Personal Wireless Audio Technologies

Showorks sells and rents a complete range of personal wireless listening devices.  These include radio frequency ( RF ), Infra-Red ( IR ) and Electromagnetic ( EM ) loop technologies.  There are advantages and disadvantages to each technology.  The discussions below attempt to explain the various options SHOWORKS offers and aid in the selection of the system that best meets your requirements.


Magnetic Induction Loop (IL) Systems:
Loop systems are the oldest wireless technology used to aid hearing impaired persons.  Until recently, these systems were thought to be archaic and production ceased in the United States.  Many users in Europe, with it's multiple jurisdictions, all with varied regulations regarding the use of RF devices, are once again advocating the use of  loop systems.  An induction loop system uses a loop of wire that encircles the listening area.  There are two types of systems, (1) direct audio frequency ( AF ) electromagnetic ( EM ) induction systems and (2) very low frequency ( VLF ) carrier modulation systems. 

In a VLF modulated loop system the audio is fed to the loop as an amplitude modulated (AM) fixed frequency carrier.  These systems are capable of providing multiple simulations channels by modulating each channel of audio on to its own frequency carrier.  Just as in all multiple channel radios, the receiver is equipped with a tuner ( A bandpass filter capable of filtering out all but the desired channel's carrier frequency ).  Once selected this modulate carrier is decoded to provide the transmitted audio signal to the listener.  Although the International Telecommunications Union (ITU) and the Federal Communications Commission (FCC) both reserve a band of frequencies at 3,155-3,195 kHz. for modulated loop systems, these systems are, to the best of my knowledge, no longer being manufactured.  It appears that at one time some hearing aid manufactures may have even included receivers for use with modulated systems. T-coil users cannot, today,  directly receive the modulated loop signals and must use a portable receiver.

Today most loop systems are the direct AF electromagnetic induction type.  In these systems the loop is attached to a driving device that is essentially an audio amplifier designed for the special challenges of driving a long loop of wire ( low impedance load, low voltage, high current ).  The IL system produces an amplitude modulated (AM) electromagnetic field around the wire.  Pick up Coils are located in the receiving device ( induction coil ) or hearing aid (T-coils).  These coils induce a small electrical signal proportional to the strength of any electromagnetic field they intersect.  A battery operated powered amplifier is used to increase the signal level of  these weak signals so as to drive a headphone or earphone.

Loop systems generally have the following advantages:

  • Require no pick up device ( receiver ) for those using hearing aids equipped with T-coils.

  • Require purchasing/maintaining/replacing fewer portable receiving units ( only enough receivers for those without T-coils ).

  • Are inconspicuous: No need to display "I am hard of hearing!" Loop systems offer an easy and invisible solution to an invisible problem when used with a T-coil.

  • Are unobtrusive:  Loop wire can be hidden under carpet or baseboards where it is not as visible as an antenna or IR emitter.

  • Signals received by a T-coil are fed directly through the internal prescribed processing of the hearing aid to better meet the individuals hearing needs ( Receiver based systems may accommodate T-coil users with neck loops that plug into the receiver's earphone jack.  Neck loop users also have all the benefits of passing the received audio through the hearing aid's internal processing ).

  • Works in areas where the signal must be localized to not interfere with other users

  • Transient situations such as ticket counters and drive-thru windows where other assistive listening systems are impractical can often be served without distributing special receivers.

  • Preclude bothering others nearby with sounds leaking from headset or earphones. Sound broadcast directly to an in-canal  hearing aid is almost entirely contained within the listeners ear.

Some disadvantages of loop systems include:

  • A relatively small percentage of the hearing impaired population ( <20% ≈ 5 million of the 28 million hearing aid users in the US ) have hearing aids which contain T-coils, and the number of  T-coil users has been decreasing with the trend toward the use of smaller "in-canal" aids.  T-coils were originally added to hearing aids to deal with the problem of acoustical feedback when the large flat plastic earpiece of a telephone handset was held close to the hearing aid.  ( As a large [ relative to the microphone diameter ] boundary is placed close to a microphone the frequency response is modified with major peaks usually appearing between  2 and 3 kHz.  This extra gain combined with normal leakage around the aid or through the vent results in the loud, uncomfortable feedback squeal. ) With the advent of in-canal hearing aids the microphone was recessed into the canal and so the hand set did not cause the feedback squeal, and the T-coil was no longer needed.  Also, as the size of the hearing aid was reduced to fit into the canal there was just not enough room to include a coil.  When used with portable receivers by users without a T-coil, almost all of the advantages over other systems are no longer relevant.

  • Loop systems are often more costly to install due to the loop installation requirements. 

  • Loop systems can be extremely susceptible to interference from other sources of magnetic fields such as electric motors and fluorescent lights.

  • Loop systems may not be a good choice for very large areas as the maximum size of the loop is limited.

  • Spill-over of the magnetic field into adjacent areas can be a problem when several systems are needed in adjacent areas such as multiplex theaters.

  • Variations in the electromagnetic field strength within the loop may cause dead spots.  This becomes a bigger problem as the size of the coverage area increases.

  • Variations in quality that result from the physical orientation of the pick-up coil in relation to the plane of the loop ( noticeable changes can even occur when the listener moves their head and with it the T-coil mounted in their hearing aid ).

Infrared (IR) Systems:
Infrared systems are the newest assistive listening technology.  These systems use harmless modulated infrared light ( slightly lower in frequency than the band of light visible to the human eye ) to transmit the audio signal to listeners.  An array of infrared light emitting diodes ( LEDs ), mounted in a panel, are modulated in intensity ( brightness ) by an electrical circuit that encodes the audio signals.  Special receivers equipped with an infrared detector ( photo detector diode and optical lens ) pick up the transmission.  The receiver then extracts the original audio information from the modulated ( encoded IR light ) signal and amplifies the audio to drive headphones or earphones. 

IR systems modulate the audio signal on a high frequency "Carrier."  Options are provided on most systems to select from two or more carrier frequencies.  Just like in RF transmission of Radio or TV signals this allows the use of several channels  at once ( on individual carrier frequencies ) or the selection of a frequency that is not interfered with by ambient light sources.  Mutable channel IR systems are often used to provide an individual channel for each language for simultaneous language interpretation.

Infrared systems generally have the following advantages:

  • Generally provide exceptional sound quality which is appreciated in applications like language interpretation where most users are not hearing impaired.

  • Provides increase security and privacy.  IR cannot, in most cases, pass through solid objects like walls and curtains.  IR transmissions can, therefore, be contained within the desired environment  This can be extremely beneficial in installations such as courtrooms, boardrooms, medical facilities or military installations where security and privacy are very important.

  • Is impervious to a wide range of electro-magnetic interference ( EMI ), including all radio and TV signals.

  • Adjacent rooms in a facility can use IR systems without fear of inter-room interference.  This becomes of prime importance in facilities such as schools, universities and multiplex theaters.

  • Available in a wide range of sizes, frequencies and number of channels to fulfill individual customer requirements.

Some disadvantages of Infrared systems include:

  • Normally speaking, IR systems require a strict uninterrupted line-of-sight between the IR emitter and the transparent lens on the receiver.  Problems with line-of-sight limitations can be mitigated in some rooms with light colored walls or ceilings ( the IR light waves are reflected off the light colored surfaces ), or by adding extra emitter panels.

  • The same properties of light that add to security and privacy make it impossible for staff to monitor the meeting or service from adjacent rooms or the use such facilities for over flow purposes.

  • IR systems are often more costly to install due to the number of emitters required and the need to provide signal and power cables to each emitter. 

  • IR emitters are relatively inefficient, several may be required to cover the listening area.  The area covered is further reduced as the number of modulated channels is increased.  IR systems may not be a good choice for very large areas due to the large number of emitters required to achieve adequate coverage and the high cost of  equipment and installation.

  • High efficiency fluorescent lighting systems produce a harmonic modulation in the infrared spectrum that can interfere with IR reception.  Until recently 95 kHz. has been the unofficial RF sub-carrier for IR hearing impaired systems.  Compatibility between venues was a major advantage of IR systems for hearing impaired persons.  The wide spread use of these lighting systems in commercial spaces has caused manufactures to offer secondary carrier frequencies( i.e., 250 kHz, 2.3, 2.8, 3.3, 3.8 MHz.).  95 kHz.  IR systems should be avoided in locations utilizing high efficiency fluorescent lighting.

  • Sunlight, which contains a great deal of infrared energy may bury the emitters IR output signal in random noise making the audio signal impossible to receive or raising the background noise in the audio to the point that the system is unusable.

  • IR light emitting diodes have a limited operating life and 'burn out" with time.

Radio Frequency (RF) Systems:
The third type of wireless listening system uses radio frequency (RF) technology to transmit the audio to the listener.  This system is simply a low power variation on the commercial FM radio services that bring music and news program into your home or auto.  The signal is "broadcast" on radio waves by a frequency modulation (FM) transmitter connected to a tuned antenna in the listening area. The radio waves radiating from the antenna are picked up by listeners using an "FM radio" receiver tuned to the transmitting frequency ( See our TechNote About Radio Transmission for more information on how RF transmitters and receivers work).

The Federal Communications Commission (FCC) allows the use of two bands of frequencies for these RF transmissions: 

(1) The first, provided by the FCC, is located between 72 and 76 MHz. ( Actually only three sub-bands are authorized 72.0 - 73.0, 74.6 - 74.8 & 75.2 - 76.0 MHz. ).  Transmissions on these bands are covered by Title 47 Part 15.237 for "auditory assistance devices" See About FCC Regulations for more information.

FCC regulations do not specify exact transmission frequencies only a maximum of 200 kHz bandwidth.  Most manufactures divide the available spectrum in to10 - 200 kHz. ( audio bandwidth to 15 kHz. ) wideband channels or 40 - 50 kHz. ( audio bandwidth to 10 kHz ) narrowband channels.  The terms wideband and narrowband refer to the amount of available RF spectrum  each channel can occupy ( Bandwidth, 200 kHz or 50 kHz respectively.   See our TechNote About Bandwidth for more detailed information on RF bandwidth).  As you might expect the wider bandwidth channels produce a higher fidelity ( Less noise [ Technically, a higher S/N ratio ] and extended flat frequency response ) audio signal at the expense of having fewer available channels ( Simultaneous transmissions ) available from the allocated band of frequencies.  See the 72 MHz Compatible Chart for a detailed listing of the channels available from various manufactures.

Their are, however, several limitations to the 72 - 76 MHz band. 

(a) The first is a result of the FCC regulations which limit the maximum radiated transmitter power in terms of field strength, not power ( Watts ), as is most often the case for RF transmitters. The rule reads: 'The field strength of any emissions within the permitted 200 kHz band shall not exceed 80 millivolts/meter at 3 meters."  This is a extremely limiting regulation on the area that we can cover effectively. If we improve the antennal the out put power must be reduced to keep the field strength within the regulation at 3 meters form the antenna.  In the absence of any local interference ( not often the case ) the legal field strength is enough to cover school rooms and most churches adequately.  The largest of churches may have to resort to locating the transmitter away form the audio control point to put the transmitting antenna closer the the users in the pews.  Larger facilities such as arenas and large auditoriums will not be adequacy covered by the standard non-licensed hearing impaired transmitters.  Fortunately the most of these larger facilities can be covered with special licensed transmitters.  SHOWORKS can facilitate the application, licensing and installation of these higher powered transmitters for qualified customers.

(b) The second limitation is that interference form local transmitters licensed for other services is very common ( especially in large cities ) on these frequencies.  There are many potential sources of such interference.  These include local TV stations on channel 4 or 5, beeper transmitters, emergency call box systems and airport approach warning beacons.  To make the situation worse, section 15.5 of the FCC regulations states: "Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator."  In other words, when a authorized frequency in this band is rendered unusable by a interference there is nothing to do but look for the availability of additional frequencies that are not interfered with.

(c) The third consideration for this band of frequencies is the maximum number of simultaneous channel transmissions possible with out interference. This can be a bit confusing because the various manufactures have differing statements on the subject....

(2) The second band of frequencies, referred to by the FCC as the Low Power Radio Service (LPRS), is located between 216 and 217 MHz.  Federal regulations provide for 19 - 50 kHz. ( audio bandwidth to 10 kHz. ) narrowband channels or 38 - 25 kHz. ( audio bandwidth to 6 kHz ) very narrowband channels. This is a little confusing because the wider of the authorized channels on the 216 MHz band is the same bandwidth as the narrower bandwidth channels on 72 MHz. 

Availability of transmitters on the 216 Band can also be an issue. Both Listen and Williams offer portable body pack transmitters in the 216 band, but only Listen offers a permanently install transmitter ( Rack mount and 115V AC wall powered ) in the 216 MHz. band.  The Williams TGS-100 belt-pack transmitter offers a choice between 4 narrowband channels (E, G, J & L).  Listen offers both portable and permanent installation transmitters that transmit on all 57 authorized frequencies.  See the 216 MHz Compatible Chart for a detailed listing of the channels available from various manufactures.

[Coming: multi-path interference problems and number of available channels]

RF systems generally have the following advantages:

  • Can be used indoors or outdoors

  • Not affected by sunlight or weather

  • Can easily cover large areas ( 216 covers larger areas ( up to 3000 Ft. from the antenna ) and repeater transmitters can be used for extremely large coverage areas like golf courses.

  • Easiest technology to provide even coverage at all seat locations.

  • Not interrupted by physical objects in room.

  • Portable battery operated transmitters practical for tour guide and other mobile applications.

  • Good fidelity available using 72 MHz. wide band channels.

  • Not susceptible to local electrical interference from electric motors or power lines.

  • Not susceptible to IR spectrum output from high efficiency fluorescent lighting devices.

  • Many options of frequency and band width to tailor system to customer's exact requirements.

  • Can be used for meeting monitoring outside of meeting room.

  • Lowest cost of installation.

Some disadvantages of RF systems include:

  • Restricted access, receiver is necessary for each user.

  • Compromised Confidentiality, complete privacy is not possible.  FM signals pass through walls and can be monitored from a considerable distance with sensitive receiving equipment and antennas.

  • Susceptible to radio frequency interference from local sources or other radio services such as beeper transmitters and certain TV channels.


SHOWORKS has experience using RF systems both indoors or outdoors, They are not affected by sunlight light or magnetic interference such as high voltage power lines.  They can cover very larger areas such as arenas of even entire golf courses using repeater transmitter and antennas located high above the terrene.

Call or e-mail for any additional information you may require.


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