Copyright 2009 © Troels Gravesen
How do we choose optimal points of crossover for 2- and 3-way constructions?
The SP38 is part of a journey
into investigating the various ways of constructing a
full range speaker covering a frequency range from
40-20,000 Hz. Above is depicted four different ways of
achieving this goal.
This classic 3-way is going to
part of my learning programme. First the TJL3W
introducing the W11/W12 between the SEAS W18E001 and the
ribbon tweeter and I might try the 18W/8531G00 and the
ScanSpeak 13M/8640 or the 12M/4531G00, the latter if I
can borrow a pair. Ive got a pair of mint
condition, second hand 13M/8640s, but the price of the
new 12M is terrible.
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 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. 2.5 kHz or a suitable 5-6" going up to e.g. 2.5 kHz. Possibly the explanation is that by adding a small upper midrange we may have a driver with significant reduced distortion in this critical area compared to two drivers that work at the very limit of their optimal operational range.
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.
Example 3 is the
SP38/13 approach and to my knowledge not much
used, if used at all. And its 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
Above the 9700 FR and yes, down to 800 Hz, but does it sound good? Ive tried it many times; a 6 woofer (8545) and the 9500/9700 down to 1,800-2,000 Hz and I dont like it. I dont like 1" tweeters fiddling with upper midrange.
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
Example 4 is the
all-time classic. A 6-7 midbass married to
a 1 dome, usually with a point of crossover around
As can be seen were
moving in octaves here: 400-800-1600-3200-12800. Well, I
have never seen a point of crossover at 6,400 Hz 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
Ill leave out this option.
The planned TJL3W will be the first time* I make a 3-way with a p.o.c. at around 800 Hz. The initial modelling doesn't look too bad. Below simulated response by measured response from 1 x W18E001 + W11CY001 + TJL ribbon. Below is seen the response with inverted polarity of middriver.
*: 26-11-2005: Events took over here and
before I'll get around to the TJL3W, I have done two
systems using small upper midrange drivers. The first one
reported here: Zahra. And read below
The simulation of the crossover took some evenings and resulted in a fairly simple crossover providing a good phase tracking between drivers as seen below. This was fortunately confirmed when I came to the actual set-up of the crossover. This is not always the case.
The speaker's SPL response has an overall tilt of approx. 3 dB from 100 Hz to 10 kHz. Close to the BBC-dip target giving the best balance of sound - from this speaker. It doesn't work for all, but here it does. Raising the tweeter level by only 1 dB make this driver clearly too high.
This speaker will
hopefully be reported in detail later but here at the
final stage of fine-tuning the benefits of having a
mid-driver handling the most difficult range where the
ear has its highest sensitivity is very clear. It's
probably the speaker with the highest level of
transparency I have made. It can be played loud and is
very revealing to the programme material, but fortunately
in a non-aggressive way. The small kevlar mid-driver with
the fabric dome is certainly a fine driver deserves a
much wider use than seen. Actually I don't know of any
commercial speaker using this excellent mid-driver.
27-11-2005: Now available: Zahra.