Helmet with a magnetostrictive actuator

Abstract

A magnetostrictive actuator comprises a rod of giant magnetostrictive material, biasing permanent magnets arranged at each end thereof and coaxially therewith so as to pass a magnetic field through the rod. An energizing electromagnetic coil is connected to an energizer so as to superimpose on the magnetic field passing through the rod an alternating magnetic field to cause variation in the length of the rod in response thereto. The permanent magnets are each in the form of a disc having a cross-sectional area substantially greater than that of the rod and are spaced from the ends of the rods by material of low magnetic permeability. The magnetic field in the rod is thereby substantially linear along the length thereof.

1 . A helmet or protective hat having an audio frequency actuator acoustically coupled therewith, and means for supplying an audio frequency drive signal to the actuator.  
   
   
       2 . A helmet or protective hat according to  claim 1 , wherein the audio frequency actuator is a magnetostrictive actuator.  
   
   
       3 . A hat comprising a headband carrying an audio frequency actuator connected to means for supplying an audio frequency drive signal thereto, the actuator being positioned so as to engage the wearer's head, in use, to couple acoustic energy directly into the wearer's skull.  
   
   
       4 . A helmet or protective hat according to  claim 2 , wherein the means for supplying an audio frequency signal comprises a radio receiver.  
   
   
       5 . A hat of  claim 3 , wherein the means for supplying an audio frequency signal comprises a radio receiver.