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Types of Speaker Enclosures
Infinite Baffle - Is defined as an enclosure that contains a larger volume of air than the Vas of the driver. You don't need to build a enclosure for this type of loudspeaker. You simply mount them in the wall and use the wall cavity as the enclosure or if were talking about car speakers the trunk becomes the enclosure. In general these enclosures have the least amount of sound coloration and they are inefficient because the rear energy which was never intended to be heard by the listener is "wasted". The important thing to remember is to have the front and rear of the speaker isolated into different air spaces so the air from the front of the speaker can't interact with the air at the rear of the speaker to avoid phase cancellation.
Sealed Enclosures (acoustic suspension) - Just like the name implies this is a sealed box with a speaker mounted in it. This type is preferred due to the simplicity of it's design which promotes a smooth frequency response, excellent cone control which translates into accurate sound reproduction. The only downside to this design is that it does this all at the expense of low efficiency. The volume of air within a sealed enclosure is less than the Vas of the driver; the air trapped in the enclosure helps control the movement of the cone somewhat like a shock absorber controlling the springs on a car.
Vented Enclosures (ported, bass reflex, tuned, or tuned ported) - This is a much harder design to "get it right" with (unless you got some good loudspeaker software). In the first two designs the wave at the front of the speaker was used while the wave at the rear of the speaker was "thrown away". Vented designs use this "thrown away' energy to enhance the overall sound of a loudspeaker (if properly designed) with better efficiency and deeper bass extension from sound produce in the port. This is done by tuning the ports (length and diameter) so the rear wave pressure will produce a lower tuned frequency in the port. Put the port somewhere close around the woofer, preferably just below it (within one foot). The direction the ports face (rear or front) is a personal design choice, and if the need arises smaller multiple ports can be used in place of a larger single port. Variations of this design include the use of multiple rear chambers tuned to different frequencies and passive radiators.
Passive Radiator - This is essentially a unpowered driver which replaces the port on a ported loudspeaker. It has the advantage over the port of less distortion and it allows the loudspeaker to act with precision of a sealed enclosure without the loss of efficiency.
Isobaric - In this design the speakers face one another, one of them is in the enclosure. It's also wired out of phase; the speaker in the enclosure is wired normally and the other one's wires get flipped around. This will allow them to move together in the same direction at the same time. This design may be used when enclosure size must be small. This enclosure can be half the size of the regular enclosure.
Dipole - This is for a "effect" rear surround sound home theater type of loudspeaker which "hides" the source of the sound very well. In this design the drivers are fired out of phase with one another. As a result their is a "nil" or dead zone due to phase cancellation in the space between the loudspeakers which unfortunately also kills a lot of the loudspeakers bass. This type of loudspeaker will give the desired effect only within a defined space and need special considerations in their placement. Not intended to be used as a general loudspeaker.
Bipole-This type differs from dipolar in that both drivers fire in phase with one another "spraying" sound everywhere. Loudspeaker placement isn't as critical as with the dipole, with a much wider listening space area and no loudspeaker bass reduction due to phase cancellation.
Bandpass - A combination of a sealed and a ported enclosure. Combining the two together will result in louder bass output than what the sealed enclosure alone would put out. This type of enclosure is not used as true subwoofer because the frequencies that come out of the port are a limited to a very narrow bandwidth. This design is often used to enhance the lower frequencies of smaller bookshelf type of loudspeakers.
Selecting The Best Drivers For Your Enclosure
This is very important and its worth repeating (Qtc 0.707 stuff); you must match your drivers to the enclosure and I am not just talking about the size of circular hole in the front of the speaker enclosure. Most drivers are now designed to be used in a specific type of enclosure with a specific volume (size). So when deciding upon your drivers you have to take into consideration the type of enclosure (ported, sealed) and the enclosure size recommended by the manufacturer. If their recommendations are not available you will have to take the other information that is given such as Fs, SPL, Vas, Qms, Qts, etc and figure that out for yourself. You can do that by using various complex mathematical equations or by simply plugging in the information about the driver into any of the many loudspeaker building software programs that are available.
Okay, you now know to match up your speaker drivers to the enclosure size and type recommended; you have to do that with the midrange speaker also if it is not-rear sealed . The reason why you build an enclosure within an enclosure for a non-rear sealed speaker is that you'll want to isolate the air spaces behind the driver and midrange from each other to prevent them from "pushing" on each other. The volume of air displaced by a large woofer moving in and out can destroy the much smaller midrange driver and even if it doesn't, they will interfere with one another causing distortions which is not a good thing to have happen.
Don't forget to add the total volume of the midrange enclosure to the speakers enclosure's total volume. Don't worry about building a separate enclosure for the tweeter; every tweeter that I have ever seen is rear sealed so the driver's air pressure won't affect it.
Two or Three Way SpeakersHas to do with the numbers of ways the frequencies are divided (the crossover network); and generally to the amount of transducers (speakers) used;
A Two-Way loudspeaker uses a driver and a Tweeter. In the Two-way design the work of reproducing the total sound spectrum is divided between the two components. The driver may take the frequencies between 45Hz-2400Hz; the tweeter then must be able to handle the frequencies from 2.4Mhz and up. Often a subwoofer is needed to enhance to lower frequencies because of the design limitations of the driver. This is because most drivers can't do a lot of different frequencies all at once well. A driver that can get low as 20Hz usually won't sound good at 1,500Hz. And most tweeters that can reach 30,000Hz or above often won't operate below 2,000Hz.
So most two-way loudspeaker are a compromise between the low and high frequency. But due to new advances in speaker technology a new generation of mid/woofers and tweeters have come along that have much wider frequency ranges than ever dreamed of before. Because of this quality two-way loudspeakers can now begin to compete with three-way loudspeaker systems.
A Three-Way loudspeaker uses a woofer, midrange and a tweeter and is still considered superior over the Two-way design. The workload of reproducing the sound is divided among the Three components. The woofer may handle between 20Hz to 800 Hz, the midrange handles the frequencies between 800Hz to around 5000Hz. The tweeter then is left to deal with everything above 5000hz. This design has no compromises like that needed in the two-way system. Three-way or higher is the only way to go for a true full range loudspeaker. Some people have taken this to the extreme and added more than three speaker components making a four or five-way loudspeaker!
The big problem with the "more drivers are better" idea is in trying to design an acceptable crossover network. Two-way crossovers are easy, three-way are much more difficult. Four-way and above higher order crossovers are usually only attempted by the "Pros" or hard core loudspeaker builders due to the "huge pain in the ass to design successfully" factor involved.