Curved and Legendre-shaded loudspeaker lines use the floor reflection to achieve uniform broadband coverage.

Evidence

In Figure 12.7(f) we see that shading the curved line using the Legendre contour yields a set of plots that have two desirable characteristics: a stable sound level at ear level out to 3 m or more from the loudspeaker, and a relatively constant directivity, meaning that it applies over a wide frequency range. This is a good result, and it has been called the Constant Beamwidth Transducer (CBT). This is not merely coping with the floor as an adjacent boundary, a low-frequency concern; it is using the floor reflection to create a sound field that has desirably uniform broadband sound level over a wide range of listening distances. … Curved loudspeakers may not be to everyone’s liking, so applying the appropriate delays to the individual drivers can, in effect, acoustically contour a physically straight line. When Legendre shading is applied to the delay-curved line the resulting CBT is very similar, as seen in Figure 12.7(g). — Toole, Floyd E, Sound reproduction- the acoustics and…, 1292

Categorization

Category: Definitional Content

• DEFINE: Defines the Constant Beamwidth Transducer concept and how Legendre shading plus curvature creates it.
• XREF: Connects directly to JBL’s free-standing CBT products discussion later in the chapter; also relates to delay-based beamforming approaches in audio.

Related

• Curved, Legendre-shaded line arrays exploit floor reflections to maintain constant beamwidth at all frequencies (similarity 0.92)
• Legendre-tapered spherical transducer caps achieve constant beamwidth above a cut-off frequency (similarity 0.83)
• Constant beamwidth transducers deliver uniform sound levels over large areas (similarity 0.82)
• Constant Beamwidth Transducer loudspeakers produce astonishingly uniform sound level across listening distance. (similarity 0.81)
• Legendre-tapered drive levels give spherical-cap arrays constant beamwidth with low side lobes (similarity 0.80)
• Constant-beamwidth-transducer speaker arrays hold stable loudness across listening distance (similarity 0.80)
• CBT arrays taper drive levels from center to edges for constant beamwidth (similarity 0.80)
• Room boundaries alter loudspeaker output through reflected image sources (similarity 0.78)

Broader Connections

• Room boundaries alter loudspeaker output through reflected image sources #expands (0.75) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2133