the wiring from my preamp through the filter and on to the amplifier and speaker.

    And the verdict?

    Didn't work. I don't know why. Impedance mismatches, most likely. So, for my next trick, I'll go ahead and breadboard an active second order low pass and try again.
    "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
    -- Bob Parks

    "A thing is not necessarily true because a man dies for it."
    -- Oscar Wilde

    "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
    -- Oscar Wilde

    "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
    -- Theodore White

  • #2
    Here's a link to an ELF project: http://sound.westhost.com/project48.htm

    He explains what he did pretty well in my opinion.
    Last edited by isaac42; 05-11-2014, 12:03 AM.
    "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
    -- Bob Parks

    "A thing is not necessarily true because a man dies for it."
    -- Oscar Wilde

    "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
    -- Oscar Wilde

    "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
    -- Theodore White

    Comment


    • #3
      Low frequencies do consume allot of power which is why most cabs roll off in the 40~100hz. When you consider a normal bass guitar has a low E string of 41Hz,, there's not a whole lot more going on down stairs that can be heard. Most frequencies below 100hz are felt not heard. Most modern recordings on both LP's and Digital use high pass filtering to remove sub lows because they eat up all the headroom.

      I did read through most of that article and I'm not sure why normal EQ or Electronic crossover properly tuned cant do the job. I'd at least start with that using a frequency generator. I'd at least know how low the speaker could go for the box size. Its not just the speaker its the power head too you have to test. One of my power heads is a 2000W Crown. I just looked up the frequency response and you can see how it rolls off the lows here.

      When you compare it with my Alesis Reference power head used for powering Passive studio monitors you can see how flat the response is even down to 1hz.
      Last edited by WRGKMC; 05-12-2014, 02:21 PM. Reason: Photo Fix

      Comment


      • #4
        Well the photos don't seem to post. The Crown began to roll off at abut 400hx to about -10db at 100hz. The chart doesn't show anything below that but my gess is its likely close to nothing once you get below 40hz. If you had a similar amp, it would be a good reason why the project failed.

        On the Alesis, the response didn't vary more than 2~3db along the entire frequency spectrum. Here again the chart ends at 200Hz. The response may drop like a lead weight below 100 I don't know. Manufacturers often doctor their specs to hide faults, but I do know the Reference head can get some mean low end if you push it.

        Comment


        • #5
          I'm surprised to hear that the Crown rolls off so much, so high. I'd expect a high fidelity response from a Crown.

          The amp I'm using is rated 20Hz to 20KHZ, though it doesn't give a tolerance. I doubt that's my limiting factor.

          One could get a similar effect using EQ, but there are a few issues. One is that a typical EQ isn't going to give you a 12dB/oct slope through the intended pass band, nor will it properly roll off the response above the pass band. Best you could do would be a -12dB shelf.
          "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
          -- Bob Parks

          "A thing is not necessarily true because a man dies for it."
          -- Oscar Wilde

          "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
          -- Oscar Wilde

          "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
          -- Theodore White

          Comment


          • #6
            Today I breadboarded a second order (12dB/oct) 20Hz low pass filter and hooked it all up again. Worked great! Massive, deep lows. Far more lows than without the circuit. Proof of concept (PoC) complete!

            Next step is to build a better filter, with a 16 or 20Hz high pass to protect the speaker. A bit of gain as well. The speaker I used for the PoC *might* be adequate for a home stereo, but doesn't have the excursion for live performance. Next step on that front is a tiny 2x15" cabinet with a couple of drivers I have laying around, each with more excursion than the one I used today.
            "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
            -- Bob Parks

            "A thing is not necessarily true because a man dies for it."
            -- Oscar Wilde

            "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
            -- Oscar Wilde

            "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
            -- Theodore White

            Comment


            • #7
              Yea, the crown is 20~20K too but you have to look at the specs to see the actual response curve. At least they don't hide it like other manufacturers do. It is a PA head so having a roll off on the low end is no big deal. You don't want bass rumble from the mics and under normal working conditions, you'd normally roll the bass off on a mixer. Instead you can leave the EQ controls flat and have the proper response.

              The Alesis is for studio mixing so its designed to produce a flat response driving flat response monitors. You don't want any enhancement so all your tweaks mixing are 1:1.

              I do use some subs in my studio but I use them for tracking purposes, not mixing. I hate using headphones when I track and using them for vocals for isolating the playback is about as much as my old ears can tolerate any more. For my instrument tracking I have several sets of speakers I can kick on. I kick my subs on especially when I track bass because you need more beef to make the instrument sound right with the rest of the mix playing back. I don't have to drive my high quality studio monitors hard for the bass to be heard tracking and I can get close to a live feel tracking.

              Mixing I just toggle the switch off and mix with the studio monitors.

              In my case I don't need allot of ultra lows. I do get that from a Harmon Kardon subwoofer which is very good. I have two cabs that are very resonant made of some antique wood. I didn't even pad them out. When you tap on them they sound like wooden drums. I used to have 6X9" car speakers in them but switched to using just 8" peavey full ranged speakers. In my case I didn't want just bass from them, I did want the full range sound. I was lucky enough the size produces just enough resonance to not be muddy.

              From there use a good soundcraftsman EQ before the amp and I used a DB meter on the speakers while I pump a Frequency Generator through the EQ to the speakers. I'll mainly focus on the EQ band frequencies and even them up the best I can from 20Hz until the speakers peak out around 10K on those. No sense boosting something the speakers cant produce.

              I do have other speakers I use similar methods of balancing them up. Some have crossovers which is all that's needed. Others may have an EQ before them. I also have a full PA in the studio and have that set up for maximum fidelity as well.

              Comment


              • #8
                The ELF is on hold again. A friend wants me to build him a sub using an 18" driver. I picked up a JBL 2240H. It arrived yesterday.

                The plan is a box 23"x36"x20", tuned to 30.9Hz. Should give him plenty of rumble.

                http://www.ebay.com/itm/JBL-2240H-PR...item461e99cdaf
                Last edited by isaac42; 05-31-2014, 02:27 PM.
                "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
                -- Bob Parks

                "A thing is not necessarily true because a man dies for it."
                -- Oscar Wilde

                "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
                -- Oscar Wilde

                "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
                -- Theodore White

                Comment


                • #9
                  Originally posted by isaac42 View Post
                  I picked up a JBL 2240H. It arrived yesterday...

                  I used to occasionally run into reconditioned 18 inch Peavey PA drivers for around $100 ~ $125. Their weight was something crazy like 60 or 80 pounds each. The giant ceramic magnet was the obvious culprit, but the speaker frame was probably significantly heavy too. I've toyed with the idea of building a front-firing semi-inexpensive 1x18 bass bin for bass guitar around one of those Peavey PA speakers. I'm thinking that decent 18 inch PA speakers tend to have a bit more frequency response than the equivalent size designed for bass guitar cabinets.

                  Comment


                  • #10
                    Generally speaking, the larger the magnet, the higher the gap flux, and the lower the Q. That translates to higher efficiency, but less low bass response. Could be good or bad, depending on the application.
                    "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
                    -- Bob Parks

                    "A thing is not necessarily true because a man dies for it."
                    -- Oscar Wilde

                    "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
                    -- Oscar Wilde

                    "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
                    -- Theodore White

                    Comment



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