| 摘要 |
<p>Burglar alarm comprises a loudspeaker with a compression chamber mounted in a housing. The opening of the chamber is closed by a panel of material which allows passage of sound vibrations but which prevents insertion of a tool or product liable to disturb its functioning. Pref. the panel material consists of completely reticulated flexible polyester foam, and is applied over the opening with interposn. of a grating. Automatic protection means may be provided, consisting of two Hg contacts each fixed to a spring blade. The two blades are attached to the two halves of the housing and are maintained under tension by lugs which coact with the blades and which are provided on the two halves of the housing. The alarm cannot be deactivated by introduction into the chamber of a liq. resin which polymerises to a foam block.</p> |
