That said, most attempts to “gently” sum the LF and HF at the crossover point through the use of asymmetrical slopes, asymmetrical rates, or even asymmetrical crossover points, are rarely successful.
You cannot make a 12- or 15-inch cone driver behave like a 1-inch throat (or 1.4- or 2-inch throat) compression driver and horn combination.
This is often a significant source of confusion for many who work hard to optimize their systems.
The best course of action, in nearly all cases, is to ‘get out’ of the crossover region as rapidly as possible (in respect to frequency), by using high-order crossover slopes. At crossover, a lobe – however small – is generated by the differing characteristics (size, shape, mass, and radiation pattern) of the LF and HF drivers.
This lobe is the result of predictable acoustical cancellations and acoustical summations. The sums and differences will constantly change as the frequency and angle of incidence (the angle to the listener, or the measurement mic) are altered.
When using an FFT to measure a loudspeaker on-axis where the woofer and tweeter meet – and then slowly sweep the microphone up and down (usually the tweeter is atop the woofer in most loudspeakers) – you will see how the frequency and phase response vary, usually greatly, in relation to the position of the mic.
You’ll also see that the frequency and phase response at a given angle above the loudspeaker will differ from the frequency and phase response at the same angle below the loudspeaker.
Certain Exceptions
There are certain special cases in which asymmetrical crossovers can be used to advantage. These exceptions are typically found in applications such as mastering labs or studios where only one tightly defined location is occupied by the listener.
In these types of situations, it may be possible to improve the response through the crossover region for that one location by using asymmetrical crossovers.
However, the effect of irregular frequency and phase energy from above and below the loudspeaker’s sweet spot, bouncing off of room surfaces and ultimately reaching the listener by reflection, should be carefully taken into consideration.
While some of these measurable variations may be caused by the loudspeaker enclosure’s geometry, and/or the HF horn geometry, the majority of the variations are the result of the dance of interference-and-summation of the HF and LF sources.
They will never be perfectly matched in arrival time – even when incremental delay is adjusted carefully – because their physical differences, location in the enclosure, size and mass all dictate that their acceleration and de-acceleration periods cannot be the same through the crossover region. Although the two sources may have radiation patterns that are quite close on paper, they will never be identical in the real world.