Finally, a Horn Loaded Subwoofer!

I've finally pulled my thumb out and decided to build a horn loaded subwoofer for my sound system. The horn is based on an 18Sound 18LW1400 driver. These drivers are very rugged and perform well. I've used them in many projects in the past. The enclosure is 650mm wide by 1000mm high and 1200mm deep. This is almost as large as the two subwoofers I currently use, but is capable of 16dB more output at 30Hz! (138dB @ 1M, Full power from 25Hz to 100Hz)

The Specs for the horn are:
Throat area450cm^2
Mouth area3750cm^2
Path Length3M
Rear Chamber Volume95 liters

The subwoofer will be corner loaded to improve low frequency extension and sensitivity. In my model using McBeans Horn Response program, I guessed that the area of the corner will be roughly five times as great as the horn mouth, at two meters out. Changing this figure significantly does not alter the resulting frequency response very much, so the error in this guess is not important.

The LAB horn was a great motivation in the design of this project. I was actually going to build one myself, but thought that I could design a horn that better suits my application. Using drivers on hand meant I didn't have to purchase any and not having to meet the truck pack dimensions of the LAB horn allowed me a little more freedom in the design. The result is a horn that is a little larger than the LAB horn, has wider bandwidth and around the same maximum output capability. It is also quite a bit easier to build, however being made out of 25mm MDF, it is also MUCH heavier!
The predicted performance of my bass horn is very good, with wide bandwidth and good power handling. I understand that the high frequency response will not be as good as predicted in practice because of the 600mm parallel walls and the tight bends. Since this subwoofer will be crossed at 65Hz, this is not a concern. The subwoofer will be powered by an Australian Monitor AM1600, bridged, so the input voltage used in this simulation shows a realistic maximum that is available. I can't honestly see myself driving this subwoofer to within 10dB of this figure.
Thanks must be given to David McBean for developing such an excellent horn design program. It has allowed many paople to optimize a horn design and view the results of tweaking a non ideal horn. Marshall Leach's AES paper "On the Specification Of Moving Coil Drivers For Low-Frequency Horn Loaded Loadspeakers" gave me an optimum design that I could use as start values for entry into Hornresponse. The resulting design is quite different to the ideal horn in many ways.

Do note that the last conical secion in this model is actually the expansion from the corner into the room. The acoustic space, especially at low frequencies, is obviously much smaller than a true corner load. At the LF limit of the sub, ALL room boudaries are less than 1 wavelength from the mouth of the sub! Modelling this is very difficult. If the room were very rigid and air tight, I'd assume you would see a 12dB/Octave rise in the low end in a similar way you see in a car interior, just at a much lower frequency.
Cutting List (All 25mm MDF):
Top600x600 (x2)
Sides1200x1000 (x2)
1st Flair600x1035
2nd Flair600x935
Top Reflector600x250
Bottom Reflector600x300

Note that this cutting list does not include the rear chamber brace or the braces in the mouth, and between the 1st and 2nd flair pieces. Cut these pieces to fit.
A dimensioned drawing will open when you click on the image to the right. (I need to find a nice way to put numbers on this image)

The top panel support needs to have a row of holes cut in it. The whole top section is needed for the rear chamber volume. The driver exits through a 400mm by 112mm hole in the driver baffle piece. The driver baffle piece also has a 15mm rebate to acomodate the suspension at full travel.

I ran some models of this horn using a few differnt drivers. There were quite a few that behaved in a very similar manner to the 18-Sound driver I'm using.

These were :
  • RCF L18P300
  • PAS LX-2800
  • JBL 2241H
  • JBL 2242H
  • Eminence Omega Pro 18
  • Eminence Kilomax 18
  • B&C 18 TBX 46
  • B&C 18PZB 46

    These drivers all showed a response that was within a dB or so from the curve shown for the 18LW1400. Note that I have not verified that these drivers can infact fit in the back chamber of the horn. I would assume they can because the 18LW1400 is one of the deepest drivers in it's class (Except maybe the Aura NRT).

    Please note that this horn loaded subwoofer design is still a work in progress. I will be building one sometime soon and will post actual measurements and photos of the construction when finished.

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