efficacy of directional microphone hearing aids: a … am acad audiol 12: 202-214 (2001) efficacy of...

J Am Acad Audiol 12 : 202-214 (2001)

Efficacy of Directional Microphone Hearing Aids : A Meta-Analytic Perspective Amyn M. Amlani*

Abstract

The literature suggests that directional microphone hearing aids (DMHAs) are a viable means for improving the signal-to-noise ratio (SNR) for hearing-impaired listeners . The amount of directional advantage they provide, however, remains relatively unclear because of variabil-ity observed among individual studies . The present investigation was undertaken in an attempt to establish the degree of advantage provided by DMHAs. Data were synthesized from 72 and 74 experiments, respectively, on omnidirectional hearing aids and DMHAs representing both favorable and unfavorable outcomes . Using a meta-analytic approach, 138 weighted averages were derived for a variety of comparable independent and dependent variables . Comparisons were made for hearing-impaired and normal-hearing listeners . Findings are discussed with regard to their clinical and research implications .

Key Words : Analog signal processing, confidence interval, digital signal processing, direc-tional microphone, meta-analysis, omnidirectional microphone, reverberation, signal-to-noise ratio

Abbreviations : BTE = behind the ear, Cl 95 = 95 percent confidence interval, DA = directional advantage, DMHA = directional microphone hearing aid, DSP = digital signal processing, HI = hearing impaired, ITE = in the ear, NH = normal hearing, ODHA = omnidirectional micro-phone hearing aid, RT = reverberation time, SNR = signal-to-noise ratio

D

irectional microphone hearing aids (DMHAs) are a means of providing lis-teners with an improved signal-to-noise

ratio (SNR) over traditional devices configured with omnidirectional microphones (Madison and Hawkins,1983 ; Hawkins and Yacullo,1984 ; Leeuw and Dreschler, 1991 ; Valente et al, 1995, 2000 ; Gravel et a1,1999; Preves et a1,1999; Ricketts and Dhar, 1999 ; Wouters et al, 1999 ; Pumford et al, 2000). Improvement of speech intelligibility occurs as a result of the microphone's ability to attenuate sounds from the rear and sides with respect to sounds originating from directly in front. Conversely, omnidirectional microphones (ODHAs) have equal sensitivity to sound inci-dent from all directions. For DMHAs, the amount of attenuation varies relative to microphone configuration and directivity measurement index

*Department of Audiology and Speech Sciences, Michigan State University, East Lansing, Michigan

Reprint requests : Amyn M . Amlani, Department of Audiology and Speech Sciences, Michigan State University, East Lansing, MI 48824

(for a review, see Preves, 1997, and Ricketts and Mueller, 1999). Using the directivity index and the unidirectional index as metrics of direc-tionality, reports in free field have suggested that DMHAs are capable of improving SNR by 4 to 6 dB and 8.4 to 11 .5 dB, respectively (For-tune, 1997 ; Preves, 1997). Under similar testing conditions, ODHAs produce a directivity index and unidirectional index of 0.

Although measures of directionality can quantify the electroacoustic performance of DMHAs, routine clinical use is yet to be imple-mented . Therefore, clinical assessment of these devices has been based on speech intelligibility tasks. Using ODHA performance values as a reference, empirical findings have yielded mixed results. For example, whereas Dybala (1996) found a directional advantage (DA; mean ODHA SNR - mean DMHA SNR) of 16.4 dB and Valente et al (1998) found little or no DA, other investi-gations have reported values ranging from 2 to 8 dB (e .g., Leeuw and Dreschler, 1991; Valente et al, 1995; Gravel et al, 1999; Preves et al, 1999; Ricketts and Dhar, 1999 ; Wouters et al, 1999; Pumford et al, 2000) in similar low rever-berant conditions .

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Efficacy of Directional Microphone Hearing Aids/Amlani

Variability across studies can be attribut-able, in part, to four factors : (1) the testing envi-ronment, (2) the signal processing scheme (i .e ., analog, digital), (3) the azimuths at which noise is presented relative to speech, and (4) the fact that treatment generalizations are derived from individual studies using small sample sizes . Additional factors that have been considered include hearing aid style (Pumford et al, 2000), monaural/binaural fitting (Hawkins and Yac-ullo, 1984), and output limiting scheme (Rick-etts and Gravel, 2000) .

A primary factor that may explain dis-crepancies among DMHA outcomes is the acoustic environment . These devices are often evaluated in environments (e.g ., anechoic room) not typical of real-world listening situations . According to Hawkins (1986), these results may "have limited meaning for clinical application" (p . 140) . However, the justification for measur-ing directionality in a highly nonreverberant room is to "reveal the directional characteristics of the hearing aid being tested independent of the characteristics of the room itself' (Stude-baker et al, 1980, p . 81) . Outcomes derived from DMHAs assessed in these nonreverberant conditions, therefore, will likely overestimate real-world performance . For example, Leeuw and Dreschler (1991) investigated the effec-tiveness of DMHAs under reverberant condi-tions (time required for sound pressure level to decrease 60 dB after the offset of the source) on speech intelligibility in noise . Using an adaptive procedure, 50 percent speech intelligibility per-formance was assessed for normal-hearing (NH) listeners in rooms with low and high levels of reverberation . Subjects listened to a speech stimulus varied in intensity presented at 0 degrees azimuth and a fixed intensity noise presented at 180 degrees azimuth recorded through a mannikin (KEMAR) fit with an omni-directional/directional device . Results for the DMHA condition revealed a favorable mean SNR of about -5 dB in the anechoic room, whereas performance in the high reverberant condition revealed an SNR of approximately 1 dB . Based on these data, the efficacy of the DMHA in the less reverberant condition over-estimated the more real-world outcome by 6 dB .

The use of analog or digital signal process-ing (DSP) is a second factor to consider. Histor-ically, most analog amplification strategies have used omnidirectional microphones and attempted to improve SNR through various out-put limiting schemes by reducing those spectral characteristics relatively unimportant to speech

intelligibility (for a review, see Fabry, 1991) . However, empirical evidence has indicated that these strategies do not provide listeners with ade-quate intelligibility in noise (Van Tasell et al, 1988 ; Tyler and Kuk, 1989 ; Fabry and Van Tasell, 1990) . More recently, researchers have focused on DSP as a means of improving speech intelligibility. Findings from these investiga-tions have suggested that DSP circuits do not fare significantly better in quiet or noise than well-fitted analog circuits (Valente et al, 1998, 1999) . The shortcoming of these circuits is related to their inability to differentiate speech from noise, which remains integrated at the input of the hearing aid microphone . This inability is further compounded by the introduction of rever-beration, which results in a spectral smearing of the acoustic signal (Nabelek et al, 1989) . These factors, despite frequency response changes to the signal as a function of the hear-ing aid's circuitry, result in the SNR remaining relatively unchanged as both speech and noise are attenuated equally. To lessen this problem, manufacturers have developed DSP devices incorporating directional microphones . The per-formance of these devices has indicated improved speech intelligibility and user preference rela-tive to analog ODHAs (Preves et al, 1999 ; Valente et al, 2000) .

A third factor responsible for individual study variations is the placement and type of noise stimulus used relative to that of the speech signal . In everyday life, noise originates from a multitude of azimuths, with variations in inten-sity and frequency and from varying distances . In the evaluation of DMHAs, however, studies have traditionally presented speech from directly in front of the listener (0 degrees azimuth) while a single noise source is pre-sented from directly behind (180 degrees azimuth) . Although this condition is important to the assessment of hearing aid directionality (e.g ., front-to-back ratio), where such compar-isons are designed to determine the micro-phone's ability to attenuate sounds from the rear of the listener, the interpretation of intel-ligibility tasks under such conditions may result in an overestimated DA. In addition, results may be further overstated by the use of non-real-world noises (e.g ., speech weighted), which have been reported to increase SNR values (Rick-etts and Dhar, 1999) . To account for the effect of random noise on the efficacy of DMHAs, recent studies have assessed presentations of real-world noises from multiple or diffuse azimuths (Preves et al, 1999 ; Valente et al,

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Journal of the American Academy of Audiology/Volume 12, Number 4, April 2001

1999). In general, results have shown a decrease in the amount of DA.

Lastly, and most importantly, generaliza-tions regarding the amount of DA are often based on findings from a single study with a small sample size, typically 10 to 30 subjects . Unfor-tunately, these results may not be representative of the population as small sample sizes yield large amounts of variability and sampling error. Meta-analysis is a method used to counter small sample size and better estimate treatment effi-cacy through a convergence of scientific evidence from many independent experiments.

Meta-Analysis

The purpose of a meta-analysis is to derive an objective, quantitative metric that addresses the magnitude of group difference relative to the population effect. Through the use of research review, analysis, synthesis, and evaluation, numerous and diverse findings are synthesized to determine predictive sample effects (Hunter and Schmidt, 1990).

Specifically, meta-analysis assesses the extent of the comparative difference, or rela-tionship, between common independent and dependent variables. Group differences or rela-tionships are obtained by deriving the amount of variability through the use of confidence inter-vals associated with such metrics as effect size or weighted average. By compiling individual study characteristics (i .e ., mean) and experi-mental error data (i .e ., standard deviation), a cumulative mean and variance can be calcu-lated to generate estimated population charac-teristics for a given treatment.

The primary aim of this analysis was to synthesize data across a variety of studies to determine the efficacy of commercially avail-able DMHAs. Through careful analysis of the available data, the following questions were posed:

1. Do DMHAs provide a beneficial SNR inde-pendent of such factors as environmental acoustics, type of speech stimulus, noise azimuth, hearing aid characteristics, and microphone configuration?

2. Does DSP provide an improvement in SNR relative to analog signal processing? Is this improvement dependent on microphone con-figuration?

3. Is the amount of SNR improvement influ-enced by the use of certain noise stimuli and/or its azimuth of origination?

METHOD

Selection of Studies

In the social-behavioral sciences, much of the published literature reports statistically signif-icant data . That is, in many periodicals, only this type of research is accepted for publication. Called the file-drawer problem, Rosenthal (1979) stated that conflicting findings were being deleted from research synthesis protocols . An attempt was made in this analysis, therefore, to locate unpublished and conflicting findings in addition to those that stood the rigors of scien-tific review.

Studies were identified by three means: (a) a systematic manual search of references in rele-vant literature sources focusing on the periodi-cals Audiology, Ear and Hearing, Hearing Instruments, Hearing Journal, Hearing Review, Journal of the American Academy of Audiology, Journal of the Acoustical Society of America, Journal of Speech and Hearing Disorders, and Journal of Speech, Language, and Hearing Research, as well as chapters, texts, and various bibliographies containing relevant references ; (b) a search of published reports through the electronic databases of Dissertation Abstracts, ERIC, MEDLINE, and PsychINFO; and (c) direct contact with hearing aid manufacturers and researchers requesting unpublished manuscripts, convention posters, and technical bulletins.

Initially, 26 studies were collected, 6 of which were unpublished at the time of data analysis . All studies were written in English except for one written in Italian. Eighteen were deemed appro-priate for this analysis, including the study writ-ten in Italian. The latter study was translated with the assistance of a fluent, multilingual interpreter. Criteria for the inclusion of studies were specifically chosen because of their clini-cal objectivity and applicability based on (a) the use of either commercially available in-the-ear (ITE) or behind-the-ear (BTE) devices, (b) single-or dual-microphone technology incorporating either analog or DSP processing schemes, and (c) adaptive assessment by means of an objec-tive SNR task using a 50 percent criterion (e .g ., Hearing in Noise Test; Nilsson et al, 1994).

Microphone configurations were defined based on previously published reports . An omnidirectional microphone configuration was characterized as a single microphone with equal sensitivity to sound incident from all azimuths . Directional microphones, on the other hand, were defined as either a single- or

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Table 1 Experiments for Omnidirectional and Directional Microphone Hearing Aids in Less Reverberant Environments (< 600 msec)

Study/Conditions (azimuths)

Reverberation Time Speech

Subjects (msec) Stimulus Noise Stimulus Directional Advantage

Madison and Hawkins (1983) Speech 0°/noise 180' 12 normal AR NU-6 Multitalker babble 10.6

Hawkins and Yacullo (1984) Speech 0'/noise 180' 12 normal 300 NU-6 Multitalker babble 3 .8 Speech 0'/noise 180' 12 normal 300 NU-6 Multitalker babble 3.6 Speech 0'/noise 180' 11 mild/moderate SNHL 300 NU-6 Multitalker babble 6 .3 Speech 0'/noise 180' 11 mild/moderate SNHL 300 Ni Multitalker babble 3 .6

Schum (1990) Speech 0'/noise 180' 16 mild/moderate SNHL ASB CID W-22 Cafeteria 3 .0

Leeuw and Dreschler (1991) Speech 0'/noise 0' 12 normal 156 Dutch Sentences SWN 0.0 Speech 0'/noise 45' 12 normal 156 Dutch Sentences SWN 1 .0 Speech 0'/noise 90' 12 normal 156 Dutch Sentences SWN 3.0 Speech O'/noise 135' 12 normal 156 Dutch Sentences SWN 4.5 Speech 0'/noise 180' 12 normal 156 Dutch Sentences SWN 6.5

Chasin (1994) Speech 0'/noise 180' 10 moderate SNHL ASB Ni SWN 8.2

Valente et al (1995) Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL ASB HINT SWN 7.4 Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL ASB HINT SWN 7.8 Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL ASB HINT SWN 7.8 Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL ASB HINT SWN 8.1

Dybala (1996) Speech 0'/noise 180' 12 normal AR NU-6 Multitalker babble 10 .4 Speech 0'/noise 180' 12 normal AR Ni Multitalker babble 16 .4

Agnew and Block (1997) Speech 0'/noise 180' 25 mild/moderate SNHL ASB HINT SWN 7.5

Larsen (1998)* Speech 0'/noise 45', 135', 19 moderate SNHL ASB Dantale ICRA 3.6

225', 315' Prosser and Biasiolo (1998) Speech 0'/noise 180' 5 Severe SNHL ASB Spondees Cocktail 5.0

Valente et al (1998) Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL ASB HINT SWN 0.1 Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL ASB HINT SWN -0.8 Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL ASB HINT SWN 0.5 Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL ASB HINT SWN 0.7 Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL ASB HINT SWN -0.4 Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL ASB HINT SWN 0.2

Gravel et al (1999) Speech 0'/noise 180' C 10 mild/MS SNHL ASB PSI, words Multitalker babble 4.7 Speech 0'/noise 180' C 10 mild/MS SNHL ASB PSI, sentences Multitalker babble 5.2 Speech 0°/noise 180' C 10 mild/MS SNHL ASB PSI, words Multitalker babble 4.8 Speech 0'/noise 180' C 10 mild/MS SNHL ASB PSI, sentences Multitalker babble 4.3

ICRA = International Colloquium of Rehabilitation Audiology noise, PSI = Pediatric Speech Intelligibility test

dual-microphone configuration. A single micro-phone with an open front inlet and a rear inlet fitted with an acoustic resistance was defined as a single-microphone directional device (Preves, 1997). A dual-microphone configura-tion consisted of two omnidirectional micro-phones using a signal-processing delay algorithm (Agnew, 1997). Other variations in microphone configuration, such as directional-

plus-omni and beam-forming arrays, were excluded from this analysis due to the small number of such studies or their commercial availability at the time of this undertaking.

Identification of Study Statistics

Eligibility for inclusive of the 18 studies was evaluated by the author and later confirmed

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Table 1 Experiments for Omnidirectional and Directional Microphone Hearing Aids in Less Reverberant Environments (< 600 msec) (continued)



Subjects (msec) Stimulus Noise Stimulus Directional Advantage

Preves et al (1999) Speech 0°/noise 115°, 245° 10 mild/severe SNHL ASB HINT Uncorrelated SWN 2.8 (UENC target)

Speech 0°/noise 115°, 245° 10 mild/severe SNHL ASB HINT Uncorrelated SWN 2.7 (UENC MCL)

Speech 0°/noise 115°, 245° 10 mild/severe SNHL ASB HINT Uncorrelated SWN 2.8 (ENC target)

Speech 0°/noise 115°, 245° 10 mild/severe SNHL ASB HINT Uncorrelated SWN 1 .4 (E/VC MCL)

Ricketts and Dhar (1999) Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL AR HINT Cafeteria 7 .5

180°, 225°, 270° (aid 1) Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL AR HINT Cafeteria 6.5

180°, 225°, 270° (aid 2) Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL AR HINT Cafeteria 0.8

180°, 225°, 270° (aid 2)t Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL AR HINT Cafeteria 6.0

180°, 225°, 270° (aid 3) Wouters et al (1999) Speech 0°/noise 90° 10 mild/moderate SNHL 450 BLU spondees SWN 3.2 Speech 0°/noise 90° 10 mild/moderate SNHL 450 BLU spondees SWN 3.9 Speech 0°/noise 90° 10 mild/moderate SNHL 450 BLU spondees Traffic 3 .6 Speech 0°/noise 90° 10 mild/moderate SNHL 450 BLU spondees Multitalker 3 .3 Speech 0°/noise 90° 10 mild/moderate SNHL 450 BLU spondees SWN 2.8

Pumford et al (2000) Speech 0°/noise 72°, 144°, 24 mild/moderate SNHL ASB HINT SWN 5.8

216°, 288° (BTE) Speech 0°/noise 72°, 144°, 24 mild/moderate SNHL ASB HINT SWN 3.3

216°, 288° (ITE) Valente et al (2000) Speech 0/noise 180 (site 1) 25 moderate SNHL ASB HINT SWN 3.7 Speech 0*/noise 45°, 135°, 25 moderate SNHL ASB HINT SWN 3.5

180°, 225°, 315° (site 1) Speech 0/noise 180 (site 1)t 25 moderate SNHL ASB HINT SWN 4.5 Speech 0°/noise 45°, 135°, 25 moderate SNHL ASB HINT SWN 4.2

180°, 225°, 315° (site 1)t Speech 0/noise 180 (site 2) 25 moderate SNHL ASB HINT SWN 3.2 Speech 0°/noise 45*, 135°, 25 moderate SNHL ASB HINT SWN 2.7

180°, 225°, 315° (site 2)

AR = anechoic room, NU-6 = Northwestern University Auditory Test No . 6, SNHL = sensorineural hearing loss, ASB = audiometric sound booth, SWN = speech-weighted noise, HINT = Hearing in Noise Test, C = child, MS = moderately severe, UE = unequalized, VC = volume control, E = equalized, MCL = most comfortable loudness .

*In May et al (1998) ; tadjustment in fitting .

by a colleague. Several studies were found to exhibit multiple experimental conditions . In the data analysis, these multiple experiments were treated independently to increase sample size (Tables 1 and 2) . As tabulated, the number of experiments for which data were analyzed increased from 18 studies (N = 345) to 72 exper-iments (N = 1057) for ODHAs and from 18 stud-ies (N = 345) to 74 experiments (N = 1081) for DMHAs. This slight discrepancy in the number

of experiments between groups occurred because one study (Dybala, 1996) compared a single ODHA with two DMHAs in both a low and a high reverberant condition. This further resulted in DA being calculated by subtracting the single ODHA device from each of the independent DMHA devices.

Each experiment was coded for a variety of variables including sample size ; subjects' type and degree of hearing loss ; hearing aid style;

206


Table 2 Experiments for Omnidirectional and Directional Microphone Hearing Aids in Reverberant Environments 1600 msec)



Subjects (msec) Stimulus Noise

Stimulus Directional Advantage

Madison and Hawkins (1983) Speech 0°/noise 180° 12 normal 600 NU-6 Multitalker babble 2 .5

Hawkins (1984) Speech 0°/noise 180° C 11 mild/moderate SNHL 600 Child's spondees SWN 2.6 Speech 0°/noise 180° C 11 mild/moderate SNHL 600 Child's spondees SWN 2.8

Hawkins and Yacullo (1984) Speech 0°/noise 180° 12 normal 600 N U-6 Multitalker babble 3 .8 Speech 0°/noise 180` 12 normal 600 N U-6 Multitalker babble 3 .6 Speech 0°/noise 180° 12 normal 1200 NU-6 Multitalker babble 1 .3 Speech 0°/noise 180° 12 normal 1200 NU-6 Multitalker babble 2 .6 Speech 0°/noise 180° 11 mild/moderate SNHL 600 NU-6 Multitalker babble 4 .7 Speech 0°/noise 180° 11 mild/moderate SNHL 600 NU-6 Multitalker babble 5 .5 Speech 0°/noise 180° 11 mild/moderate SNHL 1200 NU-6 Multitalker babble -0 .6 Speech 0°/noise 180° 11 mild/moderate SNHL 1200 NU-6 Multitalker babble 1 .5

Leeuw and Dreschler (1991) Speech 0°/noise 0° 12 normal 883 Dutch Sentences SWN 1 .5 Speech 0°/noise 45° 12 normal 883 Dutch Sentences SWN 2 .5 Speech 0°/noise 90° 12 normal 883 Dutch Sentences SWN 2.0 Speech 0°/noise 135° 12 normal 883 Dutch Sentences SWN 3.5 Speech 0°/noise 180° 12 normal 883 Dutch Sentences SWN 2.5

Dybala (1996) Speech 0°/noise 180° 12 normal 600 NU-6 Multitalker babble 6.8 Speech 0°/noise 180` 12 normal 600 NU-6 Multitalker babble 8.8

Ricketts and Dhar (1999) Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL 642 HINT Cafeteria 6.5

180°, 225°, 270° (aid 1) Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL 642 HINT Cafeteria 5.0

180°, 225°, 270° (aid 2) Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL 642 HINT Cafeteria 2.3

180°, 225°, 270° (aid 2)* Speech 0°/noise 90°, 135°, 12 mild/moderate SNHL 642 HINT Cafeteria 4.5

180°, 225°, 270° (aid 3)

NU-6 = Northwestern University Auditory Test No . 6, C = child, SNHL = sensorineural hearing loss, SWN = speech-weighted noise, HINT = Hearing in Noise Test

*Adjustment in fitting

monaural/binaural fitting; single- or dual-micro-phone configuration ; hearing aid manufac-turer/model, analog, or DSP scheme ; nominal reverberation time (RT) ; type of speech and noise stimuli; absolute mean and standard devi-ation SNR values for 50 percent speech intelli-gibility perceived with ODHAs and DMHAs; and DA. Furthermore, experiments were dichotomized based on their testing conditions and labeled as being less (RT < 600 msec) or more reverberant (RT > 600 msec). This dichotomy was based on literature reports of audiometric sound rooms exhibiting RTs ranging between 100 and 600 msec (Nielson and Ludvigsen, 1978; Stude-baker et al, 1980 ; Madison and Hawkins, 1983) and average real-world environments ranging in RT from 600 to 1500 msec (Moncur and Dirks, 1967 ; Nabelek and Mason, 1981).

Calculation of Weighted Averages

Weighted averages (mean x sample size) were used as the metric for this analysis . The rationale for using this metric, as opposed to deriving the traditional effect size, was to simplify the clinician's ability to compare performance values without having to reformulate values .

Data important to the calculation of weighted averages (mean, sample size) were determined from each experiment and entered into a spreadsheet. Subjects were dichotomized by hearing sensitivity (i .e ., hearing impaired, nor-mal hearing) to distinguish performance between groups . Furthermore, all raw data reported in a given study were recalculated for accuracy. Using meta-analytic software based on the for-mulas and procedures of Hunter and Schmidt

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(1990), weighted averages were computed for relevant independent and dependent variables to take into account individual sampling error found within each experiment . In addition, correction formulas for unequal sample sizes and repeated measures were used to prevent under- or overestimation of the treatment effect.

Weighted averages were calculated at 95 percent confidence intervals (CI 95 ) . Further-more, traditional SNR values were not used in this study. That is, a more negative value was representative of a favorable outcome because of the listener's ability to understand speech in an adverse condition. Conversely, a more posi-tive value resulted in a less favorable outcome. For DA, the inverse was true . By definition, DA is the difference between the ODHA and DMHA conditions (i.e ., mean ODHA - mean DMHA). Therefore, a negative or small positive value predicates no or little relative difference between omnidirectional and directional microphones. A large positive value, on the other hand, sug-gests that DMHAs provide a considerable dif-ference between devices.

Statistical significance was determined by comparing the upper and lower limits of CIs across variables (Durlak, 1996). Overlapping limits accept the null hypothesis and were con-sidered not to be statistically significant. Nonoverlapping limits, on the other hand, indi-cated acceptance of the alternate hypothesis and were considered significantly different at p < .05. In the case that a given CI's range over-lapped 0, the null hypothesis could not be rejected . This special case assumption is based on the premise that the null hypothesis is one of no difference (~L1- [L2 = 0) .

A

12

N=707

N = 707

O ODHA " DMHA % Dir Adv

N =a707

Y

RT < 600 RT > 600 RT < 600 RT > 600

Figure 1 Weighted average, Cl,,, and sample size for all omnidirectional hearing aids, directional microphone hear-ing aids, and directional advantage dichotomized by acoustic condition for (A) HI and (B) NH listeners.

A total of 138 weighted averages were cal-culated for this meta-analysis. A summary

of the findings follows .

Overall Directional Advantage

Using the 72 experiments (N = 1057) per-taining to ODHAs, a weighted average value of 1.1 dB with Cl 95 ± 1.0 was determined. Similarly, a weighted average for the 74 experiments (N = 1081) on DMHAs yielded an SNR of-2.6 dB with CI 95 ± 1 .1 . A comparison between the ODHA and DMHA values resulted in a statis-tically significant difference at an alpha of .05 . A calculation of DA resulted in an SNR value of 4 dB (N =1081, Cl 95 ± 0.8) . Relative to these find-ings, a post hoc analysis was performed to quan-tify the variability of individual studies. Results revealed that 31 percent (22/72) of ODHAs and DA (23/74) experiments were found to be between their respective CI ranges, whereas findings for DMHAs revealed a mere 8 percent (6/74) .

Figure 1 illustrates weighted averages syn-thesized across microphone condition and hear-ing sensitivity as a function of environmental condition. For hearing-impaired (HI) listeners, a comparison between ODHAs and DMHAs resulted in a statistically significant difference (p < .05) in the less reverberant condition. At an alpha of .05, a significant difference in the less reverberant condition was also found for ODHAs compared across hearing sensitivity (see Fig. 1) .

As expected, the more reverberant condition resulted in reduced speech intelligibility. HI and NH listeners required an additional 2.2-(CI 95 ± 2.7) and 3-dB (CI 95 ± 2.7) increase in

B 12

RESULTS

8

4

Z 0 V)

M -4

N=108

N=120

-8

-12

-16

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signal, respectively, for ODHAs when compar-ing between acoustic conditions . A similar find-ing indicated that DMHAs required an increase of 2 .4 (CI 95 ± 2.9) and 5.1 dB (CI 95 ± 7.1) for the HI and NH groups, respectively.

Signal Processing

The introduction of DSP in hearing aid tech-nology has stimulated both public and profes-sional interest . However, its impact on improving speech intelligibility with directional micro-phones remains unknown . Thus, an analysis was undertaken to determine the effect of sig-nal processing and microphone condition across listener groups (Fig . 2) . It should be noted that because of the sparse number of reported out-put limiting schemes, data could not be further dissected and analyzed . Furthermore, all results reported are representative of BTE devices unless specified .

As seen in Figure 2A, ODHA devices incor-porating DSP improved speech intelligibility over similar analog devices, independently of reverberation for HI listeners. However, the inverse was found for comparisons made between signal processing and directional microphones, resulting in a statistically significant difference (p < .05) for microphone type and signal pro-cessing in the less reverberant condition . Although not a significant finding, a similar pattern was also seen in the more reverberant condition. This trend suggests that analog pro-cessing may be better suited to improving intel-ligibility with DMHAs. To further verify this finding, a comparison was performed between similar processing scheme and microphone con-

A 16

figuration, as illustrated in Figure 2 . For ana-log processing, a pattern of improved SNR was seen for directional microphones for both groups . For the HI group, this resulted in a statistically significant difference (p < .05) . Conversely, a pattern indicating reduced speech intelligibility for DSP devices coupled with directional micro-phones, independent of room acoustics (Fig . 2A), was noted.

A post hoc analysis was performed to eval-uate signal-processing schemes. In this analysis, data were further dichotomized based on micro-phone configuration and hearing aid style. In Fig-ure 3, data are illustrated by hearing sensitivity, acoustic condition, and single-microphone designs. Figure 4 differs only in that data rep-resented are configured with dual microphones. Both figures depict data for BTE hearing aids . Results regarding ITE devices are described below. Because of the small number of reported output limiting schemes, data could not be fur-ther segmented and analyzed .

In Figure 3A, single-microphone-configured directional devices exhibited an improved SNR pattern over single-microphone ODHA devices. A comparison between microphone type and signal-processing scheme resulted in a statisti-cally significant difference (p < .05) for DSP cir-cuits in both acoustic conditions for HI listeners.

A search for single-microphone ITE exper-iments turned up a single experiment (Chasin, 1994) . Data were collected on 10 HI listeners in the less reverberant condition using analog sig-nal processing . Results revealed an SNR value of 0.5 and -7.7 dB for ODHAs and DMHAs, respectively. This finding was found to be sta-tistically significant at an alpha of .05 . This resulted in a DA of 8.2 dB .

12 N=74 12 N=120

N=36 8 N=521 N=74 N 74T 8 F

= T _ N=144

4 671 N=186J ~-N 4 N=120 N=132 Cr Z

= T N=186 N=36 j M

Z N=108 r 1 4 (n 0

TO N 0 N = 1 4

~' N = 5211 N=36

T

m 0 -4 N=186 m -o _4 1

-8 -8 O ODHA-Analog O ODHA-DSP

-12 " DMHA-Analog -12 " DMHA-DSP

-16 % Dir Adv-Analog -16 0 Dir Adv-DSP

RT < 600 RT > 600 RT < 600 RT > 600

Figure 2 Weighted average, CI95, and sample size for behind-the-ear omnidirectional hearing aids, directional micro-phone hearing aids, and directional advantage dichotomized by signal-processing scheme and acoustic condition for (A) HI and (B) NH listeners. (*In the Valente et al [19981 study, each subject's own analog device was compared to a DSP

device .)

209


A L N=48

N=62

N=48

N=48 ~N = 162T

N=162 N=162

§ T 0

13 ODHA-DSP " DMHA-Analog " DMHA-DSP % Dir Adv-Analog 0 Dir Adv-DSP

0

N=62

N=12 I I 0

N=62 T 0 %N-12

N=12

Noise Stimuli

RT < 600 RT > 600 RT < 600 RT > 600

Figure 3 Weighted average, Mg., and sample size for behind-the-ear omnidirectional hearing aids, directional micro-phone hearing aids, and directional advantage dichotomized by signal-processing schemes for single-microphone devices and acoustic condition for (A) HI and (B) NH listeners .

For HI listeners, the dual-microphone con-figuration indicated mixed trends with regard to signal-processing schemes (see Fig. 4A). DSP coupled with omnidirectional microphones improved SNR by 0.5 and 1 dB over their analog counterparts in the less and more reverberant conditions, respectively. Conversely, analog DMHAs significantly improved speech intelli-gibility over DSP directional devices in both acoustic conditions (p < .05) .

With regard to ITE dual-microphone devices, data for 11 analog experiments (N = 203) were syn-thesized and analyzed. Data are only applicable to the HI group assessed in less reverberant con-ditions . Results for ODHAs indicated an SNR value of 1.5 (CI 95 ± 1.0), whereas DMHA data (-1.9 dB SNR, CI 95 ± 0.7) revealed a statistically significant improvement (p < .05) in dB SNR. This resulted in a DA of 3 .4 dB (CI 95 ± 1 .5) .

A

W Z !n

m

N = 473 TN=24

S

O ODHA-Analog 0 ODHA-DSP " DMHA-Analog " DMHA-DSP % Dir Ad-Analog 0 Dir Adv-DSP

N=12~

RT < 600 RT > 600

0 -

Because the primary purpose of a DMHA is to reduce noise from the sides and rear of the lis-tener, the effects of various noise spectra and pre-sentation azimuth on speech intelligibility are of particular clinical interest . For this analysis, various noise stimuli were categorized into two groups : steady state and fluctuating. Noise stim-uli that remain relatively constant in ampli-tude over time (e .g ., speech-weighted noise) were defined as steady state. In contrast, fluc-tuating noise stimuli were characterized as being more real world due to their constant changes in amplitude over time . Examples of this stimulus included multitalker babble, cocktail noise, traffic noise, and cafeteria noise. Readers should refer to Schum (1996) for the spectral con-tent of noise.

N=108

N=108

N=132

N=132 Y

B

X Z 1n

m U

N=12 0

N=12 O N=12

N=12

RT < 600 RT > 600

Figure 4 Weighted average, CI95, and sample size for behind-the-ear omnidirectional hearing aids, directional micro-phone hearing aids, and directional advantage dichotomized by signal-processing schemes for dual-microphone devices and acoustic condition for (A) HI and (B) NH listeners.

N=24 N=12 T

ON=24 N=1213 N=24

210


A 16

12 N=92

8 N=199 I N=92 N=18 N = 5271 N=199

Q

T T oL 4 N=18 Z r T <n 0 N

F~

1 T N = 199

27

m ~ + T+Ty N

-4 lj N=5271 0

-8 O ODHA-SSN

-12 0 ODHA-FN " DMHA-SSN " DMHA-FN

-16 % Dir Adv-SSN 0 Dir Ad-FN

N=92 -

=18

B 16

12

8 N=48 N=60

0= 4 N

160 T

Z N 0 N=60 N=60 N=60 0] - TT 0 -4

-8

-12

-16

N=72

RT < 600 RT > 600 RT < 600 RT > 600

Figure 5 Weighted average, Cl,,, and sample size for behind-the-ear omnidirectional hearing aids, directional micro-phone hearing aids, and directional advantage dichotomized by steady-state (SSN) and fluctuating (FN) noise spectra and acoustic condition for (A) HI and (B) NH listeners.

As seen in the left panel of Figure 5A, the HI group performed slightly better in fluctuating noise than with steady-state noise independent of microphone type (i .e ., omnidirectional, direc-tional) . However, when RT increased, speech intelligibility improved in steady-state noise by 3 to 4 dB . For fluctuating noise, the inverse was true . For the NH group, data showed no consis-tent pattern (Fig. 5B) . In the less reverberant con-dition, fluctuating noises appeared to reduce speech intelligibility for omnidirectional devices . For DMHAs, the inverse was found. In the more reverberant condition, effects were found to be the opposite of those found in the less reverberant con-dition, with a statistically significant difference (p < .05) found between ODHAs and DMHAs assessed with steady-state noise .

The mixed results found in Figure 5 are believed to be the result of interaction effects from the spectral characteristics of the noise, presentation azimuth, and single- or dual-microphone configuration. Because of the sparse number of experiments on NH listen-ers, results were derived only for the HI group and are depicted in Figure 6 for ODHAs, DMHAs, and DA, respectively. In Figure 6A, results across these variables remained mixed for ODHAs. In the less reverberant condition, data revealed a decreased trend for single-microphone devices relative to dual-micro-phone devices for speech presented from 0 degrees azimuth and noise from 180 degrees azimuth . Furthermore, a statistically signifi-cant difference (p < .05) was found between noise types for multiple or diffuse noise pre-sentations and dual microphones. A compari-son between acoustic conditions indicated that fluctuating noise resulted in reduced SNR val-

ues for single microphones assessed in the tra-ditional 0/180 paradigm .

As illustrated in the left panel of Figure 613, SNR values were nearly identical when comparing within-microphone configuration (i .e ., single, dual) and within-noise azimuth presen-tation for both steady-state and fluctuating noises . SNR improvement also appears to be related to the type of microphone configuration . Dual-microphone configurations showed a 2- to 8-dB improvement in SNR over their single-microphone counterparts, with one exception. At an alpha of .05, a statistically significant dif-ference between microphone configuration, steady-state noise, and the traditional 0/180 paradigm was found . The data also showed a decrease of 3 to 5 dB SNR for dual microphones when noise is presented from multiple azimuths relative to a single azimuth of 90 or 180 degrees .

DISCUSSION

T he fact that directional microphones are capable of providing listeners with a favor-

able SNR relative to ODHAs has been well doc-umented . However, the clinician's ability to predict speech intelligibility improvement for DMHAs has been confounded because of varia-tions across individual studies . Thus, this study was undertaken to provide clinicians with more accurate information for use in the selection, fit-ting, and counseling of DMHAs while possibly revealing engineering and performance trends that might be helpful to the hearing aid indus-try and researchers .

Overall, DMHAs were found to provide a sta-tistically significant advantage over ODHAs in improving SNR when data are pooled across all

211


A 16

12 v

8 N=45 N=18

4 N=142

N=160 N = 195

a

fN=84 N=36

Z (n

= N=300 _ I N

N=12 N=44 0 - =12 I

£ m N=20 1 I 0

0 " SO/N90-SSN-DM N=38 " SO/N90-FN-DM

. SO/N180-SSN-SM * SO/N180-FN-SM

-12 * SO/N180-SSN-DM V SO/N180-FN-DM

-16 0 SO/NM-FN-SM SO/NM-SSN-OM

* SO/NM-FN-DM

RT < 600 RT > 600

C 16

12 N=195 _

N 36 - N-20 N=38T

8 N=84 TN=12 ' ' N=18 -AA .~

M 4 Il 0

0 I .

Z N 0

N 45N=142 Ill m N=160

-4

-8

-12

-16

B

N=44

Z N

381 N 42 1 1

1N=20 N=195

N=12 0

N=18

v

N=12 0

RT < 600 RT > 600

Figure 6 Weighted average, Cl,,, and sample size for HI listeners dichotomized by single- (SM) or dual-microphone (DM) configuration, steady-state (SSN) and fluctuating (FN) noise spectra, and acoustic condi-tion . Data are plotted as a function of 90° (N90), 180° (N180), and multi (NM) azimuth noise presentation for (A) omnidirectional hearing aids, (B) directional micro-phone hearing aids, and (C) directional advantage.

RT < 600 RT > 600

variables. Dichotomized by acoustic condition, the results also indicate a significant difference between ODHA and DMHA devices and listener groups but only in the less reverberant condition. The fact that DMHAs do not statistically dif-ferentiate themselves from ODHAs in the more reverberant condition may be attributable to several factors. First, higher reverberant con-ditions have been shown to affect the direc-tionality of DMHAs, making them essentially omnidirectional (Studebaker et al, 1980). Second, unlike the less reverberant room, where the target signal decreases in intensity according to the inverse-square law, energy in the reverber-ant room increases and may even exceed the intensity of the target signal (Lochner and Berger, 1964). Lastly, reflected energy may change some of the characteristics important for speech intelligibility by producing overlap-ping sounds (i .e., overlap masking) and/or caus-ing the internal energy within each sound to be temporarily smeared (i .e ., self-masking) (Nabelek et al, 1989).

It has been hypothesized that the use of DSP in amplification devices may enhance word recognition abilities of listeners, possibly even in noise. However, empirical evidence has shown that the full potential of DSP in hearing aids has

not yet been met (Valente et al, 1998, 1999 ; Ricketts and Dhar, 1999). Based on the findings in this analysis, DSP coupled with an omnidi-rectional microphone was found to improve intelligibility significantly over that observed when coupled with a directional microphone . To appreciate the significance of this finding, results from a post hoc analysis (see Fig. 4) revealed that signal processing is dependent on the hearing sensitivity of the listener and sin-gle- or dual-microphone configuration but inde-pendent of acoustic condition. It should be noted, however, that many independent experiments used similar commercial devices. Based on this rationale, further empirical evidence is needed to verify these claims. In addition, two other fac-tors may have contributed to the findings . First, data were analyzed primarily for those exper-iments using BTE devices, many of which used variations in venting. Mueller and Wesselkamp (1999) have suggested that by increasing vent size, a reduction in directivity index values occurs for frequencies below 2000 Hz. Second, findings did not take into account the effect of output limiting schemes. Without accounting for this variable, it remains unclear as to how much this variable contributes to improving SNR. Recent data, however, suggest no behavioral

~fJ=30N=160

N 45

212


differences in DA for linear or wide dynamic range circuits (Ricketts and Gravel, 2000).

Presently, there are no standardized meth-ods for the clinical assessment of DMHAs. Out-comes derived from the traditional paradigm of presenting speech at 0 degrees azimuth and noise from 180 degrees azimuth tend to inflate SNR values as a result of microphone configu-ration . Thus, an attempt was made in this analy-sis to determine possible trends for future directives in the clinical assessment of these devices . According to Ricketts and Dhar (1999), non-real-world noises should not be used as they may inflate SNR values . In this analysis, steady-state and fluctuating noises were com-pared between ODHAs and DMHAs and showed mixed results for both listener groups (see Fig. 5) . Post hoc data were compiled based on azimuth presentation of these noise types and single- or dual-microphone configurations for HI listeners (see Fig . 6) . With regard to the paradigm most suited to assess the clinical efficacy of DMHA devices, findings suggest a decreased SNR when multiple or diffuse azimuths were used in the presentation of noise . In theory, this arrangement may reduce the null effects of various polar pat-terns, but the fact that the same noise spectra are presented through all speakers has been shown to result in an artificial advantage . In 1984, Cox and Bisset found that correlated noises from multiple azimuths resulted in a release from masking relative to uncorrelated noises from the same speaker arrangement. Ricketts

and Mueller (1999) have suggested presenting multiple, uncorrelated noises in a moderately reverberant room (RT = 500 msec) with speak-ers directed in random directions and placed behind, at the sides, and in front of the listener. Additionally, findings suggested that there might be an interaction effect as a result of single- or dual-microphone configuration . Further empir-ical evidence is needed to determine such an effect, as this analysis did not account for the type of speech stimulus used .

Acknowledgment. Portions of this paper were presented at the American Academy of Audiology Convention, Chicago, IL, March 2000 . The author gratefully acknowl-

edges John Hunter's guidance and assistance in the initial design and analysis of this project. Appreciation is due

to Michael Casby, Jerry Punch, Mary Jo Cooley Hidecker, and two anonymous reviewers for their helpful comments and suggestions on various versions of this manuscript; Michael Sinclair for his evaluation of the data entry and

selection criteria variables; and Anna Gambioni for her assistance in interpreting the study written in Italian. Lastly, the author expresses his special thanks to those manufacturers and researchers who unselfishly con-

tributed their published and unpublished works for the purpose of this analysis .

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