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Speaker Building
with 1/4" poly batting on the sides. A
"pillow" made from 2" of Roxul and
1/4" poly batting is mounted to the
inside of the back panel.
The cross over transfer functions
are shaped similar to those of the
Neville Thiele Method™ (NTM).
Slopes are 48 dB per octave/8th
order, and the crossover is an
elliptical filter with ripple in the stop
band. The original cross over was
found to have excessive bass below
200 Hz and has been "tweaked" to
improve performance. The HiVi tweeter sounds best with
decent quality capacitors. The 7.5 ohm padding resistor
can be increased to 8 or 8.5 ohms for less sparkle. The
1.0 mH inductor is a steel laminate inductor with a DCR value of 0.15 ohms (or less)
The enclosure is 1/2" MDF with a separate hardwood baffle, while the sides are kerfed
1/2" MDF glued to a frame made from the front, back, top, and bottom. The MDF cabinet is
spray painted black with a satin gloss clear coat, complemented by an oak and mahogany
hardwood baffle. The baffle's grain is filled with natural color filler, which is then stained
with Minwax Early American and top-coated with Deft rattle can semi-gloss lacquer. The
hardwood baffle is attached to the MDF box with two screws behind the woofer frame
sides and two behind the tweeter at the top and bottom. There are also 2 pocket screws
at the bottom under the port.
• Retro-styled 2-way bookshelf system with 48
dB/octave, eighth order crossover slopes
• HiVi RT1.3 Planar Isodynamic Tweeter and
Dayton Audio RS180P-8 7" paper cone woofer
• Vented MDF-constructed enclosure with oak
and mahogany front baffle
This project is a retro bookshelf design using
the HiVi RT1.3 planar tweeter and Dayton Audio
Reference Series RS180P-8 7" woofer. The
Seratim design's purpose was to gain experience
with series crossovers, and experiment with steep
filter slopes to reduce distortion and improve off-
axis performance.
Driver selection was based on using the HiVi
RT1.3 planar tweeter and the RS180P-8 woofer
to get to the 3,000+ Hz crossover point in a stand
mountable cabinet. Driver cost total was about
$185. The HiVi RT1.3 WE (round face plate) is a
direct replacement for the original, but no longer
available, RT1.3.
The enclosure design is 14.5 liters vented, tuned
to about 50 Hz. The slot port has an area of 17
cm2 and is 8.5 cm long with flares on both ends.
1" Roxul lines the cabinet interior, supplemented
• 3-way TMW floor standing system with mass-loaded transmission line woofer
• Usher 8137A 8" Kevlar/carbon fiber woofer, Tang Band W4-1757SB 4" aluminum
honeycomb flat cone midrange, and Beston RT003C circular planar tweeter
• Mahogany veneer quasi-trapezoidal enclosure with carved maple driver
mounting rings
The Exponents were born from a curiosity about how to design a bass-reflex
enclosure for the best possible performance and the least amount of response
ripple. To this end, mathematical simulations of the enclosure design employed
Mass-Loaded Transmission Line (ML-TL) theory instead of traditional lumped-
parameter bass reflex modeling. The goal was to determine the best possible shape
for the enclosure, and the best possible port.
Three drivers are used in each tower: an Usher 8137A woofer (discontinued),
a Tang Band W4-1757SB midrange, and a Beston RT003C tweeter. The Usher
woofer is loaded by a lower vented enclosure of 46 liters, while the Tang Band midrange driver is loaded
by a separate sealed upper chamber. Using the ML-TL model by Martin King, this lower chamber was
modeled as a tapered ML-TL with the woofer at the bottom, and with the port placed 20" from the
bottom for the flattest possible bass response. The result is a low frequency 3 dB downpoint of 30
Hz. This enclosure was stuffed with 0.75 lbs. of fill per cubic foot of internal volume to suppress any
remaining resonances.
The Beston RT003C round planar tweeter contributes excellent detail and air, and the horn loading
helps improve dynamics. The Tang Band W4-1757SB midrange features a flat aluminum honeycomb
diaphragm for excellent off-axis and phase responses. Crossover points for the design are 275 Hz/2nd order and 4500 Hz/4th order.
Midwest Audiofest 2014
Speaker Design Competition
This year's Midwest Audiofest Speaker Design
Competition showcased a new, higher level of audio
excellence, with all entries in every category exhibiting
more talent, ingenuity, and creativity than ever. For 2014,
Parts Express hosted speaker designers and projects
from as far away as New York and Georgia, and as close
as our very own backyard here in southwest-central Ohio.
Many thanks to our expert judges: Ken Weller of
Celestion, Jerry McNutt of Eminence, and Don B. Keele Jr.
of DBK Associates.
aker Building
Under $200 1st Place Winner John H. - Seriatim
Over $200 1st Place Winner George M. - Exponents
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