An audio processing system, comprising: a slow detector configured to determine an ambient sound level associated with an audio input signal that includes environment sound; wherein the slow detector comprises a time domain detector that determines an energy level associated with the audio input signal over a first time period; a fast detector configured to determine an envelope level associated with the audio input signal, wherein the fast detector comprises a time domain detector that determines an energy level associated with the audio input signal over a second time period, and the first time period is greater than the second time period; and an alert signal detector configured to: apply an adaptive threshold function to the ambient sound level to determine an adaptive threshold level; and compare the envelope level to the adaptive threshold level to determine whether an alert signal is present in the audio input signal; wherein: determining the adaptive threshold level comprises applying a first adaptive threshold function to the ambient sound level for a first range of ambient sound levels and applying a second adaptive threshold function to the ambient sound level for a second range of ambient sound levels; the first range of ambient sound levels is lower than the second range of ambient sound levels; characterised in that: the first adaptive threshold function comprises a linear function having a first slope greater than zero and less than or equal to one; and the second adaptive threshold function comprises a linear function having a second slope that is greater than the first slope. a slow detector configured to determine an ambient sound level associated with an audio input signal that includes environment sound; wherein the slow detector comprises a time domain detector that determines an energy level associated with the audio input signal over a first time period; a fast detector configured to determine an envelope level associated with the audio input signal, wherein the fast detector comprises a time domain detector that determines an energy level associated with the audio input signal over a second time period, and the first time period is greater than the second time period; and an alert signal detector configured to: apply an adaptive threshold function to the ambient sound level to determine an adaptive threshold level; and compare the envelope level to the adaptive threshold level to determine whether an alert signal is present in the audio input signal; apply an adaptive threshold function to the ambient sound level to determine an adaptive threshold level; and compare the envelope level to the adaptive threshold level to determine whether an alert signal is present in the audio input signal; wherein: determining the adaptive threshold level comprises applying a first adaptive threshold function to the ambient sound level for a first range of ambient sound levels and applying a second adaptive threshold function to the ambient sound level for a second range of ambient sound levels; the first range of ambient sound levels is lower than the second range of ambient sound levels; characterised in that: the first adaptive threshold function comprises a linear function having a first slope greater than zero and less than or equal to one; and the second adaptive threshold function comprises a linear function having a second slope that is greater than the first slope. determining the adaptive threshold level comprises applying a first adaptive threshold function to the ambient sound level for a first range of ambient sound levels and applying a second adaptive threshold function to the ambient sound level for a second range of ambient sound levels; the first range of ambient sound levels is lower than the second range of ambient sound levels; characterised in that: the first adaptive threshold function comprises a linear function having a first slope greater than zero and less than or equal to one; and the second adaptive threshold function comprises a linear function having a second slope that is greater than the first slope. the first adaptive threshold function comprises a linear function having a first slope greater than zero and less than or equal to one; and the second adaptive threshold function comprises a linear function having a second slope that is greater than the first slope.