How do we choose optimal points of crossover for 2- and 3-way constructions?
There are many ways of combining drivers with regard to
point of crossover and there is no definitive way of doing so. It's all
about compromises, and for many people cost is an issue too. Generally I
find a small 4" driver filling the gap between bass and tweeter
beneficial, but it comes at extra cost and increased complexity of
One reader has pointed out that taking the point of crossover between mid and tweeter up to 4 kHz does not remove this "problem" out of the range critical to the ear's highest sensitivity. 4 kHz is actually right at the peak of the ear's sensitivity. True! I have no definitive explanation for the subjective benefits of adding a small upper midrange handling the 800-4000 Hz range compared to a 1" dome going down to e.g. 2 kHz other than the change in dispersion from a 6" to a 1" dome is significant and a small driver in-between helps smoothing this transition in dispersion.
Spendor, Peak Consult and Verity Audio have no trouble (?) taking a suitable midrange driver up to 3800 Hz - and with apparent success. And I'm sure that the choices they have made are very well considered. They simply want to have one single driver handling the entire midrange and lower treble to get rid of this problem. However, taking a 5" AudioTechnology driver up to 3800 Hz does have an impact on the overall voicing of a speaker. I've tried taking my AudioTechnology 4H52 up to 4500 Hz crossing over to a SS 9500 treble unit (4th order slopes) and I wasn't too thrilled. The dispersion from a 100 cm^2 cone at 4 kHz is not the best, but possibly a 2nd order filter will make a suitable blend of sounds to enhance the midrange driver's limited ability to perform well at such high frequencies.Comments are welcome.
is the SP38/13 approach and to my knowledge not much used, if
used at all. And it’s a pity, because few 6-7” drivers handle well above
1500 Hz. Well, some 6-7” kevlar and polypropylene drivers have a
tolerable presentation above 2 kHz with a smooth roll-off with no
disturbing peaks to spoil the treble quality. But large cones start
beaming when having to handle higher frequencies and very few drivers
get away with it successfully – or we may have to use low order filters
to make a blend of sounds from the midbass and treble thus overcoming
some of the shortcomings of the midbass driver.
What I have left out in this presentation is the true
middome*, although the 3” 12M/4531G00 actually is a kind of middome, a
large voice coil carrying a large paper dome and a narrow cone
connecting the dome to the surround. Fortunately this driver uses a very
small magnet (neodymium?), has a vented pole piece and should have
proper rear ventilation. Not an easy thing to make when drivers get this
Example 4 is the all-time classic. A 6” midbass and a 1” dome, usually with a point of crossover around 2,000-3,000 Hz. This is obviously the most cost-effective way of getting it all from two drivers. Simple crossover – if the drivers are performing well – simple cabinetry, no need for middriver housing, etc.
As can be seen we’re moving in octaves here:
400-800-1600-3200-12800. Well, I have never seen a point of crossover at
6,400 Hz (except for horn loaded compression drivers) as few tweeters
have trouble handling 3 octaves from 3 kHz to the limit of audibility. A
good upper-middome like the D52 may be able to handle a p.o.c. around
5.6 kHz, but I’ll leave out this option here.
You may have noticed that none of my bigger
speakers use any small 3-4" upper-mid driver between the 6" midrange and
1" dome. Neither does commercial speakers, at least the vast majority.
Chario is one
manufacturer that actually adds a 4" driver ("130 mm") in between
midrange and tweeter in their larger speakers.
The examples above are quite different. The DTQWT is basically a 2-way with subwoofers and features a large 8" driver for upper bass and midrange and a large 1½" dome. Due to the wave-guided tweeter it features and seamless integration of upper-mid/treble and holds the indisputable benefit of not having a crossover (series capacitors) to the midrange. The DTQWT has a midrange coherence even the ATS4 cannot deliver.
The ATS4 is having the bass punch of a closed cabinet speaker combined with the low-end extension of a vented construction. And this from a moderate sized speaker. Point of crossover between bass and mid is just below 200 Hz. If the mid-driver had to be taken lower the price to pay would be decreased overall sensitivity as the 6" driver on such a small baffle suffers baffle step loss.
The Jenzen Illuminator is the classic 10+6+1 set-up and points of crossover are again 200 and 2000 Hz. One problem with this kind of speaker is cost of the midrange series capacitors. We need a lot of uF and they better be good, thus 500 EUR just for these two banks of 88 uF capacitance.
The OBL-11 is special and I don't know of any commercial speaker based on these design criteria as WAF is an all-time low. Points of crossover are still 200 and 2000 Hz and with it's 15" bass driver and midrange high-pass filter it can blow your brains out.
I would like to take the point of crossover to the middriver lower, like 120-140 Hz, but the trade-offs are loss of power handling and system sensitivity. There is an awful lot of energy in the 100-200 Hz range and if I had to go down to 120 Hz I would use two 6" drivers like the Wilson Audio Alexandria XLF speaker.