2.4.2 Speech Reception Threshold and Spatial Release from Masking
Speech perception in noise was evaluated using the speech reception threshold (SRT) test. The target speech signals of disyllabic words were always presented from the front loudspeaker positioned at the ear level at 0° azimuth, with the masker competitor stimuli of SSN presented variously at the ear level at the three different azimuths (0°, -90°, and +90° azimuth). Masking of SSN was presented at a fixed level of 65 dB SPL, while the level of the target speech signals started at 65 dB SPL and varied adaptively to estimate the result of SRT depending on the participant’s response. The level of subsequent disyllabic words was adjusted adaptively in 2-dB steps. SRT was defined as the intensity level of the speech signal presented when the participant had a 50% correction of disyllabic word identification. The speech-to-noise ratio (SNR) was defined as the difference between the speech and noise levels when the patient attained 50%-word recognition (SNR dB SPL=SRT dB SPL-SSN dB SPL). During the test of speech perception in noise, both ears remained open.
For the SSD group, the following spatial speech-in-noise configurations were used: (1) Co-located condition: both the target speech and SSN were located in the front (0°, azimuth); (2) Separated condition in S0NNH (SeparatedS0NNH): the target speech was located in the front (0°, azimuth) and the SSN was at the NH side (-/+90°, azimuth); and (3) Separated condition in S0NSSD(SeparatedS0NSSD): the target speech was located in the front (0°, azimuth) and the SSN was at the impaired side (-/+90°, azimuth). For the NH group, the SSN was uniformly presented from the left side and had only two test conditions: Co-located and Separated. SRM was computed as the difference in SRT between Co-located and spatially Separated conditions (SRM in dB SPL=SRTCo-located dB SPL -SRTSeparateddB SPL).