BD-Pipes Speakers with Radio Shack 40-1197 (FE-103)
Page 1: Introduction, Background, Driver, Design
Page 2: Construction, Sound
One of the first speaker projects that I came across was a set of BD-Pipes completed by David Dlugos. At the time, I knew nothing about full-range speakers, but I really liked the sleek look of the BD-Pipes and the project costs seemed very reasonable. That was enough for me to decide that I needed a set for myself. Well, almost two years later, here they are!
The BD-Pipes are a fullrange DIY speaker project designed by Bert Doppenberg of BD-Design for use with the ACR FE103 driver. Each speaker enclosure uses two full-range drivers, one firing forward with the other firing upwards and backwards at about 45 degrees. The drivers are wired in phase (push-push) which results in higher output levels than a single driver system, but more importantly the rear driver compensates for the baffle diffraction step, and thus no electronic correction circuit or crossover is required.
A full-range driver attempts to cover the entire audible frequency spectrum using only one driver (i.e. no mid-range or tweeter). Having just full-range drivers in a speaker system eliminates the need for an electronic crossover network, which is well known for being very difficult to design properly without introducing colorations to the sound. The point-source sound from a full-range driver generally results in excellent imaging, unrivaled clarity and detail. There are many full-range drivers on the market with Lowther and Fostex being two of the more popular driver manufacturers.
The BD-Pipes are a folded, Mass Loaded Tapered Quarter Wave Pipe (ML-TQWP), also referred to as a Quarter Wave Resonator. A TQWP is often considered an optimal enclosure for use with full-range drivers for two reasons. The mid-range frequencies are absorbed by the internal stuffing / damping material (mass loading) and by the geometry of a pipe. As a result, pipes are often noted for being particularly clear across the mid-range. In addition, the TQWP can often get the most bass out of a driver, which is important since fullrange drivers are often bass challenged. A great deal of information about Quarter Wave Resonators is available from Martin J. King's website Quarter Wavelength Loudspeaker Design.
What really attracted me to this project was the rear facing driver which eliminates the need for a baffle step compensation / correction (BSC) circuit. The baffle step response phenomenon (diffraction loss) occurs when drivers are mounted on a baffle. The reason being that at high frequencies. the speaker is only radiating forward (half space). However, because bass is non-directional, at low frequencies the speaker is radiating both forward and to the rear (full space). As a result, the high frequency is about 6dB higher than the low frequency in the listening position and this is what is referred to as the baffle step. To compensate for this diffraction loss, you can use a correction circuit or use a rear facing driver.
The BD-Pipes were designed for use with the ACR FE103 fullrange driver, which I understand is very similar to the Fostex FE103E. Both of these drivers are no longer in production. However, the BD-Pipes have been successfully completed with a variety of similar drivers such as the Radio Shack 40-1197, the Foster (Fostex) FE103A and the Fostex FE108 Sigma. It would be reasonable to expect the BD-Pipes to work well with the current Foxtex equivalents such as the FE103E, FE107E and the FE108E Sigma. My version of the BD-Pipes uses four Radio Shack 40-1197 drivers.
The enclosure plans for this project are no longer available on the BD-Design web site. However, permission to host the enclosure plans on this site has been provided by Bert Doppenberg, and are provided in the figures below.
Figure 01: BD-Pipes Loudspeaker Enclosure Plan
Figure 02 shows the placement of the damping material and a passive filter design by Bert to shape the frequency response a bit. I opted not to use a filter, but if you find the speaker to bright, you could try the filter.
Figure 02: Damping Material and Passive Filter Plans
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