The fundamental frequency is the natural or resonant frequency of a system and the first harmonic of a system's oscillation.

See also Subharmonic Imaging.

Usually, multiple probes are used because most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency.
Multi-frequency probes have multiple crystals with different frequencies and the desired specific frequency can be selected. Advanced probes can emit sound waves at different frequencies for the near and far fields. The disadvantage is that multi-frequency (multifrequency) probes have slower frame rates and therefore they are only useful for imaging of static structures.

See also Dual Frequency Phased Array Transducer and Tri-Frequency Probe.

(PRF) The pulse repetition frequency is the number of pulses per second. The usual range for echocardiographs is between 200 and 5000 pulses per second. The PRF varies with the type of mode in operation.

See also Q-value.

The Doppler interrogation frequency is the frequency of the transmitted acoustic energy relative to the measured Doppler frequency shift. This frequency is most but not necessarily, the nominal transducer frequency.

See also Doppler Effect and Doppler Ultrasound.

Dual frequency phased array transducers allow performing third or fourth harmonic imaging. This array design contains two different types of elements arranged in an interleaved pattern (odd and even elements). The elements can work individually and at a distinct frequency enabling separate transmission and receiving modes.