It's becoming quite common for project ideas to start
on a message board and blossom into a plan. This design
was no different. There has been some discussion over
good driver candidates for three-ways on the
Parts
Express message board and
Mike Keenan
decided to purchase the drivers for this project,
originally as an upgrade/refurbish of some enclosures he had
sitting around, looking for a little attention. Mike also
purchased the Dayton
260-154
three-way "stock" crossover to integrate the
drivers. The stock network uses crossover frequencies of
625hz and 5000hz. At around the same time, I expressed my
interest in designing a custom crossover and enclosure
for this system, since I've been so pleased with the
performance of the
other
projects
I've
done using Dayton Drivers. Also, I felt that this
project would fill another niche in the low-price kit
collection. This time, I estimated that the final "kit"
would cost in the $200-$300 range. The final design
turned out to cost about $250 in parts, using high-quality
crossover components.
Designing the
Enclosure
There are as many theories about designing enclosures
as there are people who design them. It's hard to say
whether one theory is more right than any other, since it
doesn't really matter how you get there, as long as the
enclosure does what you want it to do. My way of going
about it is to measure the woofer's Thiele small
parameters (or in this case, both woofers and averaging
them) and then making that first "big" choice:
ported or sealed.
After breaking-in the woofers with a 2W 25hz sine wave
for about 8 hours and then allowing the drivers to cool,
I measured their T/S parameters. To determine the drivers
equivalent air compliance (Vas) I use the added mass
method (the mass is measured to within 0.1g). The woofers
measured so closely to one another that difference
between the drivers were not considered in the design
process. (Fig. 1)
Once the T/S parameters had been determined I decided
to go with a sealed enclosure with 1.87ft^3
internal volume for this design. One reason I decided to
go sealed instead of ported was that the EBP (Efficiency
Bandwidth Parameter) for this woofer was about 62.
Generally, the higher the EBP, the more suitable a driver
is for a ported enclosure. EBP's less than 50 indicate a
sealed box. In this case, a ported enclosure is not out
of the question, however, but the low group delay and
good transient response of a sealed box (which translate
into crisp, tight bass with excellent clarity and
resolution) were more what I was looking for. In
addition, a 10" woofer can provide excellent bass in
a sealed box, whereas bass-extension with small woofers
generally requires a ported enclosure. Also, since sealed
enclosures have more shallow (12 db/octave) rolloff rates
(vs. 20-24 db/octave for ported enclosures), when you
couple a sealed box with a low Fs (around 49hz in this
case) with a shallow rolloff rate, you end up with
excellent bass extension for just about any musical
application.
The enclosure volume I chose was designed for a sealed
Butterworth alignment (Qtc=0.707) and a box resonant
frequency of about 49hz. One the enclosure was
constructed and the woofer was installed, I ran an
impedance sweep of the woofer to determine how close to
the design objective I had come. The result shown below
uses 100%
Acoustastuf
fill in the box (that's about 1 lb/ft3). (Fig. 2)
As a second level of
verification, I performed a nearfield frequency response
measurement to verify the F3 frequency of the enclosure (the
point where the rolloff had achieved 3 db) and also to
verify that the rolloff rate was near the theoretical
target of 12 db/octave. Again, everything looked good. (Fig. 3)
Designing the
Crossover
Now that the enclosure
design/alignment had been selected and a successful test
box had been built, I was able to install the drivers in
the enclosure and obtain their in-box impedance and
frequency response measurements to use in Calsod, a
loudspeaker crossover design/simulation program. After
trying various crossover topologies and target slopes for
each driver, I decided that impedance correction
circuitry was not warranted for either economic or
performance reasons. Further, since the acoustic offsets
of the drivers was a significant concern, I chose higher-order
crossover slopes and set a 4th-order Linkwitz-Riley
acoustic rolloff for each driver. After considerable
experimentation, I went with the following Calsod
predicted model because 1) it provided the smoothest
overall frequency response with a slight upper-midrange
depression to give the speaker a slightly laid-back
sound; 2) it provided the best overall impedance
magnitude profile and phase angle (meaning it will be
very amplifier friendly); and 3) it provided the highest
projected cost-performance for the number of components
specified.
The next step was to build
the crossover and conduct actual frequency response
measurements. The first step is to verify the accuracy
of my Calsod model and the second is to "tweak"
the design by ear to give it the most pleasing sound. I
was very pleased to see that the Calsod model very
accurately predicted the actual frequency response of the
drivers, individually, and also the overall frequency
response of the system. (Fig. 4)
As a check of the
predicted phase alignment, I also measured the system
after reversing the phase of the midrange driver. The
reverse phase nulls at each crossover frequency are very
good, indicating good phase alignment through the
crossover regions and this translates audibly into
excellent coherence between the drivers in the overall
system. (Fig. 5)
Most importantly, the
final crossover used to achieve this response is not
overly complicated and uses a quasi-3rd order electrical
filter with a 5khz crossover frequency for the tweeter; a
traditional lowpass/highpass cascade filter for the
midrange driver; and a simple second order filter with
impedance correction for the woofer. Figures 6 and 7 show the
recommended Lyra crossover for use with the Dayton silk dome tweeter and
titanium dome tweeters. My personal preference is the silk dome,
but those who like it a bit more forward on the high-end, may prefer the
titanium dome version.
A check of the measured
impedance response shows very good performance that
should achieve a nominal 6-8 ohm rating with a minimum
dip of just over 4 ohms in the bandpass region.
(Fig 8.)
Listening Tests
and Final Thoughts
This has been a fun
project for me to build. In part, I have always like
classic three way designs. Also, good quality three-ways
at a reasonable price are not common. In fact, I'm not
aware of any three-ways out there for around $250/pr (drivers
and crossover components). This design has excellent
sound with clean, natural midrange and detailed, pleasing
highs. The bass is typical of large paper cone woofers in
sealed boxes -- accurate and deep. Overall, I'd
characterize this speaker as being extremely neutral with
just a slight "laid-back" character that makes
it eminently listenable for most types of music and for
long sessions. Although I designed this project for my
local speaker-addict buddy, I think that any of you who
have been wanting an excellent, no compromise full-range
design that doesn't break the bank should give this one
some serious consideration.
Originally posted May 29, 2001
