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Short horn to increase sub efficiency?

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  • Short horn to increase sub efficiency?

    The other day I stopped at the local big-box hardware store to pick up eight, 40 lb. bags of salt for the water softener. I threw the bags in the back of the minivan, stacking 4 vertically and squeezed in on each side of the rear-facing subwoofer. I had been jamming out on the way there and the drive home was to be at the same volume. By the time I was pulling out of the parking lot, I noticed that there was much more impact to the kick-drum transients than ever before. The back of my seat was getting kicked and I could feel the hair on my head and arms moving with it too. I couldn’t believe what I was experiencing. The 10 min drive home was filled with playing other songs just to see if I could tell the difference. I could.

    When I got home and went to unload the salt bags, I realized that how they were positioned had formed a crude expansion chamber between the baffle of the sub and the sloping rear wall of the van – a horn of sorts. Some quick measuring and math showed that the ‘mouth’ of the horn was twice that of the cone area. The bottom of the horn (throat) was roughly ½ the cone area.

    After I removed the bags, I went driving again and sure enough I could not replicate what was there just minutes before.

    Did such a crude setup make the subwoofer more efficient by better coupling it to the air? If so, how short would a horn need to be to where there wouldn’t be any (or negligible) effect from horn resonance? My thought is that if the horn length is kept shorter than the ¼ wavelength of one octave above the crossover frequency, it should be passable.

    Example… Say the sub is crossed over at 60Hz. Then one octave up would be 120Hz. So [ (13,200 / 120) / 4 ] = 27.5 inches. Keeping the overall length of the horn under 27.5” shouldn’t cause any ‘honking’ effects, yes?

    Thoughts anyone?


  • #2
    Horn loading requires a considerable path length. A 27.5" horn would only load down to roughly 150Hz. More likely the bags created a resonant chamber of sorts, in effect making a band pass sub with a peaked response that gave the effect you heard. As for horn resonance, that's not a product of the horn length. It occurs when there are parallel walls that approach 1/2 wavelength apart. It's easily avoided with midbass horns by not having parallel walls, while bass horns tend to be crossed over below where parallel walls are far enough apart to be a problem.


    • #3
      Hmmmm... I guess the only thing to do now is to recreate the setup and test to see if there is a positive SPL difference. If so, replicate it without the bags (not keen on driving around with an extra 320 lb.) and figure out how to maximize it.


      • #4
        What you did was make your vehicle more like headphones, where there is obviously not room for a significant portion of any bass wavelength.

        It's more of a push-pull on your eardrum and chest sort of thing.

        You changed your vehicle from a chunky box to more of a tunnel, and salt/sand will kill reflections, cutting effective tunnel height even more.

        For maximum slam raise your floor with a couple of Bill's Truck Tubas, just have a soft turn-on or you will have a heart attack.

        p.s. now that I think of it, that disco bass was up around 200 or so, so you had multiple factors combining for an imperfect storm of upper bass.


        • #5
          Subwoofers are omnidirectional by nature but is it pretty easy as you discovered to generate some directionality. This will give the impression of more output or greater efficiency but that is technically a bit misleading as the total output hasn't changed. There are also several sub designs that generate more output than a basic woofer in a sealed box, one is the mini scoop design which in it's smallest form is little more than a large horn shaped port, and another uses a manifold cavity. With these the "horn" is so short it's going to have a very narrow bandwidth so just like a reflex box the total output is a combination of woofer and port/horn output.
          Paul O


          • #6
            Bill – Thank you for clearing up my understanding of horn loading. If it were a band pass type resonant chamber, would this cavity be the ported side? If so, would port volume = vented enclosure volume of the band pass equation? Do you have any suggested resources I use to get more educated on BP theory?
            BTW, love your work and my AutoTuba.

            davidB and Paul O – I believe somewhere between your two responses is my original line of thought. I’m not saying Bill is wrong, only what occurred to me first. I still want to learn more about BP enclosures.

            Paul, when you mentioned the Mini Scoop, were you talking about the one below? If so, what I had created is essentially the same thing (crude side-view illustration below), but with the speaker in a sealed enclosure instead of horn loading the rear of the speaker with a wraparound horn. Simply install a wall from the top of the baffle to the rear wall of the enclosure.

            My theory is that the sloped face of the baffle in such a confined space reduces the losses around the edge of the speaker cone helping the cone get more ‘bite’ on the air – better coupled. Much like how flares on the end of a port make it more efficient by reducing the turbulence around the edges. Also how a putting a duct around a propeller reduces the tip losses by reducing the air spilling around the tips.

            As far as directionality, the sub was converted from a rear firing to upward firing setup. Did the upward firing, along with the path up the back of the van which curves toward the front, help create this effect or was it the new chamber working on its own?

            True, there is the same ‘potential input power’ available between the two setups. But if the system that the speaker is working in changes, wouldn’t the power requirements/usage also change? This thought led me to how my initial tests will be conducted – using an impedance sweep for both setups.

            To test this out, I’ll make a flare that can be added to the front of the sub that will create the same situation. Taking impedance sweeps of both setups should allow one to see what changes (if any at all) are now being imposed on the system. I think this should also show if there is a band pass resonant chamber effect as well.

            Is there much merit to this?
            Attached Files


            • #7
              Re: the pictured TOA speaker, the main source of the lowest frequencies is the port, which starts off as strait, then becomes tapered. That tapered section can't work as a low frequency horn, because it's too short. It does create a chamber that the front wave fires into that introduces the band pass function, and it is long enough to horn load in the mids, but not much, because the throat is too large. Where the lows are concerned none of it alters the radiation pattern, which will be omni-directional below 200Hz or so. Whatever is happening will show up on an impedance sweep.


              • #8
                I believe that you had several things going on. Although it is often promoted that sub-bass frequencies are omnidirectional, rotating a subwoofer enclosure can certainly change the sound in the car. The bags restricting the amount of free space the driver was playing into, and controlling its dispersion likely contributed as well. Also, it sounds like your enclosure is loose in the vehicle. If the bags both held it firmly in place, and also by their sheer weight provided some mass loading to the floor of the car, your driver may have been able to deliver more tactile impact to the vehicle.


                • #9
                  Rotating a sub in a car or for that matter in a living room will change the results, not due to the radiation pattern, but due to the change in boundary effects.