Travel safety issues related to digital hearing aids

Assessment of the effect of compression in sound localization and distance evaluation tasks by blindfolded hearing impaired adults

Agathe Ratelle¹, Julie Dufour², Tony Leroux^2,3 & Martine Gendron²

• ¹ Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal (CRIR), Institut Nazareth et Louis-Braille;
• ² Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal (CRIR), Institut Raymond-Dewar;
• ³ School of Speech-Language Pathology and Audiology, Université de Montréal
• Montréal (Québec), CANADA

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• {Logo du CRIR – Centre de recherche interdisciplinaire en réadaptation du Montréal métropolitain}
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• {Logo de l’Institut Raymond-Dewar Centre de réadaptation spécialisé en surdité et en communication }

Collaborators

• Chantal Laroche & Christian Giguère, Université d’Ottawa
• Elisabeth Têtu, Groupe Forget-Parent, Sherbrooke
• Manon Trudel & Rachel Dion, Institut Raymond- Dewar
• Annie Marcil, Phonak Canada,
• Renée Sirois, Institut Nazareth et Louis-Braille
• Danielle Cloutier, IRDPQ, Québec

Problem

• Visually and hearing impaired people have specific auditory amplification needs:
  • Communication: need to reduce noise located in low frequencies, because of its masking effect on speech components. Emphasis on speech spectrum frequencies (middle and high).
  • Mobility: meaningful traffic-related sounds are dominated by low frequencies. Sound localization cues are more precise at both ends of the spectrum.
  • Thus Dilemma for amplification
• Programmable and digital hearing aids could help meet the multiple needs. However, these aids also have signal processing algorithms that could alter sound localization and distance evaluation.

Review of the literature

• Auditory distance evaluation cues:
  • Reverberation, sound pressure level, spectrum, movement
  • Negative effect of compression in lab settings
• Sound localization & hearing aids:
  • Binaural hearing aid fitting: best performance
  • Type of molds: not conclusive
  • Training could improve sound localization
  • “Directional microphones” were designed to improve speech perception but not sound localization

Travel safety issues related to new technologica hearing aids: lack of studies

Research

• Research question: What is the impact of new technologies used in hearing aids on travel safety of visually and hearing impaired people?
• Purpose of this study: To assess the effect of compression on sound localization and distance evaluation tasks by blindfolded hearing impaired adults. (Funded by REPAR)

Participants

• 6 adults
• Sensorineural hearing loss (ranging from 25 to 80 dBHL)
• Bilateral & symmetrical (within 15 dBHL)
• Experience with hearing aids (min 1 year)
• No visual impairment

Methods

{Illustration}

• 1 – Subject selection
  • 2 – EAR canal impression
    • 3 – Double-blind hearing aid fitting and programming
      • 4 – 30 day adaptation period
        • 5a – Phase 1-Measurement sela and test site
          • Return in the stage 3
        • 5b – Phase 2-Measurement test site and sela

Hearing aids: Phonak Aero211dAZ

Program 1:  best fitting for communication (always the same)

Program 2:  linear or Wide Dynamic Range Compression (different in each phase)

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Experimental sites

• Laboratory setting: sound booth equipped with SELA (Auditory localization evaluation system)
• Field measurements: reproduction of travel situations on closed-circuit track

Laboratory procedures

• SELA evaluation
  Localization of a short (250 ms) and a long (1500 ms) duration broad band noise of 65 dBA in 3 positions (frontal, left & right lateral) while the participant is seated; pointing tool (head-mounted laser)

{Illustration}

Schéma qui représente l’installation du SÉLA qui est composé de 11 hauts-parleurs disposés en demi-cercle dont le centre est la position du sujet évalué. Le SÉLA est opéré par un PC utilisant une carte de son standard et contrôlant la sélection des haut-parleurs dans lesquels est émis le son après avoir été amplifié via un amplificateur externe.

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Field measurements procedures

• Detection of approaching vehicle from all approaches (measured with radar)
• Distance judgment of a perpendicular car while stationary (car passing lane 1 up to 4) and moving (subject stop before entering lane of the passing car, lane 1 or 2)
• Alignment with a parallel moving vehicle (signed error measured with a compass waist mounted)

{Illustration}

Photo du site extérieur où les mesures de terrain ont été recueillies pour cette étude.

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Results

• Because of varying wind conditions, group data could not be analysed. Individual data were examined and only large fitting strategies differences were retained.
• Distance judgment tasks
  • stationary condition : with WDRC, majority of subjects tend to underestimate a car in the farther lane (lane 4) while perception of distance was generally accurate when the car was in the nearest lane (lane 1).
  • moving condition : no program effect ; most subjects tend to underestimate.

{Illustration}

Diagramme à moustache « Vehicle approaching subject’s left side » représentant les résultats obtenus pour la distance de détection d’un véhicule s’approchant à gauche pour chacun des sujets selon la stratégie d’appareillage utilisée (linéaire ou WDRC).

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{Illustration}

Diagramme à moustache « Vehicle approaching subject’s right side »représentant les résultats obtenus pour la distance de détection d’un véhicule s’approchant à droite pour chacun des sujets selon la stratégie d’appareillage utilisée (linéaire ou WDRC).

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• Distance of detection : one subject showed a capability of detection larger than 200 m with the linear strategy. All the other subjects showed detection distance of about 60 m regardless of the fitting strategy.

• Localization and alignment tasks: no difference between the two fitting strategies is noted except for one subject that showed a substantial number of front-back confusions with WDRC.

{Illustration}

Diagramme à bandes représentant les résultats obtenus pour les tâches de localisation et d’alignement selon la stratégie d’appareillage utilisée (linéaire ou WDRC).

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Discussion and limits

• Constraints for participants: lenght of study (3 months), data collection is time consuming, changing schedule (climatic conditions), adaptation to new hearing aids
• Closed-circuit track is not representative of urban situation: no reverberation, open area, windy (uncontrollable variable)
• Alignment task: difficult to perform by hearing impaired blindfolded participants with the short training received
• Results seem indicate a possible adaptation with WDRC
• No travel problems was noted in daily report (with vision)
• The perceived distance evaluation was limited: Ex. the person’s capability to track a moving sound could not be evaluated

Conclusion

• Even if a general adverse effect of compression was not demonstrated in this study, potential risk was not ruled out.
• Further research is needed
• methods have to be found to measure the perceived distance including both ecological and laboratory measurements.
• Clinical approach should consider careful use of compression.