In order to prove that Marilize van Emmenis is in fact the girl on the Joostgate video, world-renowned voice analyst Dr Len Jansen compared her voice (as recorded during the interview with heat) against the voice of the girl on the video. In order to ascertain if the voices are the same, she was required to read certain audible sentences from the video soundtrack three times, and those were compared and analysed.

Listen to one of the 24 clips Dr Jansen used:

Dr Jansen, who has 45 years experience in the scientific identification of voices, uses the American “aural spectrographic” method of voice identification, in which spectrographic comparisons as well as aural recognition are used to establish whether a known and unknown voice belong to the same person. “Spectrographs are also described as ‘visible speech’ and give a visual depiction of the speech,” he explains. “If exactly the same words in the same language are repeated you can establish with a high degree of trust that the known and unknown speakers are the same person.”

Dr Jansen isolated certain words for this purpose, which were then analysed – he used a total of 24 corresponding words. This was the same test that heat had invited Joost to do, which would be a certain way to exonerate himself if he was innocent. Joost declined the offer.

Says Dr Jansen. “I compared the known and unknown female voices with each other. I found that the sound of the voices correspond, and found that the spectrograms corresponded throughout. I am totally convinced that the voice of the woman on the so-called Joost sex video does belong to ‘the lady’.”

Here's a clip of Dr Jansen confirming his findings:

NOTE: Information on forensic expert Dr Len Jansen:

Dr Len Jansen is a forensic expert in possession of several degrees obtained at the Universities of Stellenbosch and Pretoria. It includes a B.Sc. degree with Physics, Mathematics and Applied Mathematics as main subjects, a B.Sc.-Hon. degree in Applied Mathematics, a M.Sc. degree in Physics and a D.Sc degree in Physics, with Scientific Voice Identification as the subject of the thesis. During 1966/67 he received specialised training in speech analysis and voice identification at the Institute for Perception Research in Eindhoven, Netherlands. From 1964 he worked for 18 years at the Acoustics Division of the CSIR and from 1982 he worked for 6 years at the Forensic Science Laboratory of the South African Police. During 1974, 1975, 1976, 1979 and 1983 he received training in the United States of America on the scientific identification of voices. In 1974 he became a member of the International Association of Voice Identification and during 1979 he was certified by them as an expert on voice identification. He has also attended a number of their congresses in the United States. He is a retired fellow of the South African Acoustics Institute. During 1992 he joined the International Association of Forensic Phoneticians.

Here is Dr Jensen's voice comparison report: