Rear Speakers
These are the speakers I am currently using as my rear (or "surround") speakers in my home theater. This is a variation of the "Dayton Home Theater" design by Wayne J. of Speakerbuilder.net. It uses two of the speakers in his design and places them "back to back" to create a dipolar or bipolar speaker. The majority of speakers produced today are "mono polar" meaning that sound is only emitted from one side (the front) of the speaker. In bipolar and dipolar designs sound is emitted from two sides of the speaker (usually the "front" and "back") and can be used to create some interesting effects. The difference between the bipolar concept and the dipolar concept is subtle, but important. In a bipolar design both the front and the back of the speaker are "in-phase", meaning that when the drivers in the front of the speaker are being pushed outward, the drivers in the back of the speaker and also being pushed outward. Bipolar speakers can be used to widen the apparent source of the sound coming from the speaker. In a dipolar design the front and the back of the speaker are 180 degrees "out-of-phase", meaning that when the drivers in the front of the speaker are being pushed outward, the drivers in the back of the speaker are being pulled inward. Dipolar designs can be used to "de-localize" the source of sound, that is to make it difficult to tell where exactly sound is coming from.
You might now find yourself wondering how exactly these designs are relevant to home theater. The answer can be found by taking a trip to a real theater, a movie theater. Next time you're at the movies take a look at their rear speakers. You'll notice there are about 4 speakers on both the left and the right and 4 speakers across the back wall (assuming the theater supports audio standards that include a rear center channel). These individual speakers do not represent discrete channels, there is still just one channel for each of the rear left, right, and center speakers. To state this another way, all of the speakers on the rear left of the theater are playing one channel, the left surround channel, and the same is true for the rear right and rear center speakers. This is a standard speaker setup that is used to mix the audio tracks for most movies, but having 12 surround speakers just isn't practical in home theater. So the next best thing to having 4 speakers on each the left, right, and back of the room would be to have 1 speaker on each the left, right, and back of the room that produced the effect of having four. Hence the use of dipolar and bipolar designs. The dipolar speakers on the right and left create diffuse sound (that is, sound that is difficult to localize) which is similar to the effect the four speakers create in the movie theater. The bipolar rear center speaker produces sound that is easily localized as "behind-my-head", but it also appears to come from a much broader area than the sound a single speaker would produce. The orientation of this kind of speaker is different than your standard speaker. "Mono polar" speakers are almost always oriented so the they face (or "point to") the listener. With these bi/dipolar speakers you want the sides of the speaker to face the listener. In order to see this is practice take a look at my Current Theater Pics. You'll see that the sides of the speaker face the listener while the "front" and "back" face the front and back walls of the theater. Remember this, the most important thing about surround speakers is how they are placed, I encourage you to try different locations (if possible) before mounting them, this can make or break a great surround system.
Enclosure Design
Enough theory, lets get on to the actual design. As I stated at the beginning, these speakers are simply a variation of the "Dayton Home Theater" design by Wayne J. of Speakerbuilder.net. His site can be a very useful as a reference, although all the information you should need to construct them can be found here. Below is the cabinet design (in PDF format).
The baffle diagram shown in the design will be built twice for each speaker, one for the "front" and one for the "back". I built my speakers from 3/4" MDF that I purchased from Home Depot for about $6 per 2'x4' sheet. I did not recess the tweeter because I lack a circle jig for my router, but it is recommended by the designer that you do so. The point of the center wall is to seperate the front part of the enclosure from the back, you must include it or this design will not work properly. All of the walls, except the front baffles, of both the front and back enclosure are lined with the same acoustic damping material that was used to make my Acoustic Panels.
Crossover Design
The crossover is essentially identical to the original. The picture below is a modified version of one I found on Wayne J's page. In the bipolar version of this speaker both the front and the back of the speaker are wired "in-phase" with one another, that is to say that the crossover in both the front and the back are exactly as shown below. In the dipolar version of this speaker the front and back are wired 180 degrees "out-of-phase", that is to say that the crossover in the front is exactly as shown below and the crossover in the back has the tweeter connected backwards. This may be confusing, so let me be very specific. First, the front of the speaker will be the half of the speaker that faces the front of the theater, making the back the half of the speaker facing the back of the theater. In the dipolar design the tweeter in the back of the speaker will be connected 180 degrees "out-of-phase", this literally means that in the diagram shown below the lead shown connected to the positive terminal of the tweeter will then be connected to the negative terminal and the lead shown connected to the negative termianl will then be connected to the positive terminal. I'm sure you are now wondering why only the tweeter is connected out-of-phase. The reason is that when the woofer is connected out-of-phase as well it will tend to cancel the frequencies being produced by the woofer in the front of the speaker. This results in a notable lack of bass response. I much prefer the sound of this speaker when only the tweeter is wired out-of-phase.
When all the construction is done you will have two speakers in one box facing opposite directions. Both of these speakers will need to be connected to your amplifier or reciever on the same channel so that they recieve the same material. There are two ways to go about this, you can either connect the sides together in series or in parallel. Which way you choose will determine the overall impedance (in the case of speakers you can think of resistance and impedance as the same thing) of the speaker. At the bottom of the picture a resistance equation is solved to show the overall impedance of the speaker, you can read more about this equation here. If you look at the other picture below you will see I've wired my speaker in parallel (I know it might appear at first glance as if it is in series because the wires are crossed, but if you look closely you'll notice that the reds and blacks are actually directly connected). I chose to do this because I will be using seperate amps to power my speakers, if you are powering your speaker with an A/V reciever I highly recommend you wire them in series to achieve a 16 ohm load, recievers rarely react well to speakers with an impedance less than 8 ohms, but more is just fine. In the instance below I can connect the speaker wire as usual (red to red, black to black) at either of the two terminal cups, it doesn't matter which. If you wire these speakers in series you will have one wire that goes from one of the red terminals to the opposite black terminal and you will connect the red speaker wire to the unconnected red terminal and the black speaker wire to the unconnected black terminal.
Parts
This is a list of all the parts nessecary to make one dipolar or bipolar version of this speaker. They also have thier Parts Express part numbers beside them. This table is a modified version of the one found on Wayne J's page. Don't forget to also order binding posts or terminal cups for both sides of each speaker.
Quantity |
Part |
PN |
Price Each |
2 |
5.25" woofer |
295-300 |
$13.50 |
2 |
1" Tweeter |
275-055 |
$16.50 |
2 |
2.2
uF NP Cap |
027-324 |
$0.35 |
2 |
6.8 uF NP Cap |
027-336 |
$0.50 |
2 |
3.3
uF MPP Cap |
027-420 |
$1.77 |
2 |
3.0 uF MPP Cap |
027-418 |
$1.67 |
2 |
.70
mH 18awg inductor |
266-820 |
$3.75 |
2 |
.80 mH 18awg
inductor |
266-822 |
$3.75 |
2 |
.15
mH 18awg inductor |
266-804 |
$2.20 |
2 |
6 ohm NI Resistor |
004-6 |
$1.50 |
4 |
68 ohm Resistors |
016-68 |
$0.78 |
As you can see the total parts price for one of these speakers is around $100.
Crossover Construction
This is an area that always takes a little creativity. You have to find a way to take all the crossover components, solder them together so that they match the schematic, and make the whole package small enough to fit easily inside the speaker. I like to mount my crossovers on 1/8" hard board that I purchase from Home Depot. It's about $2 for a 2'x2' square. The only problem is that it is almost impossible to cut with scissors, the best way I've found to cut it to size is with a table saw. Below you can see my implementation of Wayne J's crossover design. If the speaker uses a parallel crossover I like to mount each drivers crossover on it's own piece of hardboard. The first picture is of the woofer's crossover and the second is of the tweeters crossover. I'm using blue wire to represent "positive" and white wire for "negative". I also like to mark the wires that will be connecting to a driver with a sharpie, meaning that the wires that aren't marked will be connected to the binding posts. The wire I'm using is 18ga silver-coat copper wire with teflon insulation from Apex Jr. The components are attached to the hardboard with some Liquid Nails (just general adhesive) and can be mounted in the cabinet by gluing them to the damping material. Just remember to connect the rear tweeter out-of-phase if you're building the dipolar version of these speakers.
