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1. The reader shall understand the various considerations in selecting appropriate assistive listening technologies to suit specific applications/tasks.
2. The reader shall understand how induction loop assistive listening technology has evolved to overcome past limitations, making it more appropriate for a wider range of applications.

Abbreviations:

ADA:  Americans with Disabilities Act
ALD:  Assistive Listening Device
ALS:  Assistive Listening System
IEC:  International Electrotechnical Commission

Abstract
A comprehensive review of the hearing aid telecoil would be incomplete without a discussion of induction loop assistive listening technology. Based on the classic scientific principles of alternating current and resultant electromagnetic fields, the applications for this technology continue to grow more than 60 years after the advent of the first telecoil-equipped portable hearing aid. 

In the following article, the authors present a technical primer and several scenarios that illustrate the basic principles and typical applications for induction loop assistive listening systems. They then provide a historical perspective and an overview of current standards and state of the art technologies. The authors conclude that the flexibility and performance of hearing aid telecoils will continue to evolve, along with induction-based assistive listening systems, as more hearing health care professionals demonstrate to their patients the potential of this useful technology. 

Introduction
The United States Architectural and Transportation Barriers Compliance Board (Access Board) has published accessibility guidelines for buildings, public facilities and public transportation as required by the Americans with Disabilities Act of 1990. In regard to assistive listening systems, the guidelines state the following:

“Assistive listening systems (ALS) are intended to augment standard public address and audio systems by providing signals which can be received by persons with special receivers or their own hearing aids, and which eliminate or filter background noise. The type of ALS appropriate for a particular application depends on the characteristics of the setting, the nature of the program, and the intended audience. Magnetic induction loops, infra-red and radio frequency systems are types of listening systems which are appropriate for various applications.” (1) The premise of all ALS is to maximize the desired signal relative to the background noise (signal to noise ratio) by, in effect, bridging the distance between the sound source and the hard of hearing listener.   

You say ALS, I say ALD…
In late 2001, the Access Board published new guidelines (www.access-board.gov) pertaining to assistive listening systems, defining this technology as:  “…devices which enhance the sound quality and volume of public address systems for persons who are hard of hearing, including those who use hearing aids.”  While there may not be an “official government ruling” that differentiates an assistive listening system (ALS) from an assistive listening device (ALD), professionals in the field generally refer to technology that is at least partially installed (for example, an auditorium’s fixed FM transmitter, installed infrared emitter or induction loop wire) as being part of an assistive listening system.  A body worn battery powered personal amplifier is referred to as an ALD.  Similarly, a portable battery powered FM transmitter and matching receiver are generally referred to as ALDs.  In short, “devices” are very small and “systems” are big.
  
ALS Selection Considerations
There are numerous considerations involved in the selection of the most appropriate ALS for a given application (2).  Some of these considerations include:

• Is the ALS appropriate for the size and location of the listening area?
• Are there architectural barriers that might affect ALS effectiveness?
• Is signal spill over a concern?  
• Is confidentiality important?
• Do the people who will be using the system have telecoil-equipped hearing aids?
• Is there local interference from radio stations, electrical systems and/or lighting?
• Who will install the system?
• What are the maintenance requirements and associated costs?

Induction Loop ALS Primer

Any wire that carries an electrical current inherently generates an electromagnetic field.  A more efficient generator of electromagnetic fields results when the wire is wound in a multiple turn coil. Hearing aid compatible telephones and induction loop ALS exploit this basic premise of electronics by generating electromagnetic fields, respectively, from a tiny coil inside the telephone handset, or from a loop of wire that encompasses a room.  The compatible telephone and the induction loop ALS both generate electromagnetic fields that correspond to the original audio signal.  In both cases, the basic premise of ALS remains constant…to minimize the deleterious effects of background noise by bridging the distance between the sound source and listener

 Up until the 1970s practically all telephones were compatible with hearing aid telecoils due to the substantial electromagnetic leakage generated by the handset’s magnet-coil earphone transducer.  Later telephone technologies introduced smaller and more efficient components that resulted in less leakage, thereby sacrificing telecoil compatibility.  Activism by hard of hearing people resulted in public laws (such as PL100-394) that eventually required all telephones to be hearing aid compatible.

The induction loop ALS consists of three components:  
1. The audio source (microphone or audio signal from existing public address system)
2. A special loop amplifier
3. The loop wire

In the typical induction loop set up, the loop wire is installed around the perimeter of the targeted seating area.  The audio signals from microphones or the public address system are amplified and sent through the loop wire, resulting in an electromagnetic field that mirrors that frequency and intensity characteristics of the original signal input.  By switching the hearing aid/cochlear implant or receiver to “T” (telephone/telecoil) the hard of hearing listener effectively disconnects the device’s microphone and in its place connects  a small coil of wire to the input of the hearing aid amplifier.  This coil of wire (the telecoil) is very sensitive to nearby electromagnetic fields.  The electromagnetic field generated from the telephone or induction loop system induces a corresponding signal in the telecoil, which is then amplified and delivered to the listener.  This process of inducing an electrical current in a circuit as a result of a nearby electrical current flow is called induction, hence the term induction loop system.  

Telephones and Induction Loop ALS
There are distinct similarities in performance and technical considerations between hearing aid compatible telephones and induction loop ALS:

• Both are sensitive to the distance and orientation of the hearing aid telecoil relative to the plane of the generating field.  Depending on the physical dimensions and power of the electromagnetic field, the resultant signal will be diminished with distance.  With the telephone, the listener’s hearing aid is closely coupled to the field generated by a small coil within the handset receiver, resulting in minimal signal loss as a function of distance.  With room-size induction loop ALS, sufficient power must be provided by an amplifier that is current/impedance matched to the loop wire in order to create a magnetic field of sufficient quality and strength as per specific standards discussed later.  The fields generated by telephones and induction loop ALS are perpendicular to the plane of the telephone handset coil/induction loop.  Because of the differences in field orientation between telephones and induction loops (including neckloops), the best mounting for telecoils is a near-vertical position.
• The performance of both may be affected by spurious electrical noise from  sources such as power lines, fluorescent lighting and cathode-ray tube televisions/computer monitors.  Switching a hearing aid to “T” transforms the instrument into a magnetism sensing device.  In some instances, electromagnetic fields generated by AC power lines and other electrical equipment may cause interference to hearing aid telecoil users.  In most cases, to pick up interfering noise or buzz, the listener must be situated in very close proximity (within 2 feet) of fluorescent lighting, TVs, or other electrical equipment.  A pre-installation listening check should be performed before an induction loop ALS or hearing aid compatible telephone is installed in order to identify, and if possible, remedy potential sources of interference.  Examples of easily correctable sources of interference include:  faulty lighting ballasts, damaged surge protectors, and poor quality light dimmers.  In some instances, the use of an induction loop ALS may be ruled out or a telephone relocated due to high levels of electrical noise interference.
• While telephone cross talk or spill over is not a problem due to the very small fields generated by the hand set receiver,  multiple induction loop ALS may interfere with each other.   The electromagnetic fields created by large-area induction loop ALS inherently spill out of their looped areas.  This can cause interference problems for users of other nearby induction loop ALS or telephones.  To date, a practical electrical noise cancellation/shielding method has never been successfully marketed for ALS applications.  In addition to the obvious remedy of proper planning (for example, spacing loop systems on the same floor by at least twice the distance of the longest loop side), two technical approaches do exist:  low-spill induction loops and the 3-D Loop.  The former approach usually involves the use of a secondary cancellation field that partially suppresses the signal spill over from the primary loop field.  The newest approach is the 3-D Loop that minimizes spill over by utilizing a complex grid of loop wires sandwiched within a flexible mat powered by digital signal processing.  

As detailed in the next section, international standards for induction loop ALS were published in 1981, resulting in a consistently high level of performance and a resurgence of interest in this technology. Validation was not far behind. Subsequent comparative studies proved that a well designed induction loop ALS is as effective as FM and infrared technologies for improving the intelligibility of the spoken word (10).

Beyond the technological reasons, it is interesting to explore the cultural reasons why the use of induction loop ALS in public facilities has continued to hold steady in the United Kingdom and Scandinavian countries for decades.  Most importantly, for many years the national public health care systems in these countries have made the telecoil standard equipment in most dispensed hearing aids. As a result, practically all public facilities are equipped with induction loop ALS. Supporting technical standards have been developed and enforced by national organizations in Europe such as the Royal National Institute for the Deaf. Europe’s approach to accessibility and universal design adds credence to the maxim coined in the 90’s:  “If you build it, they will come!”

Back in America, “smaller & newer is better” began to take hold in the hearing aid industry of the 1980’s. As hearing aids shrank in size, features began to disappear, and the telecoil was one of the first to go. The popularity of induction loop ALS followed suit. The venerable telecoil was gone…but not forgotten. Fueled by the Telephone Compatibility Act of 1988 (Public Law 100-394) and activist organizations such as Self Help for Hard of Hearing People, Inc., high quality telecoils and supporting technical standards began to emerge in the early 90’s. A new and improved generation of induction loop ALS standards and technologies accompanied these telecoil advancements.

Standards
One of the factors that hindered performance consistency of induction loop ALS for many years was the lack of clear technical standards. In 1981, the International Electrotechnical Commission (IEC) published:  Methods of Measurement of Electro-acoustical Characteristics of Hearing Aids: Part 4: Magnetic Field Strength in Audio-frequency Induction Loops for Hearing Aid Purposes (IEC 118-4). This standard, while not comprehensive, provided basic field strength criteria that were important to designers and installers of this technology. In Europe, other standards built on IEC 118-4 such as British Standard 7594 (1993) that expands on important factors such as site assessment and methods and tools for measuring magnetic fields.

In 1999, the United States Access Board solicited recommendations from experts in the field that provided additional specifications pertaining to the following aspects of induction loop ALS:

• Loop Wire Installation: at least 80% of the installed loop wire should be free of the influences of ferrous metals that can adversely affect the signal transmission.
• Field Strength: as per IEC-118-4 with an added recommendation that the measurement be “A” weighted to disallow the influences of inaudible 50/60 Hertz low frequency power line/electrical equipment noise, and that the measurement at the center of the loop be 100 mA/meter +/- 3 dB at a height of 1.2 meters, using a 1 kHz sine wave signal.
• Input Signal Compression: in order to achieve a long-time average field strength of 100 mA/meter, and to maximize signal intelligibility and listener comfort regardless of fluctuating signal levels, an automatic gain control, signal compression and/or adjustable non-distorting peak limiter must be employed at the input of induction loop ALS systems. Recommended compression ratios: 4:1 for music and up to 20:1 for speech.
• Frequency Response: should comply with the IEC 118-4 specifications of 100 Hz-5 kHz, +/- 3 dB.  Measurements taken with an “A” weighted field strength meter will need to be corrected to compensate for the substantial low frequency roll off characteristic of this weighting. The use of a graphic equalizer is beneficial in certain situations where the environment is affecting the system’s frequency response, and/or the listening audience is composed of people who have specific needs and/or preferences. 
• Ambient Electrical Interference: sources of electromagnetic noise that may in some instances interfere with the proper functioning of induction loop ALS include light dimmers, electrical wiring, TVs, computer monitors and faulty fluorescent lighting. As with all types of ALS an on-site pre-installation evaluation of possible sources of interference should be performed. With induction loop ALS, ambient noise should not exceed 25 milliamperes/meter or   –12 dB (“A” weighted) relative to 100 milliamperes/meter as measured at any seat within the area that is to be enclosed by the loop system.  Using an induction loop receiver or telecoil-equipped hearing aid, listening checks may be required to determine if the noise falls within the frequency response of these devices.  For example, most hearing aids and induction loop receivers do not reproduce sounds below 100 Hz.  As a result, the fundamental frequency/first harmonic of 50/60 Hz. power line noise should not cause a problem, even though it may be measurable on some linear response field strength meters, unless higher audible harmonics of the noise are present.
• Signal Spill Over: in the event adjacent areas are equipped with induction loop ALS, signal spill over must not exceed 12.5 milliamperes/meter, or –18 dB (“A” weighted), relative to 100 milliamperes/meter, as measured at any seated position within the looped areas.
• System Signal to Noise: electrical signal to noise ratio of the loop amplifier output as measured directly at 1 kHz, must be at least +30 dB (unweighted) relative to the system’s internal noise at an output level sufficient to deliver a 100 milliamperes/meter field strength as per IEC 118-4 specifications.
• Distortion: With an input signal of 1 kHz and the system set up to deliver a field strength of 100 milliamperes/meter, total harmonic distortion must not exceed 3%.
• Listening Tests: regularly scheduled listening checks of all assistive listening systems is strongly recommended, preferably before each use.

Conventional Loops
Conventional large area induction loop ALS continue to have varied applications at home, in meeting rooms, theatres, vehicles, boats, schools, houses of worship and other public facilities. These systems consist of a dedicated loop amplifier and loop wire that can accept microphones and/or easily interface with existing public address systems.  Low-voltage models are used in passenger vehicles and special systems have been used in commercial vehicles.  

Major advances in induction loop ALS technology have occurred in recent years that take advantage of digital signal processing and miniaturization of electronic components.  Two examples are a three-dimensional induction loop and a self-contained portable induction loop ALS.

3-D Loop
The two inherent limitations of induction loop technology that were not addressed by any commercially available products for many years were signal spill over and inconsistent signal uniformity. The former refers to the tendency for large area inductive loop fields to pass through walls, floors and ceilings thereby interfering with adjacent looped areas/rooms. The latter refers to fluctuating signal reception, dependent on hearing aid orientation and telecoil placement inside the hearing aid relative to the induction loop field.  

In the early 1990’s, the U.S. Department of Education’s National Institute on Disability and Rehabilitation Research supported the development of a three-dimensional (3-D) induction loop ALS. The objectives of this research and development project were to design, validate and commercialize an induction loop ALS that eliminated or minimized the problems of signal spill over and inconsistent signal uniformity (11).  Unlike conventional loop systems that use a continuous single loop wire that encircles the listening area, the patented 3-D Loop consists of four loop wires laid out in a prescribed geometric configuration, prefabricated and sandwiched into a flexible mat . This mat, measuring 9 feet x 9 feet, or 12 feet by 12 feet, is designed to be installed under an area rug or a room’s carpeting. Multiple mats may be used to cover larger areas. Digital signal processing electronics and low impedance amplifiers are employed to create diverse electromagnetic field orientations, largely confined to the area immediately above the mat. Stepping off the mat results in an abrupt signal attenuation of 30 dB. Owing to the condensed geometry of the loops, the height of the fields is also controlled, calibrated at a height of approximately 39 inches from the floor. Consequently, signal spill over to rooms above and below is controlled.

The complete 3-D Loop consists of one or more mats, amplifiers, signal processing electronics, environmental microphone(s) and wireless microphone systems contained in a singular control box. Because the wireless microphones are transmitting on different frequencies to their respective receivers contained in the  3-D Loop control box, multiple microphones may be used in team teaching and large meeting situations.

The 3-D Loop was initially field tested in a variety of settings with hard of hearing adults and children, including: two preschool to grade 12 programs for deaf and hard of hearing students, a state commission for deaf and hard of hearing people, a university specializing in meeting the needs of deaf and hard of hearing students, and an international airport. The published listener and user evaluations from these test sites were very supportive of this new development in induction loop ALS technology (12). The field test results, including subjective responses, are summarized below:
 

• Adjacent room attenuation of better than –40 dB could be achieved by distancing the 3-D Loop mats six feet from common walls. Signal spillover was judged to be inaudible by hearing aid users.  This is in stark contrast to conventional loops in which room to room attenuation may be as little as –11 dB, and signal spill over can be audible to many hearing aid users.
• Adolescents were especially motivated to use their own hearing aids, rather than more obtrusive FM auditory trainers.
• Teachers and audiologists reported that many students exhibited greater awareness of sound and attended more readily to sounds presented through the   3-D Loop as compared to their response with FM auditory trainer systems.
• Teachers were very positive about how easy it was to use the 3-D Loop.  Rather than having to check and troubleshoot FM receiver cords, rechargeable batteries and other damage prone components of FM auditory trainers, the daily task of setting up a 3-D Loop was reduced to a basic hearing aid check and test of the teacher microphone.
• When used in settings where active infants and toddlers were fitted with behind the ear hearing aids, school staff reported excellent signal uniformity irrespective of hearing aid/telecoil orientation. Lab tests revealed a maximum +/- 3 dB signal variation with 360 degrees rotation of the hearing aid over a 3-D Loop mat. In contrast, a hearing aid rotated 90 degrees in a conventional loop system will exhibit a signal variation of greater than 40 dB.
• At one site, hard of hearing staff could not use their telephones when a conventional induction loop ALS was in use in a nearby meeting room. This problem was resolved with the 3-D Loop.
• Multiple wireless microphone capability was often mentioned as a positive feature of the 3-D Loop. Also, the system’s audio mixer facilitated easy interfacing with external equipment such as VCRs, CD players, etc.
• When comparing costs of competing technologies, the anticipated cost and minimal maintenance requirements for the 3-D Loop were attractive to school administrators, audiologists and facility managers.  The primary additional cost consideration was the need for carpeting/rugs to cover the 3-D Loop mats.

A recent breakthrough in this technology is a short-range, self-contained induction loop ALS that houses the loop wire, electronics, rechargeable battery and microphone in a portable enclosure measuring approximately 10” x 11” x 4” and weighing just over 3 pounds. Except for the AC adapter/battery charger and accessory plug-in microphone, there are no external components. The effective range of this system is 3-5 feet.  Intended for use by one or two listeners at a time, applications include: information counters, work areas, reception areas, meetings, TV viewing, etc. This portable system has proven useful in hearing aid clinics, banks, stores, hotels, hearing aid quality control departments, rehabilitation centers, government and state agencies, and for personal use.  Using a microprocessor controlled power supply, noise gating and automatic gain control to maximize efficiency and performance, the system’s rechargeable battery powers the unit for 24 hours between charging.

Induction Loop ALS Applications
To illustrate the technical concepts, versatility and potential usefulness of induction loop ALS, the following three scenarios depict hearing impaired people in a variety of settings:

Scenario #1
George and Sallie are a hard of hearing couple. Both are proud of their affiliation with a hard of hearing consumer organization where they have learned about hearing loss, hearing aid technology, and assistive listening systems. They recently purchased hearing aids that have various features that maximize their communication abilities. Their favorite feature is the telecoil. By simply switching their hearing aids to “T” (telephone/telecoil), they are able to magnetically couple their hearing aids to compatible telephones and various assistive listening devices and systems that have an inductive electromagnetic output. The resultant improved signal to noise ratio benefits George and Sallie’s comprehension of speech.

George and Sallie are active members of their church and have educated the board members about the value of assistive listening systems. Using a small conventional induction loop ALS and a basic diagram, they illustrated for the church board exactly how a loop wire is placed around seating areas (typically in the ceiling, on the floor or under carpeting), and is powered by a special amplifier. All audio signals from the church’s public address system are patched into the loop amplifier where they are changed into electromagnetic signals transmitted by the loop wire. These electromagnetic signals are easily received by individuals with hearing aids and cochlear implants fitted with telephone switches/telecoils, or with portable induction receivers.

George uses his own hearing aid to demonstrate that switching to “T” disconnects the aid’s internal microphone and connects in its place a tiny coil of wire, the telecoil, thereby transforming the hearing aid into a magnetism sensing, rather than sound sensing instrument. George’s hearing aid also has a combination “MT” (microphone-telecoil) mode that enables him to hear Sallie and other nearby sounds, while at the same time receiving signals from an induction loop ALS.  Switched to “T”, acoustic feedback is eliminated and undesirable background noise is greatly attenuated. Only the desirable signal is received from a compatible telephone or ALS.  

After the induction loop ALS is installed by a local sound contracting company, George and Sallie and other hard of hearing friends enter the sanctuary, sit down and switch their hearing aids to “T”. They are now connected to the public address system, without the need for special receivers or headphones. The board also has provided low cost induction loop receivers for people who do not own telecoil-equipped hearing aids. The system’s unobtrusiveness, ease of use, cost-effectiveness and low maintenance are appreciated by the church’s board of directors as well as hard of hearing congregants.

Scenario #2
Sophie is an audiologist at a school for deaf and hard of hearing children. She is considering installing 3-D Loops in several classrooms. Each classroom has 12 students and they all own telecoil-equipped hearing aids. She realizes that it would be more expensive to equip each student with new FM or infrared technology than it would be to install a 3-D Loop. Furthermore, maintenance costs for special receivers are a significant part of long-term budget planning. Sophie is especially attracted to the 3-D Loop because the students will be able to optimize listening using their own hearing aids, the same ones they wear outside of school, rather than swapping them for special receivers each day (keeping in mind that the hearing aids’ telecoil and microphone frequency response and sensitivity may be different from each other). The system also allows for multiple wireless microphones operating on different frequencies because the respective receivers are contained within the 3-D Loop control box.  All signals are received and mixed by the control box and then directed to the 3-D mat concealed beneath the room’s carpeting. External audio equipment such as VCRs and CD players also may be connected to the main control box. Sophie looks forward to exposing the students to a variety of sounds by means of the 3-D Loop.

Scenario #3
Isabelle is the manager of a bank. She has been wearing telecoil-equipped hearing aids for several years and finds them to be very useful while conducting business over the telephone. Isabelle’s audiologist recently introduced her to the potential of using assistive listening devices and systems in conjunction with her hearing aids. She was most pleased with the results using a portable self-contained induction loop ALS smaller than a briefcase that she can use at her desk and in meetings. She plans to try these systems at teller windows and other locations throughout the bank for the benefit of hearing impaired customers.

The scenarios cited above are based on actual applications and are demonstrative of the proven flexibility, cost-effectiveness and practicality of induction loop ALS. 

Looping the Future
For the remainder of this first decade of the new millennium we foresee exciting progress for hearing aid telecoils and associated induction-based assistive listening technologies:

• Higher fidelity telecoils will continue to be developed as increasing numbers of hearing aid users demand more from dispensers and manufacturers.
• Telecoils will become more readily available in all types of hearing aids.
• Automatic telecoils that are triggered by induction loop ALS as well as  telephones will become available.
• New standards governing the installation and performance of induction loop ALS will be published in the United States, and the means for measuring and verifying the criteria will become readily available along with low cost measurement equipment and concise guidelines.
• Induction loop technologies will continue to evolve as the integration of microprocessor electronics and digital signal processing becomes more commonplace in all ALS.
• New ambient electromagnetic interference cancellation technologies will make it possible to measure hearing aid telecoils and use induction loop ALS and hearing aid compatible telephones in environments with high electrical noise.

References
1. ADA Accessibility Guidelines for Buildings and Facilities (ADAAG), United States Architectural and Transportation Barriers Compliance Board, Washington, DC 20004
2. Assistive Listening Devices in Education and Vocational Rehabilitation. Rehab Briefs, 1990. PSI International, Arlington, VA; v. XII: 10
3. Hendricks, P, Lederman, N. Looping the World: Re-discovering a hearing assistance system, Parts I and II. Hearing Health 1998; v. 14:3:32-33, v. 4:45-46
4. Ross, M, Bakke, M. Large-area assistive listening systems: An overview and some recommendations. The Hearing Journal 2000; v. 53:6:52-60
5. Ross, M. Telecoils as Assistive Listening Devices. Hearing Loss 2000, Nov/Dec: 32-33
6. Berger, KW. The Hearing Aid: Its Operation and Development, Livonia, MI: National Hearing Aid Society; 1984:103
7. Ling, D. Loop induction for auditory training for deaf children. Maico Audiological Library Series 5, Report 2; 1966
8. Matkin, ND, Olsen, WO. Response of hearing aids with induction loop amplification systems.  American Annals of the Deaf 1970; 115:73-78
9. Calvert, DR. A comparison of auditory amplifiers in the classroom in a school for the deaf. Volta Review 1964; 66:544
10. Nabelek, A. Comparison of public address systems with assistive listening systems.  Hearing Instruments 1987; 38(2): 29-32
11. Hendricks, P, Lederman, N. Development of a three-dimensional induction assistive listening system.  Hearing Instruments 1991; 42(9):37-38
12. Lederman, N, Hendricks, P. Induction Loop Assistive Listening Systems. In: Ross, M, ed. Communication Access for Persons with Hearing Loss: Compliance with the Americans with Disabilities Act. York Press, Baltimore, MD; 1994:27-32

KEYWORDS:
Assistive listening systems
Induction loop assistive listening systems
Hearing aid telecoils
CEU Questions
1. The types of assistive listening systems that are in compliance with the Americans with Disabilities Act are (circle one or more):
      a) induction loop
      b) infrared
      c) vibrotactile alerting devices
      d) FM
      e) visual displays

2. Some of the considerations in choosing an ALS are (circle one or more):
      a) average temperature of the room
      b) the need for confidentiality
      c) telecoil-equipped hearing aids in use
      d) elevation of the room
      e) maintenance costs

3. Compared to FM and infrared technologies, the biggest advantage of induction loop ALS is:
      a) ease of installing the system
      b) lowest expense
      c) telecoil-equipped hearing aid users don’t require special receivers
      d) small size of the system
      e) light weight of the system

4. What is the international standard pertaining to the technical performance of induction loop ALS?
      a) ADA 1990
      b) SHHH 104
      c) IEC 118-4
      d) ASA 675
      e) EAA 98

5. What are three types of induction loop ALS technologies? (circle)
      a) conventional
      b) three-dimensional
      c) infrared
      d) FM
      e) portable self contained

Correct answers:
1. a,b,d
2. b,c,e
3. c
4. c
5. a,b,e

Originally published in Telephones and Telecoils:  Past, Present and Future;  Editor in Chief, Catherine V. Palmer, Ph.D;  Guest Editor, Jerry L. Yanz, Ph.D, Seminars in Hearing, volume 24, number 1, February, 2003.