A method for ascertaining at least one pipe wall resonance frequency of a pipeline in the region of a measuring point by means of a field device of process measurements technology having at least a first ultrasonic transducer, which is placed on the pipeline at the measuring point, comprising steps as follows: providing a first transfer function Utransducer(f) at least of the first or a plurality of ultrasonic transducers located in the region of the measuring point; ascertaining a received spectrum Urec(f) from a received signal Urec(t) after transmission of an ultrasonic signal; ascertaining a second transfer function Umeasuring point(f) from the first transfer function Utransducer(f) of the first or the plurality of ultrasonic transducers and from the received spectrum urec(f), wherein the second transfer function Umeasuring point(f) is characteristic for the measuring point; and ascertaining the at least one pipe wall resonance frequency fres, especially a plurality of resonance frequencies, in the region of the measuring point by evaluating the second transfer function Umeasuring point(f) from step III, as well as a clamp-on, ultrasonic, flow measuring device, a method for ascertaining flow, a method for ascertaining a change of the measuring point and an identification device.