Three types of ALS are in widespread use at the present time: Induction Loop (IL) systems, Frequency Modulated (FM) radio systems, and Infrared (IR) systems. Each offers the advantages of bridging the acoustical space between the source and the listener. Thus each can potentially offer advantages not possible when listening to a live voice or in the acoustic far field of a loudspeaker. Each, however, presents its own unique set of installation and user related concerns (Ross l994). In this section, we will describe the systems, discuss standards and issues pertaining to each, and conclude with a direct comparison of the three systems.
2.1 Induction Loop (IL) Systems
2.1.1 General Description
In an IL system, the output from an amplifier is delivered to a loop of wire placed around the circumference of a designated “listening area”. The audio signals from the sound source (usually a microphone, though other audio sources can be accommodated) are amplified and sent as an alternating electric current through the wire loop. This electrical current creates an electromagnetic field around the wire. This electromagnetic field - basically the original audio signals coded in a different form - can be accessed by someone wearing a hearing aid (or special IL receiver) switched to the telecoil position. The telecoil is an “induction” coil, one in which an electrical current is “induced” when it is placed within the electromagnetic field. In other words, the information coded in the electromagnetic field is converted to an electrical current. This electrical current is then amplified by the hearing aid and converted back into audio signals. The sequence goes as follows: audio input > amplifier > electrical current in wire loop > electromagnetic field around wire loop > induced electrical current in telecoil > audio output (see figure 1).
IL systems are the oldest - and least employed - of the existing ALS in this country (they are used much more often in European countries). The loop has to be physically placed around the listening area and secured so that it will stay in place. Once properlyinstalled, and given that the listener’s hearing aids include “T” coils, an IL system is undoubtedly the most convenient and possibly the most cost effective ALS. To hear the audio, all a person has to do is enter the looped area and switch his/her personal hearing aids to the telecoil position. As long as the person’s hearing aids include “T” coils, he or she always has an assistive device “receiver” available. Wireless FM microphones can be integrated into the system and employed by talkers in order to provide instructional flexibility (see figure 2).
There are no national US standards that define the required performance of IL systems. The International Electrotechnical Commission (IEC) has developed pertinent standards for “Magnetic field strength in audio-frequency induction loops for hearing aid purposes” (IEC, 118-4, l981). Suggested modifications to these standards have been made by Oval Window Audio, a manufacturer of audio loops in this country. Standards developed at the National Technical Institute for the Deaf (Johnstone l997) are part of the NTS Uniform Fire Prevention and Building Code, Chapter 23 of the Laws of l989 (Title 9, Subtitle S, Volume 9 Executive [B] of the “Official Compilation of the Codes, Rules and Regulations of the State of New York”). Each of these will be briefly reviewed. In ensuring that the prescribed magnetic field goals are achieved, the utilization of a “magnetic field-strength” meter is assumed (see figure 3).
126.96.36.199 Summary of IEC 118-4 (1981) Standards.
With a source of 1000 Hz equal to the long-time average level of the speech signal applied to the input of the system, the resulting field strength within the loop shall average 100 mA/meter +/- 3 dB. This level should not go below 70 mA/meter or above 140 mA/meter. These values apply to the vertical component of the field strength inside the area enclosed by the loop, measured 1.2 meters above the floor. Allowing for the 12 dB peaks occurring in speech signals, peak field strength may reach 400 mA/meter. The frequency response shall be 100 Hz - 5 kHz +/- 3 dB. The document includes a statement that in schools for hearing-impaired children it may be desirable to boost the low frequencies to compensate for the decrement in the low frequency response often produced by the inductive process. The degree of such a boost, and whether and under what circumstances it should take place, is therefore a local option.
188.8.131.52. Additional Proposed Specifications (by Oval Window Audio)
- Loop Wire Installation: In order to maximize signal strength and uniformity, the loop wire shall be installed either at floor or ceiling level. At least 80% of the installed loop should be free of the influences of metal, either in front of, or immediately behind the wire.
- Field Strength: As per IEC 118-4, with the additional condition that the measurements be “A” weighted to disallow the influence of inaudible low frequency power line electrical noise.
- Input Signal Compression: To compensate for fluctuating signal input levels, an automatic gain control, signal compression, and/or adjustable non-distorting peak limited must be employed at the input of the system (Recommended compression ratios: 4:1 for music, and up to 20:1 for speech).
- Frequency Response: Frequency response measurements conducted with an “A” weighted field strength meter must be corrected to compensate for the substantial low frequency roll off characteristic of this weighting network.
- Ambient Electrical Interference: Sources of electromagnetic radiation that may interfere with the proper functioning of an induction loop system include: faulty florescent lighting, light dimmers, electrical wiring, TV and computer monitors, surge protectors that are in close proximity to the loop system. An on-site evaluation of ambient electromagnetic noise should be performed before a loop is installed in order to identify and resolve electromagnetic interference. Ambient electromagnetic noise should not exceed 25 mA/meter or –12dB (“A” weighting) re: 100 mA/meter as measured at any seat within the looped area. If potential sources of noise cannot be reduced or eliminated, then the use of a loop must be ruled out.
- Signal Spillover: When adjacent areas are equipped with an induction loop, signal spillover must not exceed 12.5 mA/meter, or –18 dB (“A” weighted) re: 100 mA/meter as measured at any location within an adjacent loop. Listening tests should also be performed to determine if signal spillover is audible.
- System Signal-to-Noise: The electrical signal to noise ratio of the loop amplifier output (measured directly, not inductively) at 1000 HZ must be at least +30 dB at an output level sufficient to deliver mA/meter as per IEC 118-4 specifications.
- Distortion: With an input signal of 1000 Hz and the system adjusted for an output of 100 mA/meter, harmonic distortion must not exceed 3%.
184.108.40.206.New York State Standards.
Ambient electro-magnetic fields should not exceed 30 mA/meter; higher levels would preclude the installation of an IL in the particular location. In the event that adjacent areas were to be looped, magnetic field “spill-over” should not exceed 15 mA/meter. Given a 1000 Hz signal at a level equal to the long-time average level of speech, the average value of the magnetic field should be 100 mA/meter +/- 3 dB between 100 to 8000 Hz, with a maximum no greater than 400 mA/meter. This same signal should produce no more than 5% harmonic distortion and provide a signal to noise (ambient magnetic field) of at least 30 dB.
The major advantage of IL systems is that they permit listeners whose hearing aids incorporate telecoils to use their own hearing aids as the receiver. All the person has to do is enter the field and switch the hearing aids to the telecoil position. Given an appropriately functioning telecoil (see issues below), wearers have the advantage of being able to utilize their own hearing aids that, presumably, provide an appropriate and individualized amplified frequency response. There are headsets and pocket receivers available that will detect and amplify an IL signal, but this defeats the major advantage of IL systems - their convenience to the user.
Only about 30 percent of modern hearing aids in the United States include a telecoil (more than twice this number use telecoils in Europe, which may help explain the popularity of IL systems there). As useful as telecoils are, with the trend toward smaller and smaller hearing aids, it seems unlikely that this 30% figure will increase in the future. Given the potential advantages of telecoils, not only for IL systems and telephones, but for all other types of ALS as well, it is unfortunate that a higher percentage of hearing aids do not routinely incorporate them. However, unless there is a drastic change in the type of hearing aids used by hard of hearing people, it does not appear that the use and popularity of IL systems will significantly increase.
Even when a hearing aid includes a telecoil, it is quite likely that its electroacoustic performance will not duplicate the hearing aid’s microphone response (Rodriguez, Holmes & Gerhardt l985; Culpepper l986; Thibodeau & Abrahamson l988). However, recent developments in hearing aids suggest that this variable can be circumvented, either by programming the telecoil to match the microphone response (Davidson & Noe l994) or by using amplified telecoils (Noe, Davidson & Mishler 1997).
Complicating telecoil usage is its physical orientation within the hearing aid. For optimal sensitivity to an IL system, the telecoil should be mounted perpendicular to the floor loop (or a neckloop). But since optimal sensitivity for detecting the electromagnetic field around a telephone occurs when the “T” coil is in the horizontal position, a positional compromise is often necessary (Preves, l994).
The newest telecoil standards (revision of ANSI S3.22) provide for measurements in which the extent of any compromise can be determined. In this new standard, the high frequency average (HFA) gain in the microphone position of the hearing aid is explicitly compared to both simulated telephone and induction coil usage (Preves 1996). The differences are expressed as Simulated Telephone Sensitivity (STS). A zero figure means that at the same gain control setting, the high frequency average gain in the microphone position was equal to that obtained in the telecoil position. Usually, the STS is negative, indicating that the gain must be increased in the telecoil position to achieve the same results as in the microphone position. Preves (l996) points out that the STS can be positive with the use of preamplified telecoils, and indeed this was found in the most recent study looking at ALS (Noe, Davidson & Mishler l998). However, it should be noted that even with an STS of zero there can still be significant differences in the frequency response between the telecoil and the microphone positions. That is, while equal high frequency average gain can be obtained at the microphone and telecoil positions, the pattern of gain may be quite different for the two conditions.
The hearing aid microphone is usually disconnected when the telecoil is switched on. Since in some applications, it is desirable that the person be able to respond to audio signals (such as being able to hear side comments by one’s partner during a lecture or stage performance), the optimal situation would be for the hearing aid to include an M/T position (both microphone and telecoil operative) and to do this without changing either the microphone or the telecoil frequency responses or output levels.
The electromagnetic field is not confined within the looped area; some of the electromagnetic field “spills over” into adjacent vicinities. This is not a problem unless induction loops are also installed in the adjacent areas (vertical or horizontal), in which case listeners can be exposed to simultaneous audio signals from different sources. Also, the intensity of the signal within large looped areas can vary; the further from the wire, the weaker the signal. A great many creative loop configurations have been used in an attempt to circumvent these problems, with mixed results (Lederman & Hendricks l994).
The “3-D” loop developed by the Oval Window company has minimized the effect of spillover and telecoil orientation. In the 3-D loop, four wires configured in a prescribed geometric pattern are embedded in a mat placed beneath carpeting. Reportedly, the resulting electromagnetic signal is not only contained within the looped area, but the “3-D” pattern of the electromagnetic field reduces the impact of the telecoil orientation upon the perceived signal. A limitation of the 3-D IL system is that the listening area must permit the installation of one or more rugs (see figure 4).
2.2. FM Radio Systems
2.2.1. General Description
There are two types of FM listening systems. One is a personal system, designed to be used by an individual on a one-to-one basis (Yuzon l994). The other, the type that concerns us in this paper, is meant to service one or more listeners in such large-area listening venues as auditoriums, classrooms, all types of theaters, and houses of worship (Ross l994). Both types are basically FM radios where the audio signal is “broadcast” to listeners wearing FM receivers tuned to the transmitting frequency. ALS can be a “stand-alone” device, or be integrated into an existing Public Address (PA) system (Compton l991). Of the three types of ALS described in this document, FM radio systems appear the easiest to install; generally, all that is required is a patch cord between an output from the PA amplifier (“line-out” or other audio output) and the input to the FM transmitter. Transmitters range in complexity from simple devices that include only basic interconnection and transmissioncapacities, to sophisticated devices that are capable of accepting a range of inputs from different sources and that incorporate many signal processing options (such as high frequency pre-emphasis, various compression options, etc.). Examples of commercially available large-area FM systems are displayed in figures 5, 6, 7, and 8.
220.127.116.11 FCC Regulations.
In 1982, the Federal Communications Commission authorized the use of frequencies.
within the 72-76 MHz band as the designated radio frequencies that could be used by people with hearing loss. This is non-exclusive, unlicensed band. Other users, such as pagers, emergency vehicles, etc. are also permitted to transmit radio signals on frequencies within this band. ALS manufacturers differ in how they allocate this band. For example: Phonic Ear makes 40 narrow-band (50 kHz) channels available; Gentner uses 37 narrow-band (50 kHz) channels; Telex, 16 narrow-band (50 kHz) channels; Drake, 10 wider (150 kHz) channels; Williams, 10 wide (200 kHz) or 40 narrow-band (50 kHz) channels, while Comtek employs 10 wide-bands (200 kHz) and narrow-band (50 kHz) channels both in the 72-76 MHZ and in the 216 to 217 MHZ bands. Some of the frequencies used by different manufacturers may be identical, while others may differ. The FCC also permits the use of the 216-217 MHz band as a low power radio source for auditory assistive devices and several manufacturers are now marketing large area transmitter using this higher frequency band. ALS in really large venues, such as football stadiums, can also employ one of the commercial FM frequencies as long as they meet the power requirements designated by the FCC.
In terms of the permitted maximum power of ALS, the FCC limits it to no more than 80 millivolts per meter at 3 meters. This can provide for an effective transmission range between 300 and 500 feet. Larger antennas can extend this range to l000 feet; however, the maximum strength cannot be exceeded regardless of antenna design. The 216-217 MHz band does permit higher signal strength and can provide greater operating distances. Within the transmission range, the field-strength of the transmitted radio signal should be adequate and equal at all seat locations within a venue. ALS are “low-power” devices that are not likely to interfere with other permitted user in the same channels (i.e. paging devices, emergency vehicles). No priority is given assistive listening devices. When interference occurs from other radio sources, the onus is on the ALS user to switch to another channel within the same frequency band.
18.104.22.168 New York State Standards.
As far as can be determined, New York is the only state that has developed written standards for large area FM assistive listening systems. Enforcement of the standards is the option of the local building code inspectors (who have, literally, thousands of items to verify on a checklist code). Information and educational sessions can help ensure their understanding and compliance with the ALS standards (Johnstone l997). The New York State standards reflect the combined operation of both the transmitter and the receiver using various coupling options.
Given an appropriate input signal to the transmitter (as specified by the transmitter’s specifications), the frequency response at the output of the receiver and transducer should not vary more than +/- 5 dB over the frequency range 100 to 8000 Hz from the value at 1000 Hz. (Note: This appears to be an overstatement; it is unlikely that such transducers as miniature earphones, as well as silhouettes and neckloops working through hearing aids would be capable of meeting this standard at the higher frequencies).
The minimal signal to noise ratio at the receiver’s output should not be less than 35 dB. Harmonic distortion shall not be more than 10%. The minimum sensitivity of the receiver shall be no greater than 1 uV at 12 dB SINAD. The maximum RF signal generated by the transmitter shall not exceed 8000 uV/meter at 30 meters (which accords with the FCC regulations).
When using a neckloop as a transducer, it should generate an average magnetic field strength of 150 mA/meter, with peaks no more than 600 mA/meter. Measurements shall be made in the center of the neckloop. If using a silhouette transducer, the average magnetic field strength should be 50 mA/meter, with peaks no more than 200 mA/meter. Measurements should be made at a distance 10 cm from the silhouette. (Note: 10 mm would appear to be a more realistic distance.) If using a miniature earphone as the output transducer, the output shall be at least 80 dB SPL and no more than 130 dB SPL.
All systems must be capable of accepting input signals at line or microphone levels and must be capable of interfacing with existing PA systems.
The receivers for FM assistive listening systems are basically FM radios “tuned” to the transmitting frequency. Most manufacturer supply a number of receivers that vary in complexity and secondary features, but all are designed to accord with the characteristics of their own transmitters. Companies that make 8, 10, 16, 37 or 40 wide or narrow band channels available in the transmitter also provide receivers that can detect any one or all of these channels. Channel selection is generally accomplished by a slide switch, push button, or wheel rotation. For some FM receivers, it is necessary to remove the back and “tune” a rotary wheel while listening to a test signal from the transmitter. This is not as “user friendly” as those receivers which permit pre-set channel changes by discrete switch adjustments.
Depending upon the specific transmitting frequency, it may be possible to interchange transmitters and receivers from different companies. Whenever, however, a receiver from one manufacturer is being employed in conjunction with the transmitter of a different manufacturer, complete compatibility may be questionable because of subtle differences in the RF and acoustic properties of the receivers. Engineers design receivers to conform to the electronic characteristics of their own transmitters.
Power to the FM receivers are supplied by disposable or rechargeable batteries. Battery life for the rechargeable batteries range from 6 to 10 hours (or up to 35 hours according to one report), while the life span of the disposable batteries (either 9 volt, AAA, or AA) vary from 18 to 70 hours depending upon volume setting and type of coupling. Convenient pocket recharging-carrying cases are available in which the receivers can be recharged while being stored. Unlike many personal FM receivers, those used with ALS generally do not include a warning light signal when the battery is weak.
The receivers come with a number of coupling options. While the ADA requires a set number of receivers in different venues, it does not stipulate the precise type or percentages of the different kinds of coupling alternatives. For people whose hearing aids contain “T” coils, the most convenient and desirable option is for them to plug a neckloop or silhouette inductor into the FM receiver, or to place electromagnetic headphones right over in-the-ear hearing aids. They are then able to take advantage of the “prescribed” characteristics of their own aids when listening through an ALS. Many BTE hearing aids can accept a direct audio input (DAI) from an FM (or IR) receiver through a wire connector. This will also permit users to benefit from the prescribed electroacoustic characteristics of their own hearing aids (keeping in mind, however, the possibility that either inductive or direct audio input coupling may not preserve the microphone response of a hearing aid).
People whose hearing aids do not include “T” coils can either remove their hearing aids and use earbuds or earphones, or place earphones right over the hearing aids (this will not work for people wearing behind-the-ear hearing aids). We cannot now predict whether such acoustical coupling will produce audible feedback or what acoustical changes this produces in the hearing aid’s response. This topic has not been investigated with the current generation of miniature hearing aids. Consumers would have to try using an earphone with their own hearing aid in order to determine if acoustic feedback occurs. While it may be difficult at first for a venue to ensure the proper “mix” and number of coupling alternatives, with time and experience venues should soon learn what type of coupling arrangements their patrons prefer.
Some manufacturers of FM systems depict a user with a monaural earbud in their promotional material. Unfortunately, this depiction sends an implied message that monaural use of an ALS is the routine and desirable listening condition. This is inaccurate when it comes to people with normal hearing and even more inaccurate for people with hearing loss using an ALS. They need all the acoustical help they can get and, unless contraindicated by audiological findings, two-ear listening should be the routine in all ALS situations.
While theoretically, an FM receiver from one manufacturer can be employed with a transmitter made by a different company, the reality is a bit more complex. As pointed out earlier, companies design their transmitters and receivers as a unit, to work together. Although several FM receivers can be tuned to any of the 10 wide or 40 narrow bands available, other factors, such as the selectivity and power of the FM receivers, may still affect the quality of the reception with different FM transmitters. The recommendation had been made that only standard transmitting frequencies with standardized electrical characteristics be used in all ALS. This would, theoretically, permit consumers to utilize their own personal FM receiver in any venue. However, this led to the objection that this requirement would stifle creativity and future developments. At the present time, it appears more feasible and realistic to stress flexibility in receiver options than to require standardized transmitter characteristics.
FM systems are subject to interference from other radio sources. While using a frequency scanner can help select a “clean” frequency at the time of installation, there is no assurance that the channel would remain clean at a later time. If the transmitting frequency is changed, this requires that the tuning of the receivers be changed accordingly (not always possible or easy, see below). Wide-band receivers are more likely to pick up radio interference and cannot be used in as many adjacent venues (e.g. a multiplex theater complex) as narrow-band frequencies. On the other hand, the acoustical output from a wide-band receiver is somewhat superior to that emanating from a narrow-band receiver, but it is an open question whether this difference has any real-world significance for hard of hearing people. We know of no evidence that supports or refutes the listening advantages of either wide or narrow band channels for hard of hearing people.
Many FM transmitters are capable of being adjusted to provide a range of pre-processing possibilities. This was one of the recurring issues discussed in the manufacturer’s focus group, and applies not just to FM systems but to IR and IL systems as well. The general recommendation was that the transmitted signal be “as pure” as possible, with processing used to maximize speech intelligibility (as opposed other types of sound stimuli such as orchestral music). However, because the dynamic range of some audio sources can exceed 80 dB, or far beyond the dynamic range of just about everybody with a significant hearing loss, it appears necessary to provide some compression in the transmitter to keep from overloading the system and to ensure at least some audibility for low-level input sounds.
Given the range of possible pre-processing strategies and the different venues and populations that would be using the ALS, it is necessary to develop and support a rationale for selecting one particular strategy over another. At the present time, each manufacturer provides its own instructions to installers regarding the necessity for a particular processing strategy. Further complicating the situation, what may be suitable in one situation and for one type of listener may not be optimal in another venue for other types of listeners. Some FM transmitters include choices for different amounts of high pass filtering, single or multiple band transmission, high frequency pre-emphasis, and different parameters of compression and modes of output limiting. Whatever rationale is developed, it is likely that the final decision would be made “on site” by a trained installer (meeting, it is assumed, the output electroacoustic recommendations that will be presented below). An example of the processing choices that is available can be seen in figure 9.
2.3 Infrared (IR) Systems
An IR system transmits audio signals via invisible infrared light waves. The frequency of infrared light falls somewhere between 700 nm and 1000+ nm; visible light waves fall between 400 and 700 nm. The specific bandwidth of the IR carrier varies among manufacturers; it may be as narrow as 50 nm wide, or considerably broader and perhaps be visible as a faint reddish glow (Laszlo l998). The audio signal, from any source, is used to frequency modulate an RF sub-carrier which in turn is impressed upon, and essentially amplitude modulates, the IR carrier. An FM/AM double modulation of the IR light wave is the result. Up to now, 95 kHz has been the RF sub-carrier most often used by manufacturers but this may now be changing (more on this below).
To achieve the best possible audio reception, the modulation characteristics of the RF carrier is typically +/- 50 kHz. IR systems are also frequently used with normal hearing people during events requiring simultaneous translation into several languages. In these instances, a number of other sub-carriers may be used, each narrower than the bandwidth used for people with hearing loss.
All IR systems are composed of three basic components: the transmitter (also called the modulator), the emitter and the IR receiver. The modulator processes the audio signal so that it can be transmitted via infrared light. This audio processing may include some kind of output limiting, companding (designed to control widely varying amplitude levels), high frequency pre-emphasis and/or high pass filtering. These appear to be similar pre-processing strategies that are used with large-area FM ALS. Examples of IR transmitter/emitters and receivers can be seen in figures 10, 11, 12, and 13.
The signal from the modulator (transmitter) is delivered to emitters that actually produce the IR light waves (these two components may be contained within the same physical unit). The emitter is composed of a number of light emitting diodes (the “light bulbs” of an IR system). Until quite recently, the more diodes contained within an emitter, the more powerful the system. However, with the development of more powerful diodes, this is no longer necessarily true. Although these light waves are invisible to the human eye, they are light waves with certain definable characteristics. This fact may help us understand some of the issues related to IR systems, such as the effects of sunlight and incandescent light (these contain a great deal of infrared energy) and the impact that the color and texture of the wall surfaces have upon IR reflections. Ensuring the appropriate number and placement of emitters is the major challenge facing an installer of IR systems.
The third basic component is the IR receiver. The transparent lens found on every IR receiver contains the photo detector diode that detects the IR light wave. An optical filter on the lens reduces at least some of the light interference from extraneous sources. The IR receiver then demodulates the RF sub-carrier and the audio signal is retrieved and amplified. If the audio signal has been pre-processed in some fashion, such as providing high frequency pre-emphasis or sent through a companding circuit, then it is at this stage that individualized compensation can occur. One reason that the IR systems of different companies are not always compatible, even though they may use the same sub-carrier frequency, is because they may differ in the nature of their pre-processing strategies and compensatory receiver characteristics. While a signal may be received, and even understood, this incompatibility would reduce its clarity.
There are no standards that we are aware of that specifically cover the unique aspects of IR systems. The New York Standards, quoted in respect to IL and FM installations, do not provide this information; rather these standards basically focus on the electroacoustic output from the IR receivers. When the NYS standards were drafted in l988-89, the manufacturers could not reach a consensus regarding minimum energy level of the IR light at a set distance from the emitters. Instead a general recommendation was made that the minimum light level must be sufficient so that receivers could produce the required output signal specifications (Johnstone l999).
2.3.2. IR Receivers
A number of different types of IR receivers are used. In one of the most common, the sound tubes are placed in each ear and the unit itself dangles under the chin. Sometimes this is called a stethoset or stethophone receiver. The lens of the receiving diode usually faces forward, toward the presumed location of an emitter. In traversing the tubing leading from the receiver to the eartips, the frequency response of the acoustic signal may display several resonant peaks (Nabalek, Donahue, & Letowski l986). Recently, however, receivers have been developed that locates the transducer at the earphone tips thus precluding the formation of resonant peaks. Some under-chin receivers include an output jack into which a neckloop can be plugged for inductive coupling to a hearing aid. However, these are generally designed for micro-mini plugs and will not accept the mini plug used with neck loops designed for body worn receivers. Some receivers include an environmental microphone to permit the direct audio reception of one’s companions while using the IR receiver. Acoustical coupling through personal hearing aids is not feasible with this type of receiver.
Some IR receivers are built into headphones. In these instances, the receiving diode can be placed within an oblong lens on the top of the headband, for presumably 360 degree reception, or appear as a rounded protuberance on the surface of each earphone. The “best” position for receiving IR signals depend upon the relative positions of the emitter and the receiving diodes. Some IR headphones allow for possible acoustical coupling, in that they can be placed right over hearing aids (not the BTE type, however). As pointed out earlier, the incidence of acoustical coupling upon the production of acoustic feedback, or undesirable changes in the frequency response of the hearing aid, has not been determined with this type of acoustical coupling. Many hearing aid users prefer to remove the aids before placing the headphones on and listening through headphone alone.
Body IR receivers look like personal listening systems or FM receivers. They are distinguished from these by the presence of the visible translucent lens on the front surface (presumably facing the light source, i.e. the emitter). This type of receiver would not be too useful when the emitter is directly overhead, in a darkened theatre for example. The other coupling choices - inductive, acoustical (headphones worn over personal hearing aids), or headphones or earbuds directly - are the same as that used with FM receivers. Some IR receivers include an environmental microphone and some do not. No IR receiver, as far as we know, provides a low battery light feature.
Up to now, 95 kHz has been the sub-carrier most frequently used by manufacturers of IR systems. When stereo reception was desired, a sub-carrier of 250 kHz was simultaneously employed. These frequencies reflected an informal agreement among manufacturers rather than any national or international standards (Lieske l994). Other frequencies are now being used as sub-carriers in ALS (300 kHz, 2.3 mHz and 2.8 mHz). One reason for switching to a higher sub-carrier frequency is because of the electromagnetic interference at 95 kHz produced by the newly introduced T-12 fluorescent ballasts. Since these are more energy efficient than the older type of ballast (type T-8), their use is likely to increase in the future. In other words, the pre-eminence of the 95 kHz sub-carrier is no longer assured. While it does not appear to be on its way out, it may be employed in fewer locations. Compatibility between venues has been an advantageous feature of IR but, with the introduction of different sub-carriers, this advantage can no longer be taken for granted.
The frequency of the sub-carrier is not the only uncontrolled factor in IR systems. Unlike RF fields, the strength of the IR field is left up to the manufacturers and installers. To reduce the interference effect of ambient light, a manufacturer may increase the radiated IR light level and decrease the sensitivity of the IR receiver. Other manufacturers may radiate less light energy from their emitters and depend upon the sensitivity and electronics of the receivers to detect an adequate signal. These factors help explain the variation in performance of different IR receivers in a specific venue, even when they all use the same sub-carrier frequency.
IR receiver diodes generally detect a broad bandwidth of light, extending over 500 nm. Every receiver uses filters built into the detector diode that is designed to accept the transmitter frequency and block the other IR light (Laszlo l998). However, the filter characteristics cannot be too narrow, else slight transmitter “drifts”, perhaps due to temperature effects may put the device outside the passband. This effect can be compensated for with more expensive electronics, but then the decision becomes a cost/benefit issue. The specific characteristics of these filters presumably differ among the receivers from different manufacturers and are another reason why, in spite of using the same sub-carrier frequency, some IR receivers are less compatible than others in the same location.
IR light waves are considered “line of sight” transmission. That is, ordinarily the “eye” of the emitter should be facing the “eye” of the receiver, with no physical obstructions placed between these two devices. Turning one’s back to the emitter, or placing an IR receiver in one’s pocket, can either eliminate or distort the perceived audio signal. However, line of sight reception is affected by surface material (light surfaces reflect more light energy), the strength of the transmitted signal, and the geometric shape and number of listening levels (e.g. balconies) in the venue. In rooms with light covered surfaces, these reflections may enhance IR coverage by filling in gaps not covered by the primary signal. Conversely, with dark surface, or checkerboard patterns, may reduce or modify IR light reflections. Reportedly, a few recently introduced IR transmitters are capable of being employed in some outdoor venues (but not in direct sunlight).
The emitters produce an ovoid IR light pattern that diminishes in strength following the inverse-square law. This means that a number of emitters must be used to ensure that all seats in a venue, such as in the corners or in the rear of the room, receive an “adequate” level of the light. Installers frequently “daisy-chain” emitters to guarantee that IR light is being directed at the audience from several directions. There are no standards which specify the radiated level of light required at seat locations. At least one manufacturer (Audex) makes a light meter available for installers. This particular lightmeter is designed only with the company’s own products in mind, in that it reads only relative light intensity for determining whether the illumination levels are satisfactory. Installers also can, and usually do, listen through a receiver at various seat locations. This is a highly subjective judgment and offers hard of hearing listeners no assurance of adequate IR field strength at all seat locations in a venue. One of the recurring complaints made by the consumers in the focus group was the variations they experienced in IR field strength and the clarity of the IR signal in different venues and at different locations in the same venue.