Improving audibility as a foundation for better speech understanding

1. Improving audibility as a foundation for better speech understanding Pamela Souza, PhD Northwestern University Evanston, IL

2. Outline • The talker: producing clear speech • The listener: effects of hearing loss • Speech audibility • Distance, noise and echoes • Improving audibility: hearing aids and cochlear implants

3. Communication: “send and receive”

4. Speech varies rapidly in time, frequency (pitch), and intensity (loudness)

5. Speech is redundant – we don’t need to hear every sound. But the more information is received, the less effort is needed to listen High redundancy: quiet, visual cues, high context Low redundancy: noisy, no visual cues, low context

6. Audibility is determined by the level of the speech, the level of any noise, and the listener’s audiogram Sensitive hearing Mild Moderate Severe

7. Understanding the audiogram

8. Pink line shows upside-down audiogram (high-frequency loss) Louder Louder Soft speech, no hearing aid, 29% audible Audibility is higher when the talker’s voice is louder (or closer) Loud speech, no hearing aid, 57% audible

9. Effects of distance and position The further the talker is from the listener, the lower audibility will be

10. Effects of background noise

11. Effects of background noise • Energetic masking: reduces audibility when noise overlaps in pitch and timing with the speech, blocking the speech from being heard • Informational masking: when noise does not overlap with speech energy, but causes confusion or draws attention away from the talker • More effort is required to listen in noise

12. Effects of reverberation (echoes)

13. Effects of reverberation • Sound reaches the listener directly • A delayed version of the same sound reaches the listener after reflecting from a hard surface • Overlap masking: the delayed sound is still present and energetically masks sounds that follow • More effort is required to listen in reverberation

14. Improving audibility • Improving signal • Reducing distance • Eliminating noise and reverberation • Hearing aids and cochlear implants • Assistive listening devices with remote microphones

15. What does the hearing aid do to improve audibility? • Customized frequency-gain response • Multichannel compression • Frequency lowering • Noise reduction • Suppressing reverberation (echoes)

16. Frequency-gain response • Provides more amplification (“gain”) at frequencies where there is more hearing loss • Like a mirror of the audiogram • Different mathematical formulas or “prescriptions” (such as DSLand NAL) are used to determine desired response

17. Improving audibility: frequency-gain response Audiogram shows high-frequency hearing loss Speech audibility is improved in high frequencies Hearing aid gain should be greater in high frequencies

18. Without hearing aid With linear hearing aid Soft speech 29% audible Louder Soft speech 51% audible Linear aids improve audibility, but may cause loudness discomfort Too loud Loud speech 79% audible Loud speech 57% audible

19. Multichannel compression • Within each frequency band, soft sounds are amplified more than loud sounds • Reduces the dynamic range from soft to loud • Soft sounds should be made more audible without making loud sounds too loud • But –more extreme compression parameters may distort speech

20. Without hearing aid With compression hearing aid Soft speech 29% audible Louder Soft speech 59% audible Compression hearing aids improve audibility and loudness comfort (without volume adjustments) Loud speech 79% audible Loud speech 57% audible

21. Audible bandwidth and frequency lowering amplification • Listeners with sensitive hearing receive sounds to 8000 Hz or higher; most listeners wearing hearing aids receive sounds up to 3000-5000 Hz • A wider bandwidth may improve speech understanding • A wider bandwidth may make it easier for children to learn new information • A wider bandwidth supports speech production Gustafson & Pittman, 2011; Pittman, 2008; Stelmachowicz et al., 2004;

22. Frequency lowering hearing aids • Used to improve high-frequency sound audibility • High-frequency sounds are shifted to a lower frequency range • May be more beneficial for children than adults (due to adults’ greater linguistic experience) • Should be used selectively (more studies are in progress) Souza et al. 2013; McCreery et al., 2014; Bentler et al., 2014

23. Hearing aid noise reduction: directional microphones • Apply less gain to noise that is from a different location than the talker • Improve the relative levels of the talker (“signal”) and the background (“noise”) • Can improve speech audibility (and understanding) if signal and noise are spatially separated

24. Hearing aid noise reduction: digital noise reduction • Attempts to determine what is “noise” and what is “speech” based on their sound patterns • Mathematically removes the pattern of the noise • May not improve speech understanding, but can reduce listening effort and improve listening comfort

25. Assistive listening devices • Overcome distance by placing the microphone close to the talker’s lips and transmitting that signal to the listener’s hearing aid • Suppress noise and reverberation by transmitting the “clean” signal direct from the talker • Can be used anywhere distance or noise is an issue: classroom, automobile, restaurant

26. How much audibility is enough? • When background noise limits audible speech, adult listeners with sensitive hearing need about 50% audibility to understand 80% of sentences Adults with sensitive hearing are similar to each other – if speech is at least 60% audible, it is usually understood Results from Souza, Boike, Witherell, Tremblay, 2007

27. How much audibility is enough? • When background noise limits audible speech, adult listeners with hearing loss need about 80% audibility to understand 80% of sentences With hearing loss, good audibility does not always mean good speech understanding Results from Souza, Boike, Witherell, Tremblay, 2007

28. Children need greater audibility than adults Adults Age 9-12 years Better understanding Age 5-8 years When children and adults are getting the same amount of audible information, children have more difficulty understanding speech than adults do Better audibility McCreery & Stelmachowicz, 2011

29. Audibility for children • Child-centric hearing aid fitting procedures emphasize audibility • Child audibility needs are assumed to be different from adults • Audibility may be addressed differently by pediatric audiologists than by adult audiologists

30. Using cognitive ability to “fill in” inaudible information • We unconsciously use memory and knowledge to extract meaningful information from a partially audible signal • Adults are good at using context; children have more difficulty (due to limited linguistic experience) • This process uses cognitive resources

31. Summary • Talkers should be close to the listener, visible, and producing clear speech • Reduce background noise! • Hearing aids (with appropriate settings and features) • Assistive listening devices for more difficult (noisy or distant) situations • Conversation is a two-part experience

32. Thank you Email contact: p-souza@northwestern.edu Research web site: halab.northwestern.edu