Design and Fabrication of Front Monitors

Table 2. Recommended upper limit for lowpass crossover frequency.2

Although it's theoretically desirable to have a loudspeaker radiate all frequencies from a single point, the vast majority of loudspeaker systems, especially those in automobile audio systems, rely upon individual, non-coincident loudspeaker transducers each radiating different frequencies. As a consequence, each loudspeaker transducer is separated both horizontally and vertically (see Figure 3). Vertical separation gives rise to a phenomenon called "lobing," the consequence of inter-transducer interference patterns, which result in a severely non-uniform vertical polar response. The extent of lobing worsens with greater vertical separation, as shown in Figure 4. The obvious solution is to minimize the vertical separation of the loudspeaker transducers as much as possible, or use a high-quality coaxial loudspeaker transducer.

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.