114 August2019nstereophile.com
KLIPSCH FORTE III
I
used DRA Labs’ MLSSA system
and a calibrated DPA 4006
microphone to measure the Forte
III’s frequency response in the
farfield and an Earthworks QTC-40
mike for the nearfield responses.
The Klipsch’s specified sensitivity
is an extraordinary 99dB/2.83V/m;
my estimate was lower, at
95.2dB(B)/2.83V/m, but this is still
8dB higher than the average sensitiv-
ity of the speakers I have measured
over the past 30 years. This speaker
will play loudly with a mere handful of
watts. Klipsch specifies the Forte III’s
nominal impedance as “8 ohms com-
patible.” I found that the speaker’s im-
pedance magnitude (fig.1, solid trace)
dropped below 6 ohms only in the
upper bass and in the valley between
the twin low-frequency peaks that
define the reflex woofer loading. The
minimum value is 3.65 ohms at 125Hz.
However, the electrical phase angle
(dotted trace) is sometimes extreme,
and there is a current-hungry combina-
tion of 5 ohms and –49° phase angle at
90Hz. Despite its high sensitivity, the
Forte III will work best with amplifiers
that are comfortable driving 4-ohm
loads. And the very large differencebetween the average impedance in the
lower midrange and that in the treble
means that the speaker might sound a
touch bright with tube amplifiers hav-
ing high output impedances.
There are small discontinuities in
the impedance traces, particularly
around 125Hz and 400Hz, that would
imply the presence of resonances of
various kinds. When I investigated the
enclosure’s vibrational behavior with
a plastic-tape accelerometer, I found a
low-level resonant mode at 120Hz and
a stronger one at 398Hz on the side
panels (fig.2), and another at 434Hz
on the top panel. Given the speaker’s
high sensitivity, these modes might notresult in audible midrange congestion,
however.
The Forte III’s impedance-magnitude
plot has a saddle in the bass centered
on 43Hz, close to the frequency of the
lowest note on the four-string electric
bass and double bass, suggesting that
this is the tuning frequency of the
drone. The blue trace in fig.3 indicates
that the output of the woofer, mea-
sured in the nearfield, has the expected
minimum-motion notch at 43Hz, while
the output of the passive radiator (red
trace) peaks between 30 and 60Hz.
Some midrange peaks are visible in
the passive radiator’s output. However,
because this radiator is mounted on703-M horn. Down low, the Forte III’s
12" K-281 treated-paper cone woofer,
which uses a 3" voice-coil, is supple-
mented with a 15" KD-15 paper-cone
passive radiator affixed to the cabinet’s
lower backside. Constructed with a San-
toprene/rubber surround, the passive
driver works purely off the pistonlike
air motion of the 12" woofer, addressing
frequencies below 650Hz.
“The radiator works the same as a
port,” explained Klipsch’s principal
engineer, Roy Delgado, a 30-year
veteran of Klipsch Audio Technologies
who initially worked under Paul W.
Klipsch himself. “Because of the smaller
air volume of the box, the drone (aka
passive radiator) offers the advantage
of allowing us to adjust compliance. If
it’s done right, the drone extends the
bottom end. Basically you’re creating aSome of the same principles embodied
in the Klipschorn—and, one hopes, many
of its performance characteristics—are
found in an old-new member of the com-
pany’s Heritage line, the Klipsch Forte III
($3998/pair). Introduced in 1985 as the
Forte and relaunched in 1989 as the Forte
II, this floorstander faded from the line
but was reintroduced in 2017, just in time
for a new SET renaissance!
Everything old is new again
The Forte III is a three-way design
measuring 36" high by 16.5" wide by
13" deep and weighing a solid 72lb. The
manufacturer’s specs include a frequency
response of 38Hz–20kHz, ±3dB, a
sensitivity of 99dB/2.83V/m, and an
impedance described as “8 ohms com-
patible.” A newly designed steep-filter
passive network crosses over at 65Hz and
5.2kHz. Tweeter and midrange drivers
are, respectively, Klipsch’s new K-100-TI
1" titanium-diaphragm high-frequency
compression driver loaded with an ABS
Tractrix K-79-T horn and a new K-70
1.75" titanium-diaphragm midrange
compression driver on a new Tractrix K-
MEASUREMENTS
Fig.1 Klipsch Forte III, electrical impedance (solid)
and phase (dashed) (2 ohms/vertical div.).Fig.2 Klipsch Forte III, cumulative spectral-decay
plot calculated from output of accelerometer
fastened to sidewall level with midrange unit (MLS
driving voltage to speaker, 7.55V; measurement
bandwidth, 2kHz).Stereophile Klipsch Forte III Impedance (ohms) &
Phase (deg) vs Frequency (Hz)The upright bass
sounded so rich,
yet so palpable, it
was irresistible.