The basic properties of acoustic wave propagation in stellar interiors can be analysed from the autocorrelation function (ACF) of intensity (or velocity) observations without measuring the resonant p-mode frequencies. We show how the strength of acoustic wave refraction in the stellar core, or forward acoustic amplitude, can be measured from a modulation in the ACF. This is the basic physical quantity which governs the so-called `small frequency separations', and its measurement from the ACF can be used for determining the small frequency separations when the data is of insufficient quality for a reliable identification of the stellar p-mode frequencies.