## 2 Materials and Methods  
The experimental system was composed of three main parts: the anechoic chamber, the sound system, and the samples. The liquid sample was placed inside a small handmade anechoic chamber of exterior dimensions (width, high, depth) 80 \(\times\) 72 \(\times\) 56, in centimeters. Its interior was isolated using 2-cm thick foam and a frequency-dependent absorbant pyramidal material of 4 cm in the base and 6 cm high. Thus, the interior volume of the chamber is 58 \(\times\) 61 \(\times\) 40 cm. A cylindrical glass with a volume of 200 mL filled with 150 mL of a water and fructose solution was placed at the center of the chamber. The glass mass was 123 g, with a diameter of 8 cm. Figure 1 shows the schematic diagram of the experimental setup. The proposed method uses differential measurements and the acoustic performance of the chamber and the environment is sufficient for this purpose. Measurements of the chamber performance were made by means of a Bruel and Kjaer 2250 acoustic analyzer, resulting in 28.2 dBA of background noise and a mean reverberation time of 0.17 s. The frequency response of the chamber is represented in Figure 2.  
Figure 1: Photograph of the experimental installation. The used microphone was the model ECM-TL3 of Sony, an electret capacitor with omnidirectional pattern, frequency response range (20 Hz-20 kHz) with sensitivity of \(-35\) dB that was placed vertically 2.5 cm over the liquid surface, 1.5 cm from the center. In parallel, in a symmetric position to the center of the glass, one earpiece model Sony MDRXB50 APB.CE7 was used as the sound source, with a frequency response range of (4-24) kHz, a sensitivity of 106 dB/mW, and an impedance of 40 ohms (1 kHz). The test signals were generated by a computer while the recordings were made by another computer and an external audio card. The measuring system, background sound, and noise sound generated by the sound card is represented in Figure 3. A maximum level of 0.0271 is measured against the levels near 1 (to full scale) of the signals. The microphone was connected to a PC sound card MAudio Fast Track Ultra 8R. An amplification factor of 70% for the channel was used to avoid adding internal noise from the card. The measurements were taken with a recording rate of 44.1 kHz by means of the free Audacity software [40]. Sound amplitude was kept below the 70% of the maximum  
Figure 3: Recording of the sound card without signal. Figure 2: Frequency response of the anechoic chamber. level in order to avoid saturation effects. Some test signals were generated by MATLAB [41] and Audacity software:  
1. MLS signal: a signal generated by MATLAB, taking into account that the maximum length is 30 s. The amplitude is 60% of the full scale (FS);
2. White noise: a signal generated by Audacity, with an amplitude of 60% of the FS;
3. A set of chirp signals generated by Audacity, with a duration of 1 s each, from 150 Hz to 15 kHz;
4. Square pulses with a period of 250 ms and 50% of duty cycle. Each audio recording had a duration of 30 s, more than enough to ensure the precision and stability of the measurements. Later analyses showed that the recordings were stable enough to allow their partition in several 2 s intervals in order to increase the number of recordings for the classification algorithm. Changes among different spectra from the same sample were so low that they were not measurable. The experiments were performed with a set of 364 samples of water solutions with different concentrations of fructose (see Tables 1-3). The volume of each sample was 150 mL. Distilled water was used as solvent. Food grade pure fructose (>99%) was used for the liquid samples. The concentrations of fructose were from 0 to 9 g/L. A more detailed study was done between 2 g/L and 3 g/L in increments of \(\pm\)0.1 g/L and between 2.01 g/L and 2.09 g/L in increments of \(\pm\)0.01, in order to explore the performance of the system.