Wireless covert earpiece
131-CD/TELE is a miniature in-ear earphone or
earpiece, manufactured between 1990 and 2010 by
in Ballerup (Denmark).
It is intended for use with covert radio systems
that are hidden under the user's clothing, and communicates with the radio
via an induction loop.
The device is derived from Danavox' 131-CD hearing aid,
but was not listed in their standard catalog.
The device consists of a pickup coil, an amplifier powered
by a miniature battery, and a speaker element, packed in a plastic
skin-coloured 1 enclosure that fits entirely inside the
From the outside, only the end-panel – with the battery compartment lid and
the volume control knob – is visible. In practice, this could easily
be concealed by combing the user's hair over it.
The device is powered by a standard G3-A (AG3) button-type battery,
that was typically swapped every day at the start of a surveillance task.
The molded enclosure was shaped for the left or right ear. 2 Depending on the
circumstances and the environment, the user would wear either the left
or the right earpiece, or both. For hygienic reasons, they were usually issued
to individuals, rather than with the equipment. The earpiece is used
in combination with an induction loop, fitted around the user's neck, or
a flat inductor that is hidden under the user's clothing, close to the
ear that holds the earpiece. The induction loop is connected to the user's
radio and sends the sound to the earpiece in the form of
The device is based on the Danavox 131-CD hearing aid, which has a built-in
microphone (M) and a telephone pickup coil (T). The latter can also be used
in theatres that have a hearing aid induction loop. 3 The 131-CD/TELE is
a simplified version, of which the microphone is missing.
Danavox in-ear units were used for covert surveillance by many special police
units throughout the world, and also by intelligence and security services,
until they were succeeded by smaller units that are fitted deeply inside the
ear canal and are completely invisible from the outside.
Different colours and materials were available on request.
The left/right variants are sometime identified by the colour of the
brand name, printed on the side.
Commonly known as an Audio Frequency Induction Loop System (AFILS).
The image below shows three Danavox inductive in-ear units of which the
rightmost two are the left and right variants. The brandname – Danavox –
is printed in blue and red respectively. At the left is a unit of which the
hinged battery compartment is open and the battery has been removed.
The units have a universal shape that fits most ear canals. The oval
end-panel measures just 10 by 16 mm. In some cases the units were fitted
inside customised enclosures that were molded specifically for the operator's
ears. Although this is more expensive, it offers a better acoustic performance,
as it fits more tightly and therefore suppresses ambient noise sources much better.
Covert radios that use this earpiece
Below is the block diagram of a typical setup. At the left is the
receiver-part of the (covert) radio, to which an induction loop with
several windings is connected instead of the loudspeaker. The loop converts
the audio frequency (AF) signals from the receiver to a magnetic field that
varies in the rythm of the AF signal (sound). If the induction loop is
worn around the neck, the magnetic field is wide enough to reach the
pickup coil inside the earpiece, where it is then amplified.
The advantage of using a neck loop, is that the magnetic field is identical
at either side of the body, so that the user can wear the earpiece in the
left or right ear, or even wear earpieces in both ears as shown in the block
diagram below. This is especially useful in noisy environments.
Although a induction loop (neck loop) offers the best performance, it is
sometimes inconvenient, as it has to be hidden under the user's clothing
before it can be connected to the radio. In such cases a flat induction
coil with a wide stray field could be used as an alternative. It can
easily be attached to the inside of, say, a coat by means of a safety pin.
The disadvantage is that the stray magnetic field is much narrower, as a result
of which it has to be placed close to the earpiece.
Flat inductors are often used with pre-wired radio harnesses or with
bullt-proof vests, that are worn under the clothing, and allow for quick
deployment. The inductor has to be placed
at a strategic position close to one of the shoulders,
near the ear in which the earpiece is worn.
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© Crypto Museum. Created: Saturday 19 June 2021. Last changed: Tuesday, 22 June 2021 - 19:04 CET.