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

Notes:

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.

Disclaimers:

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 .

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147 Comments

LP2

10 years ago on Introduction

What!??, never mind.
Bet it works Great, Thanks.

SGT. Desert

10 years ago on Introduction

Tachyon

10 years ago on Introduction

I was just wondering where you live so I can make sure I never move in next door!
;')
I'm sure this has made you real popular with the neighbours.

That said, I give you full credit for thinking outside the (speaker) box.

spacesaver

11 years ago on Step 3

One comment here; As shown in the 1st and 3rd photos, I have tried to incorporate a small, childs bike into the build. I believe I am doing it wrong as1) the assembly is totally unbalanced now and 2) the bike can no longer be riden. May have to re-think this step when making the other three assemblies.

amclaussen

13 years ago on Step 10

Just a couple of reflections (thoughs)... If Efficiency is a primordial objective, then the use of a Passive Crossover network is not the best selection. The simple first-order (6 dB/octave) crossover used, theoretically produces a 3 dB loss. They call it "Insertion Loss".- For many good reasons, the absolutely best way to go is to go with an ACTIVE crossover and arrive to a BIAMPED system. In such system the woofers are connected directly to the bass amplifier, which avoids the loss caused by the unavoidable resistance of the coil. An additional benefit is that the amp is now able to better control the heavy woofer cone (specially when the magnet size is somewhat small, like the ones in inexpensive drivers like those selected for this project). As the efficiency using an active crossover rises, the headroom becomes more ample, and the need to use a sufficiently large power amplifier is greatly reduced (a 3dB increase in efficiency means that instead of a (say)100 watt amp, we now need a 50 watt amp. Since today small car audio stereo amps are good sounding enough and relatively inexpensive, by using a, say 50 W amplifier for the low frecuency channel plus a 20 W amp for the mid+high is more than enough (I AM refering to REAL, so called RMS watts). The differences of efficiency and sound level between the low and midrange drivers are easily corrected with the input level controls at each amp. 2.- The Piezo tweeters can be connected directly to the mid+high amplifier WITHOUT any crossover components, since the capacitive nature of the transducer works exactly like a capacitor, then you don't need the high frecuency capacitor network and the load resistor; it is shown in the schematic only because this crossover network was designed for a Dynamic cone or dome tweeter, which has a typical impedance around 8 to 10 ohms at the typical crossover frecuency. The false impression that a Biamped or Tri-amped or Multiamped systems have to be costly, or complex or only for professional use is nonsense! Even a low budget system benefits a lot from at least biamping... The passive crossover is surely going to produce a widely varying impedance curve that only has the nominal 4 ohm value at two or three frecuency points, varying hugely across the full frecuency range, Phase response will certainly vary tremendously also, because the driver's impedance does vary inmensely (between lower than 4 ohm to higher than 80 ohm or more at the resonance point, and 20 or so ohms at crossover frecuencies because of driver interactions, remember, passive crossovers require resistive terminations of equal impedance to be able to produce a smooth impedance curve. Some amplifiers DO NOT like such impedance excursions. 3.-Why mount all drivers in the same panel??? Stereo image and coverage is destroyed when the speakers pair is not properly arranged to create a Stereo image, usually left and right speakers should form an equilateral triangle between them and the listener... May I humbly suggest that ONLY the woofers are mounted to the panel, creating an effective Sub-Woofer taking advantage of the nice and clever concept of putting those in a panel like you did, but installing the midranges and tweeters in much smaller panels/enclosures mounted in small pedestal bases that rise them to ear level and permit to locate them for a proper Stereo image and be very portable and lightweight. That is called a "subwoofer-satellite system" and is capable of very good results without the need for enormous tower speakers for a nice and balanced system. But for a good result, the crossover frecuency must be below 150 Hz, preferably 100 Hz, since lower frecuencies are not directionally perceived, which allows us to place the subwoofer almost anywhere in the room, and to place the left and right satellite speakers where acoustics, Stereo image and room decoration best fit together. Since now the midrange speaker will have to work from 100 Hz up, a 6" to 8" unit will be required per side (or a pair of good 5" per side) , with its necessary enclosure, which won't need to be large since only below 100 Hz a huge volume is needed (which is already solved by your woofers mounted in the large panel at the room door). Finally, where do we get a good Active crossover? May I suggest to go to "Elliott Sound Products" website and look for an excellent fourth order (24 dB/octave!) crossover network PCB (printed circuit board) which is a good way to obtain a tryed and tested solution! The owner, Mr. Rod Elliott from Australia sells these PCB's bare and ready to be populated with some really inexpensive IC's, small capacitors and resistors. ANY self respected Instructables reader should be fully capable of assembling and setting an excellent two way, stereo Linkwitz-Riley electronic (active) crossover with an outstanding frecuency and phase response, which combined with a couple of nice sounding inexpensive Alpine (or similar) brand car audio small power amps (a 2X50W RMS unit costs only 50 bucks new, the woofers 2X100 W RMS unit another 70 USD or so), The Active Crossover circuit board is 24 USD plus around 10 dls in IC's and small parts, plus a small aluminum extrusion box for a chassis. If you already have a good small stereo amp, then you can replace one of the car audio amps to save some bucks... You will also need a 12V DC power supply to feed the car amp(s), but Instructables has a few computer power supplies articles that use old PC's power supplies being rescued at no-cost, and capable of giving more than enough current Amperes to run the amps to full power! Make no mistake, the system I am describing is not a mediocre sounding pile of pieces, the results of using a Biamped system is so "sound" (no intended pun!) that it rivals ANYTHING costing several times as much. Mine has put to shame some extremely expensive (read over a couple of thousands) big amplifier plus passive large size speakers. (the reason, obviously, is that it is plainly stupid to put A LOT of cash in a large very expensive amplifier that will waste half its power just in teh passive crossover, and that a lot of famous speakers present an horrible load to the amplifier. I cannot make an Instructable of my system or design approach at the moment, because I don't have a decent digital camera and I'm in the middle of moving from an apartment to an old house and my wife is waiting our first baby... maybe next year (I hope..) Good Luck, I may suggest fully reading Rod Elliott's excellent articles (parts one and two) on Active vs Passive Crossovers, and also on the almost unsurmountable difficulties of making a properly behaving passive crossover (yes, it can be done, with a lot of effort, electronic lab equipment and some years of available time to test, retest, and test again a miriad components and crossover parts values... believe me, been there done that. The Active crossover IS the WAY to GO. A webpage reporting on the relative quality of popular car audio amps is at: <a rel="nofollow" href="http://slumz.boxden.com/f359/worst-best-car-audio-brands-419293/">http://slumz.boxden.com/f359/worst-best-car-audio-brands-419293/</a> Good Luck Alfredo Marquez Claussen, Mexico City. amarquez (aT) i m p dot m x

amarquez

Reply 13 years ago on Introduction

First of all: my comments were not intended as a critique of your nice instructable, and tried to be constructive. I believe there is a tremendous potencial in using a room as an enclosure in a way similar to an Infinite Baffle, specially as a Subwoofer one. I was just trying to imagine how to get from there to a very good-but still inexpensive system by Bi-amping it. Using Satellites instead of not-large-enough floor standing speaker boxes has been a great success for me. As I wrote, I do not like passive crossovers, because I've seen many speakers fail miserably just because of their interactions, During the years, I've built maybe two or three dozen crossover prototypes trying to succeed, but my efforts were above 90% failed ones, even using lab equipment at my work and the best books available at the time. No anymore! For me, to go ACTIVE is the way! (and I was just trying to save some fellows the same time wasting approach).I was also trying to highlight the fact that a large very expensive "esoteric" power amplifier connected to large and complex passive crossovers is the only way to go (because it has been the usual way for too many years). With Multiamping, small inexpensive amplifiers are more than enough. Now, on pasive crossovers; To properly work, impedance values must match at the crossover frecuency, which is not often the case. I have seen (and measured) too many systems assembled by various people that sounded horrible and presented a very funny impedance curve to the amplifier because the crossover used was a "generic" design where no real data for the drivers was used at all (or using the "ideal" 8-ohm (or 4 ohm nominal values) that are not presented by the drivers at the intended crossover points. With some luck, a passive crossover it will be acceptable and first order designs can be somewhat adjusted by ear, but most of the time, it won't. The impedances of the low, mid and high frec drivers should be equal for the theoretical crossover to properly work, (it is like a pipe network in a house, where if it has three bathrooms, and one of them is located ina lower floor, it is going to have more pressure (and more water flow) when its shower valve opens than the other bathrooms. A passive crossover relies on proper termination at all the outputs to behave correctly, and this is difficult at best. In the case of your design, using a Piezo tweeter solves some of the problems because it has a very high impedance at the crossover point, thus the tweeter resistor can emulate a correct impedance value. But I don't see how to match the 4 ohm woofer set to the 8 ohm midranges. Being 6 dB/octave surely helps, and using a not too low crossover frecuency also helps, but a beginner could interprete that any generic brand crossover that can be found in some store is going to work properly regardless fo the drivers impedance curves. This is where an ACTIVE crossover plays its magic! It is very easy to properly load the output branches of an active crossover since it is loaded in an almost resistively manner. No longer we depend on chance or interminable hours in the lab trying to "match" drivers impedances and relative efficiencies. With now available car amplifiers putting honest 25 watts RMS per channel at 4 ohms, we can drive midranges beautifully, and woofers can have a 100 watt per channel amp easily. By Biamping the system with a Do-It-Yourself Active Crossover, specially the very good one mentioned in my post, excellent results can be obtained easily and for shure. The Fouth Order 24 dB/oct Linkwitz-Rilley alignement gives flat frec response and phase response also, and really produces a sharp separation between drivers. I also don't like to use any cored inductor, because I've seen (and heard) them distort badly when compared to air cored ones, which necessarily become large and need large gauge wire to keep DC resistance acceptably low (but nevertheless significant). Damping factor is also going to be lowered by the DC resistance, and because the woofer is not damped by the large volume of air in the room, a better behaviour can be obtained with a direct connection to the amp (no inductor in series with the voice coil).
Lastly, I had an idea somewhat similar to your Instructable some years ago, but never tried it: the house I'm living in now, has a functional Fireplace (we call it "a chimmey") that is built of concrete and heavy bricks. I am wondering how to use that Fireplace and its long (about 19 feet) vertical duct to house a kind of Labyrinth or "Transmission Line" enclosure, that reportedly has an excellent low frecuency response. Even a low frecuency horn loaded woofer could be housed... Your Instructable gives me a very strong motivation to investigate and build something in that direction! And Congratulations on your prize winning Homewrecker. I thought of participating in the contest, but my idea was to write a kind of design summary for a Sound System, to try to show others what I have learned tru some years of making all kind of baffles and enclosures, and to try to Share my experiences, after many attempts, some terrible, many mediocre and a few barely acceptable, now my multi-amplified system really satisfies me and most of my friends (some are musicians, some are professional sound engineers, which is certainly reassuring). Sharing is essential because it sums the efforts of many people working towards a common end. Thats why I enjoy Instructables!
I am happy that some people found the idea stimulating towards using a closet as a Subwoofer enclosure (a good idea indeed), and that mikee69 even found the website of Mr. Rod Elliott (http://sound.westhost.com/projects-3.htm) even calling it "a goldmine of electronics info" which I fully and absolutely agree! And I invite you and all the gang to visit to read about the Passive vs Active crossover matters. I'm making a pair of Headphone amplifiers based on their circuit cards, a portable and a home, hi quality one.

Introduction: The Homewrecker

Step 1: Get Stuff

Step 2: Lay Out Driver Placement on Plywood

Step 3: Cut Holes

Step 4: Brace the Back

Step 5: Apply Weatherstrip

Step 6: Make Mounting Brackets

Step 7: Dry Run

Step 8: Mount Drivers

Step 9: Wiring and Crossover

Step 10: Notes and Disclaimers

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