1. A method for classifying a background noise contained in an audio signal, the method comprising: extracting from the audio signal a background noise signal, the background noise signal being a noise signal; utilizing a computer processor to calculate a first parameter relating to a time evolution of the noise signal, and a second parameter relating to a frequency spectrum of the noise signal, the first parameter being a time indicator and the second parameter being a frequency indicator; classifying the background noise contained in the audio signal amongst a predefined set of background noise classes, as a function of values of the calculated time indicator and frequency indicator, wherein, in order to classify the background noise, the method further comprises: performing a first comparison by comparing the value of the time indicator calculated for the noise signal with a first threshold, and determining, as a function of the result of the first comparison, whether or not the noise signal is stationary; when the noise signal is identified as not stationary, performing a second comparison by comparing the value of the frequency indicator with a second threshold, and determining, as a function of the result of the second comparison, whether the noise signal belongs to a first class or to a second class of background noises; and when the noise signal is identified as stationary, performing a third comparison by comparing the value of the frequency indicator with a third threshold, and determining, as a function of the result of the third comparison, whether the noise signal belongs to a third class or to a fourth class of background noises.