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The transmission-line principle
Loudspeakers which work on the transmission-line prin-
ciple have proved outstandingly effective in the past, and
has employed this principle for many years with
great success. The basic advantage of this type of con-
struction is highly dynamic bass response, extending
down to very low frequencies.
In a transmission-line cabinet the sound energy gener-
ated by the rear of the bass driver’s cone is routed
through a long tunnel (the transmission line) and exits via
a large opening.
The duct must be long to be effective, and for this reason
the cabinet must always be relatively large. In our de-
signs the transmission line is folded in order to produce
extreme length, and this results in a very low bottom limit
frequency. The great advantage of this principle is that
the low frequencies are amplified, and the resonance /
impedance peak is effectively damped, giving an ex-
traordinarily dry, dynamic and clean bass even at high
sound levels.
The complex internal construction of the TCI cabinet
allows our designers to fine-tune the transmission line
very accurately; for example, internal reflections are
reduced by additional diagonal sound guides. A by-
product of the internal ducting is a much stiffer cabinet
overall.
The bass drivers are installed in a large pressure cham-
ber with the cone surface facing out. Since this chamber
is large in volume, and terminates in any case in the
open transmission line, these units do not have to waste
much energy in acting as a compressor on the enclosed
air when the cone moves in and out.
The transmission-line principle has one inherent draw-
back: a lack of linearity in the upper bass response (100
Hz - 250 Hz). This results from superimposition of parts
of the sound (interference effects) from the drive unit
cone and the transmission line opening.
To counter this problem has developed an innova-
tory solution which exploits the advantages of the trans-
mission-line principle, but exhibits good linearity in the
frequency response.
The back of the duct is lined with a special absorbent
material in the form of an open-cell foam which pos-
sesses very high sound absorption values, but only at
certain frequencies. This material makes it possible to
damp out unwanted sounds which are radiated from the
back of the drive unit. At the same time the transmission
line damps the resonance peak of the bass unit, pro-
ducing very good timing characteristics.