Open baffle speakers are known for the transparency and clarity of their sound. This is due to their not having a full enclosure that would produce unwanted box resonances and the characteristic box boominess. Under about 100 Hertz, it is more difficult for a baffle speaker to produce realistic deep bass, though, because it needs the box to produce the low frequencies. An open baffle speaker with a ported woofer provides the best of both worlds, however, and building one is a rewarding project.
A single driver open baffle speaker is a purist's way to get precise time-alignment of the sound to the user's ear, instead of some of the sound arriving first and the rest an instant later because of the segregation of the woofer and tweeter. This time-alignment is critical in developing a sense of realism in the listener's brain as the sound information is processed. In this hybrid system, a quality coaxial automotive speaker should be used because it provides the desired time-alignment. Also, the automotive driver is already designed to work without a box, since it is more stiffly suspended and ruggedly constructed. This leaves all the lows to a separate low-frequency driver that can produce the deep bass through its port. The entire speaker should be constructed from ¾-inch-thick, medium-density particleboard.
This baffle speaker should be about 42-inches high and 16-inches wide. The bottom 30 inches of the baffle comprise the front face of a low-frequency enclosure with about 3-cubic feet of internal volume. For this reason, the back of the enclosure is about 12-inches deep. Select a quality woofer driver with a recommended box volume of about 3-cubic feet for a 35-Hertz F3 tuning. This means that the sound level will be down about 3 decibels at 35-Hertz, considered very respectable low-frequency performance with a 6-inch woofer. The woofer will come with recommended box sizes and port dimensions for different F3 frequencies in its instructions. The woofer should be mounted on the front face and the port on the rear wall of the enclosure. Use a plate type subwoofer amplifier of 50 watts, which will also mount on the rear wall of the low frequency section. These are readily available at low cost and will dramatically increase the performance of this set-up. It also frees up the receiver or amplifier in the stereo system to put all of its output to the high-frequency driver in the open baffle.
High Frequency Strategy
The high-frequency strategy is much easier. Mount the time-aligned 5-inch driver so that its centre is about 6 inches from the top of the baffle. Wire the coaxial driver to the high-frequency output of the subwoofer amplifier powering the woofer. Set the crossover frequency to about 80 Hertz for best performance. All of your main amplifier's power will be directed to your higher frequency section.
Experiment with different woofer levels and the phase relationship of the two drivers. There is usually a phase switch on the subwoofer amp that allows you to change the this, along with the subwoofer amp's output level. This hybrid design provides a great opportunity for you to get clear soaring high frequencies along with powerful bass that are the hallmarks of audio realism.