BasicsofHearingScience.pdf CSCD 4301 001
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This 6 page Study Guide was uploaded by Jillian Ehrlich on Tuesday September 15, 2015. The Study Guide belongs to CSCD 4301 001 at Temple University taught by Professor Mancano in Summer 2015. Since its upload, it has received 138 views. For similar materials see Principles of Audiology in Language at Temple University.
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Date Created: 09/15/15
Psychoacoustics Absolute Sensitivity Psychophysical Methods Relating a physical stimulus to a psychological percept A Psychophysics a The scientific study of the relation between physical events and sensations b Psychoacoustics the area of psychophysics that focuses on the perception of sound B Psychophysical Laws a Physical quantity vs perception i Intensity vs loudness ii Frequency vs pitch iii Sensitivitydetectability vs response proclivity b These are the differences between what you hear and what you say you hearthink you hear C Concept of Threshold a The smallest stimulus that can be perceived b Limen absolute threshold or absolute limen vs difference threshold or difference limen i Absolute thresholdlimen smallest stimulus a person can detect ii Difference thresholdlimen smallest difference that a person can detect between two stimuli D Psychometric function a For each level the proportion of times that the subject correctly heard the sound is determined b The plot of the proportion of correct detections as a function of level is what the psychometric function is c A psychometric function for frequency discrimination would show the proportion of times the listener could tell that the frequency of a tone for example had changed as a function of the size of the frequency changethe function would look similar in form d Important point about psychometric function there is NO common sense threshold there is no level above which the listener always hears the sound and below which the listener never hears the soundinstead it s a gradual improvement in detection with level e In this instance we define threshold as the level at which the listener achieves some arbitrary proportion of correct detections E Absolute Sensitivity shows stimulus detected in percentage over stimulus magnitude a Region below threshold threshold region and region above threshold are all labeled F Listener Bias a On a psychometric function we can see listener bias on the traditional measure of performancepercentage of correct responses PC b The absolute threshold is defined as 50 classically and although the sensitivity of two listeners may be equal identical twins for example one person may be more inclined to respond yes on any given trial and other person may be more inclined to say no to hearing stimulus c This bias could show a shift due to response bias that makes the threshold of the first listener appear to be 5 dB better that for listener 2 G Reality of threshold a The stimulus that produces an arbitrary but defined level of performance b Threshold the lowest stimulus value to which the listener responds 50 of the time this is the definition used in psychophysics even though reality doesn t exactly match common sense or our intuition of what threshold is c Threshold is influenced by many factors other than strictly sensitivity vi Response bias listener s tendency to respond yesno Attention memory 1 Psychometric function of this type of person would show that person never gets 100 correcta sign of either inattentiveness or forgetting 2 Need to see whole function rather than just threshold Motivation small effect on threshold though Ex people say yes on trials when no sound is present not because they re lying but because they really think there was a sound Sounds close to threshold are hard to tell whether there was a sound Response bias is biggest factor but memoryattention are problems in some populations like young kids they simply can t pay attention or forget that they re supposed to respond d Response bias people use the evidence their ears and brain provide but whether they say quotyesquot or quotnoquot depends on how much they like to say quotyesquot and other things they know about the situation ex if they know sounds are presented on 90 of trials H Psychological methods used to estimate thresholds a Method of constant stimuli I ii iii iv vi Each trial one of the designated levels is presented Listener is asked whether they heard the sound Levels are presented in random order each many times Another way of this method give listener many trials at one level than many trials at another level etcunti all levels were given in random order Psychometric function PF generated based on percent of time the stimulus is IDed correctly Threshold is predetermined point in psychometric function 50 or 75 usually b Method of limits The experimenter has control of the stimulus and the subject responds after each trial The point between the yesno responses is known as transition Threshold is measured as the average stimulus level of the transitions First series could start at a level listener can t hear and then level could keep increasing until listener hears soundsecond series could start above expect threshold of listener and then decrease in level until listener can no longer hear sound Staircase or updown method is a variation of this method of limits the level is changed on a trial by trial basis according to listener s response The transition C points reversals are averaged to obtain threshold This is a very EFFICIENT method for estimating threshold 1 Gives you the level that would produce 50 correct detections Method of adjustment i Similar to method of limits except that stimulus is constantly varied ii The listener controls the stimulus and adjusts the level until unable to hear the stimulus iii Examples Bekesy tracking and automated test programs I Problems with Classical Methods a Method of limits and adjustment susceptible to yes aggressive or no passive bias b Method of constant stimuli can identify yes bias by using catch trials but is still susceptible to no bias J Theory of Signal Detection TSD a Three things can affect our ability to detect a signal i External noise internal noise and internal response b Using signal detection theory can control for response bias C ranhrb TSD provides a measure of detectability d that is free from response bias i D measure of strength of a signal relative to noise ii Response bias individual influence on response Listener is presented with sound they have two optionsresponse yes if sound was detected or no if it was not i If they say yes to signal when it IS presented HIT ii Say no when signal IS presented MISS one type of error that can occur iii Sometimes there are catch trials no signal but listener may also respond 1 If listener does not respond to sound correct rejection Z Listener response to sound when there is no sound false alarm another type of error Whenever the hits exceed false alarms listener can theoretically detect the signal Threshold is the lowest intensity or level where the hits exceed false alarms When hits occur as often as false alarms infer that person is guessing Computing d i High intensity stim stands out clearly from the noised is high ii Low intensity stim noise only and then signalnoise are closer togetheroverlap between signal and noise occursd is low and more errors are likely liberal bias in signal detection many hits and many false alarms willing to take guess i Could be that listener is dishonest about what they re hearing trying to prove they have normal hearing or says yes even when not really sure conservative bias in signal detection few hits and more misses doesn t want to guess and wants to be sure the sound is there before responding i Could be that listener is dishonest about what they re hearing trying to fake a hearing loss or doesn t say yes unless absolutely sure K Minimal Audible Curve a Graph shows auditory response are and threshold of feeling sound level dB over frequency minimal audible curve is the lowest dB level that is responded to along various frequencies Frequency Limits a There will be very lowhigh frequencies to which the auditory system is insensitive no matter how intense the pure tone b Threshold lowest intensity required for detection of given tone varied by frequency delivery system and performance level c High sensitivity means the SAME thing as low threshold Threshold Curve Shape a Loss of sensitivity below 1000Hz and above 4000 Hz 6dBoctave rollbelow 1000 and 24 dBoctave rolloff above 4kHz b Probably related to loss in middle ear pressure gain at low and high frequencies explain this phenomena Minimal Audible Pressure MAP a Thresholds in sound pressure level at the observer s tympanic membrane b Sounds presented via earphones c Calibrated by a test coupler attached to the earphone during calibration Minimal Audible Pressure MAP vs Minimal Audible Field MAF a MAF looks better than others by 6 dB i bc uses binaural hearing b When estimates of MAP and MAF data are corrected by head diffraction and outer ear resonances and the type of calibration procedure used the greatest difference is reduce to 25 dB c MAF subject sits in sound field and stimulus presented by loudspeakers sound level then measured at position of subjects head without subject in sound field d MAP headphones or earphones and then measuring sound pressure in subject s ear canal using small probe microphone i Worse threshold bc doesn t use binaural hearing and there are physiological noises heard when ear is covered by headphonehear body noises heartbeat etc that may have masking effects e Upper Limits have been found via several perceptual tasks people said they felt pain tickle or uncomfortable i For 120130 dB SPL relatively frequency dependentdynamic range then narrows at both ends Psychoacoustics Absolute Sensitivity and Differential Sensitivity F Detection Range a 2020000 Hz b Below 20 Hz feel vibration but not tonal INFRASOUND c Above 20000 Hz cannot hear ULTRASOUND d 0 dB20 micropascals tympanic membrane moving the width of a hydrogen atom Frequency responses of the ear a Thresholds obtained using headphones or speaker in a sound field b Pure tones calibrated in dB SPL with a reference of 20 micropascals so 0dB SPL output20uPa c Normal hearing sensitivity varies as a function of frequency at threshold d INTENSE sound perceived as louddoesn t vary as function of frequency e Converting SPL to RETSPL reference equivalent threshold sound pressure level References a Sound pressure level 20uPa b Hearing level HL threshold values for young ontologically normal adults c RETSPL calibrated values d Sensation Level SL an individual s own threshold for speech or tones Audiogram a 0 dB HL on an audiogram refers to the SPL needed for a young ontologically normal adult YONA to just hear it b 0 dB HL at 2k Hz is about 3 dB SPL Anything above about 20 dB HL is beyond the range of normal sensitivity a threshold for 30 dB is in the range of a mild hearing loss Differential Sensitivity frequency intensity and time gaps duration a Differential sensitivity discrimination the smallest change in a stimulus to which the listener responds a certain percentage of time difference limen b Difference limen JND just noticeable difference i Can look at absolute difference change in Intensity or change in Freq ii Or can look at relative difference change in VI or change in FF iii Either waylooking for a constant iv Change in intensity in dB is logarithmic v Intensity difference limen smallest noticeable difference in intensity between two sounds vi Frequency difference limen small perceptible freq difference between two sounds Weber s Law a The just discriminable difference JND in a stimulus is a constant proportion of the stimulus value b Issue in psychophysics does this law hold true for various situations Comparison of how many candles must be added to an original amount present to produce a differentnoticeable brightness d Through this law many important concepts about intensity were learned e 3 methods of Difference Limen for intensity modulation of loudness masking of loudness or memory of loudness G Intensity Discrimination a The auditory system is sensitive to approximately a 051dB change over a broad range of frequenciescan hear difference between 90 and 91 dB b It s not clear that weber s law holds for pure tones At low levels the JND is really a measure of absolute threshold which tone did I hear JND is bigat midlevels it s pretty constant and at high levels it gets better H Near Miss to Weber s Law a Refers to the fact that intensity discrimination is a little better at high intensities than Weber s law predicts b As in weight discrimination JND is pretty constant once intensity gets above threshold but at higher frequencies we get better Frequency Discrimination a At low frequencies as small as 1 Hz b The value of threshold increases as frequency increases above 1000 Hz c Code people use to figure out what the frequency of a pure tone is Rate place code and temporal codeposition on basilar membrane and time d Weber s Law gets worse at high frequencies i In the mid ranges the Weber fraction is pretty much constant at higher and lower frequencies it is worse ii Weber s law doesn t usually hold for quotqualityquot dimensions like pitch but does for quotquantityquot dimensions iii Explanation for difference between lowish freqs and high freqs the code that is used to present the freqs in these freq ranges iv Phaselocking could be used to represent a low freq deteriorates above 1000 Hz but only the place code cn code high freqs over 5000Hz J Difference Limen Conclusions a Intensity close to Weber s Law quotnear miss b Frequency relative frequency changes increases with increasing frequency K Loudness and Pitch a Scaling Comparisons when listener judges the magnitude of a stimulus compared to the magnitude of sensation i Frequency and intensity b Phons how loud is this sound compared to another i At 40 dB signal at 1k Hz 40 phon ii Loudness level phons iii At 40 dB signal at 2 kHz c Loudness phons equal loudness contour L Frequency and Pitch a Pitch value Mels b 1000 Hz tone 1000 Mels c 1 Hz 1 mel