  Context: Rapid rotation modifies the structure of the frequency spectrum of
pulsating stars, thus making mode identification difficult.
  Aims: We look for new forms of organisation for the frequency spectrum that
can provide a basis for mode identification at high rotation rates.
  Methods: Acoustic modes in uniformly rotating polytropic models of stars are
computed using a numerical code that fully takes the effects of rotation
(centrifugal distortion and Coriolis acceleration) into account. All low-degree
modes, l=0 to 3, with radial orders n=1-10 and 21-25 for N=3 polytropic models
and n=1-10 for N=1.5 polytropic models are followed from a zero rotation rate
up to 59 % of the break-up velocity.
  Results: We find an empirical formula that gives a good description of the
high-frequency range of the computed acoustic spectrum for high rotation rates.
Differences between this formula and complete eigenmode calculations are shown
to be substantially smaller than those obtained with a third order perturbative
method valid at low rotation rates.
