  Photo-acoustic spectroscopy (PAS) is one of the most sensitive
non-destructive analysis techniques for gases, fluids and solids. It can
operate background-free at any wavelength and is applicable to microscopic and
even non-transparent samples. Extension of PAS to broadband wavelength coverage
is a powerful tool, though challenging to implement without sacrifice of
wavelength resolution and acquisition speed. Here, we show that the unmatched
precision, speed and wavelength coverage of dual-frequency comb spectroscopy
(DCS) can be combined with the advantages of photo-acoustic detection. Acoustic
wave interferograms are generated in the sample by dual-comb absorption and
detected by a microphone. As an example, weak gas absorption features are
precisely and rapidly sampled; long-term coherent averaging further increases
the sensitivity. This novel approach of photo-acoustic dual-frequency comb
spectroscopy generates unprecedented opportunities for rapid and sensitive
multi-species molecular analysis across all wavelengths of light.
