## 1 Introduction  
Audience research conducted by the British Broadcasting Corporation (BBC) has shown that approximately 60% of viewers in the UK experience some difficulties understanding speech content in programmes. This is due to a combination of factors such as background sound effects, intrinsically unintelligible speech, unfamiliar accents and loud ambient noise in the listening environment [1]. This paper focusses on masking caused by inappropriate levels of background sound. In films, TV and radio programmes, speech content is usually accompanied by background sounds and effects. These sounds (e.g., music) may be added by a programme producer for narrative purposes or to create a certain atmosphere. The sounds and effects may also exist in the original speech recordings, for example the recording might contain reverberation in a large conference room and clapping by an audience. While these background sounds and effects may improve an audience's experiences by providing'realistic' or 'being there' feelings, they can also lead to poorer speech intelligibility. In some situations, even listeners with normal hearing struggle to understand what is being said in programmes. It is even more difficult for listeners with hearing impairments or whose native language is different from that of the programme [2]. A sound designer or producer usually decides on the mixing level of the speech and background based on his/her intentions and experiences. The speech-to-background ratio (SBR) may be chosen according to certain rules of thumb. Here are some examples for broadcasting from sound producers and audio professionals [3]:  
* In the presence of background music, while the music is usually set to \(-20\) dB, the dialogue is about \(-20\) to \(-12\) dB. This makes sure that the background music does not interfere with the dialogue. Alternatively, speech level is empirically expected to be at between 4 and 6 (0 dB and +8 dB) on a peak programme meter. Music and effects are balanced to satisfy the artistic intentions. * The mixing level may greatly vary across different programmes based on the producer's experiences and taste. As such, the relative level between dialogue and other tracks may be determined solely based on the producer's preference. * The studio often represents optimal listening conditions. A responsible mixer/producer also checks mixes on other reproduction systems other than those in the studio, because there is always a compromise after the point when the mix sounds as good as it should in the studio. For example, the producer listens to the programme on TV speakers at the post-production stage in order to know how it would sound on the audiences' TVs. * An approach to check the balance between the dialogue and music or sound effects is to trigger the variation of frequency sensitivity with volume (represented well by the Fletcher-Munson equal-loudness curves) by listening to the mix at low volume. If the dialogue is sufficiently intelligible at low volume, it will work on most speaker setups. It is clear that in most cases, the decision on the final mixing level of broadcasting programmes relies to a great extent on sound producers' own perception to the mix, and is based on their personal preference. Furthermore, all the checking work has to be done manually by the sound producer or someone during the postproduction stage, in order to make sure that the mixing level does not compromise both speech intelligibility and artistic intentions. Despite precautions, the audience still often needs the assistance of on-screen captions: 10% of the BBC audience uses subtitles [4]. It is possible that the producer is so familiar with the script, and thus with the speech content, that the chosen SBR is set too low. While numbled speech may not be a problem for the producer who has repeatedly heard the dialog during editing, audiences may struggle.