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EQUIPMENT REVIEW - Acoustic Zen Crescendo<br />
There are two essential things you need to know about a<br />
transmission-line speaker. First, it is a quarter-wave resonator.<br />
Sound energy, which is reflected from the open end of the pipe,<br />
sets up multiple standing waves. As with any pipe open at only one<br />
end, its fundamental resonant frequency has a wavelength equal to<br />
four times its physical length, which is to say that the longest sine<br />
wave that fits into the pipe is four times as long as the pipe. That<br />
means that the lowest frequency such a pipe can be energized by<br />
corresponds to c/4L where c is the speed of sound and L is the length<br />
of the pipe. An example from the musical instrument world would<br />
be the clarinet. Its physical length is about two feet, but the clarinet<br />
can produce a note whose fundamental wavelength is about eight<br />
feet long, which corresponds to a frequency of 140Hz or C# below<br />
middle C. Of course, there are higher-order pipe resonances at odd<br />
multiples of the fundamental, which give the clarinet its distinctive<br />
timbre. In a transmission-line speaker, these are largely damped out<br />
by stuffing the line with absorbent material<br />
The second thing to recognize is that the transmission line acts as<br />
a delay line with respect to the backwave of the woofer. Since the<br />
backwave is 180 degrees out of phase relative to the front wave, the<br />
line needs to be sufficiently long to minimize destructive cancellation<br />
down to a specified bass frequency. Only at frequencies where the<br />
effective line length is equal to or greater than a half wavelength<br />
does the line output reinforce the woofer’s front radiation. Short<br />
lines simply can’t provide any deep bass augmentation. The<br />
Crescendo’s physical line length is about 9 feet, which is effectively<br />
stretched by the frictional effects of the stuffing material, a mix of<br />
poly-fil micro-beads and cotton fibers, to an apparent length of 13<br />
feet. This means that while the line’s output is extended to 20Hz, it<br />
only adds constructively to the woofer’s front radiation down to a<br />
frequency of about 40Hz.<br />
Most classic TL designs take advantage of the fact that the<br />
fundamental pipe resonance creates a pressure maximum at the<br />
closed end of the pipe, and of course, a pressure minimum (nearly<br />
atmospheric) at the open end. The line length is then chosen to<br />
match the line’s fundamental resonant frequency to the woofer’s<br />
free-air resonance in order to dampen the woofer’s excursion at<br />
resonance. The Crescendo woofer’s free-air resonant frequency<br />
happens to be 25Hz, and is consequently well damped by the<br />
fundamental TL resonance which is around 20Hz.<br />
The Crescendo is a three-way, five-driver design. The TL is<br />
energized by a pair of 8-inch woofers which feature coated paper<br />
cones and underhung voice coils. Although far less common than<br />
the overhung voice coil, its advantages are reduced moving mass,<br />
lower inductance, and a more linear motor strength over its excursion<br />
range, which translates into lower distortion. On the downside, an<br />
underhung design is costlier to manufacture, but that is hardly an<br />
important consideration in the context of high-end audio. The two<br />
5-inch midrange drivers and tweeter are arranged vertically in a<br />
D’Appolito configuration in order to maximize response uniformity<br />
in the vertical plane. The tweeter is a quasi-ribbon design with<br />
an aluminum coating over Kapton and incorporates neodymium<br />
magnets and horn loading. The woofers are crossed over at about<br />
250Hz using a second-order low-pass network. The mids also use<br />
coated paper cones and underhung voice coils. The tweeter is rolled<br />
in around 2.1kHz and is well protected against over-excursion by a<br />
third-order (18dB/octave) high-pass network. All internal wiring is<br />
said to be 10-gauge single-crystal copper.<br />
My measurements highlighted what in my book could only<br />
be described as an extremely successful design. The in-room<br />
frequency response was exceptionally uniform on axis, not only in<br />
the nearfield, but also at the listening seat. Bass response was flat<br />
nearfield (at about 4 feet) to about 50Hz with a strong contribution<br />
from the transmission line port (but several dB less in level) in the<br />
40 to 50Hz range. With room gain, response flatness was extended<br />
to about 40Hz at the listening seat. The minimum impedance was<br />
about 4 ohms, but the impedance magnitude and phase were<br />
quite uniform over the speaker’s entire bandwidth, the impedance<br />
magnitude only varying within a factor of two. That’s a far cry from<br />
the impedance variations of a typical bass-reflex loudspeaker, which<br />
can exceed an order of magnitude. And that makes the Crescendo<br />
very accommodating of high-source-impedance amplifiers, as it<br />
intrinsically minimizes amplifier-speaker load interactions. Zerofeedback,<br />
single-ended triode (SET) amplifiers can be safely used<br />
without impacting tonal neutrality.<br />
While I expected the pairing of the Crescendo with the Triode<br />
Corporation M845SE SET monoblocks to be compatible, I was<br />
genuinely surprised by the extent to which it turned out to be a<br />
match made in audio heaven. My first listen during CES 2012<br />
(with all Triode Corporation tube electronics) impressed me<br />
mightily, so I was pleased that Twin Audio-Video’s Santy Oropel,<br />
the Triode Corporation distributor, joined Acoustic Zen’s Robert<br />
Lee in delivering and setting up that exact system in my listening<br />
room. Just when I thought that the SET genre had been exhausted<br />
in terms of plausible design variations, the M845SE proved me<br />
wrong. The output stage consists of a pair of parallel-connected<br />
845 (or 211) directly heated triodes, driven by another 845 via an<br />
SPECS & PRICING<br />
Design: Three-way transmission line<br />
Frequency response: 20Hz to 30kHz<br />
(+/- 3dB)<br />
Nominal Impedance: 6 ohms<br />
Sensitivity: 90dB/2.83V/1m<br />
Recommended amplifier power: 50<br />
– 200W<br />
Weight: 125 lbs.<br />
Dimensions: 11" x 50" x 17"<br />
Price: $16,000 per pair<br />
aCOUSTIC Zen Technologies<br />
16736 West Bernardo Dr.<br />
San Diego, CA 92127<br />
(858) 487-4088<br />
acousticzen.com<br />
ASSOCIATED Equipment:<br />
Sony XA-5400 SACD player with<br />
ModWright Truth modification;<br />
Kuzma Reference turntable;<br />
Kuzma Stogi Reference 313 VTA<br />
tonearm; Grado Reference phono<br />
cartridge; Pass Labs XP-25 phono<br />
stage; Experience Music Passive<br />
Aggressive volume control, Pass<br />
Labs XP-30 line preamplifier; Triode<br />
Corporation M845SE, Lamm Audio<br />
M1.2 Reference, and Bob Carver<br />
Cherry 180 monoblock amplifiers;<br />
FMS Nexus-2, Wire World, and<br />
Kimber KCAG interconnects;<br />
Kimber KCAG and Acoustic Zen<br />
Hologram speaker cable; Bybee<br />
Speaker Bullets; Sound Application<br />
power line conditioners<br />
CLICK HERE TO COMMENT at www.theabsolutesound.COM<br />
76 Guide to High-Performance Loudspeakers www.theabsolutesound.com<br />
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