The Business of Technology


Innovation in Loudspeaker Technology

A major problem in high quality reproduction of sound arises from fundamental defects in high-frequency, moving coil (i.e. regular loudspeaker) transducers.

All mechanical systems have fundamental resonances in which the structure of the system adopts a high amplitude of motion in response to a driving force.  This occurs in all loudspeaker systems, and the designer strives to ensure that these resonances are well outside the audible range.  For a high frequency transducer (or "tweeter") the transduction mechanism is a magnetically driven coil of wire bonded to a dome-shaped structure around 20mm in diameter. 

Conventional wisdom states that placing the first resonance above 20kHz is good enough.  However, a high-Q resonance can easily be driven into non-linearity, giving rise to intermodulation between out of audible band signals providing a distortion signal in the audible band.  Placing the fundamental resonance as high as possible is therefore of critical importance in providing the highest quality of sound reproduction.  Conventionally, the dome is made from either aluminium or titanium.  Both these choices give a resonance which is just above the audible band, at around 25kHz.

The key to solving the problem is in the correct material choice for the loudspeaker cone.  The self resonance is determined by the ratio of Young's modulus to density; the stiffer and less dense the material is the better.  It is also determined by the shape of the dome itself.

The optimal "simple" material is beryllium, and although adopted at one stage by Yamaha is usually precluded from domestic loudspeaker systems for reasons of extreme toxicity.
Diamond is the next most appropriate choice, but is eliminated as a result of manufacturing problems in the desired dome format, and cost.

The final choice is alumina.  This material is capable of moving the first resonance out to 35kHz using a conventional dome (i.e. a sector of a sphere), and 40kHz if this is optimised using FE analysis.  It is also non-toxic and easy to adhesively bond to.

A Japanese manufacturing capability for the domes was located, and the first loudspeakers to incorporate tweeters using this material were manufactured by Wharfedale.  They were particularly noted for their sweet, extended and undistorted high frequency response.