Emotions are mental states occurring in response to external and internal stimuli and thus form an integral part of an animal's behaviour. Emotions can be mapped in two dimensions based on their arousal and valence. Whilst good indicators of arousal exist, clear indicators of emotional valence, particularly positive valence, are still rare. However, positively valenced emotions may play a crucial role in social interactions in many species and thus, an understanding of how emotional valence is expressed is needed. Vocalisations are a potential indicator of emotional valence as they can reflect the internal state of the caller. We experimentally manipulated valence, using positive and negative cognitive bias trials, to quantify changes in pig vocalisations. We found that grunts were shorter in positive trials than in negative trials. Interestingly, we did not find differences in the other measured acoustic parameters between the positive and negative contexts as reported in previous studies. These differences in results suggest that acoustic parameters may differ in their sensitivity as indicators of emotial valence. However, it is important to understand how similar contexts are, in terms of their valence, to be able to fully understand how and when acoustic parameters reflect emotional states.