SEALED BIPOLE SPEAKER WITH FOSTEX FE127E
Page 1: Introduction, FE127E Driver, Sealed Enclosures, Bipole Speakers, Design
Page 2: Construction, Sound
With the success of the Radio Shack 40-1197 BD-Pipes project, I wanted to try another full-range speaker project. In order to eliminate the need for a Baffle Step Compensation / Correction (BSC) circuit, I once again opted to go with a bipole system. Pushing these tiny Xmax drivers to produce bass tends to cause some loss of cone control, even at low SPL. This in turn results in unpleasant distortions which are present throughout the mid-range. On that basis, this project uses a sealed enclosure in an attempt to get the best mid-range performance from these drivers. Other added benefits of a sealed enclosure include a much higher attainable SPL and a better overall bass response as the speakers will be coupled with a subwoofer. The planet_10 hifi sealed FR125 provided inspiration for this project.
FOSTEX FE127E DRIVER
This sealed bipole speaker project uses the Fostex FE127E full-range driver (Photograph 01). The Fostex FE127E driver is 120cm (4.7"), features a paper cone made of banana plant fiber and magnetic shielding. The frame of the driver is stamped steel, so it will benefit from some damping.
Photograph 01: Fostex FE127E Driver
A Sealed Enclosure, often referred to as an acoustic suspension enclosure, is the simplest form of a speaker box. With a sealed enclosure, the back wave does not interfere with the front wave because it is trapped in the sealed box. As a result, sealed box systems are typically the least efficient enclosures since half of the radiation from the driver (i.e. the back wave) is wasted. Despite the reduced efficiency, sealed designs are quite popular as they promote a smooth frequency response and excellent cone control which translates into accurate sound reproduction.
For a sealed enclosure, the optimum volume can be determined by tuning the box to Q=0.707. Many designers like to use the Golden Ratio (0.62:1:1.62) for the interior cabinet dimensions. A box designed to this ratio will have smaller resonant peaks than one whose dimensions are equal. Another ratio sometimes used is 0.8:1:1.25. While these ratios may not result in aesthetically pleasing dimensions, it is best to keep the internal dimensions from being multiples of any another internal dimension. All sealed box designs ultimately roll off at 12 dB/octave (second order roll off).
In a bipole system, both drivers fire in phase with one another, but in opposite directions. Bipole speakers do an excellent job of propagating sound out to the sides and they create a lot of room reflection as well as a direct radiation toward the listener. The increased room reflections provide a good response off axis and a better sound stage (i.e. bipolar speakers have a large sweet spot). As a result, loudspeaker placement is generally not critical, but you do want to keep them off of a wall. Additionally, bipoles throw a huge wall of sound which can make them sound very dynamic. Unfortunately, with all the reflecting going on, many of the timing and phase cues are lost and thus imaging is not as good as a monopole design.
Another positive characteristic of a bipole system is that the rear driver compensates for the baffle diffraction step, and thus no electronic correction circuit is required.
FOSTEX FE127E SEALED BIPOLE DESIGN
Figure 01 shows the calculated frequency response. For two FE127E drivers, the optimum volume is about 12L. However, I chose to go with 23L for aesthetic reasons and to allow for an easy conversion to a T-Line had I been disappointed with the sealed results.
Figure 01: Calculated Frequency Response - Fostex FE127E Sealed Bipole Speaker
The enclosure design for this project is shown below in Drawing 01. See the construction photographs for more details regarding damping/stuffing, bracing and chamfering the driver cutout.
Drawing 01: Fostex FE127E Sealed Bipole Speaker Enclosure Plan
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