A vibration unit (112, 200, 1200, 2100) comprising: a base structure (202, 1202, 2102); a contact structure (204, 1206, 2106), wherein the contact structure takes the form of a plate having a contact surface configured to provide contact with an object under test and a further surface opposite to the contact surface; piezoelectric transducer array (124, 308, 1204) located between the base structure (202, 1202, 2102) and the further surface of the contact structure (204, 1206, 2106) and configured to generate vibrations; and a preload system (306, 2110) configured to apply the preload on the piezoelectric transducer array (124, 308, 1204), the preload system (306, 2110) is connected to the base structure (202, 1202, 2102) and the contact structure (204, 1206, 2106) and is configured to pull the base structure (202, 1202, 2102) and the contact structure toward each other to apply the preload on the piezoelectric transducer array (124, 308, 1204), the preload system comprising a tie rod (324, 2112) having a first end associated with one of the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102), the tie rod (324, 2112) extending through a channel (502, 2118) in the other of the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102), a nut (326, 2114) fastened to a second end of the tie rod (324, 2112) and arranged to be manipulated to pull the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102) towards each other, a biasing mechanism (328, 1214, 2116) configured to bias the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102) toward each other, and a load cell (330, 1216, 2130) configured to detect a load on piezoelectric transducer array (124, 308, 1204). a base structure (202, 1202, 2102); a contact structure (204, 1206, 2106), wherein the contact structure takes the form of a plate having a contact surface configured to provide contact with an object under test and a further surface opposite to the contact surface; piezoelectric transducer array (124, 308, 1204) located between the base structure (202, 1202, 2102) and the further surface of the contact structure (204, 1206, 2106) and configured to generate vibrations; and a preload system (306, 2110) configured to apply the preload on the piezoelectric transducer array (124, 308, 1204), the preload system (306, 2110) is connected to the base structure (202, 1202, 2102) and the contact structure (204, 1206, 2106) and is configured to pull the base structure (202, 1202, 2102) and the contact structure toward each other to apply the preload on the piezoelectric transducer array (124, 308, 1204), the preload system comprising a tie rod (324, 2112) having a first end associated with one of the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102), the tie rod (324, 2112) extending through a channel (502, 2118) in the other of the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102), a nut (326, 2114) fastened to a second end of the tie rod (324, 2112) and arranged to be manipulated to pull the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102) towards each other, a biasing mechanism (328, 1214, 2116) configured to bias the contact structure (204, 1206, 2106) and the base structure (202, 1202, 2102) toward each other, and a load cell (330, 1216, 2130) configured to detect a load on piezoelectric transducer array (124, 308, 1204).