Rather than telling the subwoofer how to perform by using controls for crossover frequency, level and phase, the THIEL subwoofer crossover unit has settings for the characteristics of the main speakers being matched, the configuration of the system and the performance desired by the user. This information is then used to calculate and provide the ideal subwoofer response and high-pass filter response that will give very accurate results with the user's main speaker.

There are two component techniques that are used to achieve these results.

1. Determining what LP filter characteristic for the subwoofer will perfectly match the HP characteristic of the main speaker, either HP-filtered in crossover mode, or not filtered in augment mode.
2. If a HP filter is supplied for the main speaker signal, determining what filter will, when combined with the response of the main speaker, provide a total response that perfectly matches the desired 4th-order, Q = 0.5 response.

Once the crossover unit knows the characteristics of the main speakers, it calculates and implements crossover rolloff shape, slope, phase and HP as needed to achieve perfect results. Not only does the crossover calculate and implement the optimum setting for the subwoofer cut point, slope and phase, but these characteristics are not limited to those of standard filter shapes and instead can be whatever filter shapes will give the desired results.

As an example of the first technique of generating subwoofer response that will blend correctly, Fig. 8 illustrates the subwoofer augmenting a 50-Hz reflex speaker, extending the bass response to 30 Hz. As is true in every case, the results in this case are essentially perfect. These excellent results are achieved because, in addition to the crossover automatically calculating the optimum phase response, the response shape of the subwoofer is exactly that shape needed to perfectly integrate.

Fig. 8. 50-Hz reflex main speaker response; SmartSub woofer response; net combined system response. Dotted is generic filter response.

For comparison, the dotted graph shows the response that a generic filter would provide. You can notice the somewhat lower output level around 30 Hz and a somewhat higher and more gentle rolloff slope. This "custom" slope is automatically generated and is exactly what is needed to match this main speaker.

Another example illustrates this "custom" slope ability of the crossover unit. If the sub is augmenting a sealed 60-Hz speaker, the crossover again provides the output that will produce exactly the desired combined output (Fig. 9). Again, the dotted line shows what a generic filter would provide and, in this case, the difference is large. The proper curve provides more output below 25 Hz and less above, with a slightly more gentle slope.

Fig. 9. 60-Hz sealed main speaker response; SmartSub woofer response; net combined system response. Dotted is generic woofer filter response.

The following example illustrates the second technique of providing a special HP response that will combine with the speaker's response so that the total response is correct. If an 80-Hz crossover is desired with a 60-Hz sealed main speaker, the HP supplied by the SmartSub crossover is an unusual shape that resembles an 80-Hz 4th-order above 60 Hz, and a 2nd order below (Fig. 10).

Fig. 10. 60-Hz sealed main speaker response; SmartSub crossover response supplied; net 80-Hz 4th-order Q 0.5 response.

If the same crossover is desired with a 60-Hz reflex speaker, you can see in Fig. 11 that the supplied HP shelves below 60 Hz so that, again, the response of the filter combined with that of the speaker will have the desired 80-Hz, 4th-order characteristics.

Fig. 11. 60-Hz reflex main speaker response; SmartSub crossover response supplied; net 80-Hz 4th-order Q 0.5 response.

About the author
Jim Thiel is a co-founder, co-owner, and product design engineer for THIEL. His interest in music began on the piano in early childhood and remains an important part of his life today. His involvement in audio can be traced to age 12 when he began to build and repair radios and other electronic gear. His academic background is in physics and mathematics. Jim pioneered the principle of time and phase accuracy in loudspeakers with the use of sloped baffles, coaxial driver mounting, and phase coherent crossover network design.

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