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These drivers are loaded in a sealed sub enclosure just big enough to contain them. Design focus is on quality through the midband though they do contribute significant energy through the midbass. Internal wiring is created in-house and also available in Zu's higher-end cable offerings. This is a possible contributing factor to widely reported synergy between Zu's cables and speakers. Sandwiched between the twin widebanders is a horn-loaded 'super' tweeter on a 1st-order high-pass network starting at 10kHz featuring Mundorf silver/oil capacitors.

Four Eminence 10" Pro woofers are mounted on the rear of each speaker, upgraded from the previous 1.5 but previously included with the Pros. These are loaded in two sealed internal chambers at a volumetric ratio of 3:1. That is, the upper two drivers occupy a box much smaller than the lower two, effectively providing four subwoofers tuned at two different frequencies with each pair of Definition 2s. Previous Definition 1.5s and Pros loaded all four woofers in one combined sealed sub enclosure. Overall fit and finish are sublime. These speaker exude quality on all fronts. I love the burning dark red paint on my pair. After almost three years, it continues to impress.

Setup & operation
The main array is an extremely sensitive low-excursion design. Very little power is needed to achieve satisfactory levels. Low power and damping factor are no problem at all. I asked Sean Casey about the rooms and gear they tested the Definitions with:
"In their last phase, the Definitions were voiced in a dozen rooms. This included from ground-up audio-specific rooms like Adam's main rig room to traditional rectangular 12' x 20' rooms without any audiophile treatments or tweaky construction details. Really about anything you'd find in 95% of U.S. homes were used as test environs to set up the Defs and find possible problems. Findings? The new Def 2 doesn't need nearly the breathing room as the original. You can set them up closer and the bass is easier to integrate with the space. Long-wall placement usually sounds better than short in smallish rectangular rooms.

Gear used during R&D included the Audiopax Model 88 with Zu mods, the new Audiopax solid-state monos, Melody's I2A3 and M300B monos with WE tubes, Denon PMA2000iv, AudioTropic gear, Crown iTech 6000 and all sorts of turntables and preamps (all tube products get rolled as part of the process). Amps make a big difference with the sound of all our loudspeakers including the Definitions. Results are very personal. While I generally prefer tubes, on bang for the buck it's tough for sand to compete with glass but those new Audiopax sand amps do rock, sing and work! With the 6-ohm load presented by the front driver array, most amps regardless of type and circuit are pretty happy but the Def 2 as configured are not ideal for an OTL amp where the sound can be lean, grainy and edgy as a function of mismatch. We do offer a 30-ohm version specifically tuned for OTL amps. We go over placement pretty well in the user's guide. Personally, the wider the better. I like no less than 90° and up to a 120° spread of the speaker/chair triangle, with pretty heavy aiming to a point a foot or two behind my head."

Amplifier power recommendations per the user guide
Average room / moderate volume 2 - 6 watts
Large room / loud 8 - 24 watts
Large room / concert level 20 - 60 watts
Max music input power 1000 watts
Max sustained RMS power 500 watts
Dynamic range 130dB

I have used the Definitions with 2 to 60 watt amplifiers so these recommendations are spot-on. A Yamamoto 45 amplifier enjoyably pressurized the room and my Red Wine 70.2s will play a dance party without breaking a sweat. My room is 26 feet by 17 by 9 and I sit about 12 feet from the speakers so there is a fair amount of space to fill. Remember that the main amplifier is not charged with heavy bass duties.

For active bass, AC powers the internal amplifier which takes a high-level feed from the main cabling. Supplied jumpers carry post-amplification output and connect 'Sub Amp Out' terminals to 'Subwoofer In'. It takes a minute to understand the system but it's intuitive thereafter like a quasi tape loop that allows insertion of additional devices. To run this arrangement, each speaker of course needs it own AC power connection.

Each amp/speaker is fitted with an attenuator to control low bass output. To optimize low bass response, simply turn the knob to where it feels good. Option 2 to drive and stop those 625 square inches of low frequency radiators is to supply your own V12 gas guzzler -, er, outboard amp/s. Remember that those drivers are pro models designed for maximum output, not maximally flat response. Turn them loose with a muscle amp and you'll have in-room response that looks like a wicked mountain stage in the Tour de France. This is the high performance option and demands commitment. Don't they always?

More bad news. You'll need an equalizer, parametric strongly encouraged. You're also going to have to learn how to use it. Crossover types, slopes, Q-factor and gain will painfully ingratiate themselves as a new language. You will become half audiophile, half pro audio technician. Well, perhaps a 12%/88% split but uncharted territory still. We're going to build your bottom end from scratch. Hey, maybe the ladies will be interested!

Many audio heads probably have an old equalizer of some sort in a dusty closet. That one won't work. The necessary implement is a sonic scalpel. Most primary room problems are in the bass, as functions of the speaker's output and its agitation of acoustic room anomalies. Most common are floor-to-ceiling modes or standing waves. These are frequencies where natural energy decay is not allowed by fundamental room barriers. Related phenomena create long resonance times at some frequencies and black holes called suckouts elsewhere.

There is an entire science devoted to this far beyond my understanding or the intent of this review. Basically, speakers and rooms do not get along. Every room/speaker interface has frequency and decay challenges to varying degrees, especially with parallel surfaces such as floor/ceiling and wall/wall: 8' ceiling standing wave = 70Hz; 9' ceiling standing wave = 63Hz. Standing waves are formed when a room cannot shed energy. 'Old' signal is not gone when 'new' arrives, compounding acoustic pressure at that frequency. Sonic effects include smearing and imbalanced boom, masking quieter nuances. Chances are almost certain that your room exercises strong influence over your sound, especially in the bass. Once we're ready to perform sonic surgery, step one is to measure in-room bass response. The simple way is to download RealTraps' free test tone package. Set an SPL meter at the listening position and graph the holes and bumps.

The purpose of the parametric equalizer is to mirror-image that picture. Where there is a room bump, insert a cut. Naturally there will be nulls as well. Filling them is less predictable than reducing peaks but can be effective. The next level of measurement sophistication requires a measurement microphone, USB preamplifier and a measurement program like Home Theater Shack's 'Room EQ Wizard'. I won't go into the specifics of using these tools but they are a powerful team with the parametric equalizer that truly puts the user in command of room response. Zu offers either a Rane analog equalizer or a DSP-based dbx Driverack 260 for EQ purposes. I recommend the dbx unit as it has incredible functionality and a tremendous user interface via computer. Especially during the steep learning curve, being able to see what corrections are doing to the end result is invaluable. Through my laptop, I can do measurements and switch applications to apply corrections. Changes made are instantaneous and correlating the graphic with audible effects is easy. The combined elements make the learning process easy and - fun?

I have become a bass-o-phile - or perhaps the condition is out of remission. Younger days running dances and going to rock shows certainly bore signs of the disease. Then I spent fifteen years repressed in denial, convinced of the superiority in purity and beauty. Question 1: What does the bass in your speaker sound like? Is it punchy, smooth, overpowering, extended, limited? Does it fill the room without overload? If so, at what volume? The Equal Loudness Curves also called Fletcher-Munson curves dictate perceived frequency balance, especially notable in the low frequencies. Your ears aren't even close to measuring flat:

This graphic portrays relative perception of sound pressure levels (SPL) such that any actual pressure level along a given line will be perceived at equal volume. Note that a baseline 80B SPL is 40dB different from 1000 to 20,000Hz. At 60dB it is 50dB different but at 100dB the disparity is only 30dB.

This is where the old 'loudness' button came from - a frequency-extreme booster taking into account nonlinearities of human hearing. Not only is hearing non-flat, the degree changes with volume. Most systems have an optimized volume range where balance is best. Our biological EL curves play a hand in this. I have to believe that each person's own ELC is similar but not identical to the averaged graph. Chalk up another one to personal preference as we're already used to in all matters hifi.

Relating back to question 1, how a speaker's bass responds in a room can be affected by positioning in a room, treatments of same and not much else. Toe-in won't affect response as lower bass acts as a nearly spherical wave. Bass response is generally
built into the speaker by the designer, whether bass reflex, sealed, open baffle, transmission line or horn-loaded. A certain amount of bass energy is designed to be generated per watt of total speaker input. The relationship between bass and other frequencies is largely built in and designed for a particular space.

A speaker's relationship to its room is tremendously important to bass success. If the match is poor, frustration likely cannot be cured. "Everything with the speaker is so terrific but I just can't fix the bass!" Into this toxic stew walks the Definition 2. It poses the simple question "What is wrong and what would you like to do about it?" In the basic configuration, the bass attenuators can simply be turned up or down. The basic character in this arrangement is not dissimilar to a well-done bass reflex setup. It's punchy, fast with attacks and seems to have a hot spot around the kick drum. It is not completely linear but doesn't have the ringing common to bass reflexes. Being able to set the attenuators to balance with the fronts not only gives tremendous range of room compatibility but volume as well. A little more bass energy can be quite welcome in quieter sessions like a variable loudness control that doesn't compromise the main signal.

The Definition owner who has optimized this system may decide they love what they have and not take the other pill. For the adventurous, glory awaits. Speakers will react to a room in nonlinear ways. John Atkinson accounts for this in his measurements by performing them 3 feet from the speaker. I am not denigrating this practice and Mr. Atkinson knows more about measuring equipment than I ever will. It would be unrealistic to publish graphs of how a speaker performs in his room because that has little relation to how it will play in yours. It would be misleading to do so. However, the followed practice does not account for true in-room response, necessary though it may be.