ometimes there are noises inside our home which, although not loud, can even disturb our sleep. They are more easily perceived in the evening or at night, when outdoor background noise decreases. Special tools are required to identify the type of noise by analysing its narrow-band frequency composition. Spectrograms, also known as sonograms, are usually used, together with a highly sensitive microphone, to show how the frequency content of a signal changes over time. Identifying the characteristics of a sound is a prerequisite for understanding where the noise comes from.