Between 200 kHz and 4,6 GHz, ultrasonic absorption spectra of aqueous solutions of urea and some of its alkyl derivatives have been measured at various solute concentrations. The derivatives comprise methylurea, ethylurea, N-propylurea, tetramethylurea, N,N-diethylurea, N,N'-diethylurea, and N-butylurea. Up to very high concentrations of the solute, solutions of urea do not show noticeable contributions from relaxation processes in their absorption spectra. Depending upon concentration, the solutions of the alkyl derivatives reveal absorption in excess of the asymptotic high-frequency part of the spectra. This excess absorption is discussed in terms of a recent unifying model of noncritical concentration fluctuations. Parameters of this model, particularly the fluctuation correlation length and the relaxation rate on the order of parameter fluctuations, reflect in an obvious manner the hydrophobic character of the solutes. Comparison with other series of aqueous solutions exhibiting precritical behavior clearly shows the hydrophilic part of solute molecules to be of low significance in determining the maximum fluctuation correlation length of the liquids. Rather, the number and steric arrangement of carbohydrate groups are the dominant factors that cause noncritical local fluctuations in the concentration of solutes.