A system comprising - an inhaler for dispensing aerosol or a dry powder, the inhaler comprising - a housing comprising an air-inlet (A_I) and a an air-outlet (A_O), wherein the housing defines a flow path between the air-inlet (A_I) and the air-outlet (A_O), and - a first sensor arranged at a first position of an external surface of the housing, and a second sensor arranged at a second position of the external surface of the housing, and wherein the first and second sensors (S1, S2) are arranged to sense sound or vibrations resulting from a flow in the flow path, and - a processor arranged to process output signals from the first and second sensors (S1, S2) according to an algorithm (A1, A2), so as to generate a measure of flow in the flow path, characterized in that: the first and second positions are spaced apart, wherein the first position is selected to have a shorter distance to the air-inlet (A_I) than the second position, and wherein said algorithm (A1, A2) comprises an event classification part arranged to analyse the sound or vibrations sensed by the first and second sensors (S1, S2), so as to allow identification of at least one event, and wherein the event classification part is arranged to analyse the sound or vibrations sensed by the first and second sensors (S1, S2), so as to allow identification of all of: a priming event, a firing event, and an inhalation event, and wherein the event classification part is arranged to distinguish between a priming and firing event on the one hand, and an inhalation event on the other based on a duration feature derived from start and end times of events, and wherein the event classification part is arranged to distinguish between a priming event on the one hand and a firing event on the other based on power differences between sound or vibrations sensed by the first and second sensors (S1, S2). - an inhaler for dispensing aerosol or a dry powder, the inhaler comprising - a housing comprising an air-inlet (A_I) and a an air-outlet (A_O), wherein the housing defines a flow path between the air-inlet (A_I) and the air-outlet (A_O), and - a first sensor arranged at a first position of an external surface of the housing, and a second sensor arranged at a second position of the external surface of the housing, and wherein the first and second sensors (S1, S2) are arranged to sense sound or vibrations resulting from a flow in the flow path, and - a processor arranged to process output signals from the first and second sensors (S1, S2) according to an algorithm (A1, A2), so as to generate a measure of flow in the flow path, characterized in that: the first and second positions are spaced apart, wherein the first position is selected to have a shorter distance to the air-inlet (A_I) than the second position, and wherein said algorithm (A1, A2) comprises an event classification part arranged to analyse the sound or vibrations sensed by the first and second sensors (S1, S2), so as to allow identification of at least one event, and wherein the event classification part is arranged to analyse the sound or vibrations sensed by the first and second sensors (S1, S2), so as to allow identification of all of: a priming event, a firing event, and an inhalation event, and wherein the event classification part is arranged to distinguish between a priming and firing event on the one hand, and an inhalation event on the other based on a duration feature derived from start and end times of events, and wherein the event classification part is arranged to distinguish between a priming event on the one hand and a firing event on the other based on power differences between sound or vibrations sensed by the first and second sensors (S1, S2). the first and second positions are spaced apart, wherein the first position is selected to have a shorter distance to the air-inlet (A_I) than the second position, and wherein said algorithm (A1, A2) comprises an event classification part arranged to analyse the sound or vibrations sensed by the first and second sensors (S1, S2), so as to allow identification of at least one event, and wherein the event classification part is arranged to analyse the sound or vibrations sensed by the first and second sensors (S1, S2), so as to allow identification of all of: a priming event, a firing event, and an inhalation event, and wherein the event classification part is arranged to distinguish between a priming and firing event on the one hand, and an inhalation event on the other based on a duration feature derived from start and end times of events, and wherein the event classification part is arranged to distinguish between a priming event on the one hand and a firing event on the other based on power differences between sound or vibrations sensed by the first and second sensors (S1, S2).