Improving Speech Intelligibility in Background Noise … · relatively low power consumption. The participants in the study were evaluated with speech in noise in a variety of conditions

J Am Acad Audiol 17:519–530 (2006)

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*Dynamic Hearing Pty Ltd, Richmond, Australia

Peter Blamey, Dynamic Hearing Pty Ltd, 2 Chapel Street, Richmond 3121, Australia; Phone: (+61 3) 8420 8500; Fax (+61 3) 8420 8599; E-mail: [email protected]

Improving Speech Intelligibility inBackground Noise with an AdaptiveDirectional Microphone

Peter J. Blamey*Hayley J. Fiket*Brenton R. Steele*

Abstract

Omnidirectional, supercardioid, and adaptive directional microphones (ADM) wereevaluated in combination with the ADRO® amplification scheme for eight participantswith moderate sloping hearing losses. The ADM produced better speech perceptionscores than the other two microphones in all noise conditions. Participants performedthe Hearing in Noise Test sentences at -4.5 dB SNR or better, which is similar tothe level achievable with normal hearing. The Speech, Spatial and Qualities of HearingScale indicated no disadvantages of using the ADM relative to the omnidirectionalmicrophone in real-life situations. The ADM was preferred over the omnidirectionalmicrophone in 54% of situations, compared to 17% preferences for the omnidirectionalmicrophone, and 29% no preference. The combination of the ADM to improve SNR,and ADRO® to keep the signal output comfortable and audible provided near-normalhearing performance for people with moderate hearing loss. The ADM is therecommended microphone configuration for ADRO hearing aids.

Key Words: Directional microphones, hearing aids

Abbreviations: ADM = adaptive directional microphone; ADRO = adaptive dynamicrange optimization; CUNY = City University of New York; HINT = Hearing in NoiseTest; PTA= average of the pure-tone threshold hearing levels at 500, 1000, and 2000Hz; SNR = signal-to-noise ratio, SSQ = Speech, Spatial and Qualities of Hearing Scale

Sumario

Se evaluaron micrófonos omnidireccionales, supercardioides y micrófonosdireccionales adaptativos (ADM) en combinación con el esquema de amplificaciónADRO® en ocho participantes con hipoacusias de pendiente moderada. El ADMprodujo mejores puntajes de percepción del lenguaje que los otros dos micrófonosen todas las condiciones ruidosas. Los participantes realizaron la Prueba deAudición en Ruido a – 4.5 dB SNR o mejor, que equivale al nivel alcanzable conaudición normal. La escala de percepción del lenguaje, percepción espacial y decalidad de la audición (SSQ) no mostró desventajas del uso del ADM en relacióncon el micrófono omnidireccional en situaciones de la vida real. Se prefirió el ADMsobre el micrófono omnidireccional en 54% de las situaciones, comparado con un17% de preferencia por el omnidireccional y un 29% sin preferencia. La combinacióndel ADM para mejorar la SNR, y el ADRO® para mantener la salida de la señalconfortable y audible aportó un desempeño auditivo cercano a la normalidad enlas personas con una hipoacusia moderada. La configuración de micrófono ADMes la recomendada para auxiliares auditivos ADRO.

Palabras Clave: Micrófonos direccionales, auxiliares auditivos

Abreviaturas: ADM = micrófono direccional adaptativo; ADRO = optimizaciónadaptativa del rango dinámico; CUNY = Universidad de la Ciudad de Nueva York;HINT = Prueba de Audición en Ruido; PTA= promedio de los niveles auditivos paraumbrales de tonos puros a 500, 1000 y 2000 Hz; SNR = tasa señal-ruido; SSQ =Escala de percepción del lenguaje, percepción espacial y de calidad de la audición

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Hearing impairment often results in aloss of frequency resolution inaddition to the elevation of hearing

thresholds (Zwicker and Schorn, 1978). Oneof the consequences of reduced frequencyresolution is that listeners with impairedhearing require a higher signal-to-noise ratio(SNR) to understand speech than people withnormal hearing (Plomp, 1986). Marketsurveys confirm that the majority of hearingaid users seek improved intelligibility ofspeech in background noise (e.g., Kochkin,2002), and even listeners with normal hearingare affected by noise if the SNR falls below0 dB. Designers of hearing instruments havedeveloped noise reduction and compressionalgorithms with the goals of making listeningin noise more comfortable and improving theintelligibility of speech in noise.Unfortunately, many noise reduction andcompression schemes that improve comfortfor loud sounds also reduce speechintelligibility in noise rather than enhancingit (Nabelek, 1983; Van Tasell and Trine, 1996;Verschuure et al, 1998; Hornsby and Ricketts,2001). There are, however, twocomplementary technologies that can achieveboth goals when used together. The adaptivedynamic range optimization (ADRO®)amplifier has been shown to providecomfortable listening in loud or noisyconditions and to improve speechintelligibility in noise compared withalternative linear and nonlinear amplificationschemes in multiple scientific studies(reviewed by Blamey, 2005). The ADROamplifier has noise reduction “built in.” Thesecond technique to improve speechintelligibility in noise is the use of adirectional microphone.

It is well known that directionalmicrophones can improve SNR when thespeech signal and the noise come fromdifferent directions in a laboratory setting, butthere is still controversy over some aspectsof this technology in real-life conditions. Inaddition, the compression amplificationschemes in most hearing aids may reducethe SNR advantage produced by a directionalmicrophone because they tend to amplifylower level sounds more than the higher levelsounds. When a directional microphonereduces the intensity of sounds from behindthe listener to a level that is lower thansounds in front of the listener, WDRC willtend to reduce this intensity difference, thus

reducing the benefit derived from themicrophone.

This study was designed to evaluatethree different microphone configurationsused together with the ADRO soundprocessing strategy. Previously publishedADRO studies for hearing aids have all usedan omnidirectional microphone. The threemicrophones in the present study wereomnidirectional, fixed supercardioid, and anewly designed adaptive directionalmicrophone with a unique combination offeatures (Steele, 2004). In addition to itsadaptive directionality, the ADM (adaptivedirectional microphone) has a flat frequencyresponse, adopts an omnidirectional modewhen it is advantageous to do so, and hasrelatively low power consumption. Theparticipants in the study were evaluatedwith speech in noise in a variety of conditionsin the laboratory, and with questionnaires toassess their performance and preferencesoutside the laboratory.

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Amplification alone does not overcomespeech perception difficulties in noisy

listening environments. Adverse signal-to-noise ratios for any type or degree of hearingloss can be improved with the use ofdirectional microphones (Ricketts andMueller, 2000). Directional microphonesoperate on the assumption that desiredspeech signals arrive from the front of thelistener and competing noise sources arriveat the microphone from other directions.Evaluations in sound-treated booths haveseen directional microphones improve SNRsfrom 3.4 dB to 8.5 dB (Valente et al, 1995;Wouters et al, 1999). Factors such as thelevel of reverberation, signal location, signaldistance, and the location of noise influencedirectional microphone performance.Performance is easier to optimize in the soundbooth than in the real world (Compton-Conleyet al, 2004; Walden et al, 2004).

In quiet situations, a directionalmicrophone has the disadvantage that soundsfrom behind are not heard as easily as soundsfrom the front. For hearing aid users to gainthe most benefit from a conventionaldirectional microphone, they must be able tocorrectly identify appropriate listeningsituations. The responsibility for manually

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switching between omnidirectional anddirectional microphones in a hearing aid tosuit listening environments requires not justphysical skills but cognitive ones as well.Automatic directional microphones eliminatethe need for manual switching ofmicrophones.

Adaptive directional microphonesautomatically adapt their directional responsepattern so that the SNR is optimized incontinuously changing environments. Variousevaluations have shown that adaptivedirectional microphones improve the SNRwhen tested in a sound booth environment(Wouters et al, 2002; Valente and Mispagel,2004). Furthermore, an investigation byRicketts and Henry (2002a) found that inmany situations, an adaptive directionalmicrophone outperformed a fixed directionalmicrophone. In that study, 20 participantswith binaural behind-the-ear hearing aidscompleted the Connected Speech Test andthe Hearing in Noise Test (HINT) in fournoise environments designed to represent thereal world. In conditions where the competingnoise was presented from the listener’s side,a significant speech recognition advantagewas measured for the adaptive mode.

In the present study, the adaptivedirectional microphone (ADM) was bothadaptive and automatic (Steele, 2004). Whenthe input sound level was below 65 dB SPL,the microphone automatically adopted anomnidirectional response pattern. For inputsound levels above 65 dB SPL, themicrophone adapted its directional responsepattern to minimize the total input soundlevel while keeping the response from thefront constant. The effect is to maximize theSNR for signals in front of the listener. TheADM has several advantages over otherdirectional microphones. It has a flatfrequency response from the front, unlikemost directional microphones that introducea 6 dB per octave slope (Ricketts and Henry,2002b). The flat response means that thehearing aid fitting can be exactly the samefor the ADM and the omnidirectionalmicrophone, and the fitting does not need toadapt as the ADM changes its responsepattern. The ADM will take up anomnidirectional pattern in windy conditions,because this minimizes the total input soundlevel. Most other directional microphonesare more susceptible to wind noise. The ADMis also very efficient, using less battery power

than other designs. The efficiency is achievedby using a fixed delay in the processing,compared to other schemes where the delayis variable, requiring more complexcalculations.

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The rationale and algorithm for ADROprocessing are described in detail

elsewhere (Blamey, 2005; Blamey et al, 1999,2004; Martin et al, 2001). Briefly, ADROsplits the sound into 64 or 32 narrowfrequency channels and statistically analysesthe output level distribution in each channel.The statistical information is used to optimizethe output levels in each channelindependently of the others, using four fuzzylogic rules. The comfort rule ensures thatthe output level does not exceed the comforttarget more than 10% of the time. Theaudibility rule ensures that the output leveldoes not fall below the audibility target morethan 30% of the time. The hearing protectionrule ensures that the output level does notever exceed the maximum output level. Thebackground noise rule ensures that very softnoises are not amplified to an annoying level,by restricting the maximum gain of theinstrument in each channel. To fit ADRO,the audiologist or dispenser finds theoptimum settings for the comfort targets,audibility targets, maximum output levels,and maximum gains. The 64-channel versionof ADRO was used in this study.

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The study was conducted in two parts.The first part of the study compared an

omnidirectional microphone with a fixeddirectional microphone. The participantswere fitted with a BTE hearing aid containingan omnidirectional microphone in oneprogram and a fixed directional supercardioidmicrophone in the second program. Bothprograms were used in speech perceptiontesting. Participants did not take the hearingaids home during this phase.

The second part of the study wasdesigned to evaluate the performance of theADM relative to the other two microphoneswith ADRO. In this phase of testing, theparticipants wore the hearing aids in their

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daily life for a period of four to five weeks. Thehearing aids they wore home featured anomnidirectional microphone in program oneand the ADM in program two. For theevaluation sessions, the omnidirectional,fixed directional supercardioid, and the ADMwere compared using two speech perceptiontests. The participants also responded to twoquestionnaires assessing their use of theomnidirectional and adaptive directionalmicrophones with ADRO in everyday life.

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Eight participants (three females and fivemales) took part in the study. Ages

ranged from 36 to 82 years (mean = 69.6,SD = 15.8). All participants had a bilateralsensorineural hearing loss. Air- and bone-conduction pure-tone thresholds wereobtained for all participants in both earsfrom 250 to 6000 Hz. Figure 1 shows themean air-conduction threshold at eachfrequency. All participants had previouslyworn hearing aids, with a mean of 12 yearsof experience (range four months to 28 years).All participants were fitted binaurally.

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BTE hearing aids containing the AMISToccata Plus open platform DSP chip

(Brennan and Schneider, 1998) wereprogrammed for ADRO processing. The hearingaids were fitted to the participants’requirementsusing the ADROfit fitting software developedby Dynamic Hearing Pty Ltd.

To start the fitting, comfort levels werepredicted from the audiogram. The comfortlevels were refined using in situmeasurements. At 500, 1000, 1500, 2000,3000, and 4000 Hz, narrow band noise waspresented through the hearing aid.Participants were asked to inform theaudiologist when each sound was at acomfortable level. Loudness was thenbalanced across the frequency range.

The ADROfit fitting softwareautomatically calculated initial output targets(maximum output level, comfort target, andaudibility target) from the measured comfortlevels. Initial values for the maximum gainin each frequency channel were derived fromthe audiogram. The hearing instrumentswere turned on, and overall volume for livespeech was adjusted to a comfortable levelthat was not too loud or too soft by varyingall four sets of fitting parameters in bothears up and down at the same time. Ifnecessary, fine-tuning of high, low, and mid-frequencies was used to avoid acousticfeedback and to set the sound quality to thepreferences of the listener. The adjustmentswere applied bilaterally while listening tolive speech and/or recorded sounds with theomnidirectional microphone.

FFiigguurree 11.. Mean air-conduction pure-tone thresholds for 16 ears.


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After fitting, two programs were saved tothe hearing aid. Program one always utilizedan omnidirectional microphone. Program twoused either a fixed supercardioid microphoneor the adaptive directional microphone. In thesecond phase of the experiment, theparticipants were informed that the hearinginstruments had two programs that mightsound different from one another in somesituations. They were asked to try bothprograms in different situations and forman opinion about the differences. They werenot told anything about the differences thatmight be expected between the two programsor how the microphones operated.

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Speech intelligibility was assessed usingthe Hearing in Noise Test (Nilsson et al,

1994). The HINT consists of 12 lists, eachcontaining 20 sentences. The sentences arephonetically balanced and are approximatelyequal in length. Steady-state noise acts as amasker for the sentences. The noise spectrumhas been shaped to correspond to the long-term average spectrum of the sentences. Thenoise was presented to participants at a levelof 65 dBA, and the noise onset was fiveseconds prior to the speech onset. Noise leveldid not vary throughout the test. The level ofthe sentences was varied adaptively to findthe signal-to-noise ratio where the participantunderstood 50% of the sentences.

Testing was performed within a sound-treated room having internal dimensions of2.7 m x 3 m. The reverberation time in theroom was 220 msec. Speech was presentedfrom a speaker located at 0° azimuth. Noisewas presented from three speaker positions:90°, 135°, and 180° on the side with the higherpure-tone average thresholds for each listener(PTA at 500, 1000, and 2000 Hz). All speakerswere positioned one meter from the center ofthe participant’s head at ear level.

The order of testing speaker positions,microphone types, and sentence lists wasrandomized between participants. For eachcondition, participants completed two lists ofsentences. The starting signal-to-noise ratiowas -5 dB for all conditions and allparticipants. The speech presentation levelwas automatically adapted up or downaccording to participant responses. For thefirst four sentences in each list, the step size

was 4 dB. For the remaining sentences,incorrect responses resulted in the speechpresentation level increasing in 2 dB steps,and correct responses resulted in the speechpresentation level decreasing in 2 dB steps.

The first part of the study involvedparticipants completing the HINT with theomnidirectional microphone and the fixeddirectional microphone. As the first sessionrequired all lists of sentences to be heard(two microphone conditions x three speakerpositions x two lists), a period of at least fourweeks elapsed between this session and laterevaluations involving the adaptive directionalmicrophone to minimize any learning effects.

The expected polar patterns for theadaptive directional microphone in the threenoise source conditions are bipolar (with nullsat 90° and 270°) for noise at 90°; supercardioid(with nulls at 135° and 225°) for noise at 135°;and cardioid (with one null at 180°) for noiseat 180°. Polar response patterns measured ina free field were in accord with theseexpectations (Fortune, 2004). When thehearing instruments are positioned on thehead, the polar pattern is significantly affectedby the acoustics of the head itself.Theoretically, there should be no differencebetween the fixed supercardioid microphoneand the ADM for noise at 135°. However, theADM may take up a different configurationif it is advantageous to do so, and it shouldnever perform worse than the fixed directionalmicrophone for noise from any angle. Theexperimental data confirmed this expectation.

Figure 2 shows that across each of thethree speaker locations, the best results(lowest SNR) were always achieved with theadaptive directional microphone. Repeatedmeasures analysis of variance showed thatthe differences in SNR between the threemicrophone configurations were highlysignificant (F [2, 56] = 93.2, p < 0.001) as werethe differences in scores between the threenoise locations (F [2, 56] = 16.8, p < 0.001).For all three microphones, the most difficultlistening situation (highest SNR) was fornoise presented directly behind the listener.In the other two conditions (noise from oneside at 90° or 135°), the head shadow effectprovided higher SNR at the ear opposite thenoise source. The most difficult listeningcondition, with noise from 180°, was alsothe condition in which the directionalmicrophones offered the largestimprovements in SNR.


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Paired t-tests were used to test thedifferences between the microphones in thethree noise conditions. These tests showedthat the fixed directional microphoneperformed significantly better than theomnidirectional microphone in every noisecondition (t = 10.15, p < 0.001, at 90°; t = 6.48,p < 0.001, at 135°; t = 8.57, p < 0.001, at180°), the adaptive directional microphoneperformed better than the omnidirectionalmicrophone in every noise condition (t = 4.7,p < 0.05, at 90°; t = 6.09, p < 0.001, at 135°;t = 8.04, p < 0.001, at 180°), and the adaptivemicrophone performed better than the fixeddirectional microphone in noise from 180° (t= 2.59, p < 0.05). Comparisons involving theADM should be considered cautiously becauseof the repetition of the HINT sentence lists.If the listeners remembered some sentencesor words from the first presentation, theymay have performed better with the ADM asa result.

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City University of New York (CUNY)Sentences (Boothroyd et al, 1985) were

also used to assess speech perception in quietand in noise. One hundred and seventy listsof sentences make up the CUNY speech test.

Each list contains 12 sentences having adifferent number of words (range from 3 to14). The sentences were recorded by a femaleAustralian speaker. Participants wererequired to repeat as many words in asentence as they could. Each list was scoredon the total number of words correctlyrepeated. The CUNY Sentences werepresented in four conditions: in quiet, in noisefrom 90°, in noise from 180°, and in movingnoise. Only the fixed and adaptive directionalmicrophones were assessed.

Speech was presented from a speakerlocated one meter away from the center of theparticipant’s head at ear level. The speakerwas located at 0° azimuth. In the quietcondition, participants completed two lists ofCUNY Sentences with the hearing aid infixed directional mode and two lists with thehearing aid in adaptive directional mode.Sentences were presented at 67 dB SPL.

In the noise conditions, eight-talkerbabble was introduced at an SNR determinedfor each individual so that they were capableof correctly repeating approximately 50% ofwords in the sentences when the speech andnoise were both presented from the front. Asfor the HINT, the speakers presenting thebabble were positioned one meter away fromthe listener on the side with their highestPTA. Each participant listened to two listswith the noise at 90° for each microphonecondition. Two lists were also presented for

FFiigguurree 22.. Mean HINT results for three microphones and three noise source locations.


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each microphone condition with the babblecoming from the speaker located at 180°. Inthe moving noise source condition, the eight-talker babble panned back and forth betweenthe speaker at 90° and the speaker at 180°,taking approximately three seconds to movefrom one speaker to another. Two lists ofCUNY Sentences were presented with thefixed directional microphone and two listswith the adaptive directional microphone inthe moving noise condition.

Figure 3 shows the means and standarddeviations for both microphones and eachnoise condition used with the CUNYSentences. In quiet, both microphonesproduced near perfect speech recognitionscores of 99% words correct on average. As forthe HINT test, the most difficult listeningcondition was for noise at 180°, and thiscondition also produced the largest differencein scores between the two microphones. Atwo-way repeated measures ANOVA showedthat the adaptive directional microphoneproduced significantly higher scores thanthe fixed directional microphone, (F [1, 53] =21.83, p < 0.001). Paired t-tests showed astatistically significant advantage for theadaptive directional microphone with noisefrom 180° (t = 4.54, p < 0.001) and withmoving noise (t = 2.76, p < 0.05). It should benoted that there were no repeated lists ofCUNY Sentences, and the pattern of resultsis similar to that obtained with the HINT.

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At the end of the second part of the study,the “Comparative Questionnaire” asked

each participant which microphone(omnidirectional or ADM) provided the bestlistening and understanding for 18 commonlistening situations encountered during theirfour-week trial. The 18 situations and theparticipants’ responses are summarized inTable 1. At the time that the participantsresponded to the “ComparativeQuestionnaire,” they did not know what thetechnical differences were between the twoprograms in the hearing aid and referred tothem only as “Program 1” and “Program 2.”In this situation, the most common responsebias is for listeners to use Program 1 becausethis is the program that is selected by defaultwhen the hearing aid is switched on. Program1 was the omnidirectional microphone, so thestrong preferences for the ADM are not dueto a response bias of this type. If participantshad not been exposed to a particular situation,they were able to select a “Not Applicable”option. If there was no perceptible differencebetween the two microphones, participantswere able to select a “No Difference” option.

A total of 144 responses were receivedfor the “Comparative Questionnaire.” Twelve“Not Applicable” responses were excludedfrom further analysis. Of the remaining 132responses, participants judged that there was

FFiigguurree 33.. Mean scores and standard deviations for CUNY Sentences in four conditions.


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“No Difference” between the omnidirectionalmicrophone and the adaptive microphone29% of the time (38 responses). They alsojudged that the omnidirectional microphonewas preferred 17% of the time (22 responses)and that the adaptive microphone waspreferred 54% of the time (72 responses). Thisdifference was highly significant (χ2 = 26.6, p< 0.0001) (see Figure 4).

There was only one situation (using thetelephone) where the omnidirectionalmicrophone was preferred by moreparticipants than the ADM. A special hearingaid program using a telecoil is commonly usedfor this situation. In all other situations in the“Comparative Questionnaire,” the ADM waspreferred by at least as many participants asthe omnidirectional microphone. As expected,the ADM was strongly preferred in noisysituations, such as a restaurant or grocerystore or when there were several peopleinvolved in a conversation. Also as expected,there were many “No Difference” responses forquiet situations where the ADM would haveadopted an omnidirectional response pattern.

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The Speech, Spatial and Qualities ofHearing Scale (SSQ; Gatehouse and

Noble, 2004) is composed of three scales thattogether cover a spectrum of potentiallistening situations involving speech, spatialaspects, and sound quality. The SSQ wasadministered in an interview format. Answerswere placed along a continuum from 0 to 10,with scores of “0” indicating the absence of anability or quality and scores of “10” indicatingperfect ability or quality. Across the scales, ahigh rating indicated fewer problems than alower rating. The questionnaire wasadministered twice, once for theomnidirectional microphone and once for theADM at the end of the study. The order ofmicrophone assessment was balanced acrossparticipants.

Responses from the SSQ were collated,and an average rating was obtained for eachparticipant for both microphone conditionsacross the speech, spatial, and quality

Table 1. “Comparative Questionnaire” Situations and Response Summary

No Not Situation Omni ADM Difference Applicable

At home, when someone is speaking to me from about 2–3 feet away 1 2 5 0

At home, when someone is speaking to me from across the room 0 4 4 0

At home, when someone is speaking to me from another room 2 2 3 1

At a restaurant, when sitting across from my spouse or friend 1 7 0 0

Several friends or family members talking around the dinner table 1 6 1 0

Talking with a friend or spouse outdoors 2 4 2 0

Talking with friends at a small social gathering 1 5 2 0

Listening to a priest/minister at church or speaker in a meeting (without the use of an FM system) 1 6 0 1

Conversation with my co-workers and supervisor at work 0 3 0 5

Conversation in a car 2 4 2 0

Talking with a particularly soft spoken person 2 4 2 0

A waitress at a restaurant 0 6 2 0

Cashier at the grocery store or department store 0 5 3 0

A child (6–10 years old) 0 3 2 3

Using the telephone 4 1 1 2

Listening to the news on the TV 1 3 4 0

Listening to the news on the radio 2 4 2 0

Detection of soft environmental sounds (i.e. microwave beeping, computer running, door bell, knocking at door) 2 3 3 0

Total 22 72 38 12


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categories (Figure 5). “Not Applicable”responses were excluded from the analysis.There were no significant differences in theoverall ratings for any of the three categoriesof ratings in the questionnaire.

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The primary purpose of this study was toevaluate the performance of a new

adaptive directional microphone inconjunction with the ADRO amplificationscheme. ADRO has already been shown toprovide improved speech perception in noisein hearing aids using omnidirectionalmicrophones and in cochlear implants(Blamey, 2005). The results of the presentstudy show that an additional improvementis achieved when ADRO is combined with asupercardioid microphone or an adaptivedirectional microphone when noise andspeech come from different directions. Theadvantage provided by the supercardioidmicrophone was 3.8 dB at 90°, 4.1 dB at 135°,and 5.5 dB at 180°. The advantage providedby the adaptive directional microphone was4.6 dB at 90°, 4.5 dB at 135°, and 7.3 dB at180°. Averaged over the three noise directions,the ADM provided 1.0 dB advantage overthe fixed supercardioid microphone. It ispossible that the relative advantage of theADM may be slightly overestimated in thepresent study because of the repetition ofthe HINT sentence lists.

The size of the advantages for the fixed andadaptive directional microphones are similarto other results reported in the literature. For

instance, Valente and Mispagel (2004) foundan advantage of between 4.5 and 7.2 dB for anadaptive microphone compared to anomnidirectional microphone depending onspeaker location. Wouters et al (1999) found a3.4 dB advantage for a directional microphoneover an omnidirectional microphone for noisefrom 90°. Powers and Hamacher (2004)reported an average of 1.5 dB advantage for anadaptive directional microphone over a fixeddirectional microphone. Fortune (2004) foundimprovements of up to 21% on the QuickSINtest in babble at 0 dB SNR comparing theADM to an omnidirectional microphone, andup to 9% improvement for the ADM over thefixed supercardioid microphone. The meandifference for CUNY Sentences in babblebetween the supercardioid microphone andthe ADM in the current study were 12.0% fornoise at 180° and 7.6% for moving noise ingood agreement with Fortune (2004).

Using ADRO and the adaptive directionalmicrophone, the participants were able toscore 50% correct on the HINT Sentences innoise at an average of -5.1 dB SNR. This issimilar to the performance of normallyhearing listeners on the HINT. The developersof the HINT reported average SNR results fornormally hearing listeners to be -2.92 dB(Nilsson et al, 1994). Bentler et al (2004)reported that normally hearing listenersachieved an average SNR on the HINT of-4.2 dB in stationary diffuse noise and -6.2 dBfor a moving noise source.

Noise source positioning influencesspeech perception even when the microphoneis omnidirectional. The phenomenon seen inthis study where speech intelligibility was

FFiigguurree 44.. The ADM was preferred three times as often as the omnidirectional microphone.


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always best when noise came from 90° can beexplained by the head shadow effect. Thehead acts as an acoustic barrier to noise fromthe side. Noise from the front or from 180° isable to reach both ears without beingattenuated and so has a greater maskingeffect than noise from other angles. A similarpattern of results has been reported in manyother studies.

The head shadow effect provides anadvantage to the listener when voice and noisecome from different directions. The bestsituation is when the noise comes from one sideat 90° and the speech comes from the oppositehemisphere. Thus, the listener should positionherself facing at right angles to the directionof the noise, and towards the talker. Facing thetalker will maximize the information fromboth lipreading and the acoustic speech signal.Note that if speech and noise are coming fromopposite directions, one of the worst listeningpositions is to be facing the talker with thelistener’s back to the noise. This advice isvalid for a bilateral hearing aid user regardlessof the type of microphone on the hearing aid.This is important because it means that alistener used to omnidirectional microphonesdoes not need to modify her behavior whenusing directional ones.

For all microphone types, noise from180° had a greater masking effect than 90°or 135°, but the effect was smallest for theADM. This means that the ADM is moreforgiving when the noise source is directlybehind the listener, and the listener does notneed to be so conscious of her body positionrelative to the noise and the speaker.

The adaptive directional microphoneused in this study differs from most othersavailable in commercial hearing aids. Mostdirectional microphones, whether they arefixed or adaptive, cannot adopt anomnidirectional response pattern. Hearingaids with these microphones usually requireat least two programs, one with anomnidirectional microphone, the second withthe directional microphone. The ADM in thisstudy is capable of moving from an omnimode to any directional mode provided theinput level exceeds the 65 dB SPL activationlevel. This means wearers of hearing aids,including those with manual dexterityproblems, benefit from the differentadvantages that omni and directionalmicrophones provide.

This advantage of the ADM is clearly

shown by the results of the comparativequestionnaire. Only 17% of responsesreflected situations where the omnidirectionalmicrophone was preferred to the ADM. Therewas a quite large proportion of responses(29%) where there was no difference. This wasexpected since the ADM adopted anomnidirectional response pattern identicalto the omnidirectional microphone wheneverthe total input level was below 65 dB SPL.Because of the predominance of conditions inwhich the ADM was preferred or there wasno difference (83%), it is highly recommendedthat the ADM should be the defaultmicrophone for the ADRO hearing aid. Thissuggestion is in accord with therecommendations of Walden et al (2004) whofound that “the vast majority of daily listeningtime is typically with the signal source infront of and relatively close to the listener, andwith the background noise source spatiallyseparated from the signal source”(p.393).Walden et al recommend an omnidirectionalmicrophone in quiet and a directionalmicrophone in most common noiseenvironments.

The only situations in which the ADM islikely to be disadvantageous compared to anomnidirectional microphone are on thetelephone or when the listener wants to hearloud sounds (greater than 65 dB SPL) attheir full volume, coming from behind. Oneexample is when listening to someone talkingfrom the back seat of the car. Theomnidirectional microphone (or a telecoil inthe case of the telephone) should be availablefor use on these occasions.

The ADM obviously changes the relativeloudness of sounds from different directions.ADRO also changes the relative loudness ofsounds in the environment to keep themaudible and comfortable. It is reasonable toask whether these changes affect listeners’abilities to make spatial and directionaljudgments about sounds in theirenvironment. The SSQ is an instrumentdesigned to answer these types of questions.There was no statistically significantdifference between the SSQ responses forthe omnidirectional microphone and theADM, indicating that the adaptive nature ofthe microphone did not produce anydiscernable difference in the subjects’ abilities.The overall level of the ratings in Figure 5indicates that the participants felt they hadgood (but not perfect) spatial and qualitative


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hearing capacities with both microphones.

CCOONNCCLLUUSSIIOONNSS

The adaptive directional microphone hasbeen shown to provide strong benefits in

the majority of listening situations. Speechperception performance in noise with theADM is comparable to performance oflisteners with normal hearing. Hearing aidusers are not required to switch programs toobtain the benefits of the ADM. Theaudiologist is not required to fit the ADMany differently from the omnidirectionalprogram. The benefits of the ADM areaccompanied by the comfort, audibility, andsound quality afforded by ADRO.

AAcckknnoowwlleeddggmmeennttss.. The authors wish to thank theparticipants in this study. The research was approvedby the Human Research and Ethics Committee of theRoyal Victorian Eye and Ear Hospital (Project03/529H). ADRO® is the registered trademark ofDynamic Hearing Pty Ltd.

RREEFFEERREENNCCEESS

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Documents

Improving Speech Intelligibility in Background Noise … · relatively low power consumption. The participants in the study were evaluated with speech in noise in a variety of conditions