Design and Fabrication of Front Monitors

The radiation pattern of a midrange or woofer transducer also affects the choice of upper crossover limit. At the crossover frequency between the midrange and the tweeter, the tweeter's horizontal polar dispersion is wide, relatively speaking, and that of the midrange is beginning to narrow. This can lead to non-uniformities in the horizontal polar dispersion at certain frequencies in the transition region from the midrange to the tweeter. Measurements have confirmed that irregularities in this transitional region can adversely affect stereo imaging. Figure 2 shows how the off-axis output of a transducer diminishes relative to the on-axis output. Table 2 provides recommended upper crossover frequencies for woofers and midranges of various diameters. Obviously, the criterion associated with -3dB attenuation at 45 off-axis is more stringent and may lead to better integration between the midrange and the tweeter at the crossover frequency.

Last month, part five introduced a few important concepts in materials science and mechanical design, and their correlation to improved audio system design and implementation. It also showed the steps of fabrication for the overhead electronics console. This article will focus on the design considerations for, and fabrication process of, the front monitors.

Table 1. Wavelengths of selected frequencies.

Design Considerations
Before discussing the choices that were made in designing and building the front monitors, it's important to consider certain principles of loudspeaker transducer performance and mid- and high-frequency loudspeaker enclosure design considerations.

When designing an automobile audio system, it's important to know the relationship between the frequency and wavelength of sound. How sound behaves in an automotive environment is largely influenced by its wavelength. For example, mechanisms affecting sound such as absorption, diffusion, and reflection of sound are dependent upon its wavelength. For instance, sound is reflected from objects that are large relative to the wavelength of the impinging sound. Sound behaves like "waves" below about 300 to 400Hz, and like "rays" above 300 to 400Hz. Table 1 shows the relationship between frequency and wavelength.

The radiation pattern of sound produced by a loudspeaker transducer narrows as the frequency increases. Figure 1 shows the -6dB off-axis points for loudspeakers of various diameters at various frequencies. Of particular importance is the line corresponding to the "1-inch speaker," which is representative of a tweeter's dispersion. At 10kHz, the sound from a typical tweeter is already diminished 6dB at a point about 80 off-axis. The radiation pattern continues to narrow as the frequency increases until the output is diminished by 6dB only 40 off-axis at 20kHz. Clearly, this rapid narrowing of dispersion with increasing frequency must be taken into consideration when locating and aiming the tweeter in order to maintain adequate frequency response.(See Figure 1)