Several methods exist to reduce unwanted enclosure noise and vibrations. Untreated MDF is known to have relatively poor damping characteristics. Constrained layer materials have been shown to exhibit superior damping performance relative to extensional (externally-applied) damping materials.8 Isolating the vibrations of a loudspeaker transducer's frame from the enclosure has also been shown to reduce enclosure noise. Finally, methods of decoupling, or isolating, the loudspeaker from the floor has been shown to reduce interfering floor vibrations.9
Based on the aforementioned science and art of loudspeaker design, I set out to design and build front monitors with the following goals:
1.mount the tweeters above the dash and aim them predominately on-axis to the listening position.
In part three, the resonant frequency of the MD130 tweeter was reported to be 850Hz. Common practice is to use a highpass crossover frequency approximately one to two octaves above the resonant frequency. The lowest reasonable highpass crossover frequency was expected to be between 1,700 and 2,550Hz. Because the tweeter was expected to potentially operate between the frequencies of 1,700Hz and 20kHz, any objects in the path between the tweeter and the listener between 0.203m (6.69 inches) and 0.017m (0.68 inch) in size would interfere with the output of the tweeter. Combined with the fact that tweeters nominally 1-inch in diameter have significantly narrowed dispersion as the frequency increases, it seemed logical to mount the tweeter above the dash aimed nearly on-axis. Conversely, if the tweeter were aimed substantially off-axis, not only would it suffer from reduced output, but also from "comb filtering" as a result of early reflections off surfaces, such as the dash, windshield, center console, or A-pillar.
2. Mount the midranges above the dash and aim them identically at the tweeters.
In part three, it was determined that the resonant frequency of the MW150 loudspeaker transducer in an appropriate sealed enclosure was about 95Hz. Common practice is to use a highpass crossover frequency approximately one to two octaves above the resonant frequency. Therefore, each MW150 would likely be tuned to play from 200Hz to as high as 2,687Hz (from Table 2). The wavelength of sound corresponding to these frequencies is 1.73m (68.11 inches) and 0.128m (5.06 inches), respectively. If the midrange transducers were located above the dash, objects between the path of loudspeaker transducer and the listener, capable of causing reflection, absorption, or diffusion, would be avoided.
An additional benefit of locating the tweeters and midranges above the dash was that no objects were between the loudspeakers to degrade the stereo image. Conversely, if the midrange loudspeaker transducers were mounted in the kick panels, a practice commonly believed to provide optimum results based on minimal pathlength differences between left and right channels, they would be subject to significant reflection, absorption, and diffusion by the listener's body before the sound was able to reach the listener's ears. Placing the midrange transducers in the kick panels also violates the desirable goal of placing the midrange as close as possible to the tweeter to minimize lobing. If the tweeter were placed close to the midrange transducer in the kick panel, my first goal would be violated. Combined with my experience that sonic sources playing frequencies greater than 100Hz are localizable, I believed the kick panels weren't optimal locations for the midrange loudspeaker transducers, and especially not the tweeters.