  In case of a of the heterostructure n-GaAs/AlGaAs with sheet density $n=2
\times 10^{11}$cm$^{-2}$ and mobility $\mu \approx 2 \times 10^6$
cm$^2$/V$\cdot$s with integer and fractional quantum Hall effect (IQHE and
FQHE, respectively) we demonstrate the wide applicability of acoustic methods
for determining the general conduction parameters of a two dimensional electron
gas. We also examine the mechanisms of low-temperature conductivity in the
minima of oscillations of high frequency conductivity in the IQHE and FQHE
regimes. In the magnetic field region where electrons are delocalized, the
parameters determined by the acoustic technique do not differ from those
determined by a direct current. However, the acoustic measurements do not
require Hall bars and electrical contacts to be fabricated. In the minima of
IQHE and FQHE oscillations electrons are localized, and ac conductivity turns
to be via hopping. An analysis of the high frequency conductivity in the QHE
regime has been carried out within a "two site" model. Furthermore,
measurements of acoustoelectric effects in a tilted magnetic field provided the
dependence of the activation energy on magnetic field in the fractional quantum
Hall effect regime at $\nu$=2/3.
