24.910 Laboratory Phonology Basic Audition AuditionAuditionLoudnessrms amplituderms amplitudeIntensitylogarithmic scalesLoudnessLoudnessLoudnessPitchPitchMasking - simultaneousTime course of auditory nerve response Interactions between sequential soundsAuditory ‘spectrograms’24.910 Linguistic Phonetics Analog-to-digital conversion of speech signals Analog-to-digital conversionSamplingSamplingWhat sampling rate should you use?What sampling rate should you use?AliasingQuantizationQuantizationQuantizationVoicing and aspirationVoicing and aspirationVoice Onset TimeVOT, closure voicingVOT, closure voicingMIT OpenCourseWare http://ocw.mit.edu24.910 Topics in Linguistic Theory: Laboratory PhonologySpring 2007 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.24.910Laboratory PhonologyBasic Audition• No class next week (Tuesday is a Monday)• Readings for 2/27: Johnson chs 5 & 6• Assignments (due 2/27):– Basic acoustics.– VOT and laryngeal contrasts in Mandarin and English.AuditionAnatomyEustachian TubeOuter EarMiddleEarInner EarAuditoryNerveCochleaEardrumEar CanalEar FlapHammerAnvilStirrupFigure by MIT OpenCourseWare.Audition• Loudness•Pitch• ‘Auditory spectrograms’Loudness• The perceived loudness of a sound depends on the amplitude of the pressure fluctuations in the sound wave.• Amplitude is usually measured in terms of root-mean-square (rms amplitude):– The square root of the mean of the squared amplitude over some time window.• Square each sample in the analysis window.• Calculate the mean value of the squared waveform: – Sum the values of the samples and divide by the number of samples.• Take the square root of the mean.rms amplitude-1.5-1-0.500.511.50 0.05 0.1 0.15 0.2timepressurepressure^2rms amplituderms amplitude.01 .020Time in secondsFigure by MIT OpenCourseWare. Adapted from Johnson, Keith. Acoustic and Auditory Phonetics. Malden, MA: Blackwell Publishers, 1997. ISBN: 9780631188483.Intensity• Perceived loudness is more closely related to intensity (power per unit area), which is proportional to the square of the amplitude.• relative intensity in Bels = log10(x2/r2)• relative intensity in dB = 10 log10(x2/r2)= 20 log10(x/r) • In absolute intensity measurements, the comparison amplitude is usually 20μPa, the lowest audible pressure fluctuation of a 1000 Hz tone (dB SPL).logarithmic scales• log xn= n log x00.20.40.60.811.21.41.61.80 1020304050xLoudness• The relationship between intensity and perceived loudness is not exactly logarithmic.500,000 1,000,000 1,500,000 2,000,0000010203040506070809010002468101214161820Pressure (µPa)dB SPLSonesdB SPLSonesFigure by MIT OpenCourseWare. Adapted from Johnson, Keith. Acoustic and Auditory Phonetics. Malden, MA: Blackwell Publishers, 1997. ISBN: 9780631188483.Loudness• Loudness also depends on frequency.• equal loudness contours for pure tones:Source: Wikimedia Commons.Loudness• At short durations, loudness also depends on duration.• Temporal integration: loudness depends on energy in the signal, integrated over a time window.• Duration of integration is often said to be about 200ms, i.e. relevant to the perceived loudness of vowels.Pitch• Perceived pitch is approximately linear with respect to frequency from 100-1000 Hz, between 1000-10,000 Hz the relationship is approximately logarithmic.0048121620241 2 3 4 5 6 7 8 9 10Frequency (kHz)Frequency (Bark)Figure by MIT OpenCourseWare. Adapted from Johnson, Keith. Acoustic and Auditory Phonetics. Malden, MA: Blackwell Publishers, 1997. ISBN: 9780631188483.Pitch• The non-linear frequency response of the auditory system is related to the physical structure of the basilar membrane.• basilar membrane ‘uncoiled’:Figure by MIT OpenCourseWare. Adapted from Johnson, Keith. Acoustic and Auditory Phonetics. Malden, MA: Blackwell Publishers, 1997. ISBN: 9780631188483. 1003007505001,1002,0003,7008,700 4,900 2,7006,5001,50011,50015,400Masking - simultaneous• Energy at one frequency can reduce audibility of simultaneous energy at another frequency (masking).• One sound can also mask a preceding or following sound.40003600320026002400200016001200Frequency of masked toneMasking1000800600400101102103104Example of masking of a tone by a tone. The frequency of the masking tone is 1200 Hz. Each curve corresponds to adifferent masker level, and gives the amount by which the threshold intensity of the masked tone is multiplied in thepresence of the masker, relative to its threshold in quiet. The dashed lines near 1200 Hz and its harmonics are estimatesof the masking functions in the absence of the effect of beats. Figure by MIT OpenCourseWare. Adapted from Stevens, Kenneth N. Acoustic Phonetics. Cambridge, MA: MIT Press, 1999. ISBN; 9780262194044.Time course of auditory nerve response Response to a noise burst:• Strong initial response• Rapid adaptation (~5 ms)• Slow adaptation (>100ms)• After tone offset, firing rate only gradually returns to spontaneous level.0128-6025600 64 128128-40256msecFigure by MIT OpenCourseWare. Adapted from Kiang et al. (1965)Interactions between sequential sounds• A preceding sound can affect the auditory nerve response to a following tone (Delgutte 1980).0 200 200 2002004000600400MATNO ATTT = 27 dB SPLDischarge rate (SP/S)AT = 13 dB SPL AT = 37 dB SPLTT0 400M0 400MFigure by MIT OpenCourseWare. Adapted from Stevens, Kenneth N. Acoustic Phonetics. Cambridge, MA: MIT Press, 1999. ISBN: 9780262194044, after Delgutte, B. "Representation of Speech-like Sounds in the Discharge Patterns of Auditory-nerve Fibers." Journal of the Acoustical Society of America 68, no. 3 (1980): 843-857.Auditory ‘spectrograms’The auditory system performs a running frequency analysis of acoustic signals - cf. spectrogram.• A regular spectrogram analyzes frequency of equal widths, but the peripheral auditory system analyzes frequency bands that are wider at higher frequencies.• Further disparities are introduced by the non-linearities of the peripheral auditory system, e.g.– loudness is non-linearly related to intensity– masking(simultaneous and nonsimultaneous)26242220181614121010203050406070808642Number of ERBs, EExcitation Level, dBVowel /I/The spectrum of a synthetic vowel /I/ (top) plotted on a linear frequency scale,and the excitation patterns for that vowel (bottom) for two overall levels, 50 and80 dB. The excitation patterns are plotted on an ERB