Equalization of the Thai Digits-in-Noise Test: A Study on Young Adults With Normal Hearing Sensitivities

Background: By the year 2050, nearly 2.5 billion individuals globally will experience some degree of hearing loss. To facilitate its systematic detection and timely intervention, it is imperative to develop a Thai digits-in-noise (DIN) test for efficient hearing screening. Objectives: To equalize the intelligibility of recorded Thai digits in the DIN test, ensuring consistent speech perception in noise. Methods: Twenty-five normally hearing young adults participated in this study (mean [SD] age, 21.68 [1.11] years). Thai digits from 0 to 9 were recorded by a female native speaker, and the best recordings were selected for the experiment. Speech-shaped noise was generated to match the long-term average spectrum of all digits. A custom MATLAB script was used to present digit triplet combinations at fixed signal-to-noise ratios from -20 to -2 dB in 2 dB steps. Antiphasic stimuli were delivered dichotically through insert earphones, and participants responded via a keypad. Speech reception thresholds (SRTs) for each digit were calculated to assess intelligibility. A repeated-measure ANOVA was used to compare the SRTs of all digits. Results: The average SRT for each digit exhibited slight variability, ranging from -18.1 dB (number 4) to -10.8 dB (number 2). The overall mean SRT across all digits was -14.7 dB. Significant differences in intelligibility were found across the Thai digits. Intensity adjustments were applied to digits, resulting in perceptual equalization with no significant differences post-adjustment. Conclusions: Variability in digit intelligibility reflects acoustic differences inherent in the Thai language. The equalized Thai digits demonstrated consistent intelligibility, so they are suitable for use in speech-in-noise testing among Thai speakers, supporting clinical and research applications.