Let’s consider the perfect speaker. It would contain just one full range driver. This driver would work down to the lowest frequency (20Hz) at full output level, and respond up to the highest frequencies (20kHz or beyond). It would have a constant directivity pattern and high efficiency to minimize amplifier requirements.
Not only does this driver not exist, it is not even possible given the laws of physics! To reproduce low frequencies we must move a large volume of air, albeit relatively slowly. This demands a large diameter driver which by necessity is quite heavy.
For high frequencies we only need to move a small volume of air, but we must do so very quickly.
These two requirements are mutually opposed. In addition we are not going to e=achieve a constant directivity response.
For these reasons we are forced to consider a two way system. One driver dedicated to high frequencies and one to low, with a xover network to direct the appropriate frequencies to the appropriate driver. This works well enough fora lot of applications, and is certainly a cost effective solution for the most part. However, it does have it’s limitations.
Firstly, the woofer must ideally have a good clean response up to a couple of octaves above the xover frequency. Likewise the tweeter must have clean response to a couple of octaves below the xover frequency. These requirements are not easy to achieve.
If we now consider a three way system, we have increased complexity, but with attendant benefits. Each drive, (bass, midrange, tweeter) now is only required to operate of a narrow frequency range. Their design can be better optimized for his range. The overall directivity response can be better engineered.
The cost in this approach is the need for a complex three way xover network. Often these are poorly designed, leading to less than perfect blending together of the drivers, and an audible discontinuity in the response. ELAC engineers however have decades of experience in computer aided xover network design, and contributed to some of the earliest research in this field. As a result we are able to engineer three way system with extraordinary driver integration.