  We estimate the levels of turbulence in the envelopes of Class 0 and I
protostars using a model based on measurements of the peak separation of
double-peaked asymmetric line profiles. We use observations of 20 protostars of
both Class 0 and I taken in the HCO+ (J=3-2) line that show the classic
double-peaked profile. We find that some Class 0 sources show high levels of
turbulence whilst others demonstrate much lower levels. In Class I protostars
we find predominantly low levels of turbulence. The observations are consistent
with a scenario in which Class 0 protostars form in a variety of environments
and subsequently evolve into Class I protostars. The data do not appear to be
consistent with a recently proposed scenario in which Class 0 protostars can
only form in extreme environments.
