cone

materials

basic properties

The cone transforms mechanical energy into acoustical (also mechanic) energy.

The cone is the part of the speaker that physically moves the air. The shape, weight and strength of the cone relate directly to the frequency response of the speaker.

Covering the voice coil assembly and attached to the speaker cone is the dust cap. This cap exists to keep foreign particles from entering the voice coil area and causing a failure. The shape, weight and strength of the dust cap also relate to the frequency response of the speaker. Dust cap of proper shape can also be called phase plug.

Every medium agitated with oscillation has at certain frequency reinforced movement (resonance frequency), which is not present in the input and is specific to material properties, physical dimensions, weight and shape of a medium. Laws of nature work everywhere; at bridges, engine's inlets (BMW's Vanos system has variable inlet lenght) and, guess what, even at speaker's cones. Theoretically a cone should perform a uniform, pistonic manner all the way through it's FR. In praxis this is allmost unachievable.

Resonances (standing waves) are inherent to any resonating system. With proper engineering they can be eliminated from audio range or equally spread through wider FR. First approach is possible only with tweeters, where cone resonances can be pushed way out of normal hearing range, second option is more suitable for full/extended range applications or systems with first order crossovers.

materials

paper

Paper is the traditional material for speaker cones and is by popular opinion considered to be an obsolete technology not suitable for high performance audio applications.

It can easily be formed into a wide variety of shapes without overly complex and expensive tooling, though it is not the easiest material to manufacture consistently, so pair-matching isn't quite as exact, which may affect imaging, depending on the precision and quality of production and its control.

Its mechanical properties can be varied over a usefully wide range. Paper cone must be treated to avoid influence of environmental conditions (humidity), in order to avoid changes in cone mass and other parameters. Paper cone properties can be easily altered with coating to achieve desired properties, though many paper cones are coated largely for aesthetic purposes and not for enhancement of acoustical performance of a driver. Paper properties may change with time more than other cone materials.

Despite the seeming low-techness involved, a well engineered paper cone can deliver a combination of bandwidth and smoothness that is at least as good as any "high tech" stunning looks material. Paper is a material that sounds better than it usually measures.

The biggeest streghts are excellent selfdamping, potentially excellent resolution and detail, very flat response without excessive peakes and dips because of a gradual cone breakup. It can be most effectivelly used for full/extended range driver's cones or for systems with low slope crossovers.

polypropylene

Polypropylene is the most common plastic material used in speaker cones. Most polypropylene cones are a combination of polypropylene and a mineral or other filler (carbon fiber and kevlar). These fillers can be used both to control costs and to alter the mechanical properties of the material. Polypropylene cones tend to be inherently well damped with the result that they can deliver smooth, if not terribly extended, frequency responses. The material itself and the methods used in manufacturing cones with it are such that tight tolerances are easily achieved.

In the early days of polypropylene use it acquired some bad reputation due to a fact that it's a difficult material to bond. Modern adhesive technology has completely solved this problem, allthough it is not completelly free of problems. There are some who feel that drivers made with polypropylene cones tend to exhibit an audible degree of hysteresis or hysteresis-like behavior; nonlinear behaveour where the parameters of a system vary depending on the system's history. The most common thinking is that it is the viscoelastic creep present in all plastic materials that is responsible. Viscoelastic creep refers to the tendency of plastic materials to slowly stretch when under stress. This process may or may not be linear and typically is related to the lossiness in the material. By some opinions that the joint between the voicecoil former and the cone may be to blame. It is suggested that the heat generated by the vc and dissipated by the former may soften either the plastic cone material or the glue at the joint. Despite these polypropylene cones remain a popular choice for high performance systems largely because of their well-behaved high-frequency response and consistent performance.

other plastics

Apart from polypropylene, there are many plastic and plastic-based materials that have appeared over the years including TPX, HD-A, and HD-I (all manufactured by Audax), Neoflex (manufactured by Focal), and Bextrene (completelly replaced with polypropylene ). All these represent attempts at finding combinations of stiffness, lossiness, density, and sound velocity that are somehow optimal for a given application. They generally have the same virtues and potential pitfalls as polypropylene.

Resin-bonded high-strength woven fibers
In this class of materials belong most carbon-fiber, fiberglass and kevlar cones. These cones are made from a fabric of fibers bonded together with an epoxy or similar resin. The fibers themselves have a high degree of tensile strength; when embedded in an appropriate resin, a material of considerable stiffness results. Not surprisingly, these woven cones tend to have extended bandwidths. However, it comes with the cost of quite a bit of roughness as the internal losses of the basic resin-bonded material are quite low. It has been suggested the random orientation of the fibers helps to break up standing wave patterns on the cone, thereby smoothing the response of the driver. In our experience, this phenomenon has at best a minor influence on the high frequency response of the driver as every woven-cone driver we've examined has exhibited a rather rough high-frequency response.

Attempts have been made to improve on the basic construction of simple woven fabric cones. One manufacturer of raw driver units employs two thin layers of kevlar fabric bonded together with a resin and silica microball combination. The laminated structure is purported to be very stiff and the core material has the potential of introducing a controllable amount of damping. Another driver manufacturer employs a similar sandwich structure but with a honeycomb Nomex core. While these technologies are very exciting, they tend to be extremely costly and suffer, to greater or lesser extent, from the same high frequency roughness as their simpler cousins.

It is highly unlikely that a woven fabric cone will have any hysteretic properties. (Although the surround and spider -- even the motor system -- may still suffer from hysteresis, but that's another issue.) So, while they may not generally be the best choice for wide-range applications, woven fabric cones are well suited to low-frequency applications owing to their inherent stiffness and immunity to hysteresis.

carbon fiber/kevlar/composites/metal

The next generation were the Japanese carbon-fiber units, which made their first appearance in the pro studio monitor (prosound) 12" TAD units with very high efficiencies and very high prices (around $300 each in 1980). Carbon fiber prices have now dropped, and Vifa and Audax make good examples of this type of driver. The Japanese make lots more of them, having pioneered the technology, but they have been difficult to obtain if you are a non-Japanese small-run specialist manufacturer. These drivers have true piston action, outstanding bass and midbass response, but also have nasty, chaotic breakup modes at the top of their range. Removing these breakup modes requires a sharp crossover slope and one or two very sharp notch filters.

Kevlar drivers made their appearance in the mid-eighties with the Focal and Eton lines, with the Eton having superior damping due to the higher-loss Nomex honeycomb structure separating the front and rear Kevlar layers. The Eton and much newer Scan-Speak Kevlar drivers now share the limelight as the worlds pre-eminent high-tech drivers.

Audax has made a surprise reappearance in the high-end market with an unusual composite technology, called HD-A. This is an acrylic gel containing a controlled mix of grain-aligned carbon-fiber and Kevlar fibers.

Metal is seeing something of a surge in popularity as a cone material. Of all the materials, it has the worst damping attributes and so suffers from extreme peakiness in the high frequency region (common 12 dB at 5 kHz for a 16cm driver). However, below their first breakup mode, metal cones tend to be very well behaved, and this is, except stylling,a major source of the attraction to metal cones. However, even with the best crossover design, the high-frequency peaks present in currently available cones make them a poor choice for wide range applications.The most common materials used in metal cones are aluminum (and its alloys) and magnesium. Rigid cones of any material have advantages, but are difficult to damp completely. After all, why does a bell, or any other rigid metal, ring so long, for many thousands of cycles? First, the metal is rigid, and formed in a shape that increases the rigidity even further. Second, the only path for the bell to release mechanical energy is to the air itself, which takes a long, long time, since the density of air and bronze are quite different, resulting in very weak coupling, and very little damping by the air load. At the present, though, even the best kevlar, carbon-fiber, or aluminum designs show at least one high-Q peak at the top of the working range, requiring a sharp crossover, a notch filter, or preferably both to control the peak. Unfortunately, this peak usually falls in a region between 3 and 5 kHz, right where the ear is most sensitive to resonant coloration and phase shifts due to a crossover. Self-damping results in an absence of coloration, as well as contributing to a relaxed, natural, and unfatiguing quality.