# Spatial Unmasking Effect on Speech Reception Threshold in the Median Plane
## 1 Introduction  
It is widely reported that the detection and comprehension of target speech in the presence of a noise masker depends on its position with respect to the masker on the horizontal plane [1; 2; 3; 4]. An increase of the angular displacement of a masker and target has been shown to increase the effect of spatial unmasking [1; 4]. Increasing the distance of a masker from a target has also been shown to increase the effect of spatial unmasking [4].  
In the horizontal plane, spatial unmasking is primarily due to the better ear and binaural effects. The better ear effect relies on increased signal-to-noise ratios (SNRs) at one ear due to directional differences, producing an improved SNR at one ear due to interaural level difference (ILD) [4; 5]. Binaural unmasking occurs as a result of different interaural time differences (ITDs) between the target and masker, resolved as phase differences between the ears at different frequencies, used to provide the brain with a less noisy representation of the auditory environment [6].  
Previous studies prove that spatial unmasking persists in the median plane without azimuthal displacement. [PERSON] et al. [7] showed the greatest mean percentage of correct target speech identification with the target at \(-50^{\circ}\) elevation and masker at \(+50^{\circ}\) elevation. This was a \(27.5\%\) increase from their worst condition, with both target and masker at \(-50^{\circ}\). [PERSON] et al. [8] showed that ITDs were not responsible for the significant spatial unmasking found in the median plane by removing the ITDs from head-related transfer functions (HRTFs) used in the experiment, finding the effect of spatial unmasking still to be present. These studies used speech-on-speech masking, and the resultant unmasking was largely informational due to the nature of the stimuli. [PERSON] and [PERSON] [9] showed that subjects were able to track a speech's fundamental pitch with respect to a speech masker to aid differentiation from the masking speech source and provide further unmasking effects, which could contribute to the release of masking in speech-on-speech conditions if fundamentals were not matched. The current study sought to discover a speech reception threshold (SRT) at each masker elevation to compare the amount of spatial unmasking available at each location. Use of a speech-shaped noise masker helped to remove the informational aspects involved in speech-on-speech masking.  
While previous research [7; 8] suggests that the binaural influence on median plane spatial unmasking exists, albeit minimal, the range of tested elevation angles of the maskers was limited. Furthermore, the localization of sound in the median plane relies on spectral cues due to pinnae and torso diffractions (i.e., HRTF) [10; 11; 12], and therefore the spectral aspects of HRTF may also be related to spatial unmasking for speech intelligibility in the median plane. From this, it is hypothesised that if spatial unmasking for speech against noise operates in the median plane, it would be associated with the spectral energy difference between the target speech and masker noise at the ears, which would vary depending on the frequency notches produced at each elevation angle.  
From this background, a subjective listening experiment was conducted to investigate the dependency of SRT in the median plane on the elevation angle of the masker, which varied with 30\({}^{\circ}\) intervals (-30\({}^{\circ}\), 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\) and 90\({}^{\circ}\), with the target speech at 0\({}^{\circ}\)). The next section details the experimental method used. Section 3 presents the results of the listening test, which are discussed in Section 4 with objective analyses.