Derivation of the BIF

We used an established protocol [13] to derive the BIF. Briefly, the established protocol used an iteration technique exploiting the relationship between the BIF and the SII. Relative SII values were determined from percent correct scores. BIF values were then derived from the relative SII values.

First, the relationship between the percent correct scores and the relative SII values were determined. The initial point for the relationship was determined by plotting the percent correct scores as a function of band-frequency at the highest SNR (+5 dB SNR) used in the present study. One curve represented the data where the high frequencies were removed using a low-pass filter and the other curve represented the data where the low frequencies were removed using a highpass filter. The intersection of these two curve was considered the initial point in a series of graphical processes and curve interpolations for data at different SNRs, generating the constant values in Equation 1 [Eq. (1)]:

In Eq. (1), which was originally proposed by Fletcher and Galt [14], S is the percent correct score, A is the SII value, P is a value for the measure of the talker's and listener's proficiency, and Q and N are fitting constants. The proficiency value, P, was assumed to be 1 because all participants and talkers had normal hearing. The BIF was determined using the computed values of fitting constants through the inverse of Eq. (1). Using the inverse of Eq. (1), all percent correct scores were transformed into associated SII values and the cumulative values were converted to separate values. The separate SII values for low- and high-pass filters were averaged, then the values for the same cutoff frequency band at the different SNRs were averaged. After expanding the averaged values to a 0.0 to 1.0, the final BIF values for each frequency band were determined. Kim and Jin [15] provided a more detailed description for the BIF protocol.