Hearing Aid Saturation, Coherence, and Aided Loudness ... · Hearing Aid Saturation, Coherence, and Aided Loudness Discomfort Todd W. Fortune* David A. Preves* Abstract Real-ear measurements

J Am Acad Audiol 3 : 81-93 (1992)

Hearing Aid Saturation, Coherence, and Aided Loudness Discomfort Todd W. Fortune* David A. Preves*

Abstract Real-ear measurements of the aided Loudness Discomfort Level (LDL) were obtained from five hearing-impaired listeners who were fit with two Class A and two Class D linear hearing aids, each with a different saturation sound pressure level (HFA SSPL90) . These measure-ments were obtained with 75 dB SPL continuous discourse to determine whether saturation-induced distortion contributes to the sensation of loudness . Real-ear coherence measure-ments made at LDL were used to determine the extent of saturation, and sound quality judgments were used to determine whether the distortion present at LDL affected sound quality . Results indicated that the SPL, coherence, and sound quality ratings obtained at LDL were all higher for Class D hearing aids with relatively high HFA SSPL90 than for Class A hearing aids with relatively low HFA SSPL90 . Overall results were generally consistent with the hypothesis that distortion affects both sound quality and the perception of loudness .

Key Words: Aided loudness discomfort level (LDL), hearing aid saturation, sound quality, real-ear coherence

n hearing aid fittings, the purpose of ob-taining the Loudness Discomfort Level (LDL) is to ensure that the maximum out-

put of the hearing aid will not exceed the level found uncomfortably loud by the user . While this objective is perfectly reasonable, the method generally used to derive the LDL, which re-quires stimuli to be presented from an audiom-eter through headphones, carries an assump-tion that the LDL obtained in this manner will accurately reflect the LDL that would be ob-tained had the listener based his judgments on stimuli heard through an actual hearing aid. Stimuli presented from an audiometer through headphones (which will be referred to as AUD-HP) are virtually distortion free, even at high sound pressure levels (SPLs) . This is rarely the case with "linear" hearing aids, which can satu-rate at relatively low SPLs, depending on the electroacoustic characteristics of the hearing aid and the acoustic properties of the stimulus passing through it . It is well known that satura-tion-induced distortion can reduce speech intel-ligibility and sound quality (Licklider, 1946 ;

*Argosy Electronics, Minneapolis, Minnesota Reprint requests : Todd Fortune, Argosy Electronics,

10300 W 70th St, Eden Prairie, MN 55344

Corliss et al, 1968 ; Tecca and Goldstein, 1984 ; Agnew, 1988; Schweitzer et al, 1991), but it is not known whether this distortion can affect the sensation of loudness . If the distortion pro-duced by a hearing aid in saturation has any influence on the sensation ofloudness, then it is reasonable to expect that the SPL in the ear canal at LDL might vary with the amount of distortion . Particularly for linear hearing aids with low HFA SSPL90 values, the aided LDL might be expected to be lower than the LDL obtained under AUD-HP conditions, due to the effects of saturation-induced distortion .

Previous research has shown that satura-tion-induced distortion can affect the way in which hearing-impaired subjects adjust the gain of their hearing aids . Hoffnung (1981) found a relationship between the HFA SSPL90 of a hearing aid and the listening level preferred by subjects . His data showed that as the HFA SSPL90 of a master hearing aid was decreased by peak clipping from 120 to 100 dB SPL, the preferred listening level of hearing-impaired subjects listening to speech decreased. This reduction in preferred listening level was thought to be due to the high amplitude peaks within speech driving the hearing aid into saturation . When this occurred, the response on the part of the listener was to reduce use gain .

81

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Journal of the American Academy of Audiology/Volume 3, Number 2, March 1992

Class D

0

C5 0.8 c 0

t 0.6 0 U m 04 w

d 0.2 96 dB RMS Output SPL

Mean Coherence : 0.92

01--200 500 1k 2k 5k

Frequency (Hz)

1 .0

c 0.8

m °' 0.6 0 U

0 .4 W

m 0.2 Q

99 dB RMS Output SPL

Mean Coherence. 0.82 0I- __ .-' -- 200 500 1k 2k 5k

Frequency (Hz)

Figure lA Real-ear coherence functions at 3 rms out-put sound pressure levels . Each panel displays coherence as a function of frequency for a linear hearing aid with a Class D output stage, as measured in the real ear of subject S2 . The input stimulus was continuous discourse, presented at 75 dB SPL. The gain of the hearing aid was adjusted to produce 96 dB output SPL (top panel), 98 dB SPL (middle panel), and 99 dB SPL (bottom panel) . The mean coherence value displayed within each panel was obtained by averaging the coherence at 238 frequencies (16-Hz intervals) between 200 and 4000 Hz .

The influence of distortion on intelligibility and sound quality cannot be predicted from the amount of harmonic or intermodulation distor-tion produced by the hearing aid (Licklider and Held, 1952 ; Peters and Burkhard, 1968). As an alternative to these distortion measures, some investigators (Reddy and Kirlin, 1979; Burnett et a1,1982; Preves and Newton, 1989 ; Bareham, 1990 ; Schweitzer et al, 1991) have suggested that coherence might correlate more strongly with speech quality. Coherence reflects that portion of an output signal that is due exclu-sively to the input signal, and is a very sensitive indicator of nonlinear distortion . A coherence of 1 .0 reflects a perfect correspondence between input and output signals, meaning that no dis-tortion or noise has been added to the input

82

Class A

1 .0

v 0.8 m L 0.6 0 U `m 0.4 w m

. Q 02

0

Figure 1B Real-ear coherence functions at 3 rms out-

put sound pressure levels for a linear hearing aid with a Class A output stage, as measured in the real ear of subject S2 . The gain of the hearing aid was adjusted to

produce 90 dB output SPL (top panel), 93 dB SPL (middle panel), and 95 dB SPL (bottom panel) . Other details as in Figure 1A .

signal . For high input levels, a coherence of 0.99 corresponds to 10 percent overall distortion and a coherence of 0.90 corresponds to 30 percent overall distortion (Kates,1990) . In hearing aids, low coherence can be due to a variety of factors, including amplifier noise as well as harmonic and intermodulation distortion .

Coherence measurements provide one way of monitoring saturation-induced distortion . As the input to or gain of a hearing aid is increased, saturation will be manifested by coherence de-terioration. Examples of this deterioration, measured in the real ear of an experimental subject, are shown in Figures lA and 1B . The upper panel of Figure lA shows the real-ear coherence function obtained with a linear hear-ing aid having a Class D output stage. The input stimulus was tape-recorded continuous dis-course presented to the hearing aid at 75 dB SPL. Gain of the hearing aid was adjusted to

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produce 96 dB output SPL in the ear canal. By averaging individual coherence values across 238 frequencies (200-4000 Hz, at 16-Hz inter-vals), a mean real-ear coherence of 0.92 was obtained . This indicates that the hearing aid was somewhat saturated. Coherence deteriora-tion may be seen primarily in the high frequen-cies . Increasing gain to produce 98 dB SPL in the ear canal (middle panel) caused mean real-ear coherence to drop to 0.86. At this level, it was likely that high-amplitude peaks within the speech material were regularly saturating the hearing aid. Coherence deterioration was more pronounced and occurred over a wider frequency range than in the upper panel of the figure . A further increase in the hearing aid gain and output SPL (lower panel) resulted in a further reduction in real-ear coherence.

Figure 1B shows comparable data for a Class A circuit. These data were acquired in the same manner, using the same subject. At 90 dB output SPL, mean coherence was 0.90, and coherence deterioration was limited primarily to the high frequencies. As output SPL was raised (middle and lower panels), coherence deterioration was more pronounced and spread to lower frequencies . By monitoring coherence in this fashion, it is possible to assess the extent of saturation-induced distortion present during various experimental conditions .

This investigation was designed to com-pare LDLs obtained under AUD-HP conditions with those obtained with linear hearing aids differing in HFA SSPL90, and to examine the extent of saturation-induced distortion present at LDL by making real-ear measurements of coherence. If the LDL is affected by saturation-induced distortion, then it is reasonable to expect a relationship to exist between the HFA SSPL90 of the hearing aid, the SPL present in the ear canal at LDL and the coherence associ-ated with this SPL. Hearing aids with rela-tively high HFA SSPL90 values that are less prone to saturate might be associated with higher aided LDLs, while those with lower HFA SSPL90 values might produce lower aided LDLs . In either case, the coherence function should indicate the extent of saturation-induced dis-tortion present at LDL. If both the aided LDL and the coherence obtained at LDL are rela-tively low for hearing aids having low HFA SSPL90 values, then support exists for the hypothesis that saturation-induced distortion contributes to the sensation of loudness . If, on the other hand, no differences in coherence are found at LDL, or if the SPL present at LDL does not vary with the HFA SSPL90 of the hearing aid, then the hypothesis can be rejected .

Saturation and Aided LDL/Fortune and Preves

METHOD

Apparatus

A specially-designed master hearing aid was constructed for the experiment . The device housed four separate linear hearing aid ampli-fiers. Two RTI Ultima II preamplifiers were used to drive Class D output stages in two integrated receivers (Knowles CK 3122 and EP 3075). Two other receivers (Knowles XL 9800 and XL 9799) were driven by two Gennum LS-505 amplifiers, these with Class A output stages . Each amplifier was configured to provide the maximum output SPL possible with a particu-lar receiver . Gain for each amplifier was ad-justed in 1 .5-dB steps with separate volume controls that were mounted on top of the master hearing aid.

High-frequency gain for each of the four amplifiers was rolled off with a single-pole low-pass filter . The decision to use low-pass filter-ing was based on pilot data that showed that acoustic feedback would occasionally occur at levels below LDL unless high-frequency gain was attenuated . During testing, these low-pass filters were adjusted to provide maximum re-duction of high-frequency gain . Each amplifier provided as much low-frequency gain as was possible with a particular receiver . No attempt was made to adjust frequency response to con-form with the configurations of individual hear-ing losses .

Four ITE hearing aid shells were custom made for each of five hearing-impaired subjects that participated in the experiment . Each shell contained a Knowles EK 3024 wide-band micro-phone and one of the four receivers . The two Class D hearing aids are hereafter designated as HAI and HA2, while the two Class A hearing aids are hereafter designated as HA3 and HA4.

Each of the 20 hearing aids was first tested on an HA-1,2-cm3 coupler. These measurements were made with the low-pass filters adjusted for actual test conditions . SSPL90, full-on gain, harmonic distortion, and frequency range meas-urements were made in accordance with ANSI S3.22-1987 ; these data are shown in Table 1. Most notable in the table are the differences in HFA SSPL90 values, which varied from a mean of 122 dB SPL for HA1 down to 96 dB SPL for HA4. Note that the Class D hearing aids, HA1 and HA2, had HFA SSPL90 values that were 12 to 28 dB higher than those of the Class A hearing aids, HA3 and HA4. HA1 and HA3 had similar HFA full-on gain values of about 38 dB . HFA full-on gain for HA2 and HA4 was approxi-mately 31 and 28 dB, respectively .

83


Table 1 2 Cm3 Coupler Data per ANSI S3.22 -1987, for Hearing Aids

Subject- Peak High-Frequency Average High-Frequency Average Total Harmonic Distortion Frequency Range Hearing Aid SSPL90 SSPL90 Full-On Gain 0.5 kHz 0.8 kHz 1.6 kHz Fl F2

(dB) (dB) (dB) (%) (%) (%) (Hz) (Hz)

S1-HA1 127 123 37 1 1 0 <200 6100 S2-HA1 126 122 38 1 2 0 <200 6900 S3-HA I 125 122 38 1 2 0 <200 6700 S4-HA1 126 122 38 1 2 0 <200 6700 S5-H A1 125 121 39 1 2 0 <200 6700

S1-HA2 122 118 29 2 3 2 <200 5900 S2-HA2 124 118 30 2 2 1 <200 9000 S3-HA2 123 118 31 2 2 1 <200 6900 S4-HA2 126 120 32 3 4 4 <200 8500 S5-HA2 123 119 32 1 2 1 <200 9000

S1-HA3 110 106 38 1 6 2 200 6400 S2-HA3 110 105 38 1 4 2 <200 4900 S3-HA3 109 104 37 1 6 2 <200 4900 S4-HA3 109 105 37 1 7 2 <200 6500 S5-HA3 110 105 38 1 5 2 <200 5100

S1-HA4 105 97 28 1 2 1 <200 5800 S2-HA4 105 96 27 1 4 1 <200 7700 S3-HA4 105 96 28 1 2 1 <200 6100 S4-HA4 105 96 28 1 3 1 <200 5800 S5-HA4 104 95 27 1 2 1 <200 5500

Representative examples of SSPL90 and full-on frequency response curves for each ofthe four circuit types are shown in Figure 2 . These data show that with low-pass filtering, HA1 and HA2 had primary frequency response peaks at 2100 and 2800 Hz, respectively . HA3 and HA4 had primary frequency response peaks at ap-proximately 1100 and 650 Hz, respectively .

Subjects

Five hearing-impaired subjects, ranging in age from 31 to 70 years, participated . As shown in Table 2, four of these subjects had sloping high-frequency hearing losses, while one (S2) had a moderate flat loss . All subjects were previous hearing aid users, and none had previ-ous experience in experimental listening tasks,

Stimulus

Procedure

Each subject was seated in the center of the test suite, facing a loudspeaker (Realistic Minimus-3 .5) at a distance of 48 inches . The subject's head was stabilized by an adjustable headrest and the nontest ear was occluded with an E-A-R plug . The LDL was defined as the level at which the continuous discourse material began to become uncomfortably loud, a crite-rion often referred to as the point of initial discomfort (Hawkins, 1980). Prior to testing, each subject was given the following written instructions :

You are about to hear a sample of pre-recorded speech . Using the dial, I would like you to increase the loudness of the stimulus to the point where it first becomes uncomfortably loud. By uncomfortably loud I mean the point at which you would not care to listen to the stimulus for more than a brief period of time . Do not stop ifthe speech merely becomes distracting, but only when it reaches a level where you would prefer to turn the volume down .

Speech passages from the Speech Intelligi-bility Rating (SIR) test (Cox and McDaniel, 1984) were presented in the sound field at 75 dB SPL, a level that has been shown to approxi-mate the level of conversational speech pro-duced at the hearing aid microphone by the hearing aid wearer's own voice (Olsen, 1991). All testing was conducted in an audiometric test booth (Tracoustics) in a quiet background .

These instructions were followed by a verbal reiteration that emphasized that neither ex-treme discomfort nor the most comfortable lis-tening level were being sought .

Using the master hearing aid described earlier, one of the four hearing aids was then randomly selected and placed in the subject's ear. The tube of a probe tube microphone

84

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1 0 20

110 1,0 ' - 0-1 0 (n 10

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1

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2 4 8

Frequency (kHz) F e (k Z)

HA2 requ ncy H

L -------------- r- --

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1 1 1 1

'20

, 20 110 110 J d

J 100 100

U) 90 90

80 80

70 70

60 60 25 .5 1 2 4 8

Frequency (kHz)

25 .5 1 2 4 8

HA4 Fre q uency (kHz)

I HA3

(Rastronics PM-10L) was inserted through a 3-mm-wide vent in the shell to a constant depth of 20 mm, measured from the peripheral vent opening. This insertion depth was selected based on estimates of the average adult ear canal length of 24 mm (Zemplenyi et al, 1985) . The output of the probe tube microphone was routed directly to channel B of a Bruel & Kjaer 2032 spectrum analyzer . To prevent feedback, and to ensure that as much low-frequency am-plification as possible was presented to the ear, the remainder of the vent opening was then sealed with putty. Using the method of adjust-ment, subjects were asked to adjust a desig-nated volume control on the master hearing aid until the 75 dB SPL continuous discourse be-came uncomfortably loud . Subjects were asked

Table 2 Pure-Tone Thresholds for Hearing-Impaired Subjects

Frequen cy (Hz)

Subject Ear 250 500 1000 2000 4000 8000

S1 R 35 40 45 40 65 75 S2 R 50 60 55 50 55 20 S3 R 35 45 50 55 55 65 S4 R 20 20 55 60 80 105 S5 L 20 20 40 60 75 80

Thresholds are in dB HL (ANSI S3.6-1989) .

Figure2 SSPL90andfull-onfre-quencyresponse curves forrepre-sentative hearing aids, HAI to HA4. These response curves were obtained per ANSI 53.22-1987 ; each curve reflects low-pass fil-tering, as described in the text . Complete tabular data for all hear-ing aids may be found in Table 1 .

to adjust both above and below the level of initial discomfort before arriving at their final volume control setting.

Once LDL had been established, a 20-sec-ond sample of hearing-aid processed speech was transduced by the probe tube microphone and the total rms SPL present in the subject's ear canal was determined. The total rms level of the averaged speech sample was determined by the 2032 analyzer over the frequency range from 200 to 5000 Hz. A coherence measurement was also made at this time . To make this measurement, a one-half inch reference micro-phone (Larson-Davis 2559) was positioned within 5 mm of the hearing aid microphone, and the output of the reference microphone was routed to channel A of the 2032 analyzer . Due to software limitations that existed during the early phases of the experiment, coherence func-tions were plotted, but could not be saved for further analysis .

Once the LDL had been reached, the subject was instructed to make sound quality judg-ments along the following dimensions : (1) dis-tinctiveness/clearness, (2) brightness, (3) loud-ness, (4) discomfort, and (5) overall impression. A ten-point rating scale was used (Gabrielsson and Lindstrom, 1981). These qualities were selected and modified from an extensive list of quality dimensions that have previously been

85


used to evaluate hearing aid performance (Gabrielsson et al, 1980; Gabrielsson et al, 1985). Each quality was defined verbally, and each subject was provided with one or more descriptive terms that might also describe the individual qualities.

This procedure, consisting of measurements of SPL, coherence, and sound quality, all made at LDL, was repeated for each of the four hear-ing aids . Hearing aids were tested in random order. Volume control positions were recorded for all LDL settings, so that use settings could be recreated at a later time .

Following the evaluation of the four hear-ing aids, similar measurements were made under AUD-HP conditions (Grason-Stadler GSI-10 audiometer, TDH-50 headphones). To make these measurements, two different psycho-physical methods were employed . First, in or-der to simulate traditional clinical practice, the LDL was derived using the ascending method of limits . (This method will be referred to as the AUD-HP-L condition, meaning that stimuli were delivered to headphones via the audiom-eter, and the method of limits was used during testing.) Starting at 60 dB HL, the level of speech was increased in 2 .5-dB steps until LDL was reported . Stimulus level was then decreased by 10 dB and another ascent was made . If a response was elicited at the same level on two different ascents, this level was recorded as the LDL. Generally, this required either two or three ascents to achieve. If LDL was not reach-ed after three ascents, a fourth ascent was made . No more than four ascents were ever required to reach LDL by this method . Once LDL had been established, a 20-second sample of speech was recorded from the probe tube microphone and the total rms sound pressure level was determined by the 2032 analyzer . Caution was used to ensure that both the probe tube microphone and the earphone were prop-erly positioned, so as to not create an additional acoustic leak .

To be consistent with the aided testing conditions, LDL was also determined using the method of adjustment (hereafter referred to as the AUD-HP-A condition) . With the subject facing away from the audiometer dial reading, the subject was instructed to adjust the attenuator dial until LDL for speech was reached. Subjects were asked to adjust both above and below LDL before making their final response, and to apply the same response crite-rion they had used during the aided conditions . While the initial presentation level was 60 dB HL, subjects used their own stepping rules, range of presentation levels and speed to deter-

mine their own level of discomfort . If a response was elicited at the same level on two different ascents, this level was recorded as the LDL. This required from two to four ascents to achieve. The LDL was originally recorded in dB HL. Using a probe microphone, real-ear sound pres-sure levels for individual subjects were later obtained with the audiometer attenuator set to the individual's mean dB HL value at LDL. The entire test session lasted approximately 2 hours, and a short break was typically allowed be-tween conditions .

Because the LDL has been reported to in-crease with repeated trials (Morgan et a1,1974), caution was used to ensure stable performance. Each of the five subjects returned for repeated testing of all conditions until a minimum of three SPL at LDL values were measured within any 4-dB range for each condition. This re-quired a minimum of four and a maximum of seven experimental sessions per subject. The data obtained during the first experimental session were considered to represent both fa-miliarization and practice effects and were not included in the analysis . All remaining values, including those that fell outside the 4-dB ranges, were included in the analysis .

Real-Ear Coherence Measurements Averaged across Frequency

Software improvements accomplished dur-ing the course of the experiment made the storage and off-line analysis of coherence func-tions possible . However, this capability was only utilized during the last experimental ses-sion in which each subject participated . During these final sessions, SPL and real-ear coher-ence measurements were made across a wide range of volume control settings for each of the four hearing aids . These data were stored for off-line analysis . As many measurements as possible were made and stored during the avail-able test time . These coherence measurements were made to determine whether a relationship existed between saturation-induced distortion (as represented by coherence) and the aided LDL.

Data Analysis

SPL and sound quality judgments obtained at LDL were analyzed both descriptively and by separate repeated-measures one-way analyses of variance, with stimulus delivery source (four aided and two AUD-HP conditions) serving as the main factor . Significant main effects were further analyzed using Newman-Keuls post hoc

86

-


analysis . Mean coherence was determined by averaging 238 absolute coherence values across the frequency range from 200 to 4000 Hz . Calcu-lations were made at 16-Hz intervals . The cor-relation between real-ear SPL and real-ear co-herence was determined by nonlinear regres-sion .

RESULTS

Overall Mean Data

Figure 3 shows the mean real-ear SPLs obtained at LDL for each of the six conditions . The figure shows three main results. First, each of the four hearing aids produced a mean LDL that was lower than that obtained during either of the AUD-HP conditions . Aided LDLs ranged from 98 dB (HA1) to 93 dB SPL (HA3, HA4), levels that were 2 to 8 dB lower than the AUD-HP LDLs. Second, HA1 and HA2, both with Class D output stages, produced mean LDLs that were 4 to 5 dB higher than those for the Class A hearing aids . Finally, the LDLs ob-tained during the two AUD-HP conditions were nearly identical, despite the fact that they were obtained using different psychophysical meth-ods.

Analysis of variance revealed a significant main effect for sound pressure level at LDL [F (5,149) = 20.4, p < .011 . Newman-Keuls post-hoc testing revealed that the SPL at LDL values for both HA1 and HA2 differed significantly from HA3 and HA4 (p < .05) . The SPL at LDL values for both AUD-HP-L and AUD-HP-A also dif-

AUD HP L AUD- HP -A HA1 HA2 HA3 HA4

Figure 3 Mean real-ear sound pressure levels ob-

tained at LDL, as measured by a probe-tube microphone

(n = 5 subjects). The input stimulus was continuous discourse, presented at 75 dB SPL. LDLs were obtained a minimum of four times per condition for each subject.

Mean LDLs obtained for stimuli presented from an audi-

ometer through headphones are referred to as AUD-HP-

L (method of limits) and AUD-HP-A (method of adjust-ment). Error bars represent one standard deviation.

Loudness Discomfort E

HAI HA2 HA3 HA4

Figure 4 Mean sound quality ratings obtained at LDL for all subjects . Ratings were obtained a minimum of four times per condition for each subject. Error bars represent one standard deviation. (Reprinted from Forture TW, Preves DA . (in press) . Hearing aid saturation and aided loudness discomfort . J Speech Hear Res.)

fered from those of HA3 and HA4. LDL for HA1 and HA2 did not differ from each other, or from either of the AUD-HP LDLs. LDL for HA3 did not differ from that of HA4.

Mean quality judgments obtained at LDL are shown in Figure 4 . HA1 and HA2 were rated higher than HA3 and HA4 in distinctiveness/ clearness (Panel A), brightness (Panel B), and overall impression (Panel C) . HA1 and HA2 were also judged lower in discomfort than HA3 and HA4 (Panel E) . In the loudness dimension (Panel D) the four hearing aids produced nearly' identical results, which suggests that subjects were in fact performing the task as instructed, and not varying their response across condi-tions .

Analysis of variance revealed a significant main effect for distinctiveness/clearness [F (3, 56) = 2 .8, p < .05], brightness [F (3, 56) = 3.2, p < .05], and overall impression [F 3, 56) = 2.8, p < .05] . Post-hoc analysis revealed a significant difference (p < .05) between HA1 and HA4 for each of these three quality dimensions. HA2 was also found to differ from HA4 in brightness . No significant differences were found among hearing aids in loudness or discomfort .

87

Journal of the American Academy of Audiology/volume 3, Number 2, March 1992

� 9 ,o=

95

B

A

90

S1

woera.uo,ou HAI HA2 HA3 HAI

S2 S3

'}I{ .{

Figure 5 Mean real-ear sound pressure levels ob-

tained at LDL for individual subjects . Each panel shows

the mean sound pressure levels measured at LDL for the

six listening conditions . Other details as in Figure 3.

Individual Mean Data

Individual LDL data for the five subjects are shown in Figure 5 . These data show that

four out of five subjects (S1-S4) reported mean real-ear LDLs for HAI and HA2 that were

higher than those obtained with HA3 and HA4. The fifth subject (S5) had the highest aided LDL

for HAI, followed by HA3 . HA2 and HA4 pro-duced the lowest aided LDLs for this subject. For each of the five subjects, the four aided

1 .0

0.9

0 .7

0 .6

0 .9

0

HA1

65 90 95 100 105

dB SPL

A

0

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0 .7 -

0 .6 1

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90 95 100 105

dB SPL

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05

HA4

65

LDLs were lower than those obtained during the AUD-HP-L condition. Four out of five sub-

jects showed aided LDLs for all four hearing aids that were below those obtained during the AUD-HP-A condition. The fifth subject (S2) showed aided LDLs for HA1 and HA2 that were 1 and 2 dB higher than the AUD-HP-A LDL, respectively .

Real-ear coherence data obtained from one subject (S1) during one session are shown in Figure 6 . Data associated with hearing aids HAI to HA4 are shown in Panels A to D, respec-

tively . The individual data points within each panel represent mean coherence values, aver-

aged across frequency, measured at various sound pressure levels . The filled data points

within each panel show the coherence meas-ured at the SPL associated with the listener's LDL, reported during this single experimental session.

Panel A of Figure 6 shows that for this subject, the SPL values measured in the ear

canal at LDL for HA1 were 95.1 and 95 .3 dB . The two filled symbols indicate that both test

and retest measurements were made (using two different speech passages) at the one vol-

ume control setting corresponding to the re-ported LDL. The real-ear coherence values at

these two LDL values were 0.94 and 0.97 . At higher sound pressure levels, coherence dropped rather rapidly. A similar pattern is seen in panel B, in which the coherence at LDL for HA2

(95 dB SPL) was 0.97. These real-ear coherence values suggest that both HAI and HA2 were in

the initial stages of saturation at LDL. Panels C and D show that for this subject,

not only were the aided LDLs for HA3 and HA4

lower than those found for HAI and HA2, but

these LDLs were also associated with lower

o o ° L7 0 8

°

B

0

O

95 100 105

dB SPL

D

° ° ° ~ o

0

0

95 100 105

dB SPL

Figure 6 Mean real-ear coher-

ence (238 points at 16-Hz inter-vals) as a function of rms ear canal output SPL for subject Sl . Panels

A to D represent data obtained with HAI to HA4, respectively, during one experimental session. The filled symbols represent the SPL and coherence values associ-ated with the LDL reported for the indicated hearing aid. Input stimulus : 75 dB SPL continuous discourse .

88

Saturation and Aided LDIJFortune and Preves

Table 3 Mean

Hearing Aid

Real-Ear Coherence at

Subject

LDL for Individual

LDL (dB SPL)

Subjects

Mean Coherence at LDL

HA1 S1 95 96 HA1 S2 97 88 HA1 S3 103 85 HA1 S4 94 92 HA1 S5 97 85

Mean 97.2 90

HA2 S 1 95 97 HA2 S2 98 75 HA2 S3 93 75 HA2 S4 96 77 HA2 S5 96 80

Mean 95.6 81


Mean 91 .6 70


Mean 91 .4 71

Data represent one session/subject

real-ear coherence . The aided LDLs (test and retest) for HA3 (Panel C) were 90.0 and 90 .1 dB SPL, and the corresponding real-ear coherence values were 0.78 and 0.88. These measure-ments also reflect any differences that existed between two speech passages ; the difference in coherence obtained probably reflects differences in the crest factors of the two speech passages, an uncontrolled variable that was not system-atically investigated . The aided LDL for HA4 (Panel D) was 92 dB SPL, and the associated real-ear coherence value was 0.77. These coher-ence data suggest that significant distortion in the processed speech signal was present at LDL for both HA3 and HA4. The presence of signifi-cant distortion, combined with the lower SPLs obtained at LDL, is consistent with the hypoth-esis that saturation-induced distortion may have contributed to the sensation of loudness, thus significantly lowering the LDL for the Class A hearing aids .

Aided LDLs (in dB SPL) and the associated coherence values obtained at LDL for all sub-jects and all hearing aids are shown in Table 3. It should be emphasized that these data were obtained during the last experimental session for each subject. For HA1, the mean SPL at LDL was 97 .2 dB, while mean coherence at LDL was

0.90. For HA2 hearing aids, the SPL and coher-ence values obtained at LDL were 95.6 dB and 0.81, respectively, values lower than those found for HAL For HA3 and HA4, the mean SPL at LDL values were 91.6 and 91 .4 dB, respectively, and the associated coherence values were 0.70 and 0.71, respectively . These data indicate that the two Class D hearing aids, each having a relatively high SSPL90, produced less satura-tion-induced distortion (as indicated by coher-ence) at LDL than the two Class A hearing aids, both having a relatively low SSPL90 . Those hearing aids with higher real-ear coherence values at LDL were also associated with higher SPL values at LDL. Based on the data in Table 3, the correlation between real-ear SPL and real-ear coherence at LDL was not significant at the .05 level (r = .36 ; p = .11).

Pooled real-ear coherence data for all sub-jects are shown in Figure 7, which, as in the previous figure, displays data for each of the four hearing aids, one per panel. As in the previous figure, each individual data point rep-resents one mean coherence value, measured at a specific SPL. Recall that these coherence data were acquired during the final experimental session for each subject. Each panel also dis-plays the mean LDLs obtained under each of

89

-p,- rq.. ."


A B d d 0 U

1 .0 HA2 Figure 7 Mean real-ear coher-

0.8

os

d

0

a

0 .4

0.2

0 .0 8o 85 90 95 100 105 110 80 85

SPL (dB) 90 95 100 105 110

SPL (dB)

ence as a function of rms ear canal output SPL, pooled across sub-jects. Coherence data were obtained during one experimental session per subject. Scatterplots have been fit with third-order poly-nomial regression curves . Mean aided and AUD-HP LDLs are in-dicated by the arrowheads posi-tioned along the abscissa . All LDLs shown represent data obtained over repeated sessions per sub-ject . Other details as in Figure 6. (Reprinted from Fortune TW, Preves DA . (in press) . Hearing aid saturation and aided loudness dis-comfort. J Speech Hear Res.)

two conditions : (1) the aided LDL for the par-ticular hearing aid, which varies from panel to panel, and (2) the AUD-HP LDL, which does not vary across panels . Because the two AUD-HP LDLs were nearly identical, the means of the two have been plotted. All LDLs shown reflect data gathered over repeated sessions for each subject.

Three trends may be seen in these data . First, as was also shown in Figures IA and 1B, the data show that as real-ear SPL increased, real-ear coherence decreased. The correlation coefficients shown within each panel of Figure 7 represent the relation between coherence and SPL. Each correlation was found to be statisti-cally significant (p = .00) . However, HAI and HA2 maintained higher coherence with SPL than HA3 and HA4. Coherence values in excess of 0.9 were often obtained with HAI and HA2, but this was rarely the case with HA3 and HA4. The slopes of the regression curves suggest that coherence deteriorated more gradually for HA 1 and HA2 than for HA3 and HA4. Second, the regression data predict coherence at the known aided LDLs for HAI to HA4 to be 0.86, 0 .78, 0.75, and 0.71, respectively . These predicted coherence values are consistent with actual coherence values (shown in Table 3) that were obtained at LDL. Finally, it can also be seen that better agreement between the aided LDLs and the AUD-HP LDLs occurred for HA1 and HA2, which retained higher coherence with SPL.

As indicated earlier, differences in frequency response existed across hearing aids, due pri-marily to the use of low-pass filtering. To exam-

90

HA1

ine whether these frequency response differ-ences might have contributed to differences in LDL, 2-cm' frequency response curves were generated for each hearing aid at the mean gain settings obtained at LDL for individual sub-jects . These data were then converted into pre-dicted insertion gain, using the KEMAR-based correction factors provided by Burnett and Beck (1987) for unvented, in-the-ear hearing aids . These predicted insertion gain values, together with directly measured SPL and LDL values are presented in Table 4. As shown in Table 4, HAI and HA2 generally produced more gain at LDL at .25, 0.5, and 3.0 kHz, than HA3 and HA4. HA3 and HA4 typically provided higher gain between .75 and 2.0 kHz than that for HAI or HA2.

Statistical analysis revealed significant positive correlations between SPL at LDL and predicted insertion gain at .25 kHz (r = .83, p = .00), .5 kHz (r = .76, p = .00), 2.0 kHz (r = .50, p = .020.0 kHz (r = .71, p = .00), and 4.0 kHz (r = .67, p = .00) . As stated earlier, HAI and HA2 produced more gain at most of these frequencies than HA3 or HA4. These data suggest that higher gain was associated with higher LDLs .

DISCUSSION

T he main results of this experiment may be summarized as follows: First, the mean LDLs produced while subjects wore hearing aids were always lower than the LDLs obtained under typical clinical conditions (i .e ., when stimuli were delivered from an audiometer through headphones). Second, the mean aided

t it

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Predicted Insertion Gain (dB) at LDL

Subject Hearing Aid .25 kHz 5 kHz 75 kHz 1 .0 kHz 1 .5 kHz 2.0 kHz 3.0 kHz 4 .0 kHz SPL (d8) at LDL

S1 HA1 30 32 30 30 32 34 21 29 97 S1 HA2 24 26 24 24 24 23 23 23 98 S1 HA3 12 21 28 32 29 25 19 17 92 S1 HA4 14 25 29 25 21 17 16 18 93

S2 HA1 30 34 32 31 33 27 26 33 98 S2 HA2 29 30 28 28 26 24 26 25 100 S2 HA3 13 21 25 32 30 27 22 27 92 S2 HA4 14 26 32 29 27 25 20 21 94

S3 HA1 32 34 34 33 35 37 25 37 100 S3 HA2 23 26 24 22 22 21 21 24 96 S3 HA3 13 20 27 30 28 24 19 25 93 S3 HA4 12 24 27 24 20 16 13 14 93

S4 HA1 24 28 28 30 32 31 22 26 94 S4 HA2 23 25 24 22 21 18 22 21 96 S4 HA3 12 20 25 29 25 20 16 22 93 S4 HA4 14 25 30 28 24 19 18 18 92

S5 HA1 27 29 30 30 34 37 33 39 98 S5 HA2 23 26 25 24 22 19 23 21 95 S5 HA3 14 23 30 34 32 27 24 29 97 S5 HA4 11 24 31 30 28 22 19 20 95

LDLs for the two Class D hearing aids (HA1, HA2), having relatively high HFA SSPL90 val-ues, were about 5 dB higher than the LDLs associated with the Class A hearing aids (HA3, HA4), having relatively low HFA SSPL90 val-ues. Third, sound quality judgments obtained at LDL were higher for the Class D hearing aids than the Class A hearing aids . Fourth, real-ear coherence decreased significantly with increas-ing output SPL near the onset of saturation .

Several explanations can be offered to ac-count for these results . First, the real-ear coher-ence data suggest that lower LDLs were associ-ated with lower coherence values . From this, the argument could be made that saturation-induced distortion contributed to the sensation of loudness . If true, this would have lowered the aided LDLs for HA3 and HA4, relative to the less-distorted Class D hearing aids, HAI and HA2. Statistical analysis of the data in Table 3, however, did not support this conclusion . This may have been due to insufficient data, since the statistical treatment was based on coher-ence and SPL values acquired during only one experimental session.

A second explanation might be that differ-ent LDLs might have resulted from the differ-ences in frequency response that existed across circuits . It could have been argued that the low-frequency emphasis of HA3 and HA4 (see Fig. 2)

might have lowered the LDL relative to HAI and HA2 . Examination of predicted insertion gain at LDL, however, suggested just the oppo-site ; HAI and HA2 were adjusted (by subjects) to produce the most low-frequency and the most high-frequency gain, and these circuits were associated with higher aided LDLs. Significant positive correlations between SPL and gain at several frequencies suggested that higher LDLs were associated with higher gain . In each case higher gain was produced by HAI and HA2. In addition, examination of individual coherence functions showed that the pattern of coherence deterioration with SPL was similar across hear-ing aid types. As demonstrated in Figures lA and B, coherence deterioration typically began in the high frequencies and progressed to lower frequencies with increasing SPL for both Class D and Class A circuit types. This pattern oc-curred despite the differences in frequency re-sponse that existed across circuit types. Thus, if coherence is a valid indicator of distortion, and if distortion influences the LDL, it would appear unlikely that the aided LDLs varied across hearing aids due to differences in fre-quency response .

These results have also been examined with regard to audiometric information and patient history. While estimates of loudness growth were not obtained, estimates of the

91

Table 4 Predicted Insertion Gain and Directly Measured Real-Ear SPL at LDL for Individual Subjects


dynamic range between threshold and loudness discomfort were made for each subject by com-paring Speech Reception Thresholds (SRTs) with the AUD-HP LDLs. These data are shown in Table 5 . The SPLs reported for AUD-HP, Class D, and Class A represent the mean ofboth conditions within each category . SRTs have been converted from dB hearing level (HL) into dB SPL by applying a 20 dB correction (ANSI S3.6-1989). The table shows that two subjects (S1 and S5) had fairly wide dynamic ranges, while three subjects (S2 to S4) had relatively narrow dynamic ranges . No particular pattern is evident between the size ofthe dynamic range for a subject and the aided LDLs . Subjects S4 and S5 showed only a 2 dB and a 1 dB mean difference between Class D and Class A LDLs, respectively, as presented in Table 5. These two subjects had very different dynamic ranges (23 vs 50 dB), but similar audiometric configura-tions; each had a steeply sloping high frequency hearing loss . Whether this result is related to good low-frequency or poor high-frequency hear-ing is unclear. The remaining subjects, having mildly sloping or flat losses, showed a clear distinction between Class D and Class A LDLs.

Patient histories revealed little that might have contributed to the results of this investiga-tion . Each subject was an experienced hearing aid user, having worn a hearing instrument for a minimum of 6 months prior to their participa-tion in this research . Three subjects (S2, S3, and S5) wore linear aids with HFA SSPL90 values of 123, 104, and 109 dB SPL, respectively . Two subjects (S1 and S4) wore automatic signal processing (ASP) aids having HFA SSPL90 values of 110 and 116 dB SPL, respectively . HFA full-on gain values for these aids ranged from 28 dB (S3) to 45 dB (S2) . Each subject reported occasional episodes of loudness dis-comfort when wearing their own hearing aids . Once again, however, no particular pattern was suggested between the characteristics of the hearing aids worn by subjects prior to their

participation in the experiment and the results reported herein .

Overall, these results suggest that linear hearing aids that saturate at relatively low SPLs will often be driven into saturation by a stimulus as innocuous as, for example, a hear-ing aid wearer's own voice (Olsen, 1991) . The level at which saturation occurs for speech will most likely be considerably lower than would be suggested by the HFA SSPL90 value for the hearing aid, a measurement that is based on pure tones. When saturation occurs, it is possi-ble that because of the combined influence of high sound pressure level and distortion, the listener will judge the stimulus not only as having poor sound quality, but also as uncom-fortably loud . If this occurs, the action taken will likely be to reduce use gain, perhaps to a level that provides reduced benefit (Hoffnung, 1981). Conversely, linear hearing aids with higher HFA SSPL90 values will likely be driven into saturation less often by speech, be judged to have higher sound quality, and produce judg-ments of uncomfortable loudness that are based on SPL alone.

The hypothesis that the low aided LDLs associated with low SSPL hearing aids were caused by excessive distortion (as measured by coherence) was not confirmed by statistical analysis . However, the finding that the aided LDL did in fact vary systematically between high SSPL and low SSPL hearing aids suggests that when speech is presented at a high SPL, it will be more tolerable if the hearing aid is not saturating. When saturation is minimized, it is more likely that the hearing aid will be worn at a gain setting that provides appropriate ampli-fication . Thus, in hearing aid fittings, it is important to prevent not only the occurrence of excessively loud sounds but also the occurrence of excessively distorted sounds .

Excessive distortion can be prevented by a variety of means, including the use of ASP or compression circuits . In linear hearing aids it

Table 5 Dynamic Range Estimates Hearing

and Mean LDL Values for AUD-HP, Class D, Aids for Individual Subjects

and Class A

Subject SRT

(dB SPL) Mean AUD-HP LDL

(dB SPL) Dynamic Range

(dB) Mean Class D LDL

(dB SPL) Mean Class A LDL

(dB SPL)

S1 55 101 46 98 92

S2 70 100 30 99 93

S3 70 102 32 97 93 S4 75 98 23 95 93 S5 50 100 50 96 95

Data are based on all sessions .

92

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can be prevented by keeping the HFA SSPL90 relatively high . Because the HFA SSPL90 of the hearing aid is usually preset as a result of the clinically-measured LDL, it is important that the clinician realize that the aided LDL for a linear hearing aid will be well below the LDL determined under headphones, unless the HFA SSPL90 of the hearing aid is relatively high .

All of the results presented here are based on one realistic environmental stimulus, single talker continuous discourse. The effects of dif-ferent stimuli on the aided LDL are not pres-entlyknown.Ifthehigh amplitude peaks present in continuous discourse reduce the aided LDL by inducing saturation, then it is possible that stimuli with lower amplitude peaks might pro-duce higher aided LDLs . Similarly, transient-type stimuli might produce lower LDLs. In addition, the effects of compression, ASP and other forms of nonlinear amplification on the aided LDL are unknown. The evaluation of these circuit types, as well as the examination of the aided LDL for different audiometric con-figurations and severities will help better de-fine the relationship between the characteris-tics of the hearing aid and the sensation of loudness discomfort .

REFERENCES

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Agnew J. (1988) . Hearing instrument distortion : What does it mean for the listener? Hear Instr 39:10-20 .

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Burnett ED, Corliss ELR, Nedzelnitsky V. (1982) . Re-search problems in coupler and in-situ measurements on hearing aids . In : Studebaker G, Bess F, eds. The Vanderbilt Report: State of the Art-Research Needs . Upper Darby, PA : Monographs in Contemporary Audiology, 67-73.

Corliss ELR, Burnett ED, Kobal MT, Bassin MA. (1968) . The relative importance of frequency distortion and changes in time constants in the intelligibility of speech . IEEE Trans Audio Electroacoust AU 16(1):36-39 .

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Olsen WO. (1991) . Clinical assessment of output limiting and speech enhancement techniques . In : Studebaker GA, Bess FH, Beck LB, eds. The Vanderbilt Hearing Aid Report II. Parkton, MD : York Press.

Peters R, Burkhard M. (1968) . On Noise Distortion and Harmonic Distortion Measurements . Report 10350-1. Franklin Park, IL : Knowles Electronics.

Preves DA, Newton JR . (1989) . The headroom problem and hearing aid performance. Hear J 42:1-5 .

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Documents

Hearing Aid Saturation, Coherence, and Aided Loudness ... · Hearing Aid Saturation, Coherence, and Aided Loudness Discomfort Todd W. Fortune* David A. Preves* Abstract Real-ear measurements