We sometimes encounter audio component descriptions proclaiming the glory of super-hard materials for component isolation or sonic benefit. As an example, describing a turntable platter: “The reason for using such an exotic and difficult to produce material? It is the hardest material known to man apart from diamond!” We're sure this is a very fine platter, but not solely because of the hardness of the material.
Hard materials are of course necessary parts of audio systems. A material’s hardness, on its own, has little or nothing to do with how that material will interact sonically with an audio system, though. Most very hard materials readily relay vibrations of virtually all frequencies. Very hard materials can be brittle. Some resonate profusely.
Herbie’s Audio Lab has experimented with hundreds of hard materials to assess their sonic qualities in audio systems. We have found no direct relation between hardness and degree of sonic neutrality.
Boron nitride, at the very elite top of hard materials along with diamond, when placed into the direct vibrational path of audio gear, introduces a strange sonic twang in the upper frequencies. So do carbon, silicon carbide, tungsten carbide and other ceramics. The sonic twang is different with each material, while some are more pronounced than others. In addition to these colorations, considerable differences in dynamic impact occur, with muddling in some frequency ranges or raspiness at the edges of dynamics at other frequencies. Each material has a unique and individual character or coloration.
One of the materials that we’ve found to be closest of all to sonic neutrality, as far as transfer of vibrations through the material, is fluorite. At only 4.5 on the Mohs hardness scale, this material is soft (though to the touch it seems as hard as glass). Ruby and sapphire, much harder (9 on the Mohs scale), are also very sonically neutral, with audible results almost identical to fluorite. Yet, a vast array of minerals that fall in-between these have sonic results that are all over the place.
Some hard materials will shatter easily but not bend; others will bend but not shatter. Some are heavy, others relatively light. We believe that these and other qualities in combination with a material’s hardness, contribute to the end result sonically. Like hardness alone, however, not one of these other qualities on its own independently determines a material’s sonic quality.
Many materials share virtually the same physical properties: same hardness, same density, same elasticity. Why is it then that they would have considerably different vibrational and sonic qualities? We believe it’s the atomic structure--not the individual atoms themselves–but the ways they are linked together in latices. Hard materials form crystals or latices that are like fantastic three-dimensional tinker toys. Vibrations pass through these latices differently from one material to another. In some materials, like graphite, vibrations of different frequencies will take different paths and follow different planes, ending up with phase relations different from where they started.
There’s no such thing as a single vibrational frequency that an audio system, component, or device has to deal with. There are usually one or more prominent frequencies, but the vibrational environment in virtually any audio system covers the whole spectrum. Vibrations that interfere with audio reproduction spread from below the lowest frequencies to way, way above the highest frequencies that audio electronics can render. We believe much of the distortion and loss of “life” in audio reproduction is caused by vibrations, both very low and very high, that are outside the audible range. These vibrations “tweak” music in the audible range by altering their phase relationships and introducing other anomalies. Very low vibrations can cause glare in upper frequencies in addition to bloopy bass; conversely, very high frequencies can adversely affect bass response.
Some component designs will dictate because of physics that a hard material be used, the harder the better. For example, a rigid-design roller bearing. Arbitrarily deciding on a tungsten carbide ball because it is harder than another kind of ball won’t necessarily deliver the best sonic result solely because of its hardness, though. Tungsten carbide will give a less bright and raspy sound than chromium steel, but not generally as neutral as some stainless and carbon steel alloys, or ruby.
In audio applications like roller bearings, any degree of hardness equivalent to carbon steel or higher should be sufficient. An audio component’s vibrational environment isn’t going to cause any compression of the material, even microscopically. Harder materials tend to be but are not always necessarily “faster” than softer materials.
Though tungsten carbide and other materials aren’t perfectly neutral sonically, their actual contribution to the audio result depends on complementary aspects. For example, the complementary influence of the ball with the “cup” material it rests in. If the cup presents a coloration we might call “blue” and the ball a coloration we might designate as “yellow,” the end result would be an audio coloration we might just call “green.” If used together with the right complementary materials, tungsten carbide can deliver superb or likeable audio results.
Carbon fiber composites are used for many audio isolation devices nowadays. The problem with carbon fiber, however, is that it has its characteristic “twang.” It’s frustrating, because carbon fiber brings out really fast dynamics and is very close to sonically pure. Not close enough for the cigar, though. And that is why so many of the carbon fiber products use other materials laminated in-between carbon fiber layers to alter the coloration.
Simply mating two or more materials that have different sonic colorations will not necessarily cause each to cancel the other out. Instead, you’ll usually get just a different color.
Steve Herbelin
Herbie’s Audio Lab
