Large Area Assistive Listening Systems: Review and Recommendations

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 theater 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.