It employs adjusting the front-to back spacing of the individual drivers so that the sound output is truly simultaneous.
In 1975 Ed Long[1] in cooperation with Ronald J. Wickersham invented the first technique to Time-Align a loudspeaker systems.
This technique relied on subjective evaluation of various square pulses swept through the crossover frequencies.
Long also manufactured near field studio monitors from the late 1970s through the 90's utilizing the Time-Align technique.
Therefore, most hi-fi loudspeakers employ multiple drivers to cover the audio spectrum satisfactorily.
Since the woofer covers the lower-end of the audio spectrum and the tweeter covers the upper-end, the dividing point between the two being the crossover frequency, it is of utmost importance that, at the crossover frequency the outputs of both drivers should acoustically sum so as to be seamless, without any peaks or dips, otherwise the loudspeaker is said to colour the sound.
A typical characteristic of a 2-way speaker is that at the crossover frequency, due to the physical distance between the centres of the woofer and tweeter, the sound that emanates from the combination is not omni-directional, but lobed.
Therefore loudspeaker designers try to make the main lobe as fat as possible, by using drivers that are as small as possible in diameter so as to allow closer spacing between them.
However, the lowest frequency of interest (bass) puts a lower limit to the woofer's diameter.
However, this technique requires that the exact depths of the acoustic centres be known at design time, so that the physical offset may be introduced in the front panel of the speaker where the drivers mount.
[7] Sloping and rounding the edges of the step helps in reducing diffraction, but it cannot be eliminated completely.