Fostex NF-1, Technical White Paper

Page: 5 / 16

- 5 -
One of the properties of the HP structure is the
unlikelihood of development of inherent timbre
because of widely dispersed resonance.
This finding has made it possible to use, without
encountering any particular major
resonance, materials such as metal and carbon
fibers having high propagation velocity and low tan
δ
(internal loss) which used to be regarded as
properties to be overcome. The Figures 2 and 3
indicate that resonance is shifted up on HP
diaphragms by one decimal point and is narrower,
when compared to a plane surface. This represents
that HP diaphragms provide higher strength and
finely dispersed resonance. This property of
dispersed resonance can be used as a good proof
that HP diaphragms are ideal where no particular
resonance disturbs reproduced sound.
3) HP diaphragms, easy to analyze and
excellent in reproducibility of simulation
HP diaphragms' complexity in configuration can be
overcome by development of a design method
which involves CAD/CAM techniques, whereby
optimal configuration design of curved surfaces and
mold machining will become possible.
Configuration of HP shells is optional, i.e., the
number of divisions and degree of torsion of curved
surfaces (height of ridges and depth of troughs) can
be determined by the aimed response
characteristics/sound quality. In addition, articulate
configuration, though the curved surfaces are
complex, provides ease in analysis and
reproducibility of simulation.
Figure 5 represents a conventional straight cone.
Cone break-up is present at 596 Hz in the axis-
symmetry mode (in the direction of circumference),
causing the 3rd harmonic distortion which is said to
be detrimental to sound quality, to develop.
Figure 6 shows an HP diaphragm. Axis-symmetry
mode resonances can be seen at neither of the
vicinity of 596 Hz nor at any other frequencies.
At this stage, we can say that it is has been
successfully verified that the use of HP diaphragms
can surmount the shortcomings of conventional
cones, eliminate speaker-specific sound
colorization, and finally, attain ideal speakers
capable of offering precise sound reproduction.
Figure 4 presents comparison data of frequency
responses taken from 16 cm woofers with an HP or
conventional straight diaphragms. The HP
diaphragm speaker is free of evidence of either dips
at around 600 Hz or peaks at the high end of around
4500 Hz due to anti-resonance of the edge. Figures
5 and 6 compare the FEM modal analysis results.
20 50 100 200 500 1000 2000 5000 10000 20000
50
60
70
80
90
100
110
(db)
Hz
Figure 4
Comparison of frequency responses of
a straight cone and HP diaphragm
Figure 5
Figure 6
FEM modal analysis of a straight cone
FEM modal analysis of an HP diaphragm
Large split resonance is seen in
the direction of the circumference.
No axis-symmetry mode
resonance is occurring.
HP diaphragm Straight cone