690 Chapter 23
modes and decrease stiffness for the concentric modes. By correctly proportioning the
number, width, and wall thickness of these corrugations, the outer edges of the cone are
progressively decoupled as frequency increases. This results in the “ working ” diameter
of the diaphragm being reduced at high frequencies, thus improving the high-frequency
performance.
23.12 Material
Hard impregnated or fi lled pressed calendered papers are used when loudness effi ciency
and apparent high-frequency response are important. The impregnant is usually a hard
thermo-setting resin. The radiation response provides very little dissipation in direct
radiator cones, hence by using paper having low internal fl exural losses, the transmission
line is made to have strong resonances. The transient response of this type is necessarily
poor, as noncenter moving modes of the cone are unappreciably damped by the motor
unit. Soft, loosely packed, felted cones are used when some loss in the high-frequency
response can be tolerated and a smooth response curve with reduced transient distortion
is required. The apparent loudness effi ciency of high loss cones of this type is anything up
to 6 dB lower than that of low loss cones of similar weight.
In an effort to overcome the intransigencies of paper cones, resort has been made to
other materials. Light-weight metal (aluminum alloys, etc.) immediately springs to
mind because of its stability, homogeneity, and repeatability but, because of the very
low internal frictional losses, strong multiple resonances occur in the upper frequencies.
A diaphragm of, say, 250 mm in diameter made from a 0.1-mm-thick aluminum alloy
with a total mass of 40 g will show a “ ruler ” level response up to approximately 2 kHz
when multiple resonances occur. These are extremely narrow band (in some cases only
1 or 2 Hz wide) with an amplitude of anything up to 40 dB and an effective Q of several
hundred. Putting a low-pass fi lter cutting very sharply at, say, 1 kHz does not eliminate
shock excitation of these resonances at low frequencies and the result is a “ tinny ” sound.
Reducing the cone diameter and making the fl are exponential reduce this effect and
also places the resonant frequency a few octaves higher, but does not entirely eliminate
the problem. Using foamed plastic materials (and sometimes coating the surfaces with
a metal to form an effective girder structure) has met with some success. There are
problems associated with the solid diaphragm in that the different fi nite times taken for
the sound wave to travel directly from the voice coil through the material to the front
and along the back edge of the diaphragm to the anulus and then across the front cause