A noise reduction device for a respiratory apparatus , comprising: a body configured to be mounted on the respiratory apparatus , the body comprising a side wall and a gas outlet ; and a cover configured to be detachably engageable with the body for forming a gas inlet and a gas passage , wherein the cover comprises a guiding member defining at least a part of the gas passage , wherein the guiding member is configured to be coupled with the side wall of the body to form the gas passage between the body and the cover , and wherein the gas passage formed directs a flow of gas to an angular rotation about a centre of the gas outlet of at least 180 degrees to 330 degrees relative to the gas inlet before discharging at the gas outlet ; characterized in that the noise reduction device further comprises: a locking mechanism for slidably locking the cover to the body , wherein the locking mechanism comprises at least two corresponding tabs arranged on the cover , and at least two slots respectively arranged on substantially diametrically opposite sides of the side wall for complementary slide locking with the corresponding tabs on the cover , and wherein the cover is engaged with the body to form the gas inlet and the gas passage by that the at least two corresponding tabs arranged on the cover are slidably locked to the at least two slots arranged on the body respectively. a body configured to be mounted on the respiratory apparatus , the body comprising a side wall and a gas outlet ; and a cover configured to be detachably engageable with the body for forming a gas inlet and a gas passage , wherein the cover comprises a guiding member defining at least a part of the gas passage , wherein the guiding member is configured to be coupled with the side wall of the body to form the gas passage between the body and the cover , and wherein the gas passage formed directs a flow of gas to an angular rotation about a centre of the gas outlet of at least 180 degrees to 330 degrees relative to the gas inlet before discharging at the gas outlet ; characterized in that the noise reduction device further comprises: a locking mechanism for slidably locking the cover to the body , wherein the locking mechanism comprises at least two corresponding tabs arranged on the cover , and at least two slots respectively arranged on substantially diametrically opposite sides of the side wall for complementary slide locking with the corresponding tabs on the cover , and wherein the cover is engaged with the body to form the gas inlet and the gas passage by that the at least two corresponding tabs arranged on the cover are slidably locked to the at least two slots arranged on the body respectively. a locking mechanism for slidably locking the cover to the body , wherein the locking mechanism comprises at least two corresponding tabs arranged on the cover , and at least two slots respectively arranged on substantially diametrically opposite sides of the side wall for complementary slide locking with the corresponding tabs on the cover , and wherein the cover is engaged with the body to form the gas inlet and the gas passage by that the at least two corresponding tabs arranged on the cover are slidably locked to the at least two slots arranged on the body respectively. <EOS>