# Spatial Unmasking Effect on Speech Reception Threshold in the Median Plane
## 3 Result and Discussion  
The results from the repeated measure ANOVA analysis, shown in Table 1, suggest a significant dependency of SNR on the masker elevation position (F = 23.170, \(p\) = 0.000), although this pattern is not linear, as can be observed in Figure 1. To examine which spatial configurations produced statistically different SRTs, paired-sample T tests with Bonferroni correction were conducted. The results are summarised in Table 2. The SNR for the \(-30^{\circ}\) condition was significantly lower than any other condition (\(p<0.01\)). It showed \(-3.5\) dB of vertical spatial unmasking gain compared to the \(0^{\circ}\) condition (\(p=0.003\)), whilst the \(30^{\circ}\) and \(60^{\circ}\) conditions had 1 dB (\(p\) = 0.038) and 1.6 dB (\(p\) = 0.008) gains, respectively. The \(90^{\circ}\) had a negative gain, but this was not statistically significant (\(p=0.446\)). This result confirms the existence of spatial unmasking in the median plane. In order to obtain insights into the potential influence of spectral cue on the perceived results, HRTFs for the masker loudspeaker positions were analysed. As the original subjects were not available after the initial testing, head-related impulse responses (HRIRs) between \(-90^{\circ}\) and \(+90^{\circ}\) (at \(30^{\circ}\) intervals) in the median plane at \(0^{\circ}\) azimuth were taken from the SADIE II database [16] using the eighteen human subjects available. From this, an average HRTF was computed for each elevation angle by taking the mean value of the magnitude spectrum in decibels (number of FFT points = 8192, sampling frequency = 96 kHz). Since it has been shown that ITD has little influence on median plane SRT [8], and ILD is considerable in the median plane only above around 10 kHz [17], which is outside the important range of speech spectrum, only left ear signals were processed for this HRTF analysis. The current experiment included only one negative elevation (\(-30^{\circ}\)) due to practical limitations in physical setup. Therefore, the subjective results presented here cannot generalise the consistency of the spectral notch influence on SRT at lower elevation angles. However, in order to provide further objective insight on the above hypothesis, \(-60^{\circ}\) and \(-90^{\circ}\) were also included in the spectral analysis. Figure 2 plots HRTF differences of each tested masker position from the reference position \(0^{\circ}\). Any positive value in the plots represents a greater energy compared to the reference, and vice versa. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type III Sum of Squares** & **df** & **Mean Square** & **F** & \(p\) \\ \hline
59.862 & 4 & 14.965 & 23.170 & 0.003 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of repeated measure ANOVA (the sphericity is assumed based on [PERSON]’s test of sphericity (\(p\) = 0.542)). Figure 1: Signal-to-noise ratio (SNR) for 50% correct speech reception as a function of the elevation angle of the masker. The plots represent the mean average of SNRs obtained from six subjects and one standard error. The subjective results showed that the \(-30^{\circ}\) masker condition produced the largest SRT gain of 3.5 dB among all conditions. Based on the delta HRTF plots, this might be explained as follows: Whilst the \(30^{\circ}\), \(60^{\circ}\) and \(90^{\circ}\) conditions commonly display a high peak at around 8 kHz, it can be observed that the \(-30^{\circ}\) and \(-60^{\circ}\) conditions display a considerable notch at around 6 kHz, showing up to about 6 dB and 10 dB less energy than \(0^{\circ}\), respectively. This is above the ranges that are considered to be most important for speech intelligibility (i.e., 2 to 4 kHz), but is still important for the reception of sibilant sounds and might therefore have been helpful in resolving confusion between words such as sat and that, which contain consonants with similar phoneme sounds. These phoneme sounds, called fricatives, display a peak between the range of 4 to 10 kHz depending on the sound pronounced [18].