Hoffman's Iron Law states that a woofer's efficiency is proportional to the volume of the enclosure it is mounted in and the cube of its low frequency cutoff. In other words, if you want a loudspeaker with very low frequency extension AND high efficiency, you need an enormous enclosure. Or you could build The Homewrecker.
This Instructable will show you how to build a loudspeaker that can mount in most standard interior doorways, using the room as the enclosure. The system is easily removable, though quite heavy. The system shown here is not a high-fidelity system, but it is very efficient (i.e. LOUD) and can reproduce very low frequencies. Based on the parameters of the woofer, this system should easily reach below 30Hz (-3dB) without including the natural boost obtained from room reflections. With this boost included, the system should reach 20Hz - the lower limit of human hearing. All of this bass extension comes at a very respectable 96dB with a 2.83V input (4 ohms).
It consists of (8) 12' woofers, (8) 5' midranges, (4) 2' x 5' tweeters, a simple crossover and (4) easy to use mounting brackets. The size and number of speakers can be just about anything you want, but this combination utilizes the available space in a doorway quite well.
Step 1: Get Stuff
The following is a list of the components I used for this system, but these exact parts may be difficult to obtain and can be replaced as described below.
- 12" woofers qty 8
- 5" midranges qty 8
- 2"x5" tweeter qty 4
- input terminal qty 2
- 10W resistors qty 2
- 3.3uF non-polarized capacitors qty 2
- 16uF non-polarized capacitors qty 2
- 0.7mH inductors qty 2
- 0.4mH inductors qty 2
- 18 or 16 awg wire qty 50 ft
- 4' x 8' plywood qty 1
- 2 x 4 studs 96" qty 5
- L-brackets qty 4
- 1.25" weather strip qty 17 ft
- 3/8" carriage bolts qty 4
- 3/8" nuts qty 4
- 3/8" wing nuts qty 4
- 3/8" fender washers qty 4
- 3/8" T-nuts qty 4
The midrange and tweeter units were chosen based solely on price. These can be replaced with any midrange and tweeter of your choosing as long as they are wired properly and the sensitivities are matched to each other and the woofers. This can be done in the crossover and will be partially explained later.
The woofer was chosen based on price and a parameter called Qts. This parameter should be available from the speaker retailer and should be between 0.65 and 0.95 for best results. The woofers I'm using have a published Qts of 1.17 which is a little high, but as I said, this particular system is not designed for high-fidelity. The picture below is from the Parts Express website and it has the same specs as the woofers I'm using.
All of these drivers were purchased from the PartsExpress factory buyout section of their website (www.partsexpress.com) for less than $120 total. Better divers would make for a better system, but things get really expensive when you have to buy 8 of each component.
EDIT - 11/23/2010 This (http://www.parts-express.com/pe/showdetl.cfm?Partnumber=292-422) is an excellent woofer to use in the Homewrecker as well. They are only $13.76 each if you buy 4 or more. They are less efficient than the original woofers used and you will have to account for this in the mid and tweeter level, but they are 8 ohms which will be easier on your amp. Plus, I think they look better without the ribs on the cone.
Step 2: Lay Out Driver Placement on Plywood
Standard interior door sizes are 30", 32" and 36" wide and 80" tall. My house is old and most doorways are 29" wide by 80" tall. With these dimensions in mind, I chose to make the overall size of the baffle 35" x 82", which should accommodate 30" and 32" doorways as well as my narrow 29" doorways. The baffle can be made as wide and/or tall as necessary for special situations.
After cutting your piece of plywood down to size (35" x 82" in this case), plan and lay out your speaker arrangement on the plywood. Use the overall driver diameters to achieve proper spacing, but make sure to leave 1.5" between woofer mounting holes to allow for 2 x 4 bracing on the backside. In my case, my woofers are exactly 12" in diameter, but require an 11" hole. For my layout, I started in the exact center of the board with the tweeters, then moved outward with the midranges, and finally placed the woofers on top and bottom. If you plan well, you can get distances between divers to be quite symmetrical.
Step 3: Cut Holes
After marking the centers of the woofer and midrange holes, I used a router with a circle cutting attachment to cut the holes. An adequate job can be done, however, by drawing the appropriate sized circles and using a jigsaw to do the cutting. This is in fact how I cut the holes for the tweeters which are rectangular in shape.
This is a good time to cut the slots for the mounting brackets as well. I made these slots 0.5" x 1.5" long to allow ample room for the bracket bolt to move while mounting. The slots are located in each of the 4 corners with exact height chosen so the mounting brackets will not run into the door hinges when mounting. In this case each slot is 5.75" away from its closest side and 5.125" away from its closest top or bottom.
Again, I used a router for these slots, but a 1/2" drill and a jigsaw could do the same job.
Step 4: Brace the Back
This design relies on four brackets to hold the entire baffle against the door trim, so it must be relatively stiff. To do this, 2 x 4 studs run 66.5" lengthwise down the middle and just outside of each column of woofers. 28" lengths run sideways at the ends of these with another 28" length 1.75" away from the first. This 1.75" channel will easily accommodate the mounting bracket 2 x 6 and keep them from spinning when tightening. In between the long spanners are short sections bracing directly around the large woofers. Everything is screwed in place with 3" multi-purpose screws.
Step 5: Apply Weatherstrip
Apply self-adhesive weatherstrip along each side edge and the top edge, which will create an airtight seal between the baffle and the door trim. I have left the bottom edge with no weatherstrip. In my house, the bottom edge will "seal" against the carpet. If you are planning on mounting this in a doorway over hardwood floor, you may need to add weatherstrip to the very bottom or even rest the baffle on a rolled up towel on the floor.
The weatherstrip I used was the widest and thickest available at the hardware store - 1.25" wide by 7/16" thick.
Step 6: Make Mounting Brackets
These brackets are designed to pull the speaker baffle tight against the doorway trim. The blue end of the bracket shown below slides in between the trim and the end of the door (on the hinge side) when it is all the way open. On the non-hinge side, the brackets work the same, but you don't need to worry about sliding them into position. The carriage bolt is inserted from the front of the baffle through the slot and screws into the 2 x 6 section to pull the brackets (and therefore the baffle) into the doorway trim. It consists of 2 sections shown screwed together in the picture below. All threaded hardware in this section is 3/8".
The "back" section is a 6" long piece of 2 x 6 with a 3.5" L-bracket screwed to it. I used blue painter's tape wrapped several times around the end of the bracket to protect the doorway trim when the bracket pulls against it. I drilled a 1" diameter hole 4" deep into the end of the 2 x 6 and drilled a 1/2" hole through the remaining 2". Then I installed a 3/8" T-nut into the 4" deep hole. This allows the carriage bolt to reach the T-nut only 2" into the assembly.
The "front" section is a 7" long 3/8" carriage bolt with a wing nut screwed all the way tight against the head and a jam nut locking it in place. This can be replaced by any type of thumb screw type fastener, but I had trouble finding a one-piece option that was this length and diameter. Then I used a 1.5" diameter fender washer and a 3" diameter by 3/4" thick particle board "washer" to completely cover the slot when in position.
Step 7: Dry Run
At this point it may be a good idea to test fit the baffle in a doorway. It will be easier to make any necessary tweaks before the drivers are mounted. Make sure the brackets work properly with the advantage of being able to see through the woofer holes.
Step 8: Mount Drivers
Mount the drivers using the appropriate screws. I used 1.25" drywall screws for the midranges and black 1" pan heads for the woofers and tweeters (available from Parts Express). The input terminals are simple surface mount binding post types, but any kind will do.
Step 9: Wiring and Crossover
The crossover, which routes the proper frequencies to the proper drivers, for this project will be very simple. Crossover design is a very complex and intricate matter when done properly, but this specific design is about deep bass extension and efficiency, not hi-fi. That said, it doesn't have to be a complete mess. Below is the schematic for 1 channel of the stereo pair which is 4 woofers, 4 midranges, and 2 tweeters. The circuits below are wired together in parallel and duplicated for the other channel. The component values are based on the impedances and relative efficiencies of these specific drivers.
The woofers are 4 ohms each with an 87dB efficiency rating. The four woofers in the series-parallel configuration raises the efficiency to 93dB. At 4 ohms total, that means a 96dB sensitivity rating (@2.83V input).
The midranges are 8 ohms each with a 90dB efficiency rating. The four midranges in the series-parallel configuration raises the system efficiency to 96dB. At 8 ohms total, that means a 96dB sensitivity rating (@2.83V input) - equal to the woofers.
The tweeters are piezoelectric units which do not behave as normal resistive loads and as such the 10 ohm resistor on them was chosen by ear.
If using different drivers, try to find woofers and midranges that are similar in sensitivity, and then a single resistor can be used to attenuate the tweeters, which are usually more sensitive than the other divers. The value of this resistor can be determined by listening - the lower the value, the higher the attenuation.
- Edited 2/22/2010 - After listening to this setup for a while, I have made some pretty serious modifications to the crossover. These modifications will only apply properly if using the exact drivers that I have used, but it may be worth a try even with slightly different drivers.
Woofer Circuit: change 0.7mH inductor to 1.5mH inductor
Midrange Circuit: remove 0.4mH inductor, change 16uF capacitor to 12uF, insert 3.0mH inductor in parallel with midrange assembly
Tweeter Circuit: remove 10 ohm resistor, change 3.3uF capacitor to 2.2uF
Step 10: Notes and Disclaimers
This system is very heavy and will probably require two people to move it. I will probably mount handles on the front to make it easier to handle and create some kind of removable cover to protect the drivers while in transit or storage.
The room does not actually act as an enclosure for the woofers as much as it just keeps the front wave separated from the back wave, which is known as an infinite baffle arrangement. If there is some way for the rear wave to reach the front wave (e.g. mounting in the doorway of a room with multiple entrances), this system will not be very effective. The two waves will be 180 degrees out of phase and at least partially cancel each other.
This is effectively a 4 ohm system. Make sure the amplifier used is compatible with this impedance.
I have not yet fully tested the capabilities (or potential dangers) of this system. The heating and cold air return ducts in most rooms in a house should be enough to relieve the pressure induced by large woofer excursions. However, if there are no ducts in a room for some reason and there are windows, be aware of the effect of the pressure on the window(s).
Very low frequencies travel well and are not easily absorbed, so if you are using this system at full or nearly full capacity and have immediate neighbors, they will hear it and probably call the police. Use good judgement .
Runner Up in the
Art of Sound Contest