## 1 Introduction
Sound designers or producers seem to lack a psychoacoustic guide that allows them to estimate the likelihood of the audience being able to understand the speech at their preferred mixing levels, or gives them minimum mixing levels for a desired intelligibility. Consequently, the mixing level decided by conventional empirical approaches may fail in maintaining intelligibility in two aspects. First, the mixing level may not work for all reproduction output configurations. While high-end spatial audio systems (e.g., 22.2 and 9.1) can preserve sufficient spatial cues (e.g., source azimuth) which listeners can use to enhance speech intelligibility, other systems with reduced spatial capabilities and often use down-mixed versions of the programme material (e.g., stereo) and so may not be able to provide the same level of intelligibility. Many studies (e.g., [5; 6; 7; 8; 9]) have suggested that spatial cues can significantly improve speech intelligibility even when the SBR remains constant. Loss of these spatial cues may lead to poor intelligibility in low-end systems. Unfortunately, stereo loudspeakers in TVs are often of poor quality and mono audition is common for radios and mobile devices. Second, background sounds vary in their masking effect. It is well documented that the type of background sound affects listener speech perception differently [10; 11; 12; 13; 14; 15; 16; 17]. Compared to fluctuating background sounds (e.g., temporally-modulated sounds), stationary background sounds (e.g., pink noise) usually introduce stronger masking effects to speech even at the same SBR level. This implies that an optimal SBR, which maintains speech intelligibility while providing a reasonable atmosphere to the audio scene, should vary over time. Some research has focused on solving this problem by enabling the audience to gain control of the foreground-background mixing level. [PERSON] and [PERSON] [18] introduced an approach for 'clean' audio in TV broadcast, which is compatible to ISO/IEC 23008 High Efficiency Coding and Media Delivery in Heterogeneous Environments. This object-based method allows the stream carrying the speech content to be separately transmitted; end users therefore can alter the SBR level to achieve better intelligibility for the speech content. [PERSON] et al. [19] proposed an object-based coding system to enhance dialogues for TV and radio broadcasts. This system enables renderer-listener interaction and permits listeners to control the mixing level for personalised intelligibility. Similar to [18], this approach essentially adjusts the gain of speech as a single object against the gain of other objects in the background. Further subjective rating experiments suggested that this system improved listeners' experience when auditioning speech content in background sounds. However, in order to make adaptations the system expects input from the listener, but the desired SBR varies as the broadcast material changes. Thus, this begs the question: is it possible to automatically adjust the mixing level? In this way, a preset intelligibility goal (set by either the sound producer or listener) can be ensured over time. Objective intelligibility models (OIMs) have been improving in accuracy for almost a century. State-of-the-art OIMs (e.g., [20; 21; 22; 23; 24]) are capable of making accurate intelligibility predictions in various masking conditions, with their binaural counterparts able to deal with more realistic situations [25; 26; 27; 28]. Compared to conventional subjective evaluations which are usually time-consuming and expensive, OIMs are computationally fast and thus easy to incorporate into a closed-loop optimisation framework for intelligibility. OIMs have been used for interim speech intelligibility evaluations for optimising speech modification algorithms developed to boost speech intelligibility in noise (e.g., [29; 30; 31]), prior to final subjective evaluation.