Courts often need expert opinion as to whether two recordings were made by the same speaker or not. Typically one voice has been recorded by an unknown speaker on a telephone answering machine and the other somehow obtained from a suspect, When such an opinion has been given, the court is bound to ask how reliable it is. There is no answer to this question. There are too many unknown factors. Were the recording conditions similar? Were both the speakers talking in the same style? Was one speaker trying for a disguise? Was there any background noise? How long was the recording? Does the suspect have family or friends with very similar voices? It would be almost impossible to set up a valid experiment that can take all the variables into account. Given two 15 minute recordings of similar material made under studio conditions an expert could state beyond a reasonable doubt whether they were made by the same speaker or not. Given a 5 second bomb threat on a noisy phone line there is no way that this could be done.