An injector including a nozzle that includes an opening and a seat, a needle movably mounted in the nozzle and having an end defining a valve in a contact area with the seat, a mechanism for vibrating the valve, a first acoustic-impedance breaking area at a first distance from the valve along the nozzle, and another first acoustic-impedance breaking area at a second distance from the valve along the needle. Each of the first and second distances is such that the respective propagation time of acoustic waves along the distance is: Ti=ni*[ζ/2], where ni is a positive integer coefficient different from zero with i=3 for the first distance and i=4 for the second distance, ζ being a period of the vibrations.