The present invention discloses an ultrasonic phased array transducer with an ultralow backfill and a method for making. The ultrasonic phased array includes a low density backfill material having an ultralow acoustic impedance. The backfill material is either an aerogel, a carbon aerogel, an xerogel,...http://www.google.ca/patents/US5655538?utm_source=gb-gplus-sharePatent US5655538 - Ultrasonic phased array transducer with an ultralow impedance backfill and a method for making
