# Listeners' Spectral Reallocation Preferences for Speech in Noise
## 3 Expt. 1: Spectral Reallocation Preferences for Full-Bandwidth Speech
### Interim Discussion  
When provided with the opportunity to adjust spectral tilt or the level of an octave band of speech in the presence of masking noise, listeners made adjustments that tended to maximise intelligibility (Figure 3), enabling the effect of the masker to be counteracted to some extent, and moreover achieved intelligibility scores well in excess of those for unmodified speech [70]. The utility of spectral energy reallocation as a strategy for improving the intelligibility of speech in noise is illustrated by the range of keyword scores across conditions at each SNR (Figure 2): under a constant energy constraint, reallocating energy to the 2-4 kHz region or flattening spectral tilt resulted in approximately half as many keyword errors as modifying speech level in the 1-2 kHz band. A transfer of energy to mid-frequencies is in line with natural modifications made by talkers when speaking in noise (e.g., [78]), or when asked to speak clearly, e.g., [36]. The finding that speech modification can lead to substantial gains in intelligibility echoes earlier studies [1; 3; 10; 13], and confirms the outcome of studies that indicated the value of spectral modification in particular [41; 44]. The present study extends previous investigations by providing a listener-centric approach, which has the potential to permit the identification of optimal values for any parameters tested, rather than relying on an a priori set of conditions chosen by the experimenter, as is the case in earlier studies e.g., [42].  
Glimpse distributions across frequency were relatively stable over a range of neighbouring adjustment values (Figure 4). This can be explained by the binary nature of glimpses: once the target speech has sufficient energy in a given region, the glimpse distribution does not change by further boosting the target speech energy in that region. This fact provides listeners with some latitude in the choice of adjustment, and it may be that other factors come into play once a glimpse pattern that supports a high level of intelligibility has been established. Indeed, listeners made adjustments at a scale commensurate with the adversity of the listening condition, evidenced by the choice of modifications whose deviation from the unaltered speech increased monotonically as SNR decreased  
Figure 4: Proportion of glimpses (spectro-temporal regions where the speech is more intense than the masker) in each spectral region as a function of listeners’ level adjustment preferences. In each panel, the x-axis represents the adjustment chosen by listeners, while the y-axis represents spectral frequency (34 channels on an ERB-rate scale). Darker areas indicate a higher proportion of glimpses. Vertical bars indicates mean listener preferences.  
(Figure 2). Listeners also spent more time selecting the appropriate adjustment in challenging listening conditions. These findings indicate a tension between maintaining a spectral balance close to that of the original speech, and making changes to overcome the masker. The distribution of adjustment values chosen by listeners also hints at bimodality in several conditions (Figure 3). This tendency is best appreciated in the high band and for spectral tilt, where, in addition to a relatively central mode, a secondary and more extreme mode exists. A similar tendency in the opposite direction is evident for the low band.  
Reallocation of speech energy to the remainder of the spectrum provides an opportunity for listeners to optimise intelligibility while simultaneously expressing a preference influenced by other factors. In Expt. 1, the entire spectral band of speech was intact, and as such, provides only a partial view into listeners' preferences. To explore choices in a different--and more challenging--scenario, in Expt. 2, listeners were presented with a single band of speech and were able to control its spectral extent via three forms of bandwidth modification, or by changing the spectral location of the band.