Heterodyne scanning probe microscopy method for imaging structures below the surface of a sample, the method including: applying, using a transducer (12, 18), an acoustic input signal to the sample; sensing an acoustic output signal using a probe, the probe including a cantilever and a probe tip, wherein the probe tip is in contact with the surface, wherein the acoustic output signal is representative of an amount of indentation of the surface by the probe tip in response to the acoustic input signal applied to the sample; wherein the acoustic input signal comprises a plurality of signal components (33, 34) that each have a unique frequency, wherein the plurality of signal components (33, 34) comprise: a base signal component at a carrier frequency (f c); and a plurality of additional signal components having further excitation frequencies; wherein the carrier frequency (f c) and the further excitation frequencies together form a group of frequencies, wherein the frequencies of the group of frequencies are distributed with an equal difference frequency between each two subsequent frequencies of the group; wherein the difference frequency is a frequency below a sensitivity threshold frequency, for being within a sensitivity frequency range of the system, for enabling said sensing of the acoustic output signal; and wherein the additional signal components are provided as sideband signal components such that in a frequency domain the further excitation frequencies of the sideband signal components are arranged at either side of the carrier frequency (f c) of the base signal component, wherein at least two sideband signal components are arranged at either side of the carrier frequency (f c). applying, using a transducer (12, 18), an acoustic input signal to the sample; sensing an acoustic output signal using a probe, the probe including a cantilever and a probe tip, wherein the probe tip is in contact with the surface, wherein the acoustic output signal is representative of an amount of indentation of the surface by the probe tip in response to the acoustic input signal applied to the sample; wherein the acoustic input signal comprises a plurality of signal components (33, 34) that each have a unique frequency, wherein the plurality of signal components (33, 34) comprise: a base signal component at a carrier frequency (f c); and a plurality of additional signal components having further excitation frequencies; wherein the carrier frequency (f c) and the further excitation frequencies together form a group of frequencies, wherein the frequencies of the group of frequencies are distributed with an equal difference frequency between each two subsequent frequencies of the group; wherein the difference frequency is a frequency below a sensitivity threshold frequency, for being within a sensitivity frequency range of the system, for enabling said sensing of the acoustic output signal; and wherein the additional signal components are provided as sideband signal components such that in a frequency domain the further excitation frequencies of the sideband signal components are arranged at either side of the carrier frequency (f c) of the base signal component, wherein at least two sideband signal components are arranged at either side of the carrier frequency (f c). a base signal component at a carrier frequency (f c); and a plurality of additional signal components having further excitation frequencies; wherein the carrier frequency (f c) and the further excitation frequencies together form a group of frequencies, wherein the frequencies of the group of frequencies are distributed with an equal difference frequency between each two subsequent frequencies of the group; wherein the difference frequency is a frequency below a sensitivity threshold frequency, for being within a sensitivity frequency range of the system, for enabling said sensing of the acoustic output signal; and wherein the additional signal components are provided as sideband signal components such that in a frequency domain the further excitation frequencies of the sideband signal components are arranged at either side of the carrier frequency (f c) of the base signal component, wherein at least two sideband signal components are arranged at either side of the carrier frequency (f c).