Increasing amounts of informal spoken content are being collected, e.g. recordings of meetings, lectures and personal data sources. The amount of this content being captured and the difficulties of manually searching audio data mean that efficient automated search tools are of increasing importance if its full potential is to be realized. Much existing work on speech search has focused on retrieval of clearly defined document units in ad hoc search tasks. We investigate search of informal speech content using an extended version of the AMI meeting collection. A retrieval collection was constructed by augmenting the AMI corpus with a set of ad hoc search requests and manually identified relevant regions of the recorded meetings. Unlike standard ad hoc information retrieval focussing primarily on precision, we assume a recall-focused search scenario of a user seeking to retrieve a particular incident occurring within meetings relevant to the query. We explore the relationship between automatic speech recognition (ASR) accuracy, automated segmentation of the meeting into retrieval units and retrieval behaviour with respect to both precision and recall. Experimental retrieval results show that while averaged retrieval effectiveness is generally comparable in terms of precision for automatically extracted segments for manual content transcripts and ASR transcripts with high recognition accuracy, segments with poor recognition quality become very hard to retrieve and may fall below the retrieval rank position to which a user is willing search. These changes impact on system effectiveness for recall-focused search tasks. Varied ASR quality across the relevant and non-relevant data means that the rank of some well-recognized relevant segments is actually promoted for ASR transcripts compared to manual ones. This effect is not revealed by the averaged precision based retrieval evaluation metrics typically used for evaluation of speech retrieval. However such variations in the ranks of relevant segments can impact considerably on the experience of the user in terms of the order in which retrieved content is presented. Analysis of our results reveals that while relevant longer segments are generally more robust to ASR errors, and consequentially retrieved at higher ranks, this is often at the expense of the user needing to engage in longer content playback to locate the relevant content in the audio recording. Our overall conclusion being that it is desirable to minimize the length of retrieval units containing relevant content while seeking to maintain high ranking of these items. 
Exploring speech retrieval from meetings using the AMI corpus