WHEN HEARING AIDS ARE
NOT ENOUGH 
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WHY ASSISTIVE LISTENING DEVICES? Many people with hearing loss who benefit from the use of hearing aid(s) can function in quiet, close or person-to-person situations. However, background noise, long distances to the sound source, and reverberation degrade intelligibility much faster for people with hearing loss, whether they wear hearing aids or not. Thus, people with hearing loss are often prevented from participating on equal terms with normal hearing people in group conversations or in larger assembly areas which are not equipped with an assistive listening system. Even the best in sound systems technology combined with the best in hearing aid technology cannot solve the intelligibility problems faced by people with hearing loss. In recognition of this, requirements for assistive listening systems were included in the Americans With Disabilities Act Information (ADA 1990). Although many assistive listening devices may also be obtained for personal use, it is important to understand the rights of people with hearing loss and theAmericans with Disabilities Accessibility Guidelines (ADAAG 1994). The purpose of an assistive listening device (ALD) is to transmit signals from a sound source as directly as possible to a sound generating transducer in the ear(s) of people with hearing loss thereby bypassing poor room acoustics and/or distance to the sound source. All ALDs have three common basic: a sound pickup, a signal transfer method, and a sound generating transducer/coupler. Within each of these elements are several options that must be carefully considered in order to provide the most effective solution for a given application. Figure 19-2 shows the most common components that applies to ALDs. As illustrated, any one signal transfer method may be combined with any one sound pickup method/source. Likewise, any one sound generating transducer/coupler can be combined with any one signal transfer method (some restrictions may apply with direct cable). In general, systems are separated into two major categories: Portable, body worn devices often referred to as "Personal ALDs," and "Installed Systems" for residential and commercial applications. SOUND PICKUP No matter what signal transfer method is used, sound pickup is the single most important factor. For installed systems, the electrical signal is routed directly from an output connector on the sound system, TV, radio or other electronic device. The wire connecting the sound source to the ALD is referred to as a "patch cord." When no direct connection can be made, the only method of sound pickup is through a microphone. For person-to-person communication, a microphone is always required for sound pickup. Depending on the nature of the problem, different applications require microphones with different acoustic characteristics. Omnidirectional and Unidirectional Microphones Every microphone has a characteristic polar pattern that determines how well it accepts or rejects signals coming from various directions around the microphone. An omnidirectional microphone treat all signals equally, regardless of wherever those signals originate (in front, behind it, or to the side). In contrast, unidirectional microphones, or commonly referred to as directional microphones, are specifically designed to accept mostly signals coming directly from the front and reject signals coming from behind or the sides. Because the amount of directionality is frequency dependent, the directional pattern for microphones are shown as polar pattern diagrams. Figure 19-3 illustrates a typical polar pattern for a directional microphone. Microphone manufacturers use terminology such as cardioid, supercardioid, and hypercardioid polar patterns to describe the characteristic properties of their individual microphone models. The most extreme variation of cardioid is the hypercardioid pattern. The hypercardioid is very effective for rejecting signals arriving from the sides at back (~ 110 and 230), but is less effective than the cardioid in rejecting signal from behind. The polar pattern also determines how prone a particular microphone is to inducing acoustic feedback. The greater rejection of signals not coming directly from the front of the microphone, the less risk of feedback. From an acoustical point of view, the directional microphones would always be preferred since most of the problems are related to background noise and reverberation. However, there are situations where omnidirectional microphones are required even at the cost of reduced signal-to-noise ratio (SNR). This would typically be in rooms where several people have to share the same microphone, (i.e., a conference room). Also, in lecture rooms where people in the audience may ask questions, the directional microphone would not pick up very well from certain directions. Thus, the people with hearing loss would not be able to understand the questions and/or conversation between the lecturer and other members of the audience. An omnidirectional microphone is often preferred in classroom settings. The directional characteristic of the directional microphones vary greatly from model to model and from manufacturer to manufacturer. But whatever the front-to-back or front-to-side relationship is in dB, the SNR will be improved by that amount. Both types of microphones are available in a variety of shapes and sizes depending on the application. Microphone Designs for ALDs Practically all microphones used with assistive listening systems (ALS) are electret or condenser type microphones. These have the advantage over the conventional dynamic microphones in that they are small in size and very lightweight. Electret microphones incorporate a small FET (field effect transistor) amplifier, thus a power source, often referred to as phantom power, is required for them to operate. Electret microphones used with ALDs typically operate on voltages between 1 and 10 volts. Most devices are designed to provide phantom power to the microphone through the microphone jack eliminating the need for additional batteries. Commercial electret microphones may have in-line battery compartments, or, require phantom power as high as 48 volts from a microphone mixer. Lapel/Hand-held Microphones The most common microphones for use with portable systems is the lapel type microphone. Available in omnidirectional and directional versions, it's small size makes it convenient and inconspicuous to attach to clothing near the speaker's mouth. Some microphones may be designed as a combination lapel/hand-held style. Other manufacturers may include a small desk stand so they also can be used as podium or desk top microphones. Head-worn Microphones In environments with continouos high noise levels, it may become necessary for the speaker to use a boom style microphone. There are two mechanically different designs: An over-the-head style and a behind-the-neck style. From an acoustical point of view, the two different designs perform alike, but the individual speaker may prefer one design to another. However, the over-the-head style may be combined with a headphone for some applications. The concept with either design is to bring the microphone as close to the mouth as possible to avoid surrounding noise from masking the speaker's voice. The microphone can be omnidirectional, directional or be a "noise-canceling" type. The noise-canceling microphone is designed for use in environments where the ambient noise level is 80 dB or greater, and is most effective for frequencies above 2 kHz. The noise-canceling characteristics are achieved by designing the microphone with a rear opening. Unwanted sounds arrive behind the microphone diaphragm out of phase and at the same time with sounds arriving from the front thereby canceling each other out. Only speech originating very close to the microphone opening is fully reproduced. The noise-canceling concept may also be found in other microphone styles. Conference Microphones At round-table discussions in conference rooms or around larger dinner tables, the lapel microphone is not a convenient option. Such a microphone would need to be passed around from person to person causing disturbances to other people. Often, it would not get to the other person in time for the person with hearing loss to hear the full sentence. Special conference (boundary) microphones are available for such applications. Figure 19-4 shows a small portable conference microphone. The basic principle in a boundary microphone is that the direct sound and the reflected sound from a hard surface below the microphone, arrive at the microphone's diaphragm at the same time and in phase. This doubles the sound pressure and increases the electrical output of the microphone by 6 dB without increasing the microphones internal noise level. Some manufacturers claim the effective pickup range of conference microphones to be up to 20 feet. However, the realized range varies greatly depending on environmental conditions and the individual user's discrimination ability. Five to15 feet is more realistic. Conference microphones designed for ALD applications are omnidirectional. However, commercial boundary microphones are also available with a directional polar pattern. Other Microphone Styles The variety of microphone designs for commercial applications is practically endless. It is usually up to a sound contractor, user, or facilities manager to select the most adequate type of microphone for a given application. One of the most interesting microphones that can be successfully used with ALDs is the shotgun microphone. This is the only type of microphone which can be aimed at, and clearly pick-up, different speaker's voices in a highly reverberant room from more than 20 feet away. A shotgun microphone has a special hypercardioid polar pattern which is achieved by it's mechanical design. The drawback of conventional shotgun microphones is their high cost and their large physical size, (i.e., one inch in diameter and usually between 15 and 20 inches long). However, recent developments in microphones utilizing two microphone elements and electronic filtering promises smaller and less expensive shotgun microphones in the future. Public Address (PA) Systems On commercial PA systems, the ALD is often connected to the sound system's mixer via the tape output, line output, or monitor/headphone output. There are many different types of output connectors for commercial sound equipment. The most typical for "tape output" is the phono (RCA) jack often used on home stereos and TVs. Line output signals may use male XLR (Cannon) or ¼" phone jacks. A monitor headphone output usually uses a ¼" phone jack. Unlike phone jacks, XLR connectors can be male or female. As a general rule, input connectors are female and output connectors male. However, to make installations more cost-effective for sound contractors, some equipment manufacturers provide screw-type terminals only. Many sound systems are rack-mounted and not always accessible without tools. It is usually up to the installer to select an appropriate patch cord. Most common patch cords are available from ALD manufacturers, but it is often necessary to convert from one type of connector to an other by means of adaptors and gender (male to female or female to male) changers. When no electrical output is available, a microphone can be placed close to the loudspeaker. Connections to personal devices such as tape decks, TVs, etc. is described in the Direct Audio Input section later in this chapter. |
The Need For Assistive Devices
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