In the previous segment, we looked at the basic process of using a high-resolution FFT (Fast Fourier Transform) analyzer to view the frequency and phase response of a 12-inch cone driver in a typical 12-inch/2-way loudspeaker.
In that segment, we established that the 30-degree off-axis response of the cone driver is substantially lower in level (12 to 18 dB), as well as highly irregular in phase and frequency above approximately 2 kHz, when compared to the driver’s on-axis response (Figure 1).
This information allows us make an educated guess at the range where the cone driver should be crossed over.
In this particular case, the 30-degree off-axis response is linear up until about 1.28 kHz, after which the output until about 2 kHz. At 2.1 kHz, the output level begins to descend rapidly as the driver enters its breakup mode (see sidebar for discussion of “breakup mode”).
Therefore, the optimal crossover could be as low as 300 to 500 Hz (for loudspeakers that employ a mid-range driver) to as high as perhaps 1.3 kHz, while still maintaining a 60-degree angle of vertical dispersion.
However, if the 12-inch cone is to be mated with a 90-degree (or wider) HF horn and driver, approximately 1 kHz should be the upper limit, as the off-axis response at 45 degrees will be much worse than at 30 degrees. Further, if the cone driver were to be 15-inch in diameter rather than 12-inch, as is common in many 2-way loudspeakers, its off-axis response will become irregular at even lower frequencies than the 12-inch cone driver, due to the larger diameter of the cone.
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