In a speech-based system, a wake word or other trigger expression is used to preface user speech that is intended as a command. The system receives multiple directional audio signals, each of which emphasizes sound from a different direction. The trigger expression is detected in an individual directional audio signal by comparing a confidence score with a confidence threshold. An individual confidence threshold is specified for each directional audio signal. The confidence thresholds are adjusted during operation of the system based on performance information that is generated during operation of the system. As an example, performance information may include the number of times that the trigger expression has been detected in each of the directional audio signals.