My Subwoofer Build

Discussion in 'Build Journals & Projects' started by Zaph, Feb 8, 2020.

  1. Zaph

    Zaph Well-Known Member

    Vehicle(s):
    2019 Ranger, A whole garage full of motorcycles
    Joined:
    Nov 28, 2019
    Location:
    Wisconsin
    Posts:
    195
    Likes Received:
    214
    Occupation:
    Engineer
    Component selection and overall design concept

    This is going into a 301A non-B&O equipment group package. I've settled on the behind the seat space for a few reasons, roughly similar to the area the B&O sub would go, and I will be custom building the enclosure to make the most use of the area. I've chosen this area for a couple reasons - 1st, I actually use the under seat storage for stuff like jumper cables and a rescue kit, and 2nd, I'm not a fan of horizontally mounted subwoofers.

    Horizontally mounted subwoofers work fine for a while. Then they start have an issue with something called "cone sag". Subwoofers have an excursion spec called Xmax. From the factory they start out +/- the same distance. Over time, the weight of the cone, suspension and voice coil causes the the coil to sag in the magnet gap, making the excursion non-linear. For example what was +/- 8mm this year becomes +4mm/-12mm next year. Then power handling becomes more limited and distortion crops up. There's more to it, but for subwoofers, vertical mounting is always better.

    I don't need a lot of additional output so for me a single 8" higher output sub will do the trick. However I'll configure it so I get the most out of it, more on that in the simulations below. I do not look at power handling ratings at all because they are generally not realistic. I look at Thiele Small parameters. Most important for a small enclosure size is a smallish Vas, and then I want a somewhat low Fs and reasonable Qts. Then after that based on experience I'll make a mental guess on how much power I *really* need. It's not going to be that much power.

    The system will get line level signal off the rear speakers. The front and rear speakers will continue to run full range off the OEM amplifier and the subwoofer will blend in on the low end to fill and enhance the missing bottom 1.5 octaves.

    My chosen components are:

    43CWRT82.jpg
    Kicker - CompRT 8" dual voice coil, 2 ohms per coil, model 43CWRT82
    The key features I need in this are low profile, decent Xmax, low Vas. It cost $128 on Amazon.

    DXA125.2.jpg
    Kicker - DXA125.2 two channel amp, model 43DXA1252
    The key features I need in this are the subsonic filter in case I go vented, line level inputs, adjustable low pass filter, and the 40hz bass boost may be useful. It cost $90 on Amazon.

    There are a lot of options out there. My range of expertise is in home audio, but home audio drivers rarely have the needed Thiele Small parameters optimized for smaller enclosures. Kicker's "Solo Baric" series is known for working well in smaller enclosures. It's a play on the old term "isobaric" which used two woofers face to face in an enclosure to get by with half the size. However I shy away from square drivers or anything non-circular as they are generally higher distortion due to uneven cone loading.

    This Kicker subwoofer is "rated" at 600 watts maximum power. That is a laughable power rating, though typical for the car audio industry. You will see how the DXA125.2, which is rated at 60 watts x 2 @ 2 ohms is a pretty good match for this woofer if I use each channel to drive each 2 ohm coil. If I wanted I could series the coils to 4 ohms and bridge the amp, which is functionally the same configuration, however there are some technical benefits to driving the coils separately so I will do that.

    I'll end this section with saying that I have very little respect for car audio. All the BS and lack of knowledge in the industry kind of makes me sick. Combine that with the worst listening room known to man, the worst speaker enclosures known to man, and most of the time I don't even bother trying. But sometimes I do anyway and I want a little more bass extension and output in my truck, so here I go. :\ Tests up next.
     
    deton8r and Cape Cruiser like this.
  2. OP
    OP
    Zaph

    Zaph Well-Known Member

    Vehicle(s):
    2019 Ranger, A whole garage full of motorcycles
    Joined:
    Nov 28, 2019
    Location:
    Wisconsin
    Posts:
    195
    Likes Received:
    214
    Occupation:
    Engineer
    #2 Feb 8, 2020
    Last edited: Feb 8, 2020
    Driver testing, enclosure simulation and design

    So, the first thing I do with this woofer is get out my testing rig and get my own real Thiele Small parameters. I paralleled the coils and tested it as a 1 ohm woofer, and also put the coils series and tested it as a 4 ohm woofer as a point of comparison, since T/S testing rigs generally lose accuracy as they go lower in impedance. I used the "delta compliance" method to test Vas, which means running a free air impedance curve, then putting it in a box of known volume and running the impedance curve again and calculating values from the changes.

    Car industry enclosure recommendations have been known to be notoriously poor, so I'll first mention what works from Kicker's own manual.
    • Sealed minimum: .2 cu ft
    • Sealed maximum: 1.25 cu ft
    • Vented minimum: .8 cu ft, tuned to 40hz
    • Vented maximum: 1.20 cu tf, tuned to 40hz
    Of these, sealed minimum and vented maximum I do not recommend. Sealed .2 cu ft will have no low end, and a honking high midbass peak. The only way that could be partway saved is with serious active parametric equalization, and even then a too small enclosure causes higher distortion for reasons way beyond the scope of this posting. Basically, it can no longer make a proper sine wave. It will make noise, and need a lot of power to do it if that's all you're after. The vented 1.2 cu ft tuned to 40hz is way too large. It results in a peaking at the tuning frequency. Any time 40hz comes around in the music there will be an unrealistic honk.

    The other 2 sealed and vented tunings kind of work though not really optimal. So now my work on enclosure simulations begins to figure out what I really want to do and where my trade offs will be between enclosure size, bass extension, and power handling, what works with cabin gain, and several other factors.

    My initial estimate of the space I have to work with behind the seat is about 14 liters, or 1/2 cubic foot. Most of my calculations will center around that volume. Starting with sealed, and a little background about what we're looking at here.

    Kicker-43CWRT82-14liter_sealed_sim_1ohm.gif

    We see a single line at the bottom with a peak and a rising portion. That is impedance in the enclosure. The peak is the resonant frequency in the box (it rises when enclosed) and the rise is due to coil inductance. The next set of 8 lines is power doubling frequency response (blue) and excursion (brown). The blue response curve rolls off on both ends - upper end because of the woofer's large inductance and this curve shows that. The brown curve is how much the cone is moving - lower in frequency is moves more in mm, and higher in power it moves more also. For the 8 line sets, bottom is 1 watt. Then 2, 4, 8, 16, 32, 64 and 128 at the top.

    The problem with this sealed box is extension. Power handling is fine, this woofer is =/- 8mm Xmax and I'm just using all of the amp's capability below 40 hz. But while the amp won't overpower the woofer in this configuration, There's not a whole lot of depth there. However there is a solution. The DXA125.2 has boost available at 40 hz, up to 12db. This could be used to bring up the drooping low end response if needed.

    What it's not going to do however is solve the problem that this woofer is woefully inefficient on the low end, and you are going to use up both your power reserves and your Xmax very fast in this configuration. It's why I stayed away from the Kicker 11HS8 all in one powered sub. It's going to have the same issues. So with this in mind I started looking more at vented options.

    Kicker-43CWRT82-14liter_vented_40hz_sim_1ohm.gif

    Now this is a vented enclosure tuned to 40 Hz. What I like about this is the massive improvement in output with the lowering of excursion in the 35-55hz range. This is effectively giving me the output of two 8" woofers in the operating area of the tuning. The downsides are that the cone is unloading below the tuning frequency at 30hz and running out of excursion, and there is still a mild amount of upper bass peaking with this enclosure size. Both of these however will be addressed.

    Kicker-43CWRT82-14liter_vented_40hz_LP.gif

    Now what I did here is added the low pass subwoofer crossover filter and integrated it into my sim. I built the active filter crossover cad section of my design software. It's a 2nd order Linkwitz Riley filter and I put the center frequency at 70 hz. Considering the rise in response there, effective crossover frequency (which would be the 6db down point) is about 120hz. At this point we're getting a very usable, deep response curve. The only concern is that I reach Xmax below the port's tuning frequency with half the amp's power. Everything is ok though. This amp has a 25hz 4th order subsonic filter. Let's see if I can model that and what it does to low end excursion with this tuning in this box.

    Kicker-43CWRT82-14liter_vented_40hz_subsonic.gif

    There we have it, with the subsonic filter this design is protected against overexcursion at the amp's maximum power and does not exceed Xmax at 30hz. Plot shows 1 watt output and 128 watt output. I think we have a winner here.

    I have not been able to get both the active subsonic filter and the main 70hz crossover onto the circuit at the same time, but this software is complicated and I haven't mastered it. Plus I don't use it much anymore. I'm a mechanical engineer, not electrical.

    I played around with the tuning frequency to see if I liked any of the options, but I'll probably stick with 40 hz. Remember that the lowest string on a bass guitar is 40 hz and there's usually not a whole lot of music below that. This is with the 70 hz sub crossover, and varied tuning from 30-45hz.
    Kicker-43CWRT82-14liter_vented_varhz_LP.gif

    I know the challenges await, like how to get the proper port length without making the vent area so small that vent mach speed becomes too high and chuffing and turbulence becomes an issue. The ports may be external of the enclosure, run along the bottom back corner of the cab and will likely have elliptical openings to smooth airflow. I may have to adjust crossover points, tuning and boost to adjust for cabin gain and for total overall response smoothness once I get an in-cab response curve.
     
    deton8r and Cape Cruiser like this.
  3. OP
    OP
    Zaph

    Zaph Well-Known Member

    Vehicle(s):
    2019 Ranger, A whole garage full of motorcycles
    Joined:
    Nov 28, 2019
    Location:
    Wisconsin
    Posts:
    195
    Likes Received:
    214
    Occupation:
    Engineer
    Reserved for:

    Enclosure design and build

    It's winter in Wisconsin. Going to take a while. :)
     
  4. OP
    OP
    Zaph

    Zaph Well-Known Member

    Vehicle(s):
    2019 Ranger, A whole garage full of motorcycles
    Joined:
    Nov 28, 2019
    Location:
    Wisconsin
    Posts:
    195
    Likes Received:
    214
    Occupation:
    Engineer
    #4 Feb 8, 2020
    Last edited: Feb 8, 2020

Share This Page