Hearing and Deafness 2. Ear as a analyzer Chris Darwin
Frequency: -Hz Sine Wave. Spectrum Amplitude against -..5 Time (s) Waveform Amplitude against time amp Hz
Frequency: 5-Hz Sine Wave. Spectrum Amplitude against -. Time.5 (s) Waveform Amplitude against time amp 5 5
Amplitude: 5-Hz Sine Wave Spectrum Amplitude against Time.5 (s) amp 5 5 Time (s).5
Phase: 5-Hz Sine Wave.9899 sine The amplitude spectrum does not show phase.99. Time (s).99 cosine amp 5 5.99. Time (s)
Phase Locking of Inner Hair Cells Auditory nerve connected to inner hair cell tends to fire at the same phase of the stimulating waveform.
Phase-locking pressure.5.2.4.6.8 -.5 - Response to Low Frequency tones Inter-spike Intervals 2 periods period voltage nerve spike time (t) Response to High Frequency tones > 5kHz voltage Random intervals time (t)
adding sine waves Spectrum of Sum Sine wave Sum amp amp.25.5.25.5 Time (s) Time (s)
-Hz fundamental Complex Wave 5. Spectrum Amplitude against -.7.5 Time (s) Waveform Amplitude against time amp 5 5
Adding nine sine waves Frequency Spectrogram Time 5s Frequency Time 5s
The linear vs log scales Linear equal distances represent equal differences 2 3 4 5 Log equal distances represent equal ratios 2 4 8 6 32-2 3 4 e.g. Piano keyboard frequencies Octave = doubling of basilar membrane has log repn of
decibel (db) scale Sound A is x db more intense than sound B when: x = *log (energy of A / energy of B) or x = 2*log (amp of A / amp of B) So if A is 2 watts and B is watts x = *log (2/) = *.3 = 3dB You can usually just hear a difference of db (jnd)
Bandpass filtering (narrow) 5. amp -.7.5 Time (s) 5 5 amp Time.5 (s) 5 5
Bandpass filtering (wide) 5. amp -.7.5 Time (s) 5 5.975 amp.975.5 Time (s) 5
Beats Repetition rate is the difference in between the two sine-wave components /3 second 55-5 = 5 Hz amp 5
Beats Repetition rate is the difference in between the two sine-wave components /th second 5-4 = Hz.975 amp.975.5 Time (s) 4 5
Reponse of basilar membrane to sine waves Each point on the membrane acts like bandpass filter tuned to a different : high freq at base, low at apex. Each point vibrates at of pure tone (-> phase locking)
Excitation patterns (envelope of excitation) amplitude of vibration high frequencies base low frequencies apex distance along basilar membrane Basilar membrane excitation pattern is like a spectrum
Excitation pattern of complex tone on bm unresolved 6 8 resolved 6 4 2 25. b m vibration 2. 5.. 5. base log (ish) apex. -5. Output of 6 Hz filter /2s = 5ms 2.5.5 -.5.2.4.6.8 - -.5-2 Output of 2 Hz filter /2s = 5ms.8.6.4.2 -.2.2.4.6.8 -.4 -.6 -.8 -
Measurement of auditory bandwidth with band-limited noise Hz 25 Hz Broadband Noise 2 Hz Amadeus
A gardening analogy
A gardening analogy
A gardening analogy Tone Noise Auditory bandwidth Noise bandwidth Detection mechanism
Wider auditory filter
Psychophysical tuning curves Masker level at threshold (db SPL) 8 6 4 2 Target Bandwidth 6 8 2 Masker center
Human auditory bandwidth Auditory bandwidth (Hz) 8 6 4 2 2 4 6 8 Center Frequency (Hz) At khz the bandwidth is about 3 Hz; at 5 khz the bandwidth is about 65 Hz. BW = freq / 8 roughly
Auditory tuning curves Normal bandwidth log amplitude (db SPL) 8 6 4 2 Inner hair-cell damage Healthy ear Normal Threshold Abnormal Threshold Normal bandwidth Characteristic Frequency log
Outer-hair cell damage Abnormal bandwidth log amplitude (db SPL) 8 6 4 2 Normal Threshold Abnormal Threshold Normal bandwidth Characteristic Frequency log
Normal auditory non-linearities Normal loudness growth (follows Weber s Law, which is logarithmic, not linear) Combination tones Two-tone suppression Oto-acoustic emissions
Combinations Tones (Tartini tones) amp amp 8 2
Two-tone suppression log amplitude (db SPL) 8 6 4 Regions for two-tone suppression Test tone at Characteristic Frequency 2 log
Conductive vs Sensori-neural deafness Mostly a combination of OHC and IHC damage Conductive Sensori-neural Sensori-neural Origin Middle-ear Cochlea (IHCs) Cochlea (OHCs) Thresholds Raised Raised Raised Filter bandwidths Normal Normal Increased Loudness growth Normal Normal Increased (Recruitment Becomes linear, so No combination tones Or two-tone suppression
Symptoms of SNHL Raised thresholds: helped by amplification Wider bandwidths: no help possible Recruitment (restricted dynamic range): partly helped by automatic gain controls in modern digital aids Often accompanied by tinnitus
Normal vs Impaired Dynamic Range
Auditory filter bandwidth (ERB)