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