If you're into audio, you probably know that the materials a speaker is composed of impact the quality of sound reproduction. One of the strongest examples of this is the speaker diaphragm, or cone.
The diaphragm of a speaker is responsible for producing the actual vibrations that create the audio waves you hear. Within the driver, a coil responds to an electromagnetic signal and pushes on the back of the cone, causing it to vibrate, somewhat like the skin of a drum. The main difference from a drum is that the actual cone itself is not meant to bend, rather it moves back and forth within the flexible surround (this is known as pistonic motion). It takes a very specific material to allow the cone to accurately reproduce all the details and frequencies in the signal without getting distorted. The three most important properties of cone material are stiffness, low density, and internal damping.
Stiffness refers to the rigidity of the cone. Good speaker cones have a high stiffness; if the cone is flexible, that leads to distortion during playback.
Low density refers to the weight of the cone relative to its size. A lightweight cone is preferred because it will offer more accurate and responsive vibrations.
Internal damping, or loss factor, is the cone's ability to reduce resonant vibrations, or vibrations caused by the diaphragm's own inertia rather than the signal. This is essential for avoiding unwanted frequencies and keeping the audio playback as accurate as possible.
Based on these key properties, four main material types have been deemed preferable: paper, plastic, metal, and fiber. There have been a number of other high-tech solutions introduced in recent years, but most materials on the market fall into one of those categories.
Paper cones are created from sheets of pulp that are made of various blends of trees or even synthetic fibers. These pulp sheets are beat into a slurry and used to form the shape of the cone. There are a wide variety of factors during the production process that can impact the quality of the cone, such as the pulp composition, slurry beating method, additives, and cone forming techniques. These decisions form the differences between speaker cones from different companies and cones made for different types of speakers (i.e., subwoofers v.s. midranges).
For example, all paper cones follow one of three cone forming methods: pressed, semi-pressed, and non-pressed. After the pulp slurry is drained, the solid fiber remains in a pressed cone are sent to a heated cone pressing machine that steams the remaining water from the pulp and forms the shape of the cone. An over-pressed cone will likely result in more noticeable breakup or distortion at high volumes. Non-pressed cones instead are chemically treated; they tend to be stiffer and have a higher sensitivity than pressed cones.
The additives used in a pulp slurry are often considered part of a company's "secret recipe". Most paper cones feature non-cellulose fibers such as carbon fiber, Kevlar, ceramics, or glass. They also often contain wool or felt. These added materials then require a binding agent to hold everything together, such as various epoxies or pastes.
Overall, paper cones constitute a wide variety of cone types and remain the hallmark for guitar amplifiers. One of the biggest limitations to paper cones is that they retain moisture from the environment, depending on if and how they were sealed. This increases the cone's mass and interferes with damping.
Polypropylene, or plastic, cones are an extremely popular material choice due to their rigidity, low-cost manufacturing, and resistance to water absorption. Just as with paper cones, there are a variety of fillers and additives used in polypropylene, the most common being glass, carbon black, mica, and talc.
Typical plastic cones are constructed using thermoforming, in which the cone is heated until malleable and then pressed into shape. Some manufacturers opt for injection molding, which results in cones with higher stiffness, but this process requires an investment in expensive tooling.
One of the most promising industry developments was Propex CURV, a self-reinforcing polypropylene introduced in 2002. While the material really landed its success in the luggage industry, it gathered a lot of interest for its performance potential in speaker cones. It offered the versatility and low cost of a thermoplastic combined with the performance of a fiber-reinforced composite. CURV cones offer high stiffness, low density, and impact resistance. The biggest limitation to the material is the challenge of forming it properly without degrading the molecular grain orientation -- a feat that few manufacturers have managed to accomplish.
Plastic cone materials can offer impressive performance attributes like high damping, but the sound they produce is often described as lifeless or bland compared to composite materials.
At first glance, metal seems like the ideal cone material, due to its rigidity and low density. However, the case for metal isn't that simple. More ideal materials like Beryllium are expensive, and therefore reserved for high-end speaker drivers (Beryllium also is too delicate for large woofer drivers). A cost-effective compromise like aluminum is typically used in the mass market. While aluminum does offer durability and detailed response, the greatest limitation is in damping. Strain and standing waves with aluminum cones results in dramatic ringing and unwanted resonance.
This is because the greatest strength of metal (rigidity) is also its greatest weakness. Although stiff speaker cones are more desirable, because flexing leads to distortion, the fact is that the cone will eventually flex. When this occurs, a more rigid material like aluminum will be more affected across the entire area of the cone, compared to a more flexible material like paper or plastic that will mitigate the distortion.
For this reason, metals are often coated with chemicals designed to absorb these unwanted sound waves and improve damping. This, however, makes the cone heavier. Creating a high-performing metal cone is a delicate balance between boosting damping abilities with other materials while minimizing the addition of mass.
Aramid fibers were mentioned previously as common additives to paper speaker cones. There are also diaphragms constructed mostly from these fibers, the most common being Kevlar. Commonly known for its use in the construction of bulletproof vests, Kevlar is a preferred material for many companies like Audioengine because it is rigid and lightweight. It responds quickly to vibration and offers a more lively tone than plastic.
Kevlar's biggest limitation is internal damping; it can have a similar sound quality to metal, with an unpleasant ringing response. To combat this, Kevlar is often coated or combined with other composite materials for a more balanced result.
Among all the materials listed above, an overarching dichotomy is created between soft and hard diaphragms. To this day, audio experts debate which type of diaphragm is superior. Hard, more rigid cones (metal and fiber) tend to exhibit fewer breakups or distortions, but when they do distort, the response is more dramatic. This contributes to the 'ringing' and 'harsh' attributes of metal diaphragms. Alternatively, soft cones (paper and plastic) exhibit more frequent distortion but have more compliance, or ability to distribute damping throughout the cone and localize disturbances when they do occur. In other words, soft cones break up more frequently, but more gently. This leads to the difference in tonal quality experienced by different diaphragms. Some people argue that a soft cone is more pleasant to hear even if it technically distorts more often than a hard cone, and soft materials are often described as silky and smooth. The limitations of each explain why the highest performing cones are often combinations of both types of materials.
Comparing the materials directly also must come with the knowledge that the overall sonic quality of a diaphragm is largely impacted by the designer's voicing decisions. The end result in the audio performance of a driver is a combination of the materials, cone geometry, the grille and baffle, and the crossover network. Eliminating these extraneous factors and isolating the impact of the material itself is a near-impossible task.
The bottom line is that there is no perfect speaker diaphragm material. A "perfect" cone would have zero mass, be perfectly rigid so that it exhibits perfect pistonic motion, would never flex, and would have immediate vibration response. Such a material will never exist, so all speaker manufacturers can do is try to get as close as possible. The best driver materials make a cone that can reproduce audio signals as accurately as possible with as little distortion as possible. The material that does the best job differs depending on the size and purpose of the driver. Smaller, high-frequency drivers have different needs and challenges than larger, low-frequency drivers.
If you're looking for the perfect speaker cone material, the best option is to listen to them yourself. Different materials have different tonal qualities, so you might find that you simply prefer one material over others.
