Design and Fabrication of Front Monitors

It's important to note that not all lateral reflections are deleterious to stereo imaging. Research shows that certain controlled lateral reflections can actually improve the imaging and sense of spaciousness in the soundstage.10 For example, the LR reflection shown in Table 3 is expected to be diminished by at least 10dB, perhaps more if one considers the effects of narrowing horizontal polar dispersion with increasing frequency, before reaching the listener. This reflection was thought to be negligible. The RL reflection, luckily, strikes the B-pillar trim piece and will eventually be treated with a sound-absorbing system. The RR reflection emerges from the monitor at an angle 108.9 off-axis, so its magnitude will be substantially reduced at higher frequencies. Table 3 also provides predictions for the periodicity of "comb filter" effects.

Image Is EverythingMany of you may question the rather analytical approach I took in building the front monitors, apparently giving less importance to aesthetics. Some will question whether the visibility out of the van was compromised. I can assure you that plenty of visibility still remains. Still, others would think the front monitors are simply ugly, too "in your face," and would not have them in any car regardless of how they sounded. Ben Oh's "Driver's Seat" column in the Aug. '08 issue of CA&E titled, "Car or Audio," addresses this topic brilliantly. And lest you think I'm crazy, I agree with most of you that the monitors I fabricated in this article are rather imposing and bulbous. But wait until you hear how they sound; I think you'll be floored. Recently, I attended, exhibited, and demonstrated at Mr. Marv's barbecue.11 At the barbecue, I met a person who was captivated by my obsession for fine craftsmanship and the sound of my system. As I explained the choices I made during the creation of an audio system dedicated purely to serving the music, I poked fun at the whole cargo van idea and especially the stamped-steel wheels. I laughed when I said I could've purchased a very expensive Bimmer with the money I used to build my van. His reply was, "With sound like you have, I'd take your van any day!" I really liked that guy. Finally, I'd like to acknowledge and thank renowned audio component designer Steve McCormack for contributing his expertise and guidance during the loudspeaker transducer placement and aiming process. Please stay tuned for Part 7, where I'll continue the fabrication process with the door-mounted woofer enclosures.

Resources1. F. A. Everest, The Master Handbook of Acoustics, 4th Ed., McGraw-Hill, p. 236, 2001. 2. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 105, 2000.3. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 113, 2000.4. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 112-116, 2000.5. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 99, 2000.6. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 108, 2000.7. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 100, 2000.8. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 101, 2000.9. V. Dickason, The Loudspeaker Design Cookbook, 6th Ed., Audio Amateur Press, p. 102, 2000.10. F. A. Everest, The Master Handbook of Acoustics, 4th Ed., McGraw-Hill, pp. 409-414, 2001.11. See http://community-2.webtv.net/MarvsPlace/MarvsPlace/index.html

Fabrication of the Dash-Mounted MonitorsThe front monitors took almost a year to design, fabricate, optimize, and finish. The following figures illustrate the fabrication process of the monitors.

7. Maximize enclosure rigidity and damping.The fiberglass composite enclosures would be fabricated to achieve maximum rigidity and treated with a variety of damping materials to minimize extraneous noise from the enclosures.

Horizontal driver separation is virtually inevitable in automotive audio systems for two reasons. First, the geometrical configuration of the automobile interior limits the available mounting locations for the individual loudspeaker transducers. Second, unintentional horizontal separation may result if care isn't taken to align the loudspeaker transducers' acoustic centers. Although the exact determination of a loudspeaker transducer's acoustic center involves sophisticated equipment and complex measurement techniques, a useful approximation of the acoustic center lies at the center of the voice coil. The task of aligning the acoustic centers is further compounded by the fact that the acoustic center of a loudspeaker changes with frequency as shown in Figure 5. Improper horizontal alignment of loudspeaker transducers can lead to an unintentional tilting of the vertical polar radiation pattern and phase errors. If it's not possible to physically align the loudspeaker transducers' acoustic centers, appropriate inter-transducer time delays have shown to be equally effective.4

Loudspeaker enclosure shape is known to significantly affect the frequency response of a loudspeaker through the mechanism of diffraction. Although a sphere has been determined to be an optimal enclosure shape, enclosures with radii larger than 50.8mm (2 inches) have been shown to be beneficial in reducing the effects of diffraction.5,6 In addition to the effects of the external enclosure shape, the internal shape of the enclosure can give rise to internal standing waves, which have also been shown to cause amplitude variations in the loudspeaker transducer response.7 Enclosures with non-parallel internal walls theoretically help to distribute the standing waves.

8. Optimize imaging.The placement and aim of the tweeter and midrange transducers would be determined by critical listening evaluations designed to arrive at the best balance between soundstage width, center focus, minimization of early reflections, and tonal balance. The ideally sloped windshield and headliner in the cockpit of the Dodge (Mercedes-built) Sprinter essentially precluded the existence of detrimental reflections from above the listening position. In addition, the high placement of the monitors above and at the front edge of the dash, along with other geometrical and physical parameters, essentially precluded the existence of detrimental reflections from below the listening position. The virtual elimination of these detrimental vertical reflections was expected to substantially improve the stereo imaging.

Figure 6 shows the geometry of the cockpit relative to the listening position. It's important to note that this geometry was measured and documented only after hundreds of hours of critical listening to establish optimal imaging. Lines of direct and reflected sound were mathematically determined and illustrated in Figure 6. The nomenclature for the reflections is as follows. The first letter indicates the source of the reflection, either the left or the right channel, indicted by an L or an R, respectively. The second letter indicates the side of reflection, L for the left side and R for the right side. The data in Table 3 characterizes the lateral reflections illustrated in Figure 6.