EFFECT OF SET SIZE AND METHOD ON SPEECH RECEPTION THRESHOLDS IN NOISE

Objective: The threshold for speech is known to improve as signal (Mil ler, Heise, & Lichten, 1951) or response (Pollack, 1959) uncertainty i s decreased, The definition of threshold as the signal level or signal to noise ratio (S/N) at which a fixed percent-correct score is obtain ed, therefore, becomes problematic when set size, M, is varied through the range for which threshold varies systematically (M = 2 to M = 16) because the meaning of that fixed percent changes as Mis increased or decreased. The goal of the present study is to examine the effect of set size, M, on speech reception thresholds (SRTs) under two testing s trategies. Design: SRTs were obtained in the presence of 80 dB SPL whi te noise for sets containing 2, 4, 8, and 16 words using two different procedures. one in which threshold was based on a fixed percent corre ct (American Speech-Language-Hearing Association, 1988) and one in whi ch threshold was defined in terms of d' = 1.00. The subjects were 12 y oung women with normal hearing and little or no experience with audiol ogic testing procedures. Results: When threshold was based an a fixed percent. correct, S/N at threshold was found to be dependent on set si ze (F = 3.333; df = 3, 33; p = 0.031). When threshold was defined in t erms of d' = 1.00, S/N at threshold was found to be independent of set size. Conclusions: If a smaller set size than that recommended by the American Speech-Language-Hearing Association (1988) guidelines is to be used for obtaining SRTs in a clinical setting, thresholds should be based on a criterion-free measure that is independent of the size of the set of words being tested, if possible.