Indistinct covert recordings admitted as evidence in criminal trials are routinely ‘enhanced’ to assist a jury in making out their contents. But just what is ‘enhancing’, and how effective is it? This paper uses two short experiments to demonstrate that a subjective impression that ‘enhancing’ has made the audio ‘clearer’ does not necessarily indicate there has been an objective improvement in intelligibility. It then outlines, in a non-technical manner, the capabilities and limitations of various ‘enhancing’ techniques, and discusses implications in relation to current legal practices around the admission and use of ‘enhanced’ audio in Australian criminal trials. Finally, it recommends that ‘enhanced’ versions of forensic recordings should only be admitted on the basis of objective evidence of the extent to which they have genuinely improved the intelligibility of the specific audio being used, noting that such evidence is easy to obtain and provide.

Many jurisdictions around the world allow an 'enhanced' version of indistinct audio to be admitted, along with a transcript, to assist the trier of fact in understanding the content of forensic recordings. Typically, ultimate evaluation of the effect of the 'enhancing' relies simply on the jury or other listeners' impression as to whether the audio sounds 'clearer' than the original. A recent article reported results of two experiments showing that listeners' subjective impressions give a surprisingly unreliable indication of the objective effects of 'enhancing'. The current article reports a new experiment that adds weight and detail to previous demonstrations that enhancing can make audio 'sound clearer' without making it more reliably intelligible. It further demonstrates how 'enhancing' can interact with priming to make phrases suggested by a transcript seem more plausible than they do in the original, even when the suggestion is unreliable and misleading. It is recommended that courts should insist on far better regulation of the use of 'enhanced' audio.

This chapter provides a general introduction to the relatively new field of forensic transcription: the theory and practice of providing reliable and useful transcripts of indistinct recordings admitted as evidence in criminal trials. It highlights problems caused by confident false beliefs within the law about the nature of speech and how speech perception works, and discusses the role that linguists can play in bringing these false beliefs to light, and in advocating for reform to legal processes for admission and use of covert recordings.

Recent research has raised concerns about legal procedures for admitting 'enhanced' versions of indistinct covert recordings used as evidence in Australian criminal trials. This paper seeks to deepen these concerns via an experimental study using two enhancements that were admitted in a trial on the grounds that the judge, listening personally, considered they assisted him to hear words from the prosecution transcript more clearly than he could in the original. Results demonstrate that, as in previous studies, the apparent clarity of the 'enhanced' audio depends on listeners having been primed by a transcript. However, this study goes further, by demonstrating that the two enhancements are not neutral, incremental improvements on the original, but incorporate unnoticed artefacts that cause them to be perceived differently from the original, and from each other. These effects were not acknowledged by the judge, who endorsed the prosecution's assurance that the enhancements had simply increased the clarity of the audio. Of course, the aim here is not to single out one particular judge, but to demonstrate that current procedures for admitting transcripts and enhancements are insufficient to protect judges, juries and defendants from potentially misleading interpretation of indistinct covert recordings.