Sunday, May 20, 2012

Cone Materials and Transient Response

Modern drivers are made from some pretty cool materials each claiming to be the next breakthrough in technology. Some do offer better characteristics in stiffness, others in damping, others in lower weight. And they all in fact do sound different. There is a sonic signature associated with certain materials that is quite revealing, not of the music, but of the contribution the material makes to the actual coloration of the sound.

The linear motor (coil in a magnetic field, AKA dynamic driver) speaker has been around since 1924 (yes folks, these are the ancestors of the present speaker technology meaning that this fundamental design has not changed in 89 years). Much like internal combustion engines, it seems like once a good idea is found, humans just stay with it and keep tweaking the fundamental design to make it better (this applies to just about everything, not just speakers and car engines).

Most advances have been made in cone materials. Originally, cones were made from paper because paper is light weight, easily molded to various shapes, and still retained good stiffness when molded into a conical shape. It also sounded a lot like the real things.

Low weight is directly proportional to transient response so back in the early 1950s, Rudy Bozak tried aluminum cones. The first generation cones rang like a bell but were incredibly snappy and showed promise. The second generation added a latex coating, bonded to the front and back with small holes through the cone helping to retain the bond, that tamed the tinniness.  A third generation replaced a foam piece placed around the edge with a edge suspension tweak to again remove edge ringing.

Lightweight drivers have impressive transient response and very low coloration bringing the dynamic driver technology closer to the real thing. Bozak woofers, made from a concoction of paper and wool, weighed in at a resounding 40 grams INCLUDING the voice coil and former, inner spider suspension, and outer edge suspension. The sound was (and still is) striking. If you ever get a chance to hear a pair of restored Bozak Concert Grands, listen to the difference in bass transients these refrigerator sized speakers create. It is quite different and extremely low in coloration. The bass drums are especially impressive even though the low frequency limit of the system is only about 40Hz.

Bozak B-410 Moorish Cabinet
Newton's second law of motion states that F=ma. This means that the amount of energy it takes to get something moving is directly proportional to its mass. Newton's second law is why low-mass drivers have such excellent transient response.

The compromise with low-mass cones is that the mechanical resonance is rather high meaning that very low frequency response is not possible (as compared to higher-mass cones with low mechanical resonance). Lower mass cones also demand a larger box and since most of the mass market did not have room for two more refrigerator-sized speakers in the listening rooms of their homes, this approach proved to be a challenge. As the public demanded deeper bass and smaller speaker boxes, this low-mass cone technology fell by the wayside.

Then came along plastics, specifically polypropylene. Such cones added their own sonic colorations (a hollowness) that was very different from paper or its composits (a thinness), although paper still retained good transient and impulse characteristics because of its lighter weight. PP so unnaturally colored the sound that another solution was needed.

The lessons learned from the paper-to-plastic transition fueled the search for unconventional cone materials. Without covering each, the memory of the transient response of light weight cones keeps coming back to the ears of their designers. Today, even exotic ceramics are being tested with excellent results.

So in summary, to get good transient response from a dynamic driver, cone mass must be kept low at the compromise of deep bass. When looking for new speakers to improve the sound of your system, your now mature ears may be telling you that it's the bass region you need to work on.

Yours for higher fidelity,
Philip Rastocny

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