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CM1 Design and Construction

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The B&W CM1’s are a small, monitor sized bookshelf speaker system. The speakers are a rear ported, two-way design utilizing a 1” aluminum dome tweeter and a 5” Kevlar mid/woofer driver. The cabinets are well made and of very sturdy construction. The CM1’s are available in several wood veneer finishes including: Wenge, Maple, and Rosenut, the review pair being the latter. I found the wood finish surface to be exceptionally smooth and the fit between veneer surfaces seamless; so smooth that I was left to wonder if the finish was man made until I confirmed it was not in product literature. Grilles for the CM1’s come in black or dark gray and are attached with internal magnets that leave no visible connection hardware on the face of the cabinets. The CM1’s are provided with a pair of gold binding posts at the rear of the cabinet to allow for bi-wiring; gold plated jumpers are also provided for those who wish to mono-wire the CM1’s to an amplifier.


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stand_000.jpgThe CM1’s come with several placement options included in the speakers design. The cabinets are constructed with threaded metal inserts that are embedded in the bottom of the cabinets that allow the speakers to be directly attached to the FS-700/CM stands. Also provided with the speakers are self adhesive rubber pads for placement on tables or other furniture.

The FS-700/CM stands are constructed of aluminum with a dark charcoal gray powder coat finish. Each stand comes in three parts for assembly: a 3/16” (approximate) top plate, a 1/16” thick tubular leg with two longitudinal chambers, and a base casting with a minimum thickness of approximately 3/16” that thickens towards corners where the mounting feet are threaded to accept carpet spikes or flat pads for hard floors. The base also has an opening into the rear chamber of the leg to allow a discrete run of speaker wire up and out an opening just below the top plate. These are substantial stands with a heavy base that when coupled to the CM1’s through the mounting screws, will keep the speakers more safely situated than many other stand options.

Inside the cabinet, ¾” MDF construction under the wood veneer can be seen. The front baffle is of thicker construction to allow the woofer frame to be mounted and recessed. The 5” woofer features a substantial magnet. Internally, the cabinet is layered in absorptive foam that covers everything, including the crossover, which is set at the back in a recess in the cabinet. The Nautilus tube is mounted to the tweeter in line with the rear port.

B&W states in their literature that the CM1 loudspeakers were designed in tandem with the 805 speaker system. Because of this, the CM1’s are said to incorporates many of the same technical features of the 805 model including driver and crossover technology.

The crossover network is a minimalist fist order design that uses only one electronic component per driver to minimize the amount circuitry and its effect on the music signal. This approach to circuit design is considered desirable by the audiophile market where it is generally believed that more electronic components in the circuit path only serves to deteriorate the signal. The downside to such a minimalist approach is that it is more difficult to meld driver behavior for smooth response that often outweighs any advantages from simplicity in the electronics to the overall speaker performance.

The tweeter uses the Natutilus tapered tube, a tube loading assembly, derived from the flagship Nautilus speaker system, which incorporates an internal inverse horn concept to improve clarity. A short conical section is used to dampen back emissions from the tweeter by reflecting the waves inwards to an absorptive lining. The intent is to eliminate phase problems associated with the rear emissions when they are reflected forward from the back of a speaker.

B&W refers to the tuned port as a ‘Flowport,’ which unlike common designs is dimpled in a similar fashion to a golf ball. What the dimples do is affect the relationship between laminar (smooth) and turbulent airflow, characterized by the Reynolds Number in fluid mechanics, through the port. The dimples increase the critical velocity at which airflow would become turbulent and begin to make those unmusical noises that are often associated with poor port designs. Also included with the speakers is a pair of short foam tubes. B&W recommends inserting these appliances into the rear ports when the speakers are to be used close to a wall to help balance reflected bass output from the port with some added absorption.

B&W is also well known for its use of DuPont’s Kevlar, a synthetic material best known for its use in bullet proof vests, as a driver material dating back to the introduction of the DM6 loudspeaker 1976. It is an orthotropic fiber, commonly used in composites, that exhibits high internal damping, considerable tensile strength, light weight with a strength to weight ratio greater than steel, but relatively poor compressive strength due to its fiberous nature. Kevlar’s high internal damping lead B&W to patent the use of the material in speaker cones to control standing waves. Kevlar is an interesting choice for application in a dynamic driver, from an engineering standpoint; when I say interesting, I mean challenging.

Much ballyhoo is made about the fact that Kevlar is used in bullet proof armor, which incidentally, has no mechanical similarity as to why Kevlar works well for stopping bullets. The high internal friction of the material that damps out free vibrations is not the mechanism by which the force of an instantaneous impact is redistributed to prevent penetration of a projective. Marketing types don’t understand the difference and they don’t really care as long as it sounds impressive, and they hope the consumer doesn’t understand either.

Chemically, Kevlar is a member of the nylon family of polymers and is known as poly-para-phenylene terephthalamide or para-aramid. During manufacturing, the material is drawn into a fiber that aligns the polymer chains giving the material its mechanical properties and orthotropic behavior. Kevlar is available in three grades of increasing tensile strength and modulus: 29, 49, and 149; Kevlar 49 is the most common. For use in composites, Kevlar fibers are often woven into a textile and laminated in a polymer matrix.

The challenge comes in the form of the substantially more complex and difficult to predict mechanical behavior of woven composites. Most materials used for cones are isotropic, meaning they exhibit behavior that is mechanically similar in all directions. Radially around the cone, the uniform behavior reduces the number of mathematical variables and equations which making them relatively simple to predict. Various plastics like polypropylene (which is homogeneous), and even paper (which is an inhomogeneous short fiber composite), behave as isotropic macroscopically. Woven long fiber composites exhibit substantial mechanical variations at angles relative to the fiber axes. The discontinuous geometry of the woven fiber structure complicates the mathematics that have to account for independent properties of the constituent components of the textile, sensitivity to weave geometry, yarn construction, and bonding between the fibers and the matrix, and anisotropic behavior of the composite with respect to loading orientation.

For more on this topic, see the forthcoming: A Primer on the Mechanics of Composite Kevlar Drivers.

At no point in the above discussion am I saying that these challenges cannot be designed for with a satisfactory acoustic result or that B&W’s expertise is not such that they cannot overcome these challenges. Quite the contrary, as B&W seems to successfully taken advantage of this behavior.

Kevlar drivers have often been attributed with a slightly richer sound by many professional and amateur listeners. With circumferential variations in stiffness, the cone will have slight variations in the uniformity of sound radiation with the more flexible areas lagging the stiffer areas. This causes time dependent variations in deformation and movement of the cone and subsequent variations in the air pressure distribution of the acoustic emissions from the cone. What this means is that there will be just a little bit of phase shifting about the frequency of any given forcing function.

My current working hypothesis is that this is the source of the perception of the slightly fuller sound associated with Kevlar drivers used for midrange. These frequencies are where instruments reveal their timbres. Current psychoacoustics research into the precedence effect suggests that the ear will integrate slight phase discrepancies and interpret this as richer sound. Perhaps this can be thought of as akin to a chorus sound processing effect where a fuller sound is generated by introducing phase offsets in the signal that, as in a real chorus of voices or instruments, gives the listener the perception of a greater number of performers.

 

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