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Week9 outline

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McMaster University
Paul Faure

This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. Detection Versus Discrimination  Absolute detection thresholds tell psychoacousticians what the limits of detection are for sound parameters (e.g. frequency, amplitude and time).  Often we are interested in the sensitivity of a listener to changes in an acoustic parameter.  Discrimination procedures measure what a listener can respond to or just detect.  S.S. Stevens correctly pointed out that psychophysicists measure what a listener does respond to, and often this measure corresponds to what a listener can respond to, but not necessarily.  We want testing procedures that provide a measure of listener sensitivity that does not change with the Subject’s (S’s) response criterion. Psychophysical Testing Procedures  Psychoacousticians employ different measurement (testing) procedures to quantify a Subject’s (S’s) perceptual sensitivity.  Detection procedures measure absolute performance of S.  Discrimination procedures measure relative performance of S.  Scaling procedures allow S to quantify subjective magnitude of stimulus.  Matching procedures determine physical aspects of stimuli that Ss use to equate different stimuli. Psychophysical Testing  Detection testing procedures measure absolute performance of Subject (S) such as Threshold of Audibility or Absolute Limen (AL).  Discrimination testing procedures measure relative performance. Ss judge if stimuli are different from each other; can use this to determine Just Noticeable Difference (JND) threshold or Difference Limens (DL) threshold.  Scaling procedures ask S to quantify stimulus magnitude on a defined scale; Ss are given a “rule” that is used to assign a number quantity to the perceptual magnitude of a stimulus.  Matching procedures ask Ss to equate different stimuli by adjusting a physical parameter of one stimulus until it appears perceptually equal to another stimulus (e.g. equal loudness matching). Psychometric Function  Most psychophysical experiments are designed to obtain a psychometric function from S.  Psychometric function relates measure of S performance to physical value of stimulus.  Estimate of a S’s threshold is obtained directly from the psychometric curve.  Threshold is often chosen to be the midpoint of the range of total possible “YES” responses. Thresholds of Audibility  In measuring or quantifying perceptual sensitivity to auditory signals, psychoacousticians adopt one of two basic threshold testing procedures.  Detection Performance (Absolute) o Detection Threshold or Absolute Limen (AL) E.g. Audibility Thresholds: Minimum Audible Field (MAF) threshold in room at 1m using method of constant stimuli. o Threshold obtained using Method of Constant Stimuli o Minimum Audible Field (MAF) thresholds are SPLs for pure tones measured in open space (free field) with loudspeaker positioned at fixed distance from S (e.g. at 1 m). o Minimum Audible Pressure (MAP) threshold at S’s tympanic membrane measured with headphones that have been calibrated with a coupler device for airspace in external auditory canal.  Discrimination Performance (Relative) o “Ask” listener if stimuli are different. o Can measure Just Noticeable Difference (JND) Threshold E.g. Frequency discrimination Beware of Response Bias  Experimenters assess the performance of subjects (Ss) on a variety of psychophysical tasks.  Experimenter is interested in determining the subject’s sensitivity to changes in the stimulus. Psych 3A03 1 November 2012 Week 9 Dr. Paul A. Faure  Experimenter is NOT interested in the subject’s ability to respond in the experiment (response proclivity).  Thus, the experimenter wants a measure of response sensitivity that is bias free (very difficult).  Many sources of bias (e.g. listener motivation, testing conditions, test instructions & feedback, experience, etc.  Response bias assessed in method of constant stimuli procedure by inserting “blank” or “catch” trials in testing sequence, and then looking for false alarm responses. Theory of Signal Detection (TSD)  Theory of Signal Detection (TSD) is used to measure the ability of a
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