## 1 Introduction  
Acoustic spectroscopy is one of the most promising techniques for nondestructive testing of many materials. This work shows that acoustic spectroscopy in the audible range is also well prepared for the study of liquid solutions. No method can claim superiority, but sound-based sensing of liquids has several advantages over optical techniques and can be easily combined with other methods, such as electroacoustic measurements, as discussed in [1]. A review devoted to describing the advantages and limitations of acoustic spectroscopy, with a particular focus on pharmaceutical applications can be seen in [2]. However, most studies on this research topic are devoted to ultrasound techniques and devices, due to their higher energy and bandwidth than sounds in the audible range. This can be seen in the monographs devoted to this topic, like [3] and [4]. The last one is very interesting because a research by [PERSON] et al. [5], page 51, describes the ultrasonic measurement of different sugar concentrations with an accuracy of 0.2% in water volume for pure sugar solutions. They measured the velocity of ultrasound and the density in solutions of D-glucose, D-trutose, and sucrose at various concentrations (0-40% \(w/v\)) and temperatures (10-30 \({}^{\circ}\)C). This conversion of acoustic data to sound velocity is the norm in most ultrasound studies of liquids. The calculation of sound velocities introduces some important problems and uncertainties due to the need of using statistical or theoretical models and the existence of other processes, as is explained in several publications, for example the [PERSON] et al. 's excellent review about the determination of the speed of sound in ionic liquids [6]. A de scription of a low-cost system for the measurement of sound velocity in liquids can be found in [7]. The use of ultrasound for the study of pure liquids and solutions is limited, com-pared to its application to colloids, suspensions, and emulsions, as reviewed in [8]. However, there have been remarkable advances in recent years, as can be seen, for example, in [9; 10; 11; 12]. The acoustic research of aqueous electrolytes was performed by [PERSON] and [PERSON] in [13] using the Fourier spectrum pulse-echo technique, which is discussed in detail in [14]. The number of publications is too large for an exhaustive literature survey, so only a handful of representative examples are shown here. For a detailed review of ultrasound spectroscopy for particle size determination see [15]. In [16] [PERSON] et al. studied polydisperse emulsions by means of acoustic spectroscopy within the frequency range of (6-14) MHz in order to measure the droplet size distribution of water-in-sunflower oil emulsions for a volume fraction range from 10 to 50%. They concluded that the methodology was suitable for polydisperse particle size characterization for moderate concentrations up to 20% and the results were in good agreement with those obtained by laser diffraction analysis. Other interesting application to food analysis can be seen in [17], where the mechanism of rehydration of milk protein concentrate powders is studied by means of broadband acoustic resonance dissolution spectroscopy. Moreover, ref. [18] describes the use of an ultrasonic pulse echo system for vegetable oils characterization. Good reviews of high-resolution ultrasound spectroscopy can be found in [19; 20]. All measurements are based on the previous determination of the speed of sound and attenuation in the samples. A number of advantages and applications of this technique are clearly described, for example, samples with very small volumes can be analyzed using different ranges of pressure and temperature. As is explained in [19], at frequencies below 100 MHz, which is clearly the case of audible frequencies, for nano-sized dispersions or solutions, the contribution of scattering to attenuation can be neglected. Thus, attenuation at this long-wavelength regime is determined by the thermal and the shear (visco-inertial) effects.