Genelec 8260A Technical Bulletin page 3

With the aim to develop a compact three-way product, the tradi-
tional waveguide simply had insufficient space. Hence Genelec
started to develop a new coaxial driver, which would combine
Midrange Frequency / High Frequency (MF/HF) waveguide for
proper directivity control. The basic solutions were developed in
the late 90's to prototype levels.
The enclosure design of the new 8260A DSP three-way loud-
speaker is based on the 8000 Series MDE™ die-cast aluminum
technology. With a 10" long throw woofer in a compact and un-
conventional enclosure the Low Frequency (LF) cut-off extends
down to 26 Hz (-3 dB). The new Minimum Diffraction Coaxial
Driver combined with a Directivity Control Waveguide provides
exceptional clarity and sound stage imaging. In addition to nor-
mal crossover functions the true three-way DSP system features
GLM™ network with flexible software controls and Genelec's
proven automatic calibration technology (AutoCal™).
Coaxial Driver Development
Coincident-source loudspeakers provide advantages over
spaced drivers, especially at the crossover region. Improvements
in imaging, response and directivity can be achieved. The basic
principle of a coaxial driver is to place the high frequency driver
at the apex of the low frequency diaphragm. Doing so, the appar-
ent source location does not depend on frequency and the direc-
tivity of both drivers may be controlled in the crossover region.
Furthermore, coaxial technology utilizes less front panel space
than comparable design with spaced drivers.
A very well known early coaxial design is Altec Lansing 604 Du-
plex from 1941, which had two magnet circuits. The tweeter horn
is protruding through the woofer magnet in front of the woofer
cone. This product design has had a very long life span. Another
very successful design was introduced in 1946, the Tannoy Dual
Concentric. It has no visible separate tweeter horn, instead the
curved woofer cone forms an essential part of the tweeter horn
originating inside the woofer magnet pole piece. Originally this
design also eliminated the tweeter magnet as the magnet circuit
had two gaps, a larger one for the woofer cone and a smaller
one in the rear for the tweeter. This design is still in production
although modified in several details. After neodymium iron boron
magnets became available, KEF, in 1988, introduced a new solu-
tion of coaxial based on Neodymium, starting a new era of radi-
cally different coaxial drivers.
BMS has designed interesting 2- and 3-way coaxial compression
drivers while Cabasse manufactured several designs, from 2-way
to 4-way direct radiators. Pioneer has refined the diaphragm ma-
terials, just to name a few companies who have been active in
this specific driver technology area.
Although conventional coaxial designs provide several key ad-
vantages, they also present drawbacks. The woofer cone move-
ment causes inter-modulation and Doppler distortion to the
tweeter radiation. The larger the cone displacement, the more
inter-modulation will be produced. The directivity, although im-
proved at crossover, may not be uniform at higher frequencies.
Also, most current common woofer-tweeter driver constructions
Figure 2: Frequency response of typical coaxial driver. Frequency
response from 1 kHz to 20 kHz.
have inherent acoustic discontinuities, which show as uneven
on-axis HF response. Whether these are judged to be impor-
tant depends totally on applications and the degree of accuracy
desired. As the history shows the benefits have exceeded the
drawbacks.
The common challenge in all coaxial designs is how to avoid the
said acoustical discontinuities and their effects on the response.
As diaphragms have to move, there have to be some gaps, but at
the same time sound is easily diffracted from these discontinui-
ties. The following picture shows the typical situation with some-
what uneven response.
The milestones in the development of coaxial drivers can be
summarized as follows:
• Altec Lansing presented their famous 604 Duplex coaxial
driver in 1941. The tweeter is horn-loaded and the horn pro-
trudes from the woofer cone apex. The frequency response
is irregular due to horn reflecting sound emitted by the cone
and due to the small dimensions of the horn itself.
• Introduced in 1946, the Tannoy Dual Concentric design over-
comes some of the problems. The tweeter horn is formed
inside an hollow pole piece and the woofer cone is an exten-
sion of the tweeter horn. Hence there are no reflections from
the horn itself. However, the tweeter response depends on
the woofer cone position and the joint between the woofer
voice coil and the cone causes diffraction.
• In 1981 Pioneer Electric Corporation introduced a 4-way
coaxial flat diaphragm loudspeaker. The diaphragms were
made from honeycomb sandwich material. This driver never
had any success.
• Tannoy published a paper called "Optimum diaphragm and
waveguide geometry for coincident source drive units". The
so-called Tannoy "Tulip" waveguide forms the internal pas-
sages for the phase plug and creates a spherical wavefront
which matches the apex of the cone. Improved bandwidth
and response are the claimed benefits.
• In 1988 KEF introduced the "Compound loudspeaker drive
unit". This design became possible having efficient magnet
materials which helped to reduce the overall dimensions of the
tweeter magnetic circuit. This design also relies on the woofer