System and method for detecting foreign objects in a product

A system for detecting a change in material composition in a product, said system comprising:

• at least one microwave transmitter, arranged so as to emit a microwave signal at a predetermined microwave frequency composition, and
• at least one ultrasonic transmitter, arranged so as to emit an ultrasonic signal at a predetermined ultrasonic frequency composition into at least a part of said product to create a propagating density displacement within the product,
• said microwave and ultrasonic transmitters are arranged so as to cause a local interaction between said microwave and ultrasonic signals in at least a part of said product, and said system further comprises:
• receiver means to receive at least one of:
• a) the microwave signal(s) emitted by the microwave transmitter(s) having passed through said part of said product,
• b) the ultrasonic signal(s) emitted by the ultrasound transmitter(s) having passed through said part of said product,
• measurement means to measure the attenuation and/or runtime between each emitted signal and each received signal, respectively, under the influence of said propagating density displacement and the associated interaction, and
• comparison means to compare the measured attenuation and/or runtime with a previously determined attenuation and/or runtime to determine a change in material composition.