**2. Materials and Methods**

An array of six broadband AE transducers (with a flat frequency response nominally in the range of 50–500 kHz, as declared by the manufacturer, i.e., well beyond the frequency range of signals propagating in the masonry) was fixed to the north-east angle of the Asinelli Tower at an average height of 9.00 m above ground level. The frequencies of interest in the old masonry, around 30 kHz, were still detected by the adopted transducers as already reported in previous papers for similar surveys [37]. Figure 1a–c show the tower and the transducers applied to a masonry wall portion.

**Figure 1.** (**a**) The Asinelli Tower (with Garisenda Tower on the left) in the city centre of Bologna; (**b**) Monitored portion of the masonry wall with the applied AE transducers, printed from [30]; (**c**) AE transducer adopted for the monitoring; (**d**) Typical AE signal formed by the sequence of P-, S- and surface waves.

The transducers were connected to a six-channel acquisition system able to store AE signal parameters such as arrival time *t* (determined with accuracy of 0.2 μs), duration, peak amplitude, and ring-down count (number of times the AE signal exceeds a preset threshold). A time accuracy of 0.2 μs, equivalent to a sampling rate of five mega-samples per second, is adequate to measure frequency components up to 500 kHz (only frequencies up to 10 times smaller than the sampling rate are usually considered), covering satisfactorily the AE frequency range.

Prior to starting the monitoring, were preliminarily performed for a representative period of time, i.e., 8 h, in order to determine the level of spurious signals. Thus, the signal acquisition threshold was set to 100 μV in order to filter out the electrical noise (see a typical signal waveform in Figure 1d). Keeping fixed the threshold made it possible to capture possible variations in the noise level due to changeable environmental conditions like traffic, weekdays, weekends, etc.

AE monitoring began on 23 September 2010 at 5:40 p.m. and ended on 28 January 2011 at 1:00 p.m. for a 2915-h period [30]. All the AE signals were found to fall in the amplitude range of 100 to 12,800 μV or, equivalently, of 40 to 82 dB, if the signal peak amplitudes *A*max are expressed in decibels (dB), *A*dB = 20 log10(*A*max/1 μV) [38]. Because of heterogeneity of masonry, the point location method of AE sources exploiting signals recorded by multiple transducers could hardly be applied.
