Guidelines for Fitting Multiple Memory Hearing Aids · Guidelines for Fitting Multiple Memory Hearing Aids Gitte Keidser* Harvey Dillon* Denis Byrne* Abstract ... a multiple memory

J Am Acad Audiol 7: 406-418 (1996)

Guidelines for Fitting Multiple Memory Hearing Aids Gitte Keidser* Harvey Dillon* Denis Byrne*

Abstract

Fitting guidelines to determine candidacy for multiple memory hearing aids and the choice of amplification for each memory are devised by reviewing studies on the selection of ampli-fication for different listening conditions . The guideline for determining candidacy comprises three factors : (1) difficulty hearing in acoustically diverse conditions, (2) an average high-frequency hearing loss greater than about 55 dB HL, and (3) ability to vary the low-frequency gain by at least 5 dB . People who meet all three criteria are highly likely to benefit from mul-tiple memories . People who meet criteria 1 and 2 or 1 and 3 are possible candidates . The guideline for determining the amplification characteristics needed for each memory also enables the fitter to determine the number of memories needed . The following recommen-dations for amplification characteristic for various listening conditions are based on the literature review : linear amplification with the prescribed National Acoustic Laboratories' (NAL) frequency response is recommended for listening to speech in quiet; substantial high-frequency compression is recommended for ease of understanding two voices that differed by 10 dB in overall level ; a linear response steeper than the NAL response is recommended for ease of understanding speech in a low-frequency background noise ; low-frequency com-pression is recommended to reduce annoyance of low-frequency background noise ; and a linear response flatter than the NAL response is recommended for listening in high-frequency background noise, and possibly for listening to music.

Key Words: Candidacy, compression characteristics, frequency responses, hearing aid fitting, multiple memory

Abbreviations: BILL = bass increase at low levels, CR = compression ratio, CT = com-pression threshold, " D" = dialogue, HF = high frequency, HL= hearing level, LF = low frequency, "M" = music, MPO = maximum power output, NAL = National Acoustic Laboratories, "Q" = quiet, RMS = root mean square, SNR = signal-to-noise ratio, SPL = sound pressure level, TILL = treble increase at low levels

M ultiple memory hearing aids offer the user access to a number of dif-ferent amplification characteristics.

The rationale for using multiple memories is that different amplification characteristics are needed to meet different listening require-ments. A logical assumption is that a change in the acoustic input (e.g ., different background noises or reverberation) requires a change in amplification to maintain the most suitable pattern of amplified sound. However, alterna-

"National Acoustic Laboratories, Chatswood, Australia Reprint requests : Gitte Keidser, National Acoustic

Laboratories, 126 Greville St ., Chatswood 2067, Australia

tive amplification schemes may also be desir-able because, at different times, the hearing aid wearer may wish to optimize different qualities of the sound such as naturalness, pleasant-ness, and ease of understanding.

Although research has shown that multiple memory hearing aids can be beneficial (Ringdahl et al, 1990; Goldstein et al, 1991 ; Kuk, 1992; Ricketts and Bentler, 1992 ; Keidser, 1995), these devices account for only a small percentage of hearing aid fittings . Cost and complexity of mul-tiple memory hearing aids have been mentioned as possible reasons for their limited acceptabil-ity (Kuk, 1992; Ricketts and Bentler, 1992). Both of these factors would probably be less important if greater and/or more universal ben-

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Fitting Multiple Memory Hearing Aids/Keidser et al

efits were established for the multiple memory devices. Furthermore, the potential benefits of multiple memory hearing aids may have been underestimated if the multiple amplification characteristics selected have not been optimal. According to various studies, summarized later in Table 2, the proportion of subjects who preferred different frequency responses in different envi-ronments varied from 0 percent to 80 percent. The conflicting observations may be partly related to the lack of an established fitting procedure.

Two approaches to fitting multiple memory hearing aids are in use (Stelmachowicz et al, 1994). One is to use the manufacturer's recom-mendations for altering amplification charac-teristics for different listening conditions . Another approach is to select a set of amplifi-cation characteristics in a largely empirical fash-ion by having the hearing aid wearer compare a range of characteristics . This approach has been favored over a more prescriptive proce-dure by some researchers who have evaluated multiple memory devices (Kuk, 1993 ; Ringdahl et al, 1993). However, as noted by Kuk (1993), this approach requires a lot of time for evaluating alternatives, as the number of possible para-meter combinations is enormous . Therefore, for clinical purposes, it is essential to develop guide-lines for selecting the initial characteristics to be programmed in each memory of a multiple memory hearing aid. The guidelines must also enable the fitter to recognize clients who will ben-efit from these aids and to identify the listening conditions (acoustic environments and/or lis-tening criteria) that require different amplifi-cation characteristics .

This paper presents guidelines for identi-fying possible users of multiple memory hearing aids and for selection of amplification for each memory. The guidelines are developed from the knowledge at present about candidacy and the choice of amplification for different listening conditions . First, we present the issues and evi-dence concerning the use of multiple memory hearing aids . The discussion in this section con-siders all available research findings but draws largely from an extensive series of studies that the present authors conducted at the NAL (Keidser, 1995, 1996 ; Keidser et al, 1995). The research included laboratory and field evalua-tions using commercially available multiple memory hearing aids and an evaluation using a digital master hearing aid. We aimed to exam-ine the potential uses of future multiple mem-ory hearing aids as well as what is possible with current hearing aids . Second, we discuss possi-ble approaches to fitting multiple memory hear-ing aids, and, finally, we present the suggested fitting guidelines .

ISSUES AND EVIDENCE

Candidacy

It appears from studies of multiple memory hearing aids that not all hearing aid users will benefit from access to multiple amplification characteristics (Ringdahl et al, 1990 ; Goldstein et al, 1991 ; Kuk, 1992 ; Ricketts and Bentler, 1992 ; Keidser, 1996). Kuk (1993) has presented some theoretical considerations about candidacy for multiple amplification characteristics. Keidser

Table 1 Overview of Factors that May Affect the Suitability of Multiple Memory Hearing Aids for an Aid User

Factor

Communication needs Wide range of communication situations (hypothesis)

Conventional hearing aid deficient in some situations (field test)

Obtainable real-ear gain Noticeable differences among memories (hypothesis)

Sufficient variation between characteristics in the low-frequency real-ear gain (laboratory test)

Audiogram Configuration Flat and gently sloping losses (hypothesis)

A high-frequency hearing loss greater than 55 dB HL (laboratory test plus field test)

Other Attitude towards the new instrument (hypothesis)

Source

Kuk (1993) Keidser (1994)

Kuk (1992) Keidser et al (1995)

Kuk (1993) Keidser et al (1995), Keidser (1994)

Kuk (1993)

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Journal of the American Academy of Audiology/Volume 7, Number 6, December 1996

et al (1995) conducted a laboratory experiment with the object of identifying candidates for mul-tiple memory hearing aids . A small field test (Keidser, 1994) was later conducted to examine whether the observations from the laboratory test would apply in real-life situations . These hypotheses and findings are outlined in Table 1. The issues are discussed below.

Communication Needs

When applied to the results of laboratory tests in which several amplification schemes were compared in a range of listening condi-tions, the indices did not help distinguish between listeners who chose different schemes for different conditions and those who did not (Keidser, 1995 ; Keidser et al, 1995). This may be because, in the laboratory, subjects may be pre-sented with listening conditions that they do not encounter in real life . Also, subjects may encounter conditions in real life that are not presented in the laboratory.

It is a logical hypothesis that candidacy for a multiple memory hearing aid depends on the variety of listening environments that the indi-vidual hearing aid user encounters (Kuk, 1993). However, some hearing aid users (typically with mild and moderate losses) do not use their hear-ing aid all day, either because they can cope without a hearing aid in some environments, or because the hearing aid offers no advantages in other environments . Such a person may be com-municatively active and move around in various acoustic environments . However, the hearing aid may only be worn when in a small range of environments for which one amplification charac-teristic may be suitable . By contrast, some peo-ple may live a very quiet life and only move around in two or three different environments . Nevertheless, the listening requirements may be very different in those environments and this person may thus benefit from a multiple mem-ory hearing aid. The above examples suggest that the number of acoustically different environ-ments in which the client has difficulty hearing may be a better guideline for deciding whether a multiple memory hearing aid will be benefi-cial than simply the number of situations expe-rienced by the client . This was confirmed in a small field test (Keidser, 1994) in which 10 hear-ing aid users wore a multiple memory hearing aid in everyday environments for 12 months . Prior to the trial, three indices were obtained for each subject using a questionnaire. The indices indicated (1) how socially active their life was, (2) to what extent they used their conventional hearing aid in the range of listening conditions they encountered, and (3) how helpful they found their hearing aid in these conditions . Only the last index showed a significant difference between subjects who used different amplifica-tion characteristics and those who did not. The trend was that multiple amplification charac-teristics were not used by people who reported that their conventional hearing aid was very helpful in all or most situations .

Obtainable Real-Ear Gain

Despite the increasing flexibility of pro-grammable hearing aids, substantial variation in real-ear gain may not be achievable for sev-eral reasons. First, feedback sometimes limits the amount of high-frequency real-ear gain achiev-able . Second, if an initial fitting results in a set-ting of one or more parameters at their upper or lower limit, further adjustments in that direction are not obtainable . Third, many hearing aids are fitted with earmolds with vents that often will result in 0-dB insertion gain at the low-fre-quencies, regardless of the electronic gain (Dil-lon, 1991). Consequently, only the high-frequency gain can be varied, and these variations may not be sufficient to make a difference to the client as may be just as easily achieved with variation of the volume control. Kuk (1992) mentioned that the alternative amplification characteristics had to be noticeably different if multiple mem-ory hearing aids are to be effective . The impor-tance of this factor was demonstrated by the results obtained in Keidser (1995b) and Keidser et al (1995) . In the first study, 25 subjects com-pared various frequency responses and com-pression characteristics simulated in a digital master hearing aid. The master hearing aid allowed all subjects, independently of degree and configuration of hearing loss, to be fitted with substantial variations in real-ear gain . In all, 84 percent of the subjects selected different characteristics in different listening conditions . In the second study, 30 subjects compared dif-ferent frequency responses using a commercial hearing aid. Only 40 percent of the subjects showed a preference for different responses. The variation achieved in the low-frequency real-ear gain helped discriminate between those who selected different responses and those who did not; those who selected different responses were fitted with greater variations-typically more than 5 dB in either direction.

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Audiogram Configuration

As indicated in Table 1, Kuk has suggested that hearing-impaired people with flat and gen-tly sloping losses are candidates for multiple memory hearing aids . Because electroacoustic changes in current programmable hearing aids occur over a broad frequency range, wearers must require amplification over a broad fre-quency range in order to notice differences between programs . However, in Keidser et al (1995) and Keidser (1994), the average loss mea-sured across 2000, 3000, and 4000 Hz helped dis-criminate between subjects who preferred multiple amplification characteristics and those who did not. Subjects who selected different characteristics for different conditions had a greater high-frequency hearing loss (typically above 55 dB HL). This excludes hearing aid users with mild and moderate flat losses, and mild, gently sloping losses .

The variation in low-frequency real-ear gain (see above) and the average high-frequency hear-ing loss were interactive parameters in a model derived by discriminant analysis (Keidser et al, 1995). Subjects who failed to meet both criteria

(i .e ., those who had a high-frequency hearing loss less than 55 dB HL and who were fitted with insufficient variations in the low-frequency real-ear gain) could be excluded as candidates, whereas subjects who met both criteria were highly likely to prefer different responses for dif-ferent conditions. However, subjects who only met one of the criteria were unpredictable. For example, subjects with mild, steeply sloping hearing losses have a great high-frequency hear-ing loss but may not achieve sufficient variation in the low-frequency real-ear gain if they have an earmold with a large vent . In Keidser et al (1995), two of five subjects with a mild steeply sloping hearing loss selected different frequency responses, whereas three did not.

Other

The last factor listed in Table 1 is the client's attitude toward the new instrument (Kuk, 1993). Although a client theoretically may benefit from multiple amplification characteristics, such a device should not be forced upon someone who believes that the hearing aid is too difficult to operate, or that it will not be any better than a

Table 2 Overview of Selected Frequency Response Characteristics, Listening Conditions, and Percentage of Subjects who Selected Different Frequency Responses in Different Listening

Conditions from Five Evaluation Studies of Multiple Memory Hearing Aids

Study Frequency Responses

Ringdahl et al (1990)

Goldstein et al (1992) Kuk (1992)

Ricketts and Bentler (1992) Stelmachowicz et al (1994)

Listening Conditions

TV news, music, speech from the adjacent room, conversation with two or three people when TV or radio is on, with two or three people in a more noisy speech situation, and at a meeting Male talker and female talker each in four conditions : in quiet, in a multivoice speech babble, in a combination of multivoice and environmental noises, and in a combination of train, car, and animal sounds Everyday listening situations categorized as easy (no difficulty in hearing), mild (some difficulty), moderate (difficult to communicate), moderately severe (very difficult to communicate), and severe (a lot of difficulty) Nonspecified, everyday listening situations Speech in quiet, in speech noise, in quiet conference room, in reverberant lecture hall, and in reverberant lecture hall in noise

NAL-R, more gain in high frequencies plus lower MPO in low frequencies in two steps, less gain and less MPO in high frequencies in two steps Four prescriptive procedures : NAL-R, Berger, Libby, 1/3, and MSU version 2 .0 Individual adjustment of low-frequency cut-off and high-frequency cut-off in a commercial hearing aid using the modified simplex procedure NAL-R plus Widex's recommendations for quiet, party noise, and music Individual adjustment of low-frequency gain and high-frequency gain (6-dB step) in a PC-based hearing-aid simulator using the modified simplex procedure

Users (01o)

81

80

68

50

0

MPO = maximum power output.


conventional hearing aid. This consideration seems relevant, and a reasonable extension of the known effect of attitude on successful hear-ing rehabilitation (see Goldstein and Stephens, 1981), but we are not aware of any specific research into the issue.

Acclimatization Effect

The acclimatization effect (Gatehouse, 1992) can complicate the identification of multiple memory candidates . Establishing, for example, the need for communication and the configura-tion of hearing loss does not guarantee that the wearer will immediately benefit from multiple memories (Kuk, 1993). Some users may take several months to acclimatize fully to the use of multiple amplification characteristics and, there-fore, to show benefit. Furthermore, a failure to select the most appropriate alternative ampli-fication characteristics could prevent, or at least delay, the recognition of benefit from the device . Those problems should be reduced by the intro-duction of an appropriate fitting procedure.

Selecting Amplification Characteristics

As suggested in the introduction, a sys-tematic procedure for selecting the alternative amplification characteristics of multiple mem-ory hearing aids may improve the effectiveness of using these devices. Although a prescriptive procedure is not sensitive to individual listen-ing preferences, such a procedure can effectively reduce the limitless number of possible options (Byrne, 1983) and probably decrease the num-ber of revisits for fine adjustment . In the early evaluation studies of multiple memory hearing aids, very different approaches were used to select the characteristic for each memory as lit-tle information was available about the condi-tions under which hearing aid users could benefit from various amplification characteristics. Table 2 summarizes, for each of five studies, the fre-quency responses selected for evaluation, the range of listening conditions in which the responses were tested, and the percentage of subjects who obtained benefit from using more than one frequency response .

When a multiple memory hearing aid is tri-aled it should not be necessary to start with a completely random selection of amplification characteristics in each memory. Some prescrip-tive hearing aid fitting procedures have proved to be effective in providing good speech intelli-gibility and comfortable listening in some com-mon listening conditions (e .g ., Byrne and Cotton,

1988). Such procedures typically prescribe a fre-quency response based on the hearing-impaired person's audiogram. It therefore seems logical to select one of the prescriptive frequency responses as a reference to compensate for the individual hearing loss and then consider variations around such a reference response to compensate for changes in the acoustic input or listening pref-erences at different times.

The issues that need to be addressed to derive a validated fitting procedure for multiple memory hearing aids include the amplification characteristic needed, the relationship of ampli-fication characteristic to environment, and the relationship of amplification characteristic to listening criterion. For example, among the para-meters that describe linear or nonlinear ampli-fication characteristics (average gain, shape of frequency response, compression ratio, com-pression threshold, time constants, and MPO), which ones need to vary to maximize benefit in different listening conditions? So far, most eval-uation studies have compared variations in lin-ear amplification only. However, nonlinear amplification may be preferable in some condi-tions, or the optimum benefit may require a com-bined adjustment of both the linear and the nonlinear characteristics. Further, when hearing aid users select an amplification characteristic other than a reference response, is this choice then related to the acoustic environment and/or to the listening criterion?

Table 3 shows an overview of studies that have demonstrated a preference for different amplification characteristics in different listen-ing environments or for different listening cri-teria. Only a limited number of characteristics was evaluated in each study and, in most cases, the selected amplification comprised different electroacoustic characteristics possible in com-mercially available hearing aids, rather than a systematic variation around a reference response .

Speech in Quiet

Among the prescriptive amplification char-acteristics suggested to optimize speech under-standing in general, the NAL response (Byrne and Dillon, 1986) is probably the one best supported by empirical data . For example, a general pref-erence for the NA prescription, compared to variations around this response, has been demon-strated for understanding speech in quiet (Byrne and Cotton, 1988 ; Keidser et al, 1995). However, in both experiments, the results were based on presentation of speech (one voice) at a "normal"

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Table 3 Overview of Studies that Have Demonstrated a Preference for Different Amplification Characteristics in Different Listening Conditions

Study Test Reference Environment/ Criterion

Preferred Response

Harford and Field test Own hearing aid Noisy environment/ Response steeper Fox (1978) "preferred" than reference

Quiet environment/ Reference response "preferred"

Punch and Paired com- Compared eight Speech in babble/ Not indicated* Parker (1981) parison test different characteristics quality

Speech in babble/ intelligibility

Tecca and Magnitude Compared three Speech at 90 dB SPL/ "Most" LF-gain Goldstein estimation aids with various intelligibility (1984) test degrees of low- Speech at 110 dB SPL/ "Least" LF-gain

frequency gain intelligibility

Byrne (1986) Paired com- NAL response Speech in quiet and in Not indicatedt parison test noise/pleasantness

Speech in quiet and in noise/intelligibility

Kuk (1990) Simplex Own voice/" preferred" "Less" insertion procedure gain

Other voice/" preferred" "More" insertion gain

Keidser (1994) Field test NAL response Heavy background Response steeper noise and in crowds/ than reference "preferred" Quiet environments Reference response and small groups/ "preferred"

Keidser et al Paired com- NAL response Speech in quiet/ Reference response (1995) parison test ease of understanding

Speech in traffic/ Response steeper ease of understanding than reference Speech in HF-noise/ Response flatter ease of understanding than reference

Keidser (1995b) Paired com- NAL response Dialogue (10-15 dB Substantial HF- parison test difference in Leq of compression

voices)/ease of understanding Speech in traffic/ LF-compression minimizing annoyance Speech in traffic/ Response steeper ease of understanding than reference Speech in HF-noise/ Response flatter minimizing annoyance than reference

*Low correlation was found between judgments of criteria. rDifferent ranking of selected responses was demonstrated .

level. In everyday environments, speech varies greatly in both spectral content and level. Tecca and Goldstein (1984), for example, showed that hearing-impaired subjects preferred relatively less low-frequency amplification as the presen-tation level of speech increased. However, the average amount of gain also decreased with the decrease in low-frequency amplification; therefore, it is uncertain whether the subjects preferred less low-frequency gain or less overall gain. A

study by Kuk (1990) indicates that hearing-impaired people prefer more insertion gain when listening to other voices than when listening to their own voice owing to the difference in level. Experience, however, suggests that few hearing aid users change the gain continually during a conversation with another person . The selected gain may be that preferred for "own" voice, for the "other" voices, or a compromise between the two. A similar situation is when the hearing-impaired

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person is among a group of people who talk at var-ious levels . A study by Keidser (1995 b) sug-gested that substantial high-frequency compression was preferred to various linear responses to optimize the ease of understanding of two voices that differed by 10 dB in overall Leq. The findings discussed above suggest that, while the NAL response should be suitable for speech in quiet at average levels, different amplifica-tion characteristics may be beneficial for listen-ing to speech at other levels, especially when there are multiple talkers.

Speech in Reverberation

Reverberation is known to cause communi-cation difficulties for hearing-impaired people because of the addition of the reflected, delayed sounds . At least three studies have investigated the choice of amplification for listening to speech in quiet as well as speech in reverberation (Harris and Goldstein, 1985 ; Stelmachowicz et al, 1994 ; Keidser, 1995b) . None of these studies found that an amplification characteristic other than the one selected for listening to speech in quiet was preferred for listening to speech in reverberation .

Speech in Low-frequency Weighted Noise

Two studies (Keidser, 1995b; Keidser et al, 1995) have suggested that a response steeper than the linear reference response (NAL) was preferred for ease of understanding speech in traffic noise. Low-frequency compression was preferred to a linear reference to minimize the annoyance of the traffic noise . The latter find-ing confirms a hypothesis presented by Sty-pulkowski (1993) that BILL (bass increase at low levels) processing is suitable for listening in low-frequency noise. A preference for a response steeper than a linear reference response for lis-tening in noisy environments was also demon-strated in two field tests (Harford and Fox, 1978 ; Keidser, 1994). Keidser (1995a) has shown that common environmental noises, such as traffic noise, motor noise from inside different means of transport, and babble noises as measured in reverberant environments such as shopping centers, cafes, and restaurants, primarily mask the low-frequency components of speech . The preferences, observed in the two field studies, for steeper responses are therefore in agreement with the results obtained for ease of under-standing speech in the low-frequency weighted traffic noise.

Speech in Babble Noise

Babble noise is one of the most common background noises and is therefore often included in the set of listening conditions used to evalu-ate hearing aids . Research (cf. Stelmachowicz et al, 1994 ; Keidser, 1995b) has indicated that the frequency response preferred for speech in quiet has also been best for listening in babble noise. An explanation could be that the overall spec-trum of the babble noise is very similar to that of speech, which means that, if some of the noise is filtered out, then some of the speech infor-mation is taken away too. It is, however, ques-tionable whether real-life babble noise has a long-term spectrum similar to that of speech. In real-life environments, a babble noise that reaches the ear of a listener is most often affected by reverberation and/or baffle effects. Both of these factors are likely to increase the sound pressure level of low-frequency components rel-ative to that of high-frequency components . Thus, in many situations, babble noise can be categorized as a low-frequency weighted back-ground noise when compared to speech. This is in agreement with findings of Keidser (1995a). Further, during the field test reported by Keidser (1994), subjects reported that they used a response steeper than the NAL response for listening in noisy environments as well as in crowds (babble noise) .

Speech in High-frequency Weighted Noise

Some specific background noises or sounds (e.g., bandsaw, printer, paper rustling, and dishes rattling) are high-frequency weighted with ref-erence to the speech signal . Stypulkowski (1993) has suggested that TILL (treble increase at low levels) processing is suitable for reducing the annoyance of high-frequency sounds . However, one study (Keidser, 1995b) that compared both linear frequency responses and compression characteristics found that hearing aid users pre-ferred a linear frequency response flatter than the NAL response to minimize the annoyance of a fluctuant, high-frequency weighted background noise.

Other Signals of Interest

The stimuli discussed above constitute some of the more common everyday environments . The main difference in the acoustic input to the hearing aid is in the spectral shape of the speech alone or speech and noise combined . There are,

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however, other acoustic inputs and other char-acteristics of the signal to consider. Franks (1982), for example, determined that hear-ing-impaired people preferred extended low-frequency gain when listening to music . However, the finding was not related to other acoustic environments or any prescribed refer-ence response and thus cannot be used to con-clude whether an alternative amplification characteristic is beneficial for listening to music.

Besides being categorized according to their spectral content, signals can be categorized with respect to their temporal and intensity charac-teristics. Such variations may require further or different adjustments of amplification charac-teristics to optimize listening. At the moment, there is no evidence that suggests a possible relationship between amplification character-istics and signals with various temporal char-acteristics .

Amplification Requirements for Different Listening Criteria

Most studies that have examined the use of different amplification characteristics in differ-ent listening conditions have only investigated changes in the acoustic input. It is possible that, for the same environment, different amplifica-tion characteristics may be useful to achieve different qualities such as "pleasantness," "intel-ligibility," and "naturalness ." Afew studies con-firm this hypothesis (Punch and Parker, 1981 ; Byrne, 1986; Keidser, 1995b) (see Table 3) . In Keidser (1995b), when listening in traffic noise, subjects tended to select different characteris-tics for ease of understanding speech than for minimizing the annoyance of noise. Similarly, Byrne (1986) found that, in both quiet and noise, some subjects preferred different frequency responses, depending on whether the criterion was intelligibility or pleasantness . Punch and Parker (1981) found a low correlation between quality judgments and relative intelligibility judgments of eight electroacoustic characteris-tics when listening to speech in babble noise. Nei-ther Byrne (1986) nor Punch and Parker (1981) have discussed the differences in the amplifi-cation characteristics preferred for the different listening criteria .

In the studies mentioned above, the listen-ing criteria were selected by the experimenters . Further research in this area should include a determination of what qualities the hearing aid user aims for when trying different character-istics in real-life environments .

POSSIBLE APPROACHES TO FITTING MULTIPLE MEMORY HEARING AIDS

Types of Selection Procedures for Candidacy

Audiologists need to advise a client whether a multiple memory hearing aid is appropriate . There are at least three ways in which this could be determined . First, a multiple memory hearing aid could be fit to all clients, on the grounds that those who can benefit from mul-tiple memories will definitely get the opportu-nity to try them. Second, those most likely to use the hearing aid could be identified on the basis of readily available information, such as their audiogram, and/or the range of situations in which they require assistance with hearing. Third, a client's preferences for different ampli-fication, while listening to various combinations of speech and noise, could be determined in the clinic with either a fitted hearing aid or a mas-ter aid of some type . These preferences could be found for all clients or just for those who meet specified criteria for hearing loss or hearing problems .

Of the three approaches, the second is the most suitable . The factors, presented earlier in this paper, that have been found to identify pos-sible users of multiple memory hearing aids would be used to preselect candidates for these devices. The first approach will cause unneces-sary expense and confusion to too large a pro-portion of the clients, and the third approach seems to be an unnecessary step (if done solely for the purposes of determining candidacy), as most people who meet our suggested criteria will choose different amplification characteris-tics for different listening environments and/or for different listening criteria . Conversely, people who do not meet any of the criteria are unlikely to choose different amplification characteristics .

Types of Selection Procedures for Alternative Amplification

The observations presented in Table 3 and the discussion of the various findings emphasize the potential for multiple memory hearing aids . There is, however, still a need to determine the optimum amplification characteristics for par-ticular listening conditions . Although it is not yet possible to derive a specific formula for choos-ing different amplification characteristics, the

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information available can and should be used to provide guidelines that are practical and easy to use in the clinic . If the base response of the hearing aid has been prescribed in some man-ner related to the hearing loss of the client, there are then at least three different ways in which the alternative amplification character-istic(s) could be selected .

First, the same variations in the amplifica-tion characteristic (e .g., a low cut relative to the base response) could be prescribed for all clients. The variations that are most useful to most clients would be used for all clients. The only decision required of the aid fitter would be how many amplification characteristics to provide. This approach represents a "one-size-fits-all" philoso-phy and is only prescriptive in that the variations in amplification are all modifications of the orig-inal, individually prescribed, base response .

Second, the fitter could determine in which acoustic environments the aid wearer most fre-quently listened, or most frequently required improved listening ability, and, for each of these, which listening criteria were most important. Armed with this information, the fitter could then use known relationships between listening condition and the best amplification character-istics (such as those shown in Table 3) to select the characteristics that are most likely to sat-isfy these needs. This approach is obviously more prescriptive than the first .

Third, the first or second approach could be used to produce a short list of potentially useful

amplification characteristics . The hearing aid wearer could then listen to appropriate signals processed by each of these variations and choose the characteristics that best meet the listening preferences. The test signals could be a stan-dard combination of speech (or music) and back-ground noise or reverberation, or the test signals could be individually selected to represent those most important to the client . This third approach involves both prescriptive and evaluative com-ponents and is the most time consuming.

Of the three approaches, we reject the first, "one size fits all," because it ignores individual listening requirements, which may differ to a considerable extent . For example, some people may not be confronted with high-frequency weighted background noises for long periods at a time, some people may not have a particular interest in listening to music, and some people may not be very sociable and thus may not need to communicate with many people at a time or need to communicate with other people in very noisy environments . The third approach most completely considers individual requirements and is, therefore, ideal scientifically. However, because it is time consuming, it is less practical than the second approach . We feel that there is sufficient consistency in the choice of amplifi-cation for specific listening conditions, both within the NAL study and among different stud-ies, that the second approach is more suitable than the third for general clinical use. Nonethe-less, a good case can be made for favoring the

Table 4 Overview of Variations in Frequency Response or Compression Characteristics (re NAL Response) that May Be Suitable in a Range of Listening Conditions

Listening Environment Listening Criterion

Lin

NAL

ear Variation

FLAT STEEP LF

Compre

HF 1

ssion

HF2 BOTH

Speech in quiet Ease of 2 3 (70 dB SPL) understanding "Dialogue" Ease of 2 1 - (70 dB SPL - loud voice) understanding Speech in LF-weighted noise Ease of 1 2

understanding Speech in LF-weighted noise Reduce annoyance 3 1 2

of noise Speech in HF-weighted noise Ease of 1 3 2

understanding Speech in HF-weighted noise Reduce annoyance 2 1 - - -

of noise Music Pleasantness (?) 2 1 - - - - - (not confirmed)

FLAT: a linear response with a 5-dB boost at 500 Hz and a 5-dB cut at 3000 Hz re the NAL response ; STEEP: a linear response with

a 5-dB cut at 500 Hz and a 5-dB boost at 3000 Hz re the NAL response ; LF : compression in the low frequencies only (CR = 2:1, CT = 55

dB); HF 1 : compression in the high frequencies only (CR = 4:1, CT = 70 dB); HF 2: substantial compression in the high frequencies only

(CR = 4:1, CT = 55 dB), BOTH : compression in both the low (CR = 2:1, CT = 55 dB) and the high CR = 2:1, CT = 70 dB) frequencies .

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third approach for experimental purposes or clinically where time permits.

The second approach requires that the fit-ter knows the relationships between listening conditions and the best amplification charac-teristics . With reference to the NAL response, Table 4 shows an overview of variations in the amplification characteristic that research has demonstrated to be suitable for a range of lis-tening conditions . Table 4 is largely based on the findings of Keidser (1995b), who tested 16 ampli-fication schemes in 15 listening conditions . Two or three amplification schemes are indicated for each listening condition. This is because the various multiple memory hearing aids vary with respect to features and options so that not all of the variations can be achieved with every hear-ing aid, and because some variations cannot be achieved for some hearing aid users owing to venting, feedback, or a limited range of adjust-ment . The amplification schemes are ranked according to their suitability, with the most suit-able scheme labelled "1." It should be noted that the relationships between amplification schemes and listening conditions given in Table 4 may be related to the overall level of speech and the sig-nal-to-noise ratios (SNRs) in background noise . Presumably, hearing aid users will not need an alternative amplification characteristic in noisy environments if the SNR is either very good or very bad. In very good SNRs, the amplification scheme preferred for speech in quiet may still be efficient, and it is doubtful whether any amplification scheme will be satisfactory in extremely poor SNRs .

SUGGESTED PROCEDURE

Procedure for Selecting Candidates

indicate that help is needed for acoustically different environments, it is considered unlikely that the client will need multiple memories, and we suggest that the client be excluded as a candidate. If the client reports difficulty hearing in at least one group of environments other than "speech in quiet," continue with the following steps.

2. Calculate the client's average high-fre-quency hearing loss (across 2000, 3000, and 4000 Hz) .

3. Determine whether the client's real-ear gain at 500 Hz can be varied about 5 dB in either direction around the fitted reference response . As discussed above, feedback and a vented earmold will limit the amount of variation possible.

Our data suggest that clients who present a low degree of high-frequency hearing loss (< 55 dB HL) and who cannot be fitted with sufficient variation in the low-frequency real-ear gain will not benefit from a multiple mem-ory hearing aid and can be excluded as candidates . All other clients who report a need to optimize listening in different listening con-ditions should be considered as possible can-didates for a multiple memory hearing aid. However, the possibility decreases if the client does not meet the criterion in either step 2 or step 3. Figure 1 shows a schematic overview of the procedure.

Selecting Amplification Characteristics

In a previous section, different approaches for selecting the alternative amplification char-acteristics were discussed and it was concluded

In the previous section, three approaches for selecting candidates for multiple memory hear-ing aids were presented. It was concluded that the most suitable approach would be to use the factors that have been demonstrated to charac-terize users of multiple amplification charac-teristics (see Table 1) . A clinical procedure is outlined below.

1. Use a checklist (see Appendix A), or some other needs-based approach, to determine the client's need for a hearing aid in different conditions . The checklist, which has yet to be evaluated, is designed to identify listen-ing conditions for which the client experi-ences difficulty hearing. If the client does not

cr~

no Needs hearing help in acoustically diverse conditions

yes

no Average HF-loss > 55 dB HL

Yes

Sufficient variation no Sufficient variation in LF real-ear gain in LF real-ear gain is achievable yes ~ is achievable

no yes

Probable candidate

Figure 1 Schematic overview of the stepwise procedure for indentifying candidates for a multiple memory hear-ing aid.

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that the second approach would be most appro-priate . This approach requires the following steps :

1. Determine the range of listening conditions in which the client needs to optimize lis-tening. The checklist used to identify can-didates for multiple memory hearing aids will provide data to assist with this .

2. Match the listening conditions identified above with the listening conditions listed in Table 4. Use Table 4 to determine the num-ber of memories needed and to select the appropriate amplification characteristic for each memory.

3. Program the amplification characteristics into the selected multiple memory hearing aid and perform an informal paired-com-parison test to determine whether the client can distinguish between the different char-acteristics (see below).

It is possible that some clients may indicate that they only need amplification for listening in noisy environments. In such cases, we rec-ommend that the audiologist also include a response suitable for listening in quiet. This is because although clients most frequently initially report difficulties with hearing in noise (Dillon et al, 1991), they report that most benefit from the hearing aid occurs in quiet situations (Byrne and Green, 1972), and that hearing aids are most frequently used in quiet situations (Keidser, unpublished data).

Table 4 indicates that hearing aid users may benefit from at least five different amplifi-cation schemes. However, as discussed earlier, it is unlikely that any individual would have a need for all of these five schemes. Nevertheless, if a client appears to need all five schemes, it is possible to compromise with a smaller number of responses by making use of the rank order-ing information. For example, a response steeper than the NAL response is suggested as the best choice for ease of understanding speech in a low-frequency weighted background noise, whereas low-frequency compression is preferred to reduce the annoyance of low-frequency weighted noise . However, compression in both frequency bands is a suitable alternative for both conditions and could be selected to reduce the number of memories needed . Similarly, the NAL response and the "flatter" response, or the NAL response and substantial high-frequency compression, would be acceptable solutions for the remaining five listening conditions . This

means that, overall, the seven listening condi-tions listed in Table 4 can be covered by three memories .

The desired amount of variation in the amplification characteristics cannot be speci-fied exactly at this stage. However, Byrne (1992) has demonstrated that, for some hearing aid users, a 3-dB root mean square difference in the shape of the insertion gain curves is notice-able . In the studies conducted by Keidser et al (1995) and Keidser (1995b), a variation of 4 to 8 dB in one direction in the low frequencies (aver-aged across 250, 500, and 1000 Hz), and a cor-responding variation in the opposite direction in the high frequencies (averaged across 2000, 3000, and 4000 Hz), proved to be readily dis-criminable for most subjects without being so extreme that the alternative frequency response was always unacceptable . In obtaining the "flat-ter" response, it is therefore suggested to aim for a 5- to 6-dB boost in the low-frequency real-ear gain at 500 Hz, and a 5- to 6-dB cut in the high-frequency real-ear gain at 3000 Hz . Similarly, to obtain a "steeper" response, a 5- to 6-dB cut in the low-frequency real-ear gain at 500 Hz and a 5- to 6-dB boost in the high-frequency real-ear gain at 3000 Hz should be achieved .

For all of the compression schemes defined in Table 4, the time constants were 5 msec for the attack time and 50 msec for the release time . Because very few studies have compared the effect of using different compression char-acteristics in different listening conditions, the suggested parameters are tentative and should be used as guidelines only.

Finally, those clients who appear to be can-didates for multiple memory hearing aids should take part in an informal paired-comparison test . The test is used to determine if the client can dis-tinguish between alternative amplification char-acteristics . During the paired-comparison test, the client should compare each of the selected amplification characteristics with a reference response in a listening condition that is consid-ered to be appropriate for the alternative char-acteristic in question . If the client cannot clearly distinguish between the various amplification characteristics, it is recommended that the fit-ter attempt to increase the variation in real-ear gain . It should be noted that some hearing aid users may show a strong preference for the same amplification scheme for all listening con-ditions, even when offered alternative amplifi-cation schemes containing large variations in amplification characteristics. Such a client may not benefit from a multiple memory hearing aid

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or will need further encouragement to try the multiple memory feature.

CONCLUSION

intelligibility and pleasantness of speech for hearing-impaired listeners. JAcoust Soc Am 80:494-504 .

Byrne D. (1992) . Key issues in hearing aid selection and evaluation . J Am Acad Audiol 3:67-80 .

This paper has discussed issues and evi-dence concerning the use of.multiple memory hearing aids based on a review of literature and findings in an extended study about multiple memory hearing aids conducted at NAL.

We found strong evidence that hearing-impaired people prefer different amplification characteristics for different acoustic inputs or to meet different listening requirements . Those who are most likely to benefit from a multiple memory hearing aid have a need for help in dif-ferent groups of environments, a high-frequency hearing loss greater than about 55 dB HL, and can be fitted with more than 5-dB variation in the low-frequency real-ear gain .

We found relatively consistent relationships between preferred amplification characteristics and listening conditions . This information can be used to determine the number of memories needed and the most appropriate characteristic to be programmed into each memory. A guide-line for fitting multiple memory hearing aids using a checklist and a table is presented.

Based on the knowledge gathered so far about multiple memory hearing aids, we suggest that future research should aim at (1) deter-mining further listening conditions for which there is a need for a specific set of amplification characteristics ; (2) determining the optimum degree of variation in amplification character-istics around a linear reference response in the various listening conditions ; (3) confirming, through extended field tests, the conclusions reached on the basis of laboratory experiments; and (4) evaluating and refining the guidelines presented in this paper.

Acknowledgment. We would like to thank Widex ApS, Resound Cooperation, and Harry Levitt from the City Uni-versity of New York for providing us with hearing aids, software, and other equipment to complete a series of stud-ies about multiple memory hearing aids. We would also like to thank Roger Lovegrove for his helpful comments on an earlier version of this paper.

REFERENCES

Byrne D. (1983). Theoretical prescriptive approaches to selecting the gain and frequency response of a hearing aid. Monogr Contemporary Audiol 4(1) .

Byrne D. (1986) . Effects of frequency response charac-teristics on speech discrimination and perceived

Byrne D, Cotton S . (1988) . Evaluation of the National Acoustic Laboratories' new hearing aid selection proce-dure . J Speech Hear Res 31:178-186 .

Byrne D, Dillon H. (1986) . The National Acoustic Laboratories' (NAL) new procedure for selecting the gain and frequency response of hearing aid . Ear Hear 7:257-265 .

Byrne D, Green AC . (1972) . Hearing Aids : Their Use.by the Aged. Commonwealth Acoustic Laboratories Report No 59 . Sydney: Commonwealth Acoustic Laboratories .

Dillon H. (1991) . Allowing for real ear venting effects when selecting the coupler gain of hearing aids . Ear Hear 12:406-416 .

Dillon H, Koritschoner E, Battaglia J, Lovegrove R, Ginis J, Mavrias G, Carrie L, Ray P, Forsythe L, Towers E, Goulios H, Macaskill F. (1991) . Rehabilitation effective-ness I: assessing the needs of clients entering a national hearing rehabilitation program. Aust J Audiol 13 (2):55-65 .

Franks JR . (1982) . Judgments of hearing aid processed music. Ear Hear 3:18-23 .

Gatehouse S. (1992) . The timecourse and magnitude of perceptual acclimatisation to frequency responses: evi-dence from monaural fitting of hearing aids . J Acoust Soc Am 92:1258-1268 .

Goldstein DP, Shields AR, Sandlin RE . (1992) . A multi-ple memory, digitally-controlled hearing instrument. Hear Instr 42(1):18,20-21 .

Goldstein DP, Stephens SDG. (1981) . Audiological reha-bilitation: management model 1. Audiology 20:432-452 .

Harford ER, Fox J. (1978) . The use of high-pass ampli-fication for broad-frequency sensorineural hearing loss . Audiology 17 :10-26 .

Harris RW Goldstein DP (1985) . Hearing aid quality judgments in reverberant and nonreverberant environ-ments using a magnitude estimation procedure. Audiology 24:32-43 .

Keidser G. (1994) . Selecting Amplification for Different Listening Conditions . Report No . 61 . The Technical University of Denmark. The Acoustics Laboratory.

Keidser G. (1995a). Long-term spectra of a range of real-life noisy environments . Aust J Audiol 17:39-46 .

Keidser G. (1996a). Selecting different amplification for different listening conditions. JAm Acad Audiol 7:92-104.

Keidser G. (1995b). The relationship between listening conditions and alternative amplification schemes for mul-tiple memory hearing aids. Ear Hear 6:575-586 .

Keidser G, Dillon H, Byrne D. (1995) . Candidates for multiple frequency response characteristics . Ear Hear 6:562-574 .

Kuk FK . (1990) . Preferred insertion gain of hearing aids in listening and reading-aloud situations . J Speech Hear Res 33:520-529 .

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Kuk FK. (1992) . Evaluation of the efficacy of a multi-memory hearing aid. J Am Acad Audiol 3:338-348 .

Does the client have difficulty understand-ing speech in the following situations :

Kuk FK. (1993). Clinical considerations in fitting a mul-timemory hearing aid. Am J Audiol 11:23-27 .

Punch JL, Parker CA . (1981) . Pairwise listener prefer-ences in hearing aid evaluation . J Speech Hear Res 24:366-374 .

Ricketts TA, Bentler RA. (1992) . Comparison of two dig-itally programmable hearing aids . J Am Acad Audiol 3:101-112 .

Ringdahl A, Eriksson-Mangold M, Israelsson B, Lindkvist A, Lejlon A, Mangold S. (1990) . Clinical trials with a pro-grammable hearing aid set for various listening environments. Br JAudiol 24:235-242 .

Ringdahl A, Mangold S, Lindkvist A. (1993) . Does the hearing aid user take advantage of different settings in multi-programmable hearing aids in acoustically vari-ous listening environments? In: Beilin J, Jensen GR, eds. Recent Developments in Hearing Instrument Technology . Copenhagen: Stougaard Jensen, 453-465.

Stelmachowicz PG, Lewis DE, Carney E. (1994) . Preferred hearing-aid frequency responses in simulated listening environments . J Speech Hear Res 37:712-719 .

Stypulkowski PH . (1993) . Fitting strategies for multiple-memory programmable hearing instruments. Am JAudiol 7:19-29 .

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APPENDIX A

Checklist

A copy of the checklist developed to deter-mine whether clients are likely candidates for a multiple memory hearing aid is shown below. At the initial fitting appointment, the audiolo-gist conducts an interview of the client to deter-mine whether the client has problems hearing in each of the listed situations . The audiologist then ticks the appropriate cells. The various situations on the checklist are grouped to match the different listening conditions listed in Table 4. If, at the end of the interview, there is a tick for at least two different groups (two different columns or two different tables), the conclusion is that the client may benefit from multiple amplification characteristics .

CHECKLIST (NEED FOR MULTIPLE MEMORIES)

Name : Audiologist: (Abbreviations for the different groups : "Q"/Quiet, "D"/Dialogue, "LF"/LF-weighted background noise, "HF"/HF-weighted back-ground noise, "M"/Music)

Listening Situation "Q" "D" "LF" "HF" Intelligibility Intelligibility

Conversation with ////// one person in quiet Listening to TV news Conversation with small group in quiet Listening to speaker in the distance Conversation while //// //// riding in a car, bus, //// ////// or train Conversation while walking in the city or along a busy road //// Conversation while surrounded by many people Conversation while exposed to prolonged "impact" or other high- pitched noises Other

Does the client want to improve the sound quality in the following situations :

Listening Situation

Listening to music on the stereo Listening to music at a concert Playing an instrument

If the client has experience with hearing aid amplification: Does the client ever spend time alone in the following situations and find that the background noise is very annoying :

Listening Condition "LF" "HF" Annoyance Annoyance

While riding in a car, bus, or train Shopping (shopping center or supermarket) In a restaurant, cafe or canteen, or among a crdwd of people Exposed to prolonged "impact"or other high-pitched noises Other

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Documents

Guidelines for Fitting Multiple Memory Hearing Aids · Guidelines for Fitting Multiple Memory Hearing Aids Gitte Keidser* Harvey Dillon* Denis Byrne* Abstract ... a multiple memory