1D ultrasonic transducer unit for area monitoring, comprising a housing, at least three ultrasonic transducers and a control unit, wherein - the control unit is configured for individually activating each ultrasonic transducer, - the housing comprises a communications interface, - each ultrasonic transducer comprises a respective transducer housing, a piezoelectric body arranged in the transducer housing and a sound decoupling layer, which is arranged at an open end of the transducer housing, for decoupling in a gaseous medium, the transducer being arranged at a fixed position in the housing, - each ultrasonic transducer is configured for the purpose of radiating and/or receiving a sound wave with a corresponding working frequency, - the working frequency of the sound waves lies in a range of 20 kHz to 400 kHz, - each two ultrasonic transducers directly adjacent to one another have in the housing a spacing from sound decoupling layer centre to sound decoupling layer centre of at most 10 centimetres or at most 5 centimetres or at most 2 centimetres, - the 1D ultrasonic transducer unit has one sound channel per ultrasonic transducer, - each sound channel has an entry opening and an exit opening, - exactly one of the entry openings is associated with each sound decoupling layer, - the exit openings are arranged along a straight line, - the exit openings are each arranged in a wall of the housing or the sound channels penetrate the wall of the housing, - a spacing from the centre of one of the exit openings to the centre of a directly adjacent exit opening corresponds with at most the wavelength in the gaseous medium or at most half the wavelength of the gaseous medium, - wherein the spacing between two directly adjacent exit openings is smaller each time than the spacing between ultrasonic transducers associated with the corresponding entry openings, - a quotient of a surface area of the exit opening to a surface area of the entry opening has a value between 0.30 and 1.2 and - each sound channel has at least a length corresponding with the diameter of the entry opening, characterised in that - the housing has fastening means for fastening to a surface, - the control unit is arranged at least partly in the housing, - the housing has a movable cover device, wherein the cover device is configured to close the exit openings of all sound channels, - the exit openings of all sound channels lie in a curved area. - the control unit is configured for individually activating each ultrasonic transducer, - the housing comprises a communications interface, - each ultrasonic transducer comprises a respective transducer housing, a piezoelectric body arranged in the transducer housing and a sound decoupling layer, which is arranged at an open end of the transducer housing, for decoupling in a gaseous medium, the transducer being arranged at a fixed position in the housing, - each ultrasonic transducer is configured for the purpose of radiating and/or receiving a sound wave with a corresponding working frequency, - the working frequency of the sound waves lies in a range of 20 kHz to 400 kHz, - each two ultrasonic transducers directly adjacent to one another have in the housing a spacing from sound decoupling layer centre to sound decoupling layer centre of at most 10 centimetres or at most 5 centimetres or at most 2 centimetres, - the 1D ultrasonic transducer unit has one sound channel per ultrasonic transducer, - each sound channel has an entry opening and an exit opening, - exactly one of the entry openings is associated with each sound decoupling layer, - the exit openings are arranged along a straight line, - the exit openings are each arranged in a wall of the housing or the sound channels penetrate the wall of the housing, - a spacing from the centre of one of the exit openings to the centre of a directly adjacent exit opening corresponds with at most the wavelength in the gaseous medium or at most half the wavelength of the gaseous medium, - wherein the spacing between two directly adjacent exit openings is smaller each time than the spacing between ultrasonic transducers associated with the corresponding entry openings, - a quotient of a surface area of the exit opening to a surface area of the entry opening has a value between 0.30 and 1.2 and - each sound channel has at least a length corresponding with the diameter of the entry opening, - the housing has fastening means for fastening to a surface, - the control unit is arranged at least partly in the housing, - the housing has a movable cover device, wherein the cover device is configured to close the exit openings of all sound channels, - the exit openings of all sound channels lie in a curved area.