However, as the acoustic properties of the oranges are not altered by the change in force (i.e. the first term of Eq. (3) remains constant), the results from each remain constant relative to one another regardless of the force which is applied. As long as the transducer coupling force is applied consistently, any number of samples can be directly compared based on their acoustic properties alone, as the ultrasonic energy returned from the imperfect coupling boundary will remain constant for each.