The effect of the lens is to distribute energy emitted from the driver (which is facing upward) across 180° horizontally and approximately 30° vertically (angled approximately 15° upwards). Other properties of the lens permit the lower crossover point of the driver to be lowered in frequency, extending the effective range of the driver and permitting the relative directivity of the two drivers at the crossover frequency to be evenly matched.

	Figure 2. Horizontal Directivity Plot of conventional tweeter

In this plot, the vertical axis represents horizontal polar angle from 180° (top) to –180° (bottom) while the horizontal axis represents frequency (from 1 kHz. to 20 kHz.). Each contour line represents .5 dB change. This plot clearly reveals the increasing directivity and diminishing power response of a conventional loudspeaker system as the frequency increases. Also note the interference effects in the 2-4 kHz. region. Close examination reveals that by +/-20° off-axis, amplitude varies by as much as 3.5 dB, which is clearly audible.

	Figure 3. Horizontal Directivity Plot of Acoustic Lens

This plot, when compared to Figure 2, reveals the functional effect of the Acoustic Lens. Variance in amplitude over +/- 20° is 1 dB, and at +/-40° is still only 3 dB. Beyond these angular limits, while the amplitude falls off, it does so with comparative uniformity, so that the frequency response does not change greatly, while with a conventional tweeter, the frequency response deteriorates significantly. A more conventional way to show this is involves displaying the frequency response curves at various angles normalized to on-axis response.

	Figure 4. Directivity Curves of a conventional system. (2 dB/increment)

Figure 4 shows the frequency response curves at 10° to 60°, normalized to on-axis, for a conventional high-quality loudspeaker system. This performance is considered excellent.

	Figure 5. Directivity Curves for the BeoLab 5.

Figure 5 shows the same data for a BeoLab 5.

	Figure 6. BeoLab 5 Amplitude and Power Response. (2 dB/incremental line)

Figure 6 shows the on-axis amplitude response and power response of the BeoLab 5.

The net result of this performance is that early lateral reflections from the loudspeaker to the listener have essentially the same frequency response as the direct sound, with constrained high-frequency content from reflections from the floor, ceiling, and wall behind the loudspeakers, in support of the criteria given above.

The full reasoning regarding why such performance is desirable is well beyond the scope of this paper. Suffice it to say that there is a significant and diverse body of evidence to support the use of these criteria. It is also worthy of note that similar behavior is obtained from many other musical instruments.