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Hearing Aid Technology:
What Consumers Need to Know.
By Harriet Kaplan
Reproduced with permission from Self Help for Hard of Hearing People, Inc., Copyright 1999 SHHH.
During the past few years, much new hearing aid technology has become available. We now have digital hearing aids, compression technology, telecoils, multiple channels, multiple memories, remote controls, and new microphone technology.These innovations come at increased cost. However, it is common for a top-of-the-line hearing aid to sell for $2,000 [to $3000] or more, including, of course, the services of the audiologist.
Many consumers are asking whether these features are worth the money. The answer to that question must be based on the communication needs of the individual hearing aid user. It is the responsibility of the consumer working with the audiologist and/or hearing aid specialist to determine which features are worth the investment. In order to participate in the decision-making, however, consumers need to understand the function, advantages and disadvantages of the new technology.
The purpose of this article is to help hearing aid users become better-informed consumers of advanced hearing aid technology.
The Telecoil Telecoils are valuable for most hearing aid users for use with the telephone, and/or assistive listening devices. For most hearing aid users, the telecoil facilitates telephone use by eliminating room noise and hearing aid feedback, and reducing distortion. Some people with mild hearing losses, however, find telephone use with only the hearing aid microphone acceptable. In addition, individuals wearing hearing aids fitted deeply into the ear canal sometimes use the hearing aid microphone without difficulty.
Still, people with mild losses and deep canal fittings can benefit from a telecoil for use with assistive listening devices. The telecoil is an important way to interface an assistive listening device such as an FM or infrared receiver to a hearing aid via a neckloop. The sound from the assistive device enters the neckloop, radiates from it in the form of electromagnetic energy, and is picked up by the hearing aid telecoil.
The telecoil can also receive sound from room loops which are amplification systems found in some auditoriums. As with telephone use, the desired signal is not contaminated by room noise or distortion. Such amplification systems are not accessible to hearing aids lacking telecoils. Therefore, telecoils should be considered not only for telephone use, but also by any hearing aid user who desires access to interpersonal assistive listening systems or to large area systems in theatres, auditoriums, or houses of worship.
Most hearing aid users have hearing loss in both ears which can and should be fitted with hearing aids. Telecoils are not necessary for both hearing aids, however. The individual should select the ear in which telephone use is most comfortable.
All hearing aid telecoils are not equivalent. An optimal telecoil is strong and located in a vertical or oblique position within the hearing aid. Location and strength of telecoils vary from hearing aid to hearing aid, however. Therefore, it is wise to try the telecoil in the dispenser's office before purchase of the hearing aid.
Although the telecoil eliminates room noise, it also eliminates the voices of other talkers in the room. Sometimes, as in a lecture-discussion situation, it is desirable to hear comments from the audience in addition to the voice of the talker. There are several strategies to deal with this issue.
First, one hearing aid may be kept on telecoil to hear the talker and the other on microphone to hear audience comments.
Second, some hearing aids provide an MT switching option which allows both the telecoil and the microphone to function simultaneously. Combining an open microphone with a telecoil, however, will also bring in room noise.
Third, a non-technology strategy may be used to deal with this communication situation. If the talker repeats all audience questions and comments, the listener will be able to hear everything through the telecoil.
Microphone Technology Directional or dual microphones can improve speech understanding in some noisy situations. These microphones favor sound entering the hearing aid from the front and provide some attenuation to sound coming from the rear or the side of the listener.
In most communication situations, the talker is positioned in front of the listener, while competing noise comes from the rear or side. In such situations, the speech entering the directional or dual microphone system is louder than the environmental noise. If noise and speech come from the same direction, however, the directional or dual microphone loses its advantage.
Directional microphones have some disadvantages. First, they do not attenuate high frequency sound coming from the rear very well. Therefore, they are not effective with high frequency noise. Second, important sound coming from the rear may be attenuated too much. Some hearing aid users prefer to use directional microphones in noise and omnidirectional microphones, which pick up sound from all directions equally well, in quiet areas. Most directional hearing aids do not allow consumers to switch from one mode to another. To meet this need, some manufacturers have introduced hearing aids that allow the user to switch from omnidirectional to directional mode as needed.
Dual-microphone design is a recent innovation that provides superior performance to the older directional microphone. These hearing aids contain two microphones which provide greater reduction of sound arriving from the rear than the older directional microphone. In contrast to the directional microphone, the dual-microphone design is sensitive to high frequency sound.
Advanced microphone technology has the potential to provide better understanding of speech in noise, at least in some communication situations. These microphones may be helpful in enough communication situations to be considered for purchase or at least a trial. As with all other hearing aid features, however, their value varies with the individual. In any case, restoration of normal listening in noise should not be expected.
Compression Technology Most new hearing aid technology involves the amplifier. All hearing aids provide a loudness ceiling called maximum power output (MPO) to protect the ear from excessively loud sounds. It is based on the individual's uncomfortable listening level which is determined during basic testing. The MPO represents the highest level of amplified sound to enter the ear canal regardless of the level of sound entering the hearing aid.
The strength or gain is also based on the individual's hearing loss. When soft sound enters the hearing aid, it is amplified based on the prescribed gain. When very loud sound enters the amplifier, however, the hearing aid must reduce the gain to avoid exceeding the MPO. For many years the only strategy available for this purpose was called peak clipping; the hearing aid circuitry literally clipped intense peaks of sound that exceeded the maximum power output. Peak clipping resulted in signal distortion which was more evident in noisy environments than in quiet. The more powerful the hearing aid, the greater the amount of peak clipping and the greater the distortion.
Compression technology is a desirable alternative to peak clipping. When the sound level within the hearing aid, a combination of sound input level and gain, reaches a pre-determined level, the gain is automatically reduced (compressed). When the sound level drops below that predetermined level, compression stops and gain is increased. The level at which compression begins (compression threshold) is set by the hearing aid dispenser. Loud sound is not distorted as occurs with peak clipping. When compression is used to maintain maximum power output, compression threshold is set as a high level, slightly below MPO.
Compression is being used for another purpose in many hearing aids, however. The aid starts compressing at a medium sound level, allowing maximum amplification of soft sounds, less amplification of louder sounds, and none at all to very loud sounds. This type of compression, called wide dynamic range compression, improves sound quality, comfort and ease of listening in most cases.
Compression is available in traditional analog aids which use circuitry, and digital aids which use computer technology. The dispenser can determine for each user the appropriate compression threshold, how much compression to use, and how fast compression starts and stops.
Most hearing aid users function well with compression. The major exception is the individual with a profound loss who has become accustomed to a great deal of power and finds that peak clipped instruments, despite distortion, better meet power needs. Many people with profound losses, however, function well with compression instruments. There is little justification today for use of a hearing aid that does not have compression.
Volume Control When a hearing aid uses wide dynamic range compression, it automatically controls volume. The volume increases as the sound level increases until the compression threshold is reached. Then the volume is automatically decreased. When the sound level drops below compression threshold, the volume is automatically increased. Theoretically, manually operated volume control is not necessary. Because many hearing aid users like a volume control on the hearing aid, some compression hearing aids provide volume controls. Other hearing aids, particularly very small instruments that fit deeply into the ear canal, do not. If the aid is properly fitted and has a good compression circuit, automatic volume control can work well.
Still another volume control option is the remote control which is similar to the remote control used for the TV. The remote control is an alternative to the volume control on the hearing aid and usually allows the user to manipulate not only volume, but also the telecoil and the on-off switch. The major problem with remote controls is that many people tend to lose them and replacement is expensive as well as a nuisance. Some hearing aids require the use of a remote control and cannot be considered if the user does not want this feature.
Analog Versus Digital Technology Analog hearing aids are traditional instruments that use electronic circuitry rather than computer microchips in the amplifier. They are adjusted by the audiologist with a small screwdriver. They are the least expensive hearing aids. They can offer compression, excellent telecoils, manual or automatic volume controls or remote controls. They cannot offer multiple channels or multiple memories which will be discussed in following sections. [Today they can offer multiple channels or multiple memories.] Properly fitted analog instruments can provide good amplification with good sound quality. (See Figure 1) 
Only a few all-digital hearing aids are currently on the market. Most of the instruments that are called digital are actually analog hearing aids with digital controls. These instruments contain electronic circuitry but the adjustments are programmed using computer software. Eventually, these hybrids will be replaced with all-digital instruments. (see Figures 2 and 3).
 
Digital hearing aids convert electronic sound information from the microphone into computer code which is then processed by an amplifier consisting of microcomputer chips instead of electronic circuitry. All adjustments are programmed using computer software.
The primary advantage of digital signal processing (DSP) is that it has far greater processing power than analog signal processing (ASP) allowing many more adjustments and greater precision for the listener. Research, however, has not yet identified which processing adjustments are optimal for various listening situations. Further, research data does not clearly indicate that DSP provides superior speech understanding in noise than ASP. Anecdotal reports, however, indicate subjective preference for the sound quality and comfort of digital instruments.
There are a number of potential advantages of digital hearing aids:
- Reduced feedback
- Improved sound quality
- Increased flexibility of programming or fine tuning. The digital instrument can be adjusted far more precisely than the analog instrument, resulting in improved sound quality.
- The digital aid can be reprogrammed if adjustments are needed or if hearing changes (not dramatically). Adjustments are possible in some analog aids, but the possibilities are far less extensive.
- The digital instrument eliminates circuit noise, which can be a problem for some clients, particularly those with normal or near normal low frequency hearing.
Digital hearing aids will not restore hearing to normal, nor will they make listening in noise comparable to listening in quiet. Digital technology can produce cleaner and more natural sound, but will not produce CD sound quality. An excellent discussion of these issues can be found in an article by Michael Valente, Ph D., and David Fabry, Ph. D. ("Responses to Manufacturers' Claims of Digital Technology in Hearing Aids," Hearing Loss, March/April 1998.)
The major disadvantage of digital technology is cost. Because of the high cost, consumers must decide whether the price differential between analog and digital is worthwhile.
Multiple Channels It is possible to obtain either analog or digital hearing aids that can amplify a wide range of frequencies. All types of hearing aids are capable of providing more amplification to high frequencies, to low frequencies, or equal amplification to all frequencies, depending on the hearing loss of the individual.
The amount of amplification across frequency (frequency response) can be adjusted by the audiologist. The digital or digital-analog hybrid aids, however, can divide the frequency response into two or more channels and adjust amplification characteristics, including compression, independently for each channel. This is an important advantage of digital technology, particularly for individuals who hear significantly better in one part of the frequency range than in others.
Listening in Noise None of the noise reduction strategies currently used in analog or digital hearing aids can reduce noise in typical listening situations where offending noise is really [with] competing speech. Some hearing aid users, however, report that their hearing aids allow them to understand speech in noise better than in the unaided condition. What is probably happening is that their hearing aids are allowing them to hear speech more clearly. Such reports have involved analog as well as digital and hybrid instruments, and may be related more to good compression characteristics rather than the type of hearing aid.
Directional or dual microphones have been helpful to some extent to speech understanding in noise because of the enhancement of the speech signal relative to the noise. Assistive technology has proven to be useful in noisy communication situations because the talker's voice at the ear of the listener is significantly stronger than background noise. Assistive devices may be used independently of the hearing aid or may be directly coupled to it by a neckloop and the hearing aid telecoil.
Which Hearing Aid Features Should an Individual Choose? Obviously some hearing aid features are selected by the audiologist based on the hearing loss. These features include amount of gain across frequency and desired maximum power output. Most audiologists strongly recommend compression technology and select the appropriate compression characteristics. Most audiologists recommend binaural amplification for clients with hearing loss in both ears that can be fitted with hearing aids. In addition, a strong, properly oriented telecoil for one hearing aid for use with the telephone and/or with assistive devices is desirable for almost all hearing aid users.
Other hearing aid options involve choices that need to be made by the hearing aid user with the help of the audiologist. One of the first decisions involves choice of analog or digital hearing aids. If cost is a factor, analog aids may be a wise choice. High quality, properly fitted analog instruments with compression can meet communication needs well for many hearing aid users.
If cost is not a factor, all-digital or hybrid hearing aids may be considered because they provide features such as multiple channels, greater processing options, and greater processing precision not available with analog instruments. Directional microphones are available with some analog instruments, but the superior dual microphone technology is available only in digital instruments. It is possible to try analog instruments first and then move up to digital aids if the analog aids prove unsatisfactory during the home trial. In order to use this strategy, however, it is necessary to work with an audiologist who dispenses both types of instruments.
A decision needs to be made about the type of volume control that is acceptable. Not all hearing aids provide manual volume controls. Many of the digital or hybrid instruments require remote controls or automatic volume control. Requirement of a specific type of volume control limits the number of instruments to be considered.
Are Multiple Memories Needed? With most multiple memory hearing aids, the second [or third, or fourth] program[s] involve[s] some type of noise reduction processing strategy. Most of these strategies have not been effective in real-life communication situations. On the other hand, if there are special communication needs such as the ability to hear subtle musical differences or to differentiate between bird calls, a second program may be important. [My dispensing experience differs from the author's. They are often helpful, especially if one memory is for a telecoil or directional microphone. On many of today's programmable instruments, alternate memories can be accessed through a button the aid itself, rather than via a remote control.]
The optimal fitting of a hearing aid often requires more than one visit to the audiologist and/or hearing aid specialist. As the individual adjusts to a new hearing aid during the home trial, the need for fine tuning may become apparent. All aids are adjustable; digital and hybrid instruments may be reprogrammed. The sale price of a hearing aid usually includes at least a 30-day home trial, a year's worth of service including unlimited walk-in appointments.
It is the audiologist's responsibility to explain the use of all features provided by the hearing aids and resolve any problems that may occur. It is the client's responsibility to communicate with the audiologist, report problems that occur, and follow through with the audiologist's recommendations. An optimal hearing aid fitting occurs when audiologist and client work together to select and adjust to the instruments that best meet the lifestyle needs of the client.
Reproduced with permission from Self Help for Hard of Hearing People, Inc., January/February, Copyright 1999 SHHH. www.hearingloss.org
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