Co-Axial Drivers

To reduce the off-axis effects of the addition of sound from spaced drivers, compound units are and have been produced with two or more drivers mounted co-axially. Restricting the examples to the two driver case, Figs 4, 5, 6 and 7 illustrate the effect that the relative positions of the woofer and tweeter have on the system impulse response.

A common arrangement, with the tweeter in front of a mf/lf unit, suffers from the acoustic obstruction formed by the hf unit to sound from the other unit. Because of this degradation in frontal response, this configuration is often used just to save baffle space rather than to reduce the variation of time delay differences in the arrival of sound from different sources off-axis, as for example in in-car applications.

In this case the sound from the hf unit will arrive before the other unit(s) at any listening position in the front hemisphere.

Another, more successful approach, is to place the tweeter behind the mf/lf unit. The sound from the hf unit must then be channelled through the magnet pole piece of the mf/lf unit. The holes or ducting through the pole piece and the cone profile of the mf/lf unit must be designed as an integral acoustic waveguide or horn system in order to achieve the intended response [12,13].

In this case the sound from the hf unit will arrive after the other unit(s) at any listening position in the front hemisphere. This can be readily seen in the impulse response where the high frequency spike in the time record arrives some time later than the more rounded lf part of the impulse.

Both approaches can produce symmetrical off-axis responses but there is still an additional positive or negative time delay to consider in the target filter designs.

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