Let Me Count the Ways....
What’s the difference between a 2-way and a 2.5-way or a 2-way and a 3-way? This is a question that I’ve seen come up with a lot speaker buyers online, so this article will cover what the difference is and what the benefits of each are.
Let’s start with some basics. An ideal speaker would be one that has a single driver that can play flat from 20 Hz to greater than 20 kHz and have all the output you would ever need. Unfortunately, physics dictates that this is impossible to achieve. To play low with any authority, you need to move a lot of air, which means you need a big cone. But a big cone has problems playing high frequencies. Even ones that have a fairly extended frequency response suffer from fast off axis roll off, which can make them sound dark if used full range. Conversely, the drivers that play high frequencies well do a terrible job at outputting bass. Now, you will find a lot of single driver systems on the market, but in our opinion, they are full of compromises for anything other than background or desktop use.
So this is where a crossover comes in to play. A crossover matches two or more drivers that have their own ideal performance bandwidth together to play as one. This is obviously easier said than done, and it takes a lot of knowledge and experience to seamlessly integrate drivers with a crossover. However, when done correctly, you can end up with amazing results.
When someone talks about the number of “ways” a speaker is, they are referring to the crossover design and driver configuration. A 2-way is a combination of drivers, for instance a single woofer and single tweeter, which splits the frequencies going to each in two. Thus, you have some of the frequency bandwidth going to one of the drivers and the rest going to the other. Now, part that can get confusing about this is that you can have multiple woofers and/or tweeters (although we don’t recommend multiple tweeters in most instances) and still only have a 2-way loudspeaker. The number of ways is not necessarily based on the number of drivers, although to have a 3-way speaker, you would need at least 3 drivers. For instance, I might design an MTM style speaker that has two woofers and one tweeter and it is still a 2-way. The key point here is that the number of “ways” is largely a function of the crossover design.
So what about .5-way configurations? What is half of a way? A .5-way (as in 2.5-way or 3.5-way) is a crossover topology which has a second lower end driver covering less than the full bandwidth of the first. To explain it better, let’s use the example of a 2.5-way speaker with one tweeter and two woofers. The tweeter is crossed over as normal. Then there is a crossover for the woofer section. This section causes the first woofer to play as if it were a normal 2-way. Then there is an extra inductor added in series with the second woofer. This causes the second woofer to “see” all of the crossover parts that the first woofer does, but also an additional inductor, which causes it to roll off faster on the top end than the first woofer. So you’ll basically end up with something that looks like below. The black line is the system response, red is the tweeter, blue is the first woofer, and magenta is the second woofer.
Now this also begs the question of why you would do this, and this is where I’ll discuss the merits of the different designs. Let’s start with a 2-way. As I mentioned, a single driver has a hard time playing the full frequency spectrum of music without some serious compromises somewhere. Add a second driver helps alleviate that, but does not necessarily make us an ideal speaker. Because of their size, most tweeters are limited to outputting frequencies only above 1.5-2.5 kHz or so (even higher for cheap or poorly designed tweeters. This means our woofer needs to play up a little bit higher than that to be able to integrate the two drivers.
Now you have a trade-off. Use a woofer with an extended response and an off-axis that can match well with the directivity of the tweeter at the crossover point, which would typically mean less than 7” in size, or use a larger woofer that doesn’t match as well. The smaller woofer doesn’t have as much output capabilities as the larger because it can’t displace as much air. In addition, given that the woofer is being asked to play into the midrange, large excursion from higher output levels can cause Doppler and IM distortion that becomes audible. If you have a smaller room or don’t listen at concert levels, a 2-way might be plenty of output. However, you might decide that neither of these options is a great tradeoff.
That leaves you with a couple of options. The first is you could add more of the same woofers. This can be done as a 2-way or a 2.5-way. Adding more woofers increases the surface area and decreases the amount of excursion required from each woofer to hit the same volume level. This does not necessarily mean that it makes the speaker more sensitive. And I definitely want to stress that adding more woofers does not lead to deeper bass. This is a common misconception I’ve seen people use when deciding on what speaker to go with.
The decision on whether to go with a 2-way or 2.5-way comes down to the overall design of the speaker. If the distance between the second woofer and tweeter is very large, like having the second woofer mounted close to the floor, or the crossover point is high, you’d want to choose a 2.5-way. This helps eliminate some of the interference from the second woofer that can lead to comb filtering. Most of the time, if I’m only using two woofers, the crossover point is low enough that a 2.5 way is not necessary and often makes the speaker worse by adding a bass boost in the low frequencies while at the same time not adding enough to the midrange. Again, this can occur if you use low crossover points. I just make sure my two woofers are close in either a TMM or MTM format.
Going this route takes some stress off the woofers, but they are still playing up into the midrange and handling bass duties at the same time. This can still lead to the same issues discussed earlier at high volumes. This leads us to the second option, and that is going with more “ways.” A 3-way or 4-way adds more drivers to break up the frequency range into smaller parts between each one. This lets each driver excel at what it does and lets you use a smaller midrange since you aren’t trying to get full range bass out of it.
When stepping up to a 3-way, you get one woofer handling bass frequencies, a midrange in the middle, and a tweeter for the highs. This can allow you to take stress of the midrange (which would have been the woofer in a 2-way) and tweeter by raising their crossover points and reducing excursion, and reducing the excursion leads to lower distortion. And you can select a much bigger woofer now since you don’t have to match to a 1” tweeter. This makes it easier to get deeper bass with more output capabilities.
It’s not all roses though, as there are still tradeoffs when going to a 3-way or larger design. The first is the size. 3-ways are most likely going to be bigger because you are wanting to use a bigger woofer to get more output capability. The second is cost. Not only are you adding an additional driver, but you are adding a lot of additional crossover components and because they are used to cross the drivers at a much lower frequency response than in a 2-way, they are much larger values and therefore more expensive. The third is complexity. Both the cabinets and crossovers are more complex. Cabinets need an internal chamber to seal off the midrange. Crossovers will likely be double the parts count of a 2-way, and if you know anyone who has designed them before, they will tell you they are much more difficult to design and get right.
Now the all-important question of which one is right for you. Well, like all things audio, everything is a set of compromises. If you have a big room and like to crank it loud, you probably want a bigger speaker. A 3-way would probably be the way to go. However, if your room is small, a 3-way might be too big. Or if you don’t listen very loud, it might be much cheaper to get a nice sounding two way than 3-way using drivers with similar performance. In the end, as I always say, do what makes you happy. If you have a small room but won’t be happy without a monster sized speaker, go for it. The enjoyment you get is more important than someone on the internet telling you what the “correct” way to do things is.