  Acoustic oscillations in the baryon-photon fluid leave a signature in the
matter power spectrum. The overall shape of the spectrum and the wavelength of
the oscillations depend upon the sound horizon scale at recombination. Using
the $\Lambda$ cold dark matter Hubble Volume simulation, we show that the
imprint of baryons is visible in the power spectrum of cluster-mass dark matter
haloes, in spite of significant differences between the halo power spectrum and
the prediction of linear perturbation theory. A measurement of the sound
horizon scale can constrain the dark energy equation of state. We show that a
survey of clusters at intermediate redshift ($ z\sim1 $), like the
Sunyaev-Zeldovich survey proposed by the South Pole Telescope or a red sequence
photometric survey with VISTA, could potentially constrain the sound horizon
scale to an accuracy of $\sim 2%$, in turn fixing the ratio of the pressure of
the dark energy to its density ($w$) to better than $\sim 10%$. Our approach
does not require knowledge of the cluster mass, unlike those that depend upon
the abundance of clusters.
