Detector and methods of detecting

A broadband ultrasonic sensor for detecting a source of airborne ultrasound within a broadband detection frequency range, the source of airborne ultrasound in an external environment external to the ultrasonic sensor, the ultrasonic sensor comprising:

• a housing providing an internal environment;
• a piezoelectric crystal having:
• two sensing faces, a first sensing face exposed to said environment for receiving said airborne ultrasound and a second sensing face behind said first sensing face and exposed to said internal environment;
• and
• at least one outer face between said first and second sensing faces;
• the piezoelectric crystal mounted in said housing and mechanically isolated from said housing;
• said mechanical isolation provided by a material holding and in contact with said piezoelectric crystal which permits substantially uninhibited deformation of said piezoelectric crystal in response to incident ultrasound within said broadband detection frequency range so that a voltage is generated between said two sensing faces thereof, but which material inhibits vibration of said piezoelectric crystal when subjected to vibrations outside said broadband detection frequency range;
• wherein said material surrounds and is in contact with substantially all of said at least one outer surface of said piezoelectric crystal, and wherein said material comprises a closed cell foam to provide a pressure barrier between said internal and external environments, whereby said airborne ultrasound is received by said first sensing face of the piezoelectric crystal but is inhibited from reaching said second sensing face of the piezoelectric crystal.