Hi Aaron,
That's a good question. I don't know if I can give a short answer to that...
When coming up with a loudspeaker design, the first thing that's decided, is usually the number and type of drivers. This decision is informed from the science of electroacoustic comprise and the art of balancing those compromises. Many people (including me at one point) decide that the best loudspeaker is a full range driver in a sealed cabinet. This is maybe the "purest" loudspeaker architecture. All the sound
actually comes from a single drive unit, and the binding posts are directly connected to the terminals of the driver. However, as many on here would point out, a speaker like that is not very good at many aspects of sound reproduction - Can't play loud, can't play low, can't really play high, can't deliver an even on and off-axis sound. That being said, they've always had a great following from their "point-source-like" qualities.
I believe the LS50 is so popular for similar reasons, albeit with fewer compromises. It can play lower, it has a nice tweeter for HF, and the sound is very spatially consistent. This has only been possible with around 40-45 years (at time of writing) of focussed development. As such, the drive units are very specialised, pushing the 2-way coaxial acoustic concept as far as possible. There are, of course, penalties to pay for this. One facet of the performance of the LS50 that can really benefit from having an extra drive unit is the distortion. The THD is pretty reasonable, but, the IMD suffers. This is the same for all 2-way coaxs, and aside from using a larger coax, such as an 8" or a 10", there's nothing to be done. As the little 5.25" LMF (with a hole in the middle) on the LS50 is told to reproduce bass, they can be overdriven relatively easily, especially with modern high power amplifiers. This, again, would benefit from handing over to a "real" bass driver that's much more resilient.
Once a third driver is added, the design direction opens up. The main decision at this point, IMO, is whether your midrange will use an underhung or overhung voice coil. Quite a specific thing, but here's where that choice could lead.
Overhung:
- More excursion so can be crossed lower (More like a point source + sub)
- Heavier coil (less efficient)
- More motion (more IMD)
- More motor non-linearities (due to more non-linear BL(x))
- More expensive due to the extra copper in the coil
- Higher power handling
- Taller motor (physically bigger, and you have to use larger shorting rings etc.)
OR
Underhung:
- Less excursion so must be crossed higher
- Light coil (efficient)
- Light surround
- Less motion (less IMD)
- Very linear motor (Very consistent BL(x))
- Cheaper (all other things being equal)
- Lower power handling
- Smaller
So if we can use an underhung design, we will, in which case the only outstanding problems/points to consider are:
Higher crossover - solved (mostly) by designing a really good crossover using the absolute best measurements we can get our hands on.
Lower power handling - Usually not a problem on midranges in hifi, due to the pretty high crest factor. (See the LS60W Whitepaper for more details on research we've done in this area). Basically if you were pumping enough music power into a Q Concerto Meta such that the midrange burnt, you'd fry your eardrums.
Back to your original question, taking an LS50 and adding a sub, but crossing it quite high (150-200) would certainly work, and certainly improve the sound. I believe YouTuber Jay's Iyagi did just that. However, when presented with the opportunity to make a 3-way from scratch, the crossover points usually fall about an octave above that due to some of the specifics of what makes a good dedicated midrange driver.
darko.audio
