Handbook for Sound Engineers

Assistive Listening Systems 1551

• High immunity to unwanted or external radio signals.


• More than ten simultaneous channels.


• Good fidelity for voice applications.

FM broadcast is much like wireless microphones;
more information can be found in Chapter 16.
An FM assistive listening system by Listen Technol-
ogies LS-04 Installed System includes programmable
receivers, and a charging case and rechargeable
batteries, Fig. 42-9. The LR-500 receiver can be
programmed to receive only the channels available at
the venue. This system helps public venues to meet
Americans with Disabilities Act (ADA) guidelines. It is
easy to add receivers and a wireless speaker/receiver.

The system has an SNR of 80 dB and is available for
72 MHz, 216 MHz, or 863 MHz band. The system
includes one LT-800 stationary transmitter, an antenna
kit, rack mount kit, and four LR-500 programmable
display receivers with ear speakers.

The Personal System by Listen Technologies
includes a portable transmitter and receiver in a soft-
sided carrying case. The Personal System’s LT-700
portable transmitter, lapel mic, LR-400 display receiver,
and ear speaker all fit in a soft case so they can be taken
to school, house of worship, or theater. The listener
gives the transmitter with its microphone to the
presenter and the listener uses the receiver with an
LA-166 neckloop or LA-164 earphone. The neckloop
generates a magnetic field that is picked up by hearing
aids that are equipped with a T-coil, Fig. 42-10.
The Sennheiser Mikroport 2015 is suited for class-
room use and allows a hearing impaired student to have
an improved learning experience via wireless audio
connection to the teacher. The system includes a wire-
less transmitter with a lavalier microphone worn by the

teacher and a body worn wireless receiver for the

student. Direct audio input cables are available for use

with cochlear implants and hearing aids and induction

neck loops for use with T-coil hearing aids. The systems

can also be used with standard headphones or ear buds.

Multiple receivers can be used with a single transmitter

and, because there are hundreds of discrete frequencies

available, systems can be used in adjoining classrooms

without crosstalk or interference, Fig. 42-42-11.

42.2.4 AM Broadcast

AM broadcast systems are largely unregulated by the

FCC in the United States. The International Standard

IEC118 Parts 1 and 4 define the performance, field

strength, frequency response, and spurious levels for

induction loop and hearing aids. The European Standard

EN 60118 is also applicable in Europe. The basic rules

(from FCC Bulletin OEC 12 dated July 1977) are:

• The system must not cause any interference to an
  existing licensed service.


• The operating frequency must be in the AM broad-
  cast band or below (10 Hz–490 kHz and 510–
  1600 kHz).


• An open-wire antenna may be used in the 510–
  1600 kHz band providing it does not exceed 10 ft in
  length and providing the transmitter is restricted to an
  input power of 100 mW. Any type of antenna and
  transmitter power may be used provided the field
  strength does not exceed:
  •(2400/F)μV/m at 300 m in the 10–490 kHz band.
  •(24,000/F)μV/m at 30 m in the 510–1600 kHz band
  where F is the carrier frequency in kHz.

Figure 42-9. An installed FM assistive listening system.

Courtesy Listen Technologies Corporation.

Figure 42-10. Neckloop and earphone. Courtesy Listen

Technologies Corporation.

A. Neckloop.

B. Earphone.

Rear view Front view