To limit the influence of the fact that the speaker is aware that his speech is being recorded one can hide the microphone and the recorder. Provided that nothing else in the environment alerts the speaker of something unusual, such clandestine or surreptitious recordings, sometimes also called candid microphone speech, should be maximally natural.
The question arises whether the possible advantage of slightly better naturalness outweighs the disadvantages of hidden recordings, the most important of which probably are the following two very different types: the ethical question of whether clandestine recording is permissible, and the risk of loss of recording quality because on-line monitoring of recording level is extremely difficult. Another possible drawback is that much effort may be spent in recording speakers who, after debriefing, refuse to give consent for the use of the recordings, even for purely scientific research.
A substantial part of the British National Corpus (BNC) has been recorded in situations where at least some persons who participated in a conversation were not aware of the fact that they were being recorded [Crowdy (1993)]. As far as we know, only a very small number of persons demanded that the recordings in which they participated were erased. It is not known whether the material in the BNC has been analysed to investigate possible differences in recording quality and vocabulary between secret and open recordings .
One obvious way to make sure that the speaker cannot know that he is being recorded is to tape telephone speech. But here again, publishing these recordings as part of a corpus requires that the speaker must be debriefed and asked for consent to keep the recordings.