1. Katherine Morrow, Sarah Williams, and Chang Liu Department of Communication Sciences and Disorders The University of Texas at Austin, Austin, TX 78712 Abstract Compare two methods to measure consonant perception in noise A common problem to measure consonant perception in noise is the unpredictable audibility of each consonant (e.g., give the same signal level, some consonants are well audible, while others are not). The purpose of this project is to examine two methods in speech perception to solve this problem and also to evaluate their time efficiency and data reliability. These two methods, frequently used in psychological measures, are method of limit and method of constant stimuli. Preliminary results showed that the method of limit was more time efficient while both methods provided quite reliable data. The advantage and disadvantage of the two methods will be discussed. Pros and Cons of Each Method Results  Thresholds of stop consonant identification and slopes of the psychometric functions were somewhat similar between the two methods with some large variations in the slopes.  The variations between the two methods may be due to the relatively small dynamic range in the method of limit, making it difficult to include very high and low performance in the psychometric function.  Altogether, the method of limits provides a reliable way (e.g., threshold measure) to measure speech perception, especially phonemic perception in noise.  One potential problem with the method of limit: listeners may never reach the target percent score (e.g., 70.7%), in which case an identification threshold cannot be reached. Why and What is Speech Perception? Comparison of psychometric functions between the two methods Basically, the two methods showed similar psychometric functions for the six stop consonants, with some variations in slopes and threshold (boundary). Discussion and Conclusion Speech is one of the most important sounds for humans. Speech perception is to measure human listeners’ identification and/or discrimination of speech sounds under a variety of listening conditions. Speech sounds: phonemes (vowels and consonants), words, phrases, and sentences. Listening conditions: quiet, noise, and reverberation Human listeners: normal-hearing vs. hearing-impaired; young vs. old How to Measure Speech Perception In general, two common methods have been used to measure speech perception Method of constant stimuli Each speech token is played with a certain number of repetitions (e.g., 20 times) at a given condition (e.g., speech level). Perception performance is measured at several conditions. S-shaped psychometric function of speech perception is obtained, for example, perceptual scores range from 0% to 100%. Method of limits The level of speech varies trial by trial, depending on whether the listener’s response is correct. Perception is measured as a threshold, e.g., the speech level to reach a certain percent correct of speech perception. Our Approach Use the Method of Limits in consonant perception for its time efficiency to see if it will yield the same results in determining the features of psychometric functions as would the Method of Constant Stimuli. If the Method of Limits provides reliable psychometric function information of speech perception comparable to the Method of Constant Stimuli, the Method of Limits can be widely used in hearing clinics and research laboratories concerning speech perception because of its quick and reliable manner. Speech level Trial number Method of Constant Stimuli Method of Limits (adaptive procedure) Pros  Detailed results  Provides the full picture of a person’s hearing from low to high performance  Time efficient  Quickly approach the target performance Cons  Time consuming  Less efficient  Does not show the full picture of a person’s hearing  May be predictable Method 1. Stimuli 6 stop consonants (/b, d, g, p, t, k/) in the /aCa/ phonetic context spoken by one adult male talker Two types of noise fixed at 70 dB SPL: 4-talker babble and cafeteria noise 2. Listeners Four young American English native listeners (20-23 years old) with normal hearing (ANSI, 2004) 3. Procedure a. Method of Limits Closed-set consonant identification based with a two-down, one-up adaptive procedure. A total of 200 trials for each consonant. Identification thresholds of each consonant were measured (pointing to the speech level at which listeners identify the consonants 70.7% accurately). The order of the conditions (noise type) was randomized. b. Method of Constant Stimuli Closed-set consonant identification. Identification scores were measured at a series of signal-to-noise ratios: -14, -12, -10…4, 6, 8 dB. The order of the conditions (noise type and SNRs) was randomized. c. General Procedure Stimuli were presented via Tucker-Davis Technologies (TDT) signal processing hardware, TDT SykofizX software, and an ER-2 insert earphone to a single ear. Method of Constant Stimuli (psychometric function) Method of Limits (adaptive procedure) Figure 1. Psychometric functions for identification of the bilabial (left), alveolar (middle), velar (right) stop consonants in the presence of cafeteria noise (upper panel) and 4-talker babble (lower panel) Comparison of thresholds and slopes of the PM between the two methods Thresholds obtained from the two methods appeared to be moderately matched with each (Pearson r > 0.70, r < 0.05). Slopes of the psychometric functions from the two method did not match with each other well (r 0.05). Such low correlations of slopes between the two methods may be due to some outliers in the data. Figure 2. Comparison of Threshold (left) and slope (right) of the PM functions