This invention utilizes the fact that sound can not propagate in a vacuum but can propagate in shell material in order to offer a method and apparatus for detecting if there exists any gaseous element inside a closed space encircled by a shell. Both the sound transmitter and the sound receiver are connected with the shell material but are separated by the closed space. By calculating the traversing times and/or amplitudes of the sound waves through the closed space and the shell material, it is possible to determine if there is any sound passing through the closed space, thereby to judze if any gas leakage exceeding the tolerable threshold has occurred inside the said closed space. |