Device for passage of an air stream through a component of an air-conditioning system or a heating system, in particular of a heat-pump heat-transfer unit, in particular for free outflow of an air stream into surroundings, comprising • an air channel with an air-channel axis and with an opening into the surroundings, and • a protective grid, arranged in the air channel transversely to a main flow direction of the air stream, in the region of the opening as an air-permeable cover of the air channel, in particular with respect to the surroundings, wherein the protective grid has a first, in particular radially inner, protective-grid portion close to the air-channel axis and has a second, in particular radially outer, protective-grid portion remote from air-channel axis, • wherein the device is configured to direct the air stream to the second protective-grid portion, wherein the air stream flows, in particular completely, through the second protective-grid portion, • wherein, with regard to their/its flow cross-sectional areas/area, the air channel and/or the second protective-grid portion (120- 2) have/has a noise-emission-optimized geometry, the device having a fan, in particular an axial fan, for conveying the air stream, wherein the fan is arranged in the air channel upstream of the protective grid, characterized in that a clear flow cross-sectional area transverse to the actual air stream in the air channel between fan and protective grid, in particular including a clear flow cross-sectional area of the second protective-grid portion, is continuous and constant along the main flow direction or is continuous and increases slightly along the main flow direction. air-conditioning system or a heating system, in particular of a heat-pump heat-transfer unit, in particular for free outflow of an air stream into surroundings, comprising • an air channel with an air-channel axis and with an opening into the surroundings, and • a protective grid, arranged in the air channel transversely to a main flow direction of the air stream, in the region of the opening as an air-permeable cover of the air channel, in particular with respect to the surroundings, wherein the protective grid has a first, in particular radially inner, protective-grid portion close to the air-channel axis and has a second, in particular radially outer, protective-grid portion remote from air-channel axis, • wherein the device is configured to direct the air stream to the second protective-grid portion, wherein the air stream flows, in particular completely, through the second protective-grid portion, • wherein, with regard to their/its flow cross-sectional areas/area, the air channel and/or the second protective-grid portion (120- 2) have/has a noise-emission-optimized geometry, • an air channel with an air-channel axis and with an opening into the surroundings, and • a protective grid, arranged in the air channel transversely to a main flow direction of the air stream, in the region of the opening as an air-permeable cover of the air channel, in particular with respect to the surroundings, wherein the protective grid has a first, in particular radially inner, protective-grid portion close to the air-channel axis and has a second, in particular radially outer, protective-grid portion remote from air-channel axis, • wherein the device is configured to direct the air stream to the second protective-grid portion, wherein the air stream flows, in particular completely, through the second protective-grid portion, • wherein, with regard to their/its flow cross-sectional areas/area, the air channel and/or the second protective-grid portion (120- 2) have/has a noise-emission-optimized geometry, the device having a fan, in particular an axial fan, for conveying the air stream, wherein the fan is arranged in the air channel upstream of the protective grid, characterized in that a clear flow cross-sectional area transverse to the actual air stream in the air channel between fan and protective grid, in particular including a clear flow cross-sectional area of the second protective-grid portion, is continuous and constant along the main flow direction or is continuous and increases slightly along the main flow direction.