1D ultrasonic transducer unit, wherein the ultrasonic transducer unit is configured for material detection, comprising a housing, at least three ultrasonic transducers and a control unit, wherein - the ultrasonic transducers are of discrete construction, - the control unit is designed for the purpose of individually controlling each ultrasonic transducer, - the housing has 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, and is arranged at a fixed position in the housing, - each ultrasonic transducer is designed for the purpose of emitting and/or receiving a soundwave with a corresponding working frequency, - the working frequency of the soundwaves lies in a range of 20 kHz to 400 kHz, characterised in that - the housing comprises fastening means for fastening to a surface, - the control unit is arranged at least partly in the housing, - each two ultrasonic transducers, which are arranged directly adjacent to one another, in the housing have a spacing from centre of sound decoupling layer to centre of sound decoupling layer of at most 10 cm or at most 5 cm or at most 2 cm, - the 1D ultrasonic transducer unit has one sound channel per ultrasonic transducer, - each sound channel has an inlet opening and an outlet opening, - exactly one of the inlet openings is associated with each sound decoupling layer, - the outlet openings are arranged along a straight line, - the outlet 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 outlet openings to the centre of a directly adjacent outlet opening corresponds with at most the wavelength in the gaseous medium or at most with half the wavelength in the gaseous medium, - wherein the spacing between two directly adjacent outlet openings is in each instance smaller than the spacing between the ultrasonic transducers associated with the corresponding inlet openings, - a quotient of an area of the outlet opening with respect to an area of the inlet opening has a value between 0.30 and 1.2 and characterised in that - each sound channel has at least a length corresponding with the diameter of the inlet opening, - the housing comprises a movable cover device, wherein the cover device is designed for the purpose of closing the outlet openings of all sound channels, - the outlet openings of all sound channels lie in a curved surface. - the ultrasonic transducers are of discrete construction, - the control unit is designed for the purpose of individually controlling each ultrasonic transducer, - the housing has 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, and is arranged at a fixed position in the housing, - each ultrasonic transducer is designed for the purpose of emitting and/or receiving a soundwave with a corresponding working frequency, - the working frequency of the soundwaves lies in a range of 20 kHz to 400 kHz, characterised in that - the housing comprises fastening means for fastening to a surface, - the control unit is arranged at least partly in the housing, - each two ultrasonic transducers, which are arranged directly adjacent to one another, in the housing have a spacing from centre of sound decoupling layer to centre of sound decoupling layer of at most 10 cm or at most 5 cm or at most 2 cm, - the 1D ultrasonic transducer unit has one sound channel per ultrasonic transducer, - each sound channel has an inlet opening and an outlet opening, - exactly one of the inlet openings is associated with each sound decoupling layer, - the outlet openings are arranged along a straight line, - the outlet 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 outlet openings to the centre of a directly adjacent outlet opening corresponds with at most the wavelength in the gaseous medium or at most with half the wavelength in the gaseous medium, - wherein the spacing between two directly adjacent outlet openings is in each instance smaller than the spacing between the ultrasonic transducers associated with the corresponding inlet openings, - a quotient of an area of the outlet opening with respect to an area of the inlet opening has a value between 0.30 and 1.2 and characterised in that - each sound channel has at least a length corresponding with the diameter of the inlet opening, - the housing comprises a movable cover device, wherein the cover device is designed for the purpose of closing the outlet openings of all sound channels, - the outlet openings of all sound channels lie in a curved surface.