Dr. Ross on Hearing Loss
by Mark Ross, Ph.D.
Recent developments in hearing aid technology confront hearing aid consumers and audiologists with an increasing and bewildering variety of choices. Hearing aids can be programmed to offer a large number of different user controlled operating characteristics, and can be reprogrammed to offer still other possibilities if and when necessary. With the advent of digital hearing aids, the number of possible electroacoustic variations that can be made are simply mind-boggling.
Some modern hearing aids are able to change their characteristics in "real-time", that is the amount and shape of amplification will vary depending upon the level and nature of the input sounds. Multiple memories give users access to, in a sense, a different "hearing aid" each time they toggle a switch or press a remote control. With directional microphones on the hearing aids, the sounds coming from the front are given priority compared to sounds arriving from other directions.
While these are all impressive and encouraging technical developments, the greatest increase in speech comprehension in the presence of noise can probably still be achieved in the same simple way it was possible to do it prior to these developments, and that is by reducing the distance between the speaker and a microphone. Essentially, this is what is meant by the title of this paper--microphone technique.
One does not have to go very far to see good examples of microphone technique. Every television newsperson knows that it is necessary to place the microphone close to a speaker if speech is to be easily understood. The mechanism for this advantage is easy to explain: by placing the microphone close to a talker's mouth, the speech signal is louder at the microphone than are the background noises. This relationship--between the loudness of the desired speech signals and the undesired noise background--is called the speech to noise ratio (S/N), and it is probably the single most important factor determining speech comprehension for everybody, whether one is hearing-impaired or not. If the speech signal is too soft or is masked by noise, it can hardly be understood very well. Given this fact, it is distressing to note how often the speech to noise concept is ignored or minimized by professionals and the public alike. Good microphone technique will increase both the speech loudness and the speech to noise ratio. In this paper, I'll give some examples of common problems with microphone technique and suggest how they can be minimized or overcome.
The Public Lecture
This section applies in any listening situation in which the speaker is using a microphone. The amplification device can be a traditional public address (PA) system, an infra-red (IR) device, or an FM radio unit specifically designed for hearing-impaired people. The first step for people to benefit from any these auditory assistive devices is to ensure that the speech level reaching the microphone is appropriately loud. If it is weak, the amplified signal reaching the hearing-impaired person will also be weak. If the listener attempts to compensate by turning the IR or FM receiver volume up, the level of the background noise will also increase and mask the desired speech signals. Over and over again, one can observe speakers moving away from the microphone during a lecture or sermon. I've seen speakers actually move the microphone away from them on the podium, evidently viewing it as an obstacle between them and their attempts to "connect" with the audience. That they're also losing an audience that can't hear them doesn't seem to register.
It doesn't take much distance from the microphone to significantly drop the loudness level of the speech received by a listener. Many microphones are designed to operate most efficiently when the mouth is less than six to eight inches away. At any distance greater than this, and the microphone may hardly register the speech signals. Some microphones are so insensitive that when a speaker is about two or more feet away, hardly any speech signal at all is converted to an electrical current (the basis operation of all microphones).
Other more sensitive microphones will respond to distant speech sounds, but they also pick up background noises as well, and thus decrease the speech to noise ratio. Particularly disturbing is the speaker who alternately moves all around the microphone, sometimes coming so close that one can hear and be disturbed by the bursts of the plosive consonants (such as /p/ and /t/), and sometimes so far that hardly any sound at all can be heard. One can but assume that such speakers get so involved in delivering their message that they ignore the other half of a communication situation--the recipients. Sometimes the speaker will hold the microphone in his/her hand and instead of talking into it, use it as a baton or pointer in during the talk. When faced with poor microphone technique, hearing-impaired people have a number of options that can improve their speech comprehension.
The Committee Meeting
The factors that must be considered here are the number of people at the meeting, the shape of the table, and the background sounds. In all of them, perhaps the most difficult, and the most necessary, condition to arrange is speaker discipline; if more than one person speaks at a time, the effectiveness of all the alternatives listed below will be reduced.
The Social Gathering
The optimal listening condition for a hearing-impaired person is a one-on-one conversation in a small quiet room. The most difficult listening condition is probably trying to follow a group conversation in a large noisy room. Examples of this latter situation are wedding receptions while the band is playing, an office cocktail party, a large dinner party in a restaurant, or a multi-generational family reunion. The best hearing aid in the world, with the most advanced digitital circuitry on the market, will only going to be marginally useful in these kinds of situations. (Still, I hasten to add, for most hearing-impaired people, it is usually easier to communicate with a hearing aid than without it in any kind of situation).
The best solution in these types of situations is for the hearing-impaired person to personally control an external microphone connected to his or her hearing device. The microphone, which can be of the wireless FM variety transmitting to personal FM systems (or BTE FMHA units), hard-wired into the hearing aid via a direct audio input (DAI), or incorporated in a personal listening device such as a body hearing aid, must be held by the listener and placed close to the mouth of the talker. As the conversational partner shifts, then so must the position of the microphone From an acoustic point of view, there is no doubt that this will enhance the ability to understand speech. Audiologists have available an array of microphone possibilities which can be employed in this fashion.
For many hearing-impaired people, however, this is not an acceptable solution. Some will object to any visible indication that they have a hearing loss and that they require additional auditory assistance; others find it difficult to change the habits of a lifetime and publicly assert their rights in a communicative exchange. This is an issue which extends beyond microphone technique; if a hearing-impaired person is unable or unwilling, for whatever reason, to acknowledge the reality of a hearing problem, then the effectiveness of any subsequent remediation procedure is going to be limited. For hearing-impaired people, self-acceptance is the key that can unlock a fuller participation in the wider society in many situations. Given self-acceptance, one can comfortably acknowledge the presence of a hearing loss if that is what it takes to improve speech comprehension. At the present state of the art, there simply is no better way of understanding speech in noisy places than locating an external microphone close to the lips of the person talking.
Classroom teachers in special schools have been using microphones or auditory training systems for decades and, with the assistance of an on-site audiologist, are generally skilled in their use. New and special teachers (shop or art, for example), and classroom aides, on the other hand, do need instructions in the effective use of a microphone (and even skilled teachers, occasionally still need some reminding). Some of the common problems in classroom use of an FM microphone (the most common type) will be discussed in this section.
The most important principle in using an FM microphone in a classroom is for the user to ensure that it is switched on when the recipient is supposed to hear the utterance and tumed off when the message is not intended for the FM user (either individually or as a member of the class). Keeping the FM microphone turned on while the teacher circulates in a classroom exposes the hearing-impaired person to irrelevant and disturbing speech inputs. No matter how visibly distant the teacher is from the student, the sounds in the student's ears do not diminish in loudness (as would normally be true for distant speech sounds in a classroom). With an understanding of this principle and some conscientious practice, teachers soon automatically switch the FM microphone on and off appropriately. Optimally, the students themselves should be self-confident to be able to remind a teacher when he or she is using the microphone inappropriately.
The FM microphone must be worn to be effective. Some teachers may lay it on the desk while sitting there, thinking that the sound detection qualities will remain unimpaired. This is not so. The FM microphones are most effective when situated close to a talker's lips. Placing an FM microphone over a heavy necklace or chain is not a good idea. The slight collisions which occur at the microphone sound like thunderous explosions in the hearing-impaired person's ear. The antenna of the FM microphone (a wire extending below a lavellier FM microphone/transmitter, or the cord connecting a lapel microphone with the transmitter) is not for nervous hands to twiddle; these motions create noises in the ears of the listener. It is also not a good idea to place the microphone next to a motor driven movie projector or tape recorder. The sounds of the motor will obliterate the speech sounds; there are auxiliary inputs on FM microphones that may be used in this type of situation. When viewing a video program in school, however, it is not only acceptable but recommended that the microphone be placed close to the TV loudspeaker.
More and more we are seeing FM systems being utilized in regular post-secondary educational programs. College-age hearing-impaired students are learning the advantages of using a personal FM system in large college classrooms. For the most part, these young men and women have to take personal responsibility for convincing the college instructor to wear the FM system, and ensure that it is working appropriately. Some college instructors may at first be reluctant to wear the device, but this can usually be resolved in a personal meeting. Even if there is a P.A. system in the classroom or lecture hall, it is still advisable to use the FM microphone (perhaps placed next to the podium microphone). While the speaker may use a podium microphone appropriately in a P.A. system, if the loudspeakers are some distance from the listener, speech reception will still be negatively affected. An FM system will ensure that the signal the person receives is essentially no further than the distance between the speaker and the microphone.
There is an increasing recognition that good microphone technique is especially critical for young hearing-impaired children. Most of these children have potentially usable residual hearing. Because loud ambient noises are such a common occurrence in our society, it is difficult to provide these children with a high-quality amplified speech signals. If the children cannot clearly distinguish the desired speech sounds from the unwanted background noises (a foreground-background distinction), then their auditory development of speech and language will be affected. This problem can be reduced or circumvented by parental use of an FM auditory training system.
As noted above, when used appropriately, these systems help ensure a consistently high speech to noise ratio. Parents do have to learn how to use them correctly, in much the same manner that teachers do. The same general principles apply, both in the classroom and at home:
In summary, good microphone technique cannot be taken for granted. When a microphone is used appropriately, it helps ensure that hearing-impaired people can more effectively use their residual hearing regardless of the kind and type of amplification system that is being employed.
This paper was supported, in part, by Grant #RH133E30015 from the U.S. Department of Education, NIDRR, to the Lexington Center.