HK Audio CDR 210 F - Manual (Page 8)

Manual
COHEDRA
® & COHEDRA ® Compact
3.1 The High Frequency
Wave Front
A specially developed and patented acoustic lens
serves to curve the wave front. It transforms the
spherical wave emitted by the high frequency driver
into an inward curving wave front.
To shape a curved wave front, the sound wave's
inner components are re-channeled into a bisected,
centrically symmetrical array. In contrast to an axial
symmetrical array, a centrically symmetrical array
has the advantages of lower manufacturing tole-
rances. What's more the subdivided cavity enables
fewer standing waves, thereby reducing interference.
Typically, these constructions are relatively long
to minimize the differences between S1 and S2.
However, acoustically speaking this creates some-
thing approaching an organ pipe. The only remedy
is to connect a very large horn, an option that for
technical reasons is often infeasible. A covering
with a porous absorber is another possibility. Desi-
gned to reduce the velocity jump in sound particle
acceleration, this covering must be acoustically very
transparent.
To this end, COHEDRA ® employs a special synthetic
fiber fleece with defined aerodynamic resistance.
This absorber reduces the velocity jump in sound
particle acceleration, thereby helping considerably
to homogenize frequency response. Figure 8 il-
lustrates the fleece's effect. The red curve shows
amplitude without fleece, the blue curve with
fleece. The two curves are depicted in staggered
formation only for the purpose of illustration.
Figure 7: The COHEDRA® acoustic lens
Figure 8: Effect of the synthetic fiber fleece
Figure 6: The COHEDRA® acoustic lens’ mode of operation