Rabu, 01 April 2009

DIY Audio Speaker FAQ - Tutorial

Why build your own speakers? Basically, you can get great sounding speakers at a low price. Granted, you won't achieve the quality of a $10,000 speaker, but for a couple hundred dollars, you can build speakers that will sound much better than anything you can buy at that price.

So what's the big deal? Build a box, cut a couple of holes, throw in some speakers, and wire them together. Well, it is almost that simple, but also much more complicated. You can make an ok sounding system by doing this and it could be done in less than a day. To do it right, you should spend some time with speaker selection, finding drivers that work well together. The box must be built to a specific size, possibly with multiple chambers. The box must be braced to the point that when the bass hits, the only thing in the room not vibrating is the speaker box. A well-designed crossover is also necessary so that each driver can perform optimally. The NHT Model 3.3

Why should it be a 3-way speaker system? With your basic 2-way system, you have a tweeter and a mid-range speaker. A 2-way system will not produce deep bass, although a good mid can provide more bass than you would expect. A 3 (or more) way system adds a woofer for full bass response. I do not believe it is worth the effort involved in building a system that isn't full range. Others may disagree, and there are many great 2-way kits out there. But, there are many great 2-way systems on the market that you can get for about $200/pair, and it would be hard to beat that price doing it yourself. PSB, Totem, & NHT are a few of the companies that make good 2-way bookshelf speakers. Von Schweikert VR-8 I have seen prebuilt 2-way boxes and premade crossovers that could make building a 2-way system very easy. The boxes were made of plywood, were stapled together, and were unfinished. Sometimes, cheaper wood is acceptable for a speaker system that doesn't have a powerful woofer. These speaker boxes ran for about $10. A 1st order high & low crossover was built into a binding post, and was about $15. For $50 plus cost of drivers, you could have a cheap 2-way system. Also, you could just paint the box to make it look better. If you were looking for a cheap simple speaker, this could be the answer. The problem is that usually the person that made the box and crossover had a specific pair of speakers in mind. The box size and crossover point must match what the drivers are designed for. At this point, it becomes more of a kit building exercise rather than a DIY project.
On the other hand, some high end drivers cost hundreds of drivers and there are bookshelf speakers on the market that cost more than $10,000. If you are an experienced builder using high end components, then a 2-way system might be right for you.

Use multiple speaker boxes, or one large box? Many people who do their own speaker projects separate the woofer from the rest of the system, putting it in its own box. Often, people also bi-amp the system, using a separate amplifier for the woofers. This greatly simplifies crossover construction, and gives you more control over bass. The mid and tweeter should be at roughly the ear level of the listener (when sitting) so that the sound doesn't seem to come from above or below. You could build a box that was 4 feet tall, but this is a lot of work and wood. There would probably be an unused chamber inside the box if it was that tall. It is cheaper and simpler to build a large sub box, with a much smaller bookshelf sized speaker box resting on top of it. Another option, which is becoming more and more popular with manufacturers, is to make a single, thin, deep box with a side firing woofer, as in the NHT 3.3 on the right. The front of the box is just wide enough to fit the mid and tweeter, and the box is just deep enough to fit the woofer on the side. The necessary volume for the woofer comes from the box's height, which is tall enough to allow the mid and tweeter to be at ear level. Since the box is thin, there is no wasted area (or wood) in the box. NHT, Klipsch, and Definitive Technologies are a few of the companies that design speakers like this. The problem with this method is that it ignores the phase shift that occurs when the speakers are not aligned properly. The back of each cone (where the dust cap is) should be aligned on the same axis. Otherwise, the high notes will reach the listener before the low notes. There are The Gallo Nucleus Solo also possible problems with cancellation. There are two ways to solve this problem. The first is once again to use multiple boxes, and position each box so that the rear of the cones align, as in the Von Schweikert VR-8 on the left. The other method is to slope the front of the box slightly so that the drivers align. This makes construction more difficult, but it looks nicer.

What is the best shape for the box? Internal reflections in the box combined with the vibration of the box itself can cause spikes The B&W tapered tube in the frequency response of the system. Different box shapes have a different effect, with perfect cubes being the worst and spherical or egg shaped boxes being the best. Although spheres have advantages, it is very difficult to create a spherical speaker box that is as strong as a typical rectangular box. One good example of a spherical speaker is the Gallo Nucleus Solo, on the right, which is made of rolled steel or brass. An even better design is the sphere/tube concept by B&W shown on the left. This design gets the benefits of a spherical design, but also adds a tapered tube at the back to eliminate all internal resonances.
Cube+-5.0 db
Rectangle+-3.0 db
Cylinder+-2.0 db
Beveled Cube+-1.5 db
Beveled Rectangle+-1.5 db
Sphere+-0.5 db

This is not something you would likely build yourself. The general rule is stay away from perfect cubes, and use a beveled rectangle if possible.

Why Individual Chambers? The mid and woofer both need their own separate chambers in the box. Both the mid and woofer are designed to work in an enclosure of a specific size. If they are both in the same chamber, like most cheap speaker systems, then the enclosure size for the mid will be too large and performance will be lost. Also, the sound waves from the woofer can overpower the mid and distort it. The tweeter is indepentently sealed and doesn't need it's own chamber.

Sealed or Ported? In my opinion, for a home application all chambers should be sealed. This improves the power handling of the driver, produces a smoother low end roll off, and basically sounds tighter. The advantage to a ported box is that is will be louder (about 3db), but this increase in db will only be at certain frequencies, depending on how the port tunes the box. Below those frequencies, the speaker will not function as well at all. Sealed boxes also allow more room for error in design.
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