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Auditory encoding of speech features: Effects of hearing impairment C315/A6: A Faulkner. Jan 30 2004 Readings: Auditory processing of Speech (Faulkner 2003 chapter) Chapter 16 (by Revoile) in Pickett “The acoustics of speech communication” 1999. 1 The role of audibility Psychoacoustic effects of sensori-neural (cochlear) hearing loss • Much of the impact of hearing loss is thought of in terms of AUDIBILITY • HOW much of the information in speech is audible? • Loss of sensitivity (most often at higher frequencies) – – – – Words recognised from simple sentences in quiet by aided hearing impaired adults as a function of average hearing loss at 0.5, 1 and 2 kHz. Circles are scores from individual subjects 2 (After Boothroyd, 1990) Raised thresholds Reduction of dynamic range Abnormal loudness growth LIMITS AUDIBILITY OF INFORMATION – Over frequency – Over intensity • Impaired frequency selectivity typical in frequency regions with significant sensitivity loss • Consider the audible area of frequency and intensity in relation to the range of frequencies and intensities in speech – Due to damaged OHCs – LIMITS ANALYSIS OF INFORMATION 3 Speech range re audibility The blue area represents the dynamic range of speech over frequency relative to the normal threshold of hearing. 4 Frequency importance weightings: AI I (2000 Hz) The red curves represent the range of audible levels over frequency for a typical moderate sloping hearing loss. Intelligibility is reduced when not all of the speech range is audible. W (2000 Hz) Audible portion can be increased with a hearing aid 5 A is the Articulation Index (predicted intelligibility). A is determined by adding up W x I over frequency bands, where I is the band importance weight and W is the proportion of a 30 dB dynamic range of speech 6 in that band that is audible. 1 AI predictions AI predicts intelligibility rather well for mild and moderate hearing losses. But not for severe and profound losses – here the effects of audibility are not enough to explain limits to speech recognition Caveat to audibility accounts • Good predictions of speech intelligibility from audibility hold only for mild to moderate hearing losses • Complete restoration of audibility with more severe losses cannot restore intelligibility • And these predictions only hold for speech in the quiet 7 8 Coding spectral shape • 1st key property of auditory coding – The cochlea acts like a bank of band-pass filters Impaired frequency selectivity – Each filter has a place (position on basilar membrane) – At basal places band-pass filters have high frequency pass-bands – At apical places band-pass filters have low frequency pass-bands 9 Auditory filtering in normal hearing 10 Auditory filtering in impaired hearing 35 dB loss at 2 kHz 11 12 2 Normal compared to impaired excitation patterns Impaired frequency selectivity • Broadened auditory filters lose detail in the auditory representation of the spectrum of speech leading to … – a reduced ability to resolve spectral detail (for example, where formants are). – lessened ability to separate speech from noise - reduced signal to noise ratio at filter output. • Because hearing loss typically affects higher frequencies, selectivity is also typically more impaired at higher frequencies Impaired excitation pattern – auditory place coding of spectrum retains much of the formant structure in quiet 13 14 Listening to speech with reduced frequency selectivity Normal compared to impaired excitation patterns Acoustic spectrum: Speech-to-Noise ratio is +6 dB – Noise fills valleys in the spectrum Both noise and reduced selectivity affect a range of speech tasks Normal excitation pattern retains much of formant structure in noise Impaired excitation pattern - shows reduced formant structure as noise spills into filters tuned to formant frequencies 15 16 Speech sound confusions threshold Threshold Discomfort Consonant place cues completely unavailable Frequent confusions of consonant place: Severe hearing loss 17 18 3 Effects of reduced spectral resolution Stimulus Consonant confusions with reduced spectral information Response (4 bands) l m u y q k v i a c f cY S e r R o s sR j l m u y 27 1 2 20 7 1 22 9 1 8 2 1 1 1 q k v i a 3 8 9 1 5 12 1 c 2 2 f cY e r R 1 10 11 4 27 S o s sR j 1 1 3 6 30 25 3 2 1 1 7 20 3 24 1 3 1 29 1 1 2 1 1 27 27 7 4 • Even though all frequencies are AUDIBLE loss of spectral resolution affects speech intelligibility • Primarily through loss of consonant place of articulation cues and vowel formant cues 3 5 15 1 23 1 1 29 2 19 1 1 8 1 30 3 1 7 20 18 Summary: 1 Summary: 2 • Three main consequences of cochlear hearing loss • Reduced audibility limits speech recognition – Reduced audibility: unable to hear the quieter parts of speech. – Loudness recruitment: dynamic range of speech exceeds dynamic range of hearing – Reduced frequency selectivity: spectral smearing of speech signal. 21 • Broadened auditory filters – Poorer resolution of spectral features of speech in excitation pattern – especially in noise – Fewer harmonics resolved, limiting the usefulness of temporal coding of frequency 22 4