When A. R. Bailey unveiled his novel “non-resonant loudspeaker enclosure” in 1965, commonly referred to today as a classic transmission line (TL), he took direct aim at the popular bass-reflex speaker design. Bailey’s measurements and listening tests highlighted the poor transient response of a bass-reflex enclosure. Such an enclosure is clearly resonant, even if tuned for a maximally flat response, due to its reliance on a Helmholtz resonator to invert the phase of the woofer’s back wave. The problem, as Bailey saw it, was that when an impulse stopped the bass-reflex port would continue to radiate for many milliseconds. His solution for tight and natural bass response was an acoustic line loosely packed with long- fiber wool. The TL became a hot topic for DIY experimentation throughout the 70s and 80s and was commercially available from several companies, most notably IMF and Fried in the U.S. For commercial reasons, the TL never displaced the bass- reflex enclosure since, for a given bass cutoff frequency, the TL consumes a much larger volume and is more costly to construct. And while designing a bass-reflex enclosure for a given woofer is pretty much a cookbook process in this day and age of Thiele- Small parameters, up until very recently there wasn’t sufficiently reliable TL design software available. In fact, Bailey in his seminal articles only described the overall design principles and failed to specify a process for matching a TL to a given woofer. Today, a TL is a rare bird in a forest of bass-reflex designs. It has been ages since a commercial TL visited my listening room; as I recall, it was one of Bud Fried’s designs circa the mid-80s. And so I was really looking forward to the transmission-line-loaded $16,000 Crescendo, especially in view of its stellar performance at past audio shows since its introduction at CES 2006.
There are two essential things you need to know about a transmission-line speaker. First, it is a quarter-wave resonator. Sound energy, which is reflected from the open end of the pipe, sets up multiple standing waves. As with any pipe open at only one end, its fundamental resonant frequency has a wavelength equal to four times its physical length, which is to say that the longest sine wave that fits into the pipe is four times as long as the pipe. That means that the lowest frequency such a pipe can be energized by corresponds to c/4L where c is the speed of sound and L is the length of the pipe. An example from the musical instrument world would be the clarinet. Its physical length is about two feet, but the clarinet can produce a note whose fundamental wavelength is about eight feet long, which corresponds to a frequency of 140Hz or C# below middle C. Of course, there are higher-order pipe resonances at odd multiples of the fundamental, which give the clarinet its distinctive timbre. In a transmission-line speaker, these are largely damped out by stuffing the line with absorbent material
The second thing to recognize is that the transmission line acts as a delay line with respect to the backwave of the woofer. Since the backwave is 180 degrees out of phase relative to the front wave, the line needs to be sufficiently long to minimize destructive cancellation down to a specified bass frequency. Only at frequencies where the effective line length is equal to or greater than a half wavelength does the line output reinforce the woofer’s front radiation. Short lines simply can’t provide any deep bass augmentation. The Crescendo’s physical line length is about 9 feet, which is effectively stretched by the frictional effects of the stuffing material, a mix of poly-fil micro-beads and cotton fibers, to an apparent length of 13 feet. This means that while the line’s output is extended to 20Hz, it only adds constructively to the woofer’s front radiation down to a frequency of about 40Hz.
Most classic TL designs take advantage of the fact that the fundamental pipe resonance creates a pressure maximum at the closed end of the pipe, and of course, a pressure minimum (nearly atmospheric) at the open end. The line length is then chosen to match the line’s fundamental resonant frequency to the woofer’s free-air resonance in order to dampen the woofer’s excursion at resonance. The Crescendo woofer’s free-air resonant frequency happens to be 25Hz, and is consequently well damped by the fundamental TL resonance which is around 20Hz.
The Crescendo is a three-way, five-driver design. The TL is energized by a pair of 8-inch woofers which feature coated paper cones and underhung voice coils. Although far less common than the overhung voice coil, its advantages are reduced moving mass, lower inductance, and a more linear motor strength over its excursion range, which translates into lower distortion. On the downside, an underhung design is costlier to manufacture, but that is hardly an important consideration in the context of high-end audio. The two 5-inch midrange drivers and tweeter are arranged vertically in a D’Appolito configuration in order to maximize response uniformity in the vertical plane. The tweeter is a quasi-ribbon design with an aluminum coating over Kapton and incorporates neodymium magnets and horn loading. The woofers are crossed over at about 250Hz using a second-order low-pass network. The mids also use coated paper cones and underhung voice coils. The tweeter is rolled in around 2.1kHz and is well protected against over-excursion by a third-order (18dB/octave) high-pass network. All internal wiring is said to be 10-gauge single-crystal copper.
My measurements highlighted what in my book could only be described as an extremely successful design. The in-room frequency response was exceptionally uniform on axis, not only in the nearfield, but also at the listening seat. Bass response was flat nearfield (at about 4 feet) to about 50Hz with a strong contribution from the transmission line port (but several dB less in level) in the 40 to 50Hz range. With room gain, response flatness was extended to about 40Hz at the listening seat. The minimum impedance was about 4 ohms, but the impedance magnitude and phase were quite uniform over the speaker’s entire bandwidth, the impedance magnitude only varying within a factor of two. That’s a far cry from the impedance variations of a typical bass-reflex loudspeaker, which can exceed an order of magnitude. And that makes the Crescendo very accommodating of high-source-impedance amplifiers, as it intrinsically minimizes amplifier-speaker load interactions. Zero-feedback, single-ended triode (SET) amplifiers can be safely used without impacting tonal neutrality.
While I expected the pairing of the Crescendo with the Triode Corporation M845SE SET monoblocks to be compatible, I was genuinely surprised by the extent to which it turned out to be a match made in audio heaven. My first listen during CES 2012 (with all Triode Corporation tube electronics) impressed me mightily, so I was pleased that Twin Audio-Video’s Santy Oropel, the Triode Corporation distributor, joined Acoustic Zen’s Robert Lee in delivering and setting up that exact system in my listening room. Just when I thought that the SET genre had been exhausted in terms of plausible design variations, the M845SE proved me wrong. The output stage consists of a pair of parallel-connected 845 (or 211) directly heated triodes, driven by another 845 via an interstage transformer. I tried both output stage configurations and clearly preferred the sound of the 845 as being more vivid harmonically, better focused, and more dynamic. I experimented with speaker toe-in angle and preferred to intersect the tweeter axes in front of the listening seat in order to obtain the widest sweet spot and soundstage lateral extension.
While I usually leave any discussion of bass performance toward the end of a review, preferring to start with the midrange, there’s a compelling reason to reverse that order in the case of the Crescendo. To confess, it became painfully obvious that I had been living in a state of perpetual sin listening to bass reproduction all of these years through bass-reflex loudspeakers. The Crescendo made that crystal clear as it recalibrated my expectations in the bass range. The attack and decay of an impulsive input signal such as a kettledrum strike is stretched in time by a phase-inverter speaker because a resonator takes time to build up and then decay the signal. It’s a well-known psychoacoustical fact, and a critical performance factor, that our ears interpret transient signals primarily in the time domain. A classic experiment involves reversing the signal’s attack and decay by playing a transient backwards in time. The result is total auditory confusion. As a consequence, it’s fair to say that a bass-reflex speaker reproduces an impulsive signal in slow motion. It may not matter as much with organ music, which lacks crisp attack and decay, but as the Crescendo made clear, even when driven by the M845SE, tympanic strikes on a properly loaded transmission line are peerless in terms of control and definition. What the Crescendo lacked in ultimate bass extension it made up for with superlative time-domain performance.
The transition from the bass to the midrange was seamless and without any audible discontinuity. The Crescendo maintained realistic tonal weight while doing justice to the power range of an orchestra. In these respects it performed with greater conviction than the similarly priced MartinLogan Summit X electrostatic hybrid, which tends to sound leaner through the upper bass. Of course, the Crescendo lacked the midrange transparency and textural delicacy of the Summit X, but it wasn’t that far behind. It was also adept at re-creating a persuasive spatial impression with excellent depth, width, and nicely focused image outlines. However, my personal preference is for a dipole midrange, which I find, at least in my listening room, to provide an enhanced spatial impression and a more immersive you-are-there experience.
In speaker land, what separates the men from the boys is typically how well a tweeter is integrated with a mid or woofer. It’s often not so much about the choice of tweeter as it is about selection of an optimal crossover frequency and a sufficiently steep high-pass network to adequately protect a tweeter from over-excursion. For me the sonic kiss of death is a tweeter whose distortion spectrum rises with signal level. In my many years of audio reviewing, I’ve endured so many ruthless-sounding tweeters that I’ve developed an extreme sensitivity, an allergic reaction if you will, to any upper- midrange and treble harshness, grit, or gratuitous brightness. I’m happy to report that the Crescendo’s ribbon tweeter is a winner, capable of reproducing sweet and refined harmonic textures with convincing transient finesse. Its level of purity gives full scope to violin overtones and female voice even when driven to loud playback levels. The treble is so well integrated with the corpus of the midrange that I found it hard to believe that it was actually crossed over in the upper midrange around 2kHz.
The overall tonal balance was quite neutral sounding, and did not display an inherent bias. Of course, the balance could easily be tilted toward midrange warmth by a tube front end or overly tubey power amp. But to its credit, this is a speaker that allows the end user to make those sorts of editorial decisions. The Crescendo was just as comfortable with solid-state amplification, though it was at its microdynamic best, able to plumb the emotional depth of a recording, when partnered by the M845SE monoblocks. However, the macrodynamic range was best served by a higher-power amplifier such as the Bob Carver Cherry 180. This was a partnership that made it possible for the Crescendo to live up to its name. Orchestral crescendi were scaled effortlessly without compression or distortion. In fact, the Crescendo brought out the best in the Cherry 180. The resultant soundstage was transparent, dimensional, and bubbling with kinetic energy—the essential ingredients for a goosebump- producing experience. It’s fair to say that the Carver amplifier with its pentode-connected output stage and a 1.7-ohm source impedance benefitted from the Crescendo’s uniform impedance magnitude and associated linear phase. Pentode amps in general welcome a resistive load, but unfortunately most real-world loads are inductive and/or capacitive in nature. As a consequence, pentode amps are difficult to match successfully. The Crescendo comes about as close to being an ideal resistive load as one can expect from a box speaker. It’s the sort of dream load every pentode amp would appreciate.
Acoustic Zen’s Robert Lee has crafted a magnificent transmission-line speaker, truly a perfectionist labor of love. The Crescendo is eminently musical and supremely well-integrated from top to bottom. It certainly pushed of all my emotional buttons and is currently my favorite box speaker under $30k. Make no mistake about it: The Crescendo is a fantastic value at its asking price. An enthusiastic five-star recommendation!
SPECS & PRICING
Design: Three-way transmission line
Frequency response: 20Hz to 30kHz (+/- 3dB)
Nominal Impedance: 6 ohms
Recommended amplifier power: 50 – 200W
Weight: 125 lbs.
Dimensions: 11″ x 50″ x 17″
Price: $16,000 per pair
ACOUSTIC ZEN TECHNOLOGIES
16736 West Bernardo Dr.
San Diego, CA 92127
Sony XA-5400 SACD player with ModWright Truth modification; Kuzma Reference turntable; Kuzma Stogi Reference 313 VTA tonearm; Grado Reference phono cartridge; Pass Labs XP-25 phono stage; Experience Music Passive Aggressive volume control, Pass Labs XP-30 line preamplifier; Triode Corporation M845SE, Lamm Audio M1.2 Reference, and Bob Carver Cherry 180 monoblock amplifiers; FMS Nexus-2, Wire World, and Kimber KCAG interconnects; Kimber KCAG and Acoustic Zen Hologram speaker cable; Bybee Speaker Bullets; Sound Application power line conditioners
By Dick Olsher
Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.More articles from this editor
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