Two and a half way layout

Loudspeaker sets (loudspeakers) have long been based on the principle of combining loudspeakers specialized in processing different parts of the acoustic spectrum. Hence the essential meaning of the very concept of "loudspeaker", i.e. groups of (different) loudspeakers (converters) that complement each other and cover the widest possible bandwidth, with low distortion.

Leaving low-budget or exotic single-way speakers aside, the simplest speaker is bilateral command. Known for many small rack-mount designs as well as more modest freestanding loudspeakers, it typically includes a 12 to 20 cm midrange driver covering a bandwidth of up to about 2-5 kHz, and a tweeter dealing with a range above that. the limit determined by the intersection of the characteristics (the so-called crossover frequency). Its definition takes into account the "natural" features and capabilities of individual speakers, but in the end is most often the result of the so-called electric crossover, i.e. a set of filters - low-pass for the midwoofer and high-pass for the tweeter.

Such a system, in the basic version, with one mid-woofer and one tweeter, using modern solutions, allows you to achieve even more power and good bass extension. However, its end is determined by the conditions imposed on the low-frequency speaker. The size of this speaker should not exceed the limit for correct processing of mid frequencies (the larger the speaker, the better it processes the bass, and the worse it handles the mid frequencies).

Looking for another layout

The classic way out of this limitation tripartite arrangementwhich allows you to freely increase the diameter of the woofer, because the midrange is transferred to another specialist - the midrange speaker.

However, there is another solution that can significantly expand the boundaries of the competence of the bilateral system, primarily to increase capacity and efficiency. This is the use of two midwoofers (which, of course, requires a correspondingly higher volume, so they are found in free-standing speakers). The triple mid-woofer design is no longer used, due to too much adverse phase shifts that would occur between the furthest drivers, outside the main axis of the assembly. A system with two midwoofers (and one tweeter), although containing a total of three drivers, is still called a two-way system because the band is divided into two parts by filters; it is the filtering method, not the number of speakers, that determines "clearness".

Understand way two and a half

The last statement is critical to understanding how it works and how to define it. double leaf system. The best starting point is the already described two-way system with two mid-woofers. Now it is enough to introduce only one modification - to differentiate low-pass filtering for midwoofers, i.e. filter one lower, in the range of a few hundred hertz (similar to the woofer in a three-way system), and the others higher (similar to the low-mid range in a two-way system).

Since we have different filters and their operating ranges, why not call such a three-band scheme?

Not even because the speakers themselves can be (and most often are, but far from always) identical. First of all, because they work together in a wide range of low frequencies, which is not inherent in a three-way system. In a two-and-a-half system, the bandwidth is divided not into three bands handled "only" by three converters, but into "two and a half bands." The independent "path" is the path of the tweeter, while the rest of the mid-woofer is driven partly (bass) by both speakers and partly (mid) by just one speaker.

Among the five free-standing speakers from the test in the magazine "Audio" in a group that well represents the price range of PLN 2500-3000, she found

there is only one three-way construction (second from the right). The rest are two-and-a-half (first and second from the left) and two-way, although the configuration of the speakers on the outside is no different from the two-and-a-half. The difference that determines the "patency" lies in the crossover and the method of filtering.

Such a system has the "efficiency" features of a two-way, two-midwoofer system, with the added benefit (at least in the opinion of most designers) of limiting midrange processing to a single driver. avoids the aforementioned problem of phase shifts. It's true that with two mids close together, they don't have to be big yet, which is why some people settle for a simpler two-way system, even using two mids.

It is worth noting that both a two-and-a-half and a two-way system, on two midwoofers with a diameter (total), for example, 18 cm (the most common solution), has the same membrane area in the low frequency range as one speaker with a diameter of 25 cm (a three-way system based on such a speaker) . Of course, the diaphragm surface is not enough, large drivers are usually capable of more amplitude than small ones, which further enhances their low-frequency capabilities (where exactly is the volume of air that the speaker can "pump" in one cycle, counts). Ultimately, however, two modern 18-inch speakers can do so much while still allowing for a thin cabinet design that such a solution is now breaking popularity records and ousting three-way designs from the mid-sized speaker segment.

How to recognize layouts

It is impossible to distinguish between a two-way system that used the same types of drivers as woofers and midrange drivers, and a two-way system with a pair of midrange-woofers. Sometimes, however, it is clear that we are dealing with a two-way system - when the differences between the two speakers are visible from the outside, even though they have the same diameter. A loudspeaker acting as a woofer may have a larger dust cap (strengthening the center of the diaphragm). The loudspeaker works as a midwoofer and - a lighter diaphragm, etc. a phase corrector that improves the processing of medium frequencies (with such a differentiation of structures, it would be a mistake to use a common filtering and a two-way scheme). It also happens, although quite rarely, that the woofer is slightly larger than the midwoofer (for example, the woofer is 18 cm, the midwoofer is 15 cm). In this case, the system begins to look like a three-way design from the outside, and only an analysis of the operation of crossovers (filters) allows us to determine what we are dealing with.

Finally, there are systems whose “patency” difficult to clearly definedespite knowing all the features of the structure. An example is a loudspeaker, which is initially considered a woofer-midrange speaker due to the lack of a high-pass filter, but it is not only smaller, but also processes low frequencies much worse than the accompanying woofer, as due to its "predispositions" , as well as to the method of application in the home - for example, in a small closed chamber.

And is it possible to consider a three-way scheme in which the midwoofer is not filtered by high frequencies, but its characteristics intersect, even at a low crossover frequency, with the characteristics of the woofer? Isn't that two and a half more ways? These are academic considerations. The main thing is that we know what the topology of the system and its characteristics are, and that the system is somehow well tuned.