The person is suffering from conductive hearing loss ,so his hearing will not be interfered by the surrounding noise .
In a normal person the surrounding noise would interfere with the vibrations of tuning fork and hence he will hear the vibrations for lesser time as compared to that of patient of conductive hearing loss
In Schwabach test we test the loudness of the tuning fork placed on the mastoid ( Bone conduction ie SN pathway ) while the EAC is open ie the Tragus is not pressed .
In case of CHL the external sounds are not able to enter through the ear to reach the SN pathway and hence they do not interfere with the tuning fork sound which is travelling through the SN pathway ( this interference occurs when there is no CHL )
Hence the patient with CHL will hear the sound longer than a person who does not have CHL .