Efficacy of Directional Microphone Hearing Aids: A

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 variability 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, directional 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 microphone hearing aid, RT = reverberation time, SNR = signal-to-noise ratio

irectional microphone hearing aids (DMHAs) are a means of providing lisD 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 incident 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

202

(for a review, see Preves, 1997, and Ricketts and Mueller, 1999). Using the directivity index and the unidirectional index as metrics of directionality, 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 (Fortune, 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 implemented . 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 investigations 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 reverberant conditions .

Efficacy of Directional Microphone Hearing Aids/Amlani

Variability across studies can be attributable, in part, to four factors : (1) the testing environment, (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 Yacullo, 1984), and output limiting scheme (Ricketts and Gravel, 2000) . A primary factor that may explain discrepancies 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 measuring 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' (Studebaker 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 effectiveness of DMHAs under reverberant conditions (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 performance 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 omnidirectional/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 overestimated the more real-world outcome by 6 dB . The use of analog or digital signal processing (DSP) is a second factor to consider. Historically, most analog amplification strategies have used omnidirectional microphones and attempted to improve SNR through various output 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 adequate 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 investigations 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 reverberation, 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 hearing 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 performance of these devices has indicated improved speech intelligibility and user preference relative 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 intensity 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 presented 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 comparisons are designed to determine the microphone's ability to attenuate sounds from the rear of the listener, the interpretation of intelligibility 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 (Ricketts 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, 203

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, generalizations regarding the amount of DA are often based on findings from a single study with a small sample size, typically 10 to 30 subjects . Unfortunately, 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 efficacy 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 relationship, between common independent and dependent variables. Group differences or relationships are obtained by deriving the amount of variability through the use of confidence intervals associated with such metrics as effect size or weighted average. By compiling individual study characteristics (i .e ., mean) and experimental error data (i .e ., standard deviation), a cumulative mean and variance can be calculated to generate estimated population characteristics 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 available DMHAs. Through careful analysis of the available data, the following questions were posed: 1.

2.

3.

204

Do DMHAs provide a beneficial SNR independent of such factors as environmental acoustics, type of speech stimulus, noise azimuth, hearing aid characteristics, and microphone configuration? Does DSP provide an improvement in SNR relative to analog signal processing? Is this improvement dependent on microphone configuration? Is the amount of SNR improvement influenced 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 significant 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 scientific review. Studies were identified by three means: (a) a systematic manual search of references in relevant literature sources focusing on the periodicals Audiology, Ear and Hearing, Hearing Instruments, Hearing Journal, Hearing Review, Journal of the American Academy ofAudiology, 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 appropriate for this analysis, including the study written 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 clinical objectivity and applicability based on (a) the use of either commercially available in-the-ear (ITE) or behind-the-ear (BTE) devices, (b) singleor dual-microphone technology incorporating either analog or DSP processing schemes, and (c) adaptive assessment by means of an objective 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


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

Study/Conditions (azimuths) Madison and Hawkins (1983) Speech 0°/noise 180'

Hawkins and Yacullo (1984) Speech 0'/noise 180' Speech 0'/noise 180'

Speech 0'/noise 180' Speech 0'/noise 180'

Schum (1990)

Speech 0'/noise 180' Leeuw and Dreschler (1991) Speech 0'/noise 0' Speech 0'/noise 45' Speech 0'/noise 90'

Subjects

Reverberation Time Speech (msec) Stimulus

Noise Stimulus

Directional Advantage

12 normal

AR

NU-6

Multitalker babble

10 .6

12 normal

300

NU-6

Multitalker babble

3 .8

11 mild/moderate SNHL 11 mild/moderate SNHL

300 300

NU-6 Ni

Multitalker babble Multitalker babble

6 .3 3 .6

12 normal

300

Sentences Sentences Sentences Sentences Sentences

SWN SWN SWN SWN SWN

0 .0 1 .0 3 .0 4 .5 6 .5

ASB

Ni

SWN

8 .2

ASB ASB ASB

HINT HINT HINT

SWN SWN SWN

7 .4 7 .8 7 .8

12 normal

AR

NU-6

Multitalker babble

10 .4

25 mild/moderate SNHL

ASB

HINT

SWN

7 .5

19 moderate SNHL

ASB

Dantale

ICRA

3 .6

ASB

Spondees

Cocktail

5 .0

Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL

ASB ASB ASB ASB

HINT HINT HINT HINT

SWN SWN SWN SWN

0 .1 -0 .8 0 .5 0 .7

Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL

ASB

HINT

SWN

0 .2

Valente et al (1995)

12 normal 12 normal 12 normal

12 normal 12 normal

156 156 156 156 156

10 moderate SNHL

Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL

Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL Dybala (1996)

Speech 0'/noise 180'

Speech 0'/noise 180' Agnew and Block (1997)

Speech 0'/noise 180' Larsen (1998)* Speech 0'/noise 45', 135', 225', 315' Prosser and Biasiolo (1998)

12 normal

Speech 0'/noise 180' 5 Severe SNHL Valente et al (1998) Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL Speech 0'/noise 180' (site 1) 25 mild/moderate SNHL

Speech 0'/noise 180' (site 2) 25 mild/moderate SNHL Gravel et al (1999)


C 10 mild/MS SNHL


C 10 mild/MS SNHL

Speech 0'/noise 180' Speech 0°/noise 180'

C 10 mild/MS SNHL C 10 mild/MS SNHL

ASB

AR

ASB

CID W-22

3.6

3 .0

Chasin (1994) Speech 0'/noise 180'

ASB

Multitalker babble

Cafeteria

Speech O'/noise 135' Speech 0'/noise 180'


NU-6

Dutch Dutch Dutch Dutch Dutch

HINT

Ni

HINT

SWN

Multitalker babble

SWN

8.1

16 .4

-0 .4

ASB ASB

PSI, words PSI, sentences


4 .7 5 .2

ASB

PSI, sentences

Multitalker babble

4 .3

ASB

PSI, words

Multitalker babble

4 .8

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

dual-microphone configuration. A single microphone 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 configuration consisted of two omnidirectional microphones 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 205


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

Study/Conditions (azimuths) Preves et al (1999)

Speech 0°/noise 115°, 245°

(UENC target) Speech 0°/noise 115°, 245° (UENC MCL)

Speech 0°/noise 115°, 245°

(ENC target) Speech 0°/noise 115°, 245° (E/VC MCL) Ricketts and Dhar (1999) Speech 0°/noise 90°, 135°, 180°, 225°, 270° (aid 1) Speech 0°/noise 90°, 135°, 180°, 225°, 270° (aid 2) Speech 0°/noise 90°, 135°, 180°, 225°, 270° (aid 2)t Speech 0°/noise 90°, 135°, 180°, 225°, 270° (aid 3) Wouters et al (1999) Speech 0°/noise 90° Speech 0°/noise 90° Speech 0°/noise 90° Speech 0°/noise 90° Speech 0°/noise 90°

Pumford et al (2000) Speech 0°/noise 72°, 144°, 216°, 288° (BTE)

Speech 0°/noise 72°, 144°,

216°, 288° (ITE) Valente et al (2000) Speech 0/noise 180 (site 1) Speech 0*/noise 45°, 135°, 180°, 225°, 315° (site 1) Speech 0/noise 180 (site 1)t Speech 0°/noise 45°, 135°, 180°, 225°, 315° (site 1)t Speech 0/noise 180 (site 2) Speech 0°/noise 45*, 135°, 180°, 225°, 315° (site 2)


Subjects

Noise Stimulus


10 mild/severe SNHL

ASB

HINT

Uncorrelated SWN

2 .8

10 mild/severe SNHL

ASB

HINT

Uncorrelated SWN

2 .7

10 mild/severe SNHL

ASB

HINT

Uncorrelated SWN

2 .8

10 mild/severe SNHL

ASB

HINT

Uncorrelated SWN

1 .4


AR

HINT

Cafeteria

7 .5


AR

HINT

Cafeteria

6 .5


AR

HINT

Cafeteria

0 .8


AR

HINT

Cafeteria

6 .0

10 10 10 10 10

SNHL SNHL SNHL SNHL SNHL

450 450 450 450 450

spondees spondees spondees spondees spondees

SWN SWN Traffic Multitalker SWN

3 .2 3 .9 3 .6 3 .3 2 .8


ASB

HINT

SWN

5 .8


ASB

HINT

SWN

3 .3

25 moderate SNHL 25 moderate SNHL

ASB ASB

HINT HINT

SWN SWN

3.7 3.5


ASB ASB

HINT HINT

SWN SWN

4 .5 4 .2


ASB ASB

HINT HINT

SWN SWN

3 .2 2 .7

mild/moderate mild/moderate mild/moderate mild/moderate mild/moderate

BLU BLU BLU BLU BLU

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 experiments (N = 1057) for ODHAs and from 18 studies (N = 345) to 74 experiments (N = 1081) for DMHAs. This slight discrepancy in the number 206

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;


Table 2

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

Study/Conditions (azimuths) Madison and Hawkins (1983)

Speech 0°/noise 180° Hawkins (1984)

Subjects


12 normal

Speech 0°/noise 180° Speech 0°/noise 180°

C 11 mild/moderate SNHL C 11 mild/moderate SNHL

Speech 0°/noise 180° Speech 0°/noise 180`

12 normal 12 normal

Speech 0°/noise 180° Speech 0°/noise 180°

12 normal 11 mild/moderate SNHL

Speech 0°/noise 180°


Hawkins and Yacullo (1984) Speech 0°/noise 180° Speech 0°/noise 180° Speech 0°/noise 180°

Leeuw and Dreschler (1991) Speech 0°/noise 0° Speech 0°/noise 45°

Speech 0°/noise 90° Speech 0°/noise 135° Speech 0°/noise 180°

Dybala (1996)

Speech 0°/noise 180° Speech 0°/noise 180`

Ricketts and Dhar (1999) Speech 0°/noise 90°, 135°,

180°, 225°, 270° (aid 1) Speech 0°/noise 90°, 135°, 180°, 225°, 270° (aid 2)

Speech 0°/noise 90°, 135°, 180°, 225°, 270° (aid 2)* Speech 0°/noise 90°, 135°,

180°, 225°, 270° (aid 3)

12 normal

11 mild/moderate SNHL 11 mild/moderate SNHL 12 normal

12 normal 12 normal

12 normal

600

NU-6

600 Child's spondees 600 Child's spondees

Noise Stimulus


Multitalker babble

2 .5

SWN SWN

2.6 2.8

600

N U-6

Multitalker babble

3 .8

1200 1200

NU-6 NU-6

Multitalker babble

1 .3

600

600

600 1200 1200

N U-6

NU-6

NU-6 NU-6 NU-6

883 Dutch Sentences 883 Dutch Sentences

883 Dutch Sentences

Multitalker babble Multitalker babble Multitalker babble

3 .6 2 .6 4 .7


5 .5 -0 .6

SWN SWN

1 .5 2 .5

SWN SWN

3 .5 2 .5

Multitalker babble

SWN

1 .5

2.0

12 normal

883 Dutch Sentences 883 Dutch Sentences

12 normal 12 normal

600 600

NU-6 NU-6


6 .8 8 .8


642

HINT

Cafeteria

6.5


642

HINT

Cafeteria

5 .0


642

HINT

Cafeteria

2 .3


642

HINT

Cafeteria

4 .5

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-microphone configuration ; hearing aid manufacturer/model, analog, or DSP scheme ; nominal reverberation time (RT) ; type of speech and noise stimuli; absolute mean and standard deviation SNR values for 50 percent speech intelligibility 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 ; Studebaker 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, normal 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 formulas and procedures of Hunter and Schmidt 207


(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 ). Furthermore, 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 positive 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, suggests that DMHAs provide a considerable difference 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 considered not to be statistically significant. Nonoverlapping limits, on the other hand, indicated acceptance of the alternate hypothesis and were considered significantly different at p < .05. In the case that a given CI's range overlapped 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

RESULTS total of 138 weighted averages were calA culated for this meta-analysis. A summary of the findings follows . Overall Directional Advantage Using the 72 experiments (N = 1057) pertaining to ODHAs, a weighted average value of 1.1 dB with Cl95 ± 1.0 was determined. Similarly, a weighted average for the 74 experiments (N = 1081) on DMHAs yielded an SNR of-2 .6 dB with CI95 ± 1 .1 . A comparison between the ODHA and DMHA values resulted in a statistically significant difference at an alpha of .05 . A calculation of DA resulted in an SNR value of 4 dB (N =1081, Cl95 ± 0.8). Relative to these findings, a post hoc analysis was performed to quantify 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 synthesized across microphone condition and hearing 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(CI95 ± 2.7) and 3-dB (CI95 ± 2.7) increase in

B 12

8

N =a707 N=707

12

4

Y N = 707

Z V)

0

M

-4

N=120 N=108

-8 O " %

-12

ODHA DMHA Dir Adv

RT < 600

-16

RT > 600

RT < 600

RT > 600

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

208


signal, respectively, for ODHAs when comparing between acoustic conditions . A similar finding 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 technology has stimulated both public and professional interest . However, its impact on improving speech intelligibility with directional microphones 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 output 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 incorporating 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 processing 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 processing may be better suited to improving intelligibility with DMHAs. To further verify this finding, a comparison was performed between similar processing scheme and microphone con-

A

8

(n m 0

A search for single-microphone ITE experiments turned up a single experiment (Chasin, 1994) . Data were collected on 10 HI listeners in the less reverberant condition using analog signal processing . Results revealed an SNR value of 0 .5 and -7 .7 dB for ODHAs and DMHAs, respectively. This finding was found to be statistically significant at an alpha of .05 . This resulted in a DA of 8 .2 dB .

16 12

Cr Z

figuration, as illustrated in Figure 2 . For analog 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 microphones, independent of room acoustics (Fig . 2A), was noted. A post hoc analysis was performed to evaluate signal-processing schemes. In this analysis, data were further dichotomized based on microphone configuration and hearing aid style. In Figure 3, data are illustrated by hearing sensitivity, acoustic condition, and single-microphone designs. Figure 4 differs only in that data represented 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 further 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 statistically significant difference (p < .05) for DSP circuits in both acoustic conditions for HI listeners.

N=74 N=521

F

4 ~-N = 671 0 -4

-8

-12 -16

N=186J T N=186 ~' N = 5211

N=36 N=74 T _ N = 74T

N=36 j O T

N=36

N=186

" % 0

M Z N m -o

4

N=144 N=108

N=120 r

0 _4

N=132 N =T1 44

1

1

-12

DMHA-Analog

DMHA-DSP Dir Adv-Analog Dir Adv-DSP

RT < 600

8

-8

O ODHA-Analog O ODHA-DSP

"

N=120

12

-16

RT > 600

RT < 600

RT > 600

Figure 2 Weighted average, CI95, and sample size for behind-the-ear omnidirectional hearing aids, directional microphone 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=62

N=48 ~N = 162T N=162

T 13 " " % 0

N=62 T 0 %N-12

0 N=162

N=108

N=12 I I 0

N=132

N=108 N=132

Y

N=12

§

0 ODHA-DSP DMHA-Analog DMHA-DSP Dir Adv-Analog Dir Adv-DSP

RT < 600

RT > 600

RT < 600

RT > 600

Figure 3 Weighted average, Mg., and sample size for behind-the-ear omnidirectional hearing aids, directional microphone 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 configuration 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 intelligibility 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 synthesized and analyzed. Data are only applicable to the HI group assessed in less reverberant conditions . Results for ODHAs indicated an SNR value of 1.5 (CI95 ± 1.0), whereas DMHA data (-1.9 dB SNR, CI95 ± 0.7) revealed a statistically significant improvement (p < .05) in dB SNR. This resulted in a DA of 3 .4 dB (CI95 ± 1 .5).

Noise Stimuli

A

B N = 473

ON=24 N=1213

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0

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Because the primary purpose of a DMHA is to reduce noise from the sides and rear of the listener, the effects of various noise spectra and presentation 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 stimuli that remain relatively constant in amplitude over time (e .g ., speech-weighted noise) were defined as steady state. In contrast, fluctuating 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 content of noise.

0

-

N=12 O

N=12

N=12

X Z 1n m U

ODHA-Analog ODHA-DSP DMHA-Analog DMHA-DSP Dir Ad-Analog Dir Adv-DSP

RT < 600

RT > 600

RT < 600

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

210


A

B 16

16

12 8 oL Z 600

RT < 600

RT > 600

Figure 5 Weighted average, Cl,,, and sample size for behind-the-ear omnidirectional hearing aids, directional microphone 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, directional) . 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 consistent pattern (Fig. 5B) . In the less reverberant condition, 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 condition, 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 dualmicrophone configuration. Because of the sparse number of experiments on NH listeners, 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 singlemicrophone devices relative to dual-microphone devices for speech presented from 0 degrees azimuth and noise from 180 degrees azimuth . Furthermore, a statistically significant difference (p < .05) was found between noise types for multiple or diffuse noise presentations and dual microphones. A comparison between acoustic conditions indicated that fluctuating noise resulted in reduced SNR val-

ues for single microphones assessed in the traditional 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 presentation 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 singlemicrophone counterparts, with one exception. At an alpha of .05, a statistically significant difference 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 he fact that directional microphones are T capable of providing listeners with a favorable SNR relative to ODHAs has been well documented . However, the clinician's ability to predict speech intelligibility improvement for DMHAs has been confounded because of variations across individual studies . Thus, this study was undertaken to provide clinicians with more accurate information for use in the selection, fitting, and counseling of DMHAs while possibly revealing engineering and performance trends that might be helpful to the hearing aid industry and researchers .

Overall, DMHAs were found to provide a statistically significant advantage over ODHAs in improving SNR when data are pooled across all 211


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" SO/N90-SSN-DM " SO/N90-FN-DM . SO/N180-SSN-SM * SO/N180-FN-SM * SO/N180-SSN-DM V SO/N180-FN-DM 0 SO/NM-FN-SM SO/NM-SSN-OM * SO/NM-FN-DM

C 12

v

N=18

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 condition . Data are plotted as a function of 90° (N90), 180° (N180), and multi (NM) azimuth noise presentation for (A) omnidirectional hearing aids, (B) directional microphone hearing aids, and (C) directional advantage.

-8 -12 -16

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 differentiate themselves from ODHAs in the more reverberant condition may be attributable to several factors. First, higher reverberant conditions have been shown to affect the directionality 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 reverberant 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 overlapping sounds (i .e., overlap masking) and/or causing 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 212

not yet been met (Valente et al, 1998, 1999 ; Ricketts and Dhar, 1999). Based on the findings in this analysis, DSP coupled with an omnidirectional 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 single- or dual-microphone configuration but independent 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 factors may have contributed to the findings . First, data were analyzed primarily for those experiments 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


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

Presently, there are no standardized methods for the clinical assessment of DMHAs . Outcomes 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 configuration . Thus, an attempt was made in this analysis 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 compared 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 patterns, 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 speakers 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 empirical 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 acknowledges 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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