## 2 1/_f_ Noise in Damaging Structures: Laboratory Tests  
Acoustic Emission (AE) is a Non-Destructive Inspection Technique that utilizes the transient elastic waves produced by fracture phenomena. These waves are picked up by piezoelectric sensors positioned on the external surface of the monitored body [22; 23]. In laboratory tests and in-situ campaigns, the authors exploited a leading-edge monitoring equipment [24] for multichannel data processing, and consisting in six synchronized \"Units for Storage Acoustic Emission Monitoring\" (USAM). Each of these units analyzes in real time, and transmits to a computer, all the characteristic parameters of an ultrasonic event. Specific waveform parameters are set as in the following: Peak Definition Time (PDT) = 50 \(\upmu\)s; Hit Definition Time (HDT) = 150 \(\upmu\)s; Hit Lockout Time (HLT) = 300 \(\upmu\)s. In this manner, each AE event is identified by a progressive number and characterized by a series of data giving the amplitude and time duration of the signals and the number of oscillations. Moreover, the absolute acquisition time and signal frequency are also given, so that, through an analysis of signal frequency and time of arrival, it is possible to identify the group of signals belonging to the same AE event and to localize it. Each unit is equipped with a pre-amplifiedwide-band piezoelectric sensor, which is sensitive in a frequency range between 50 kHz and 800 kHz. The signals acquisition threshold can be set in a range between 100 \(\upmu\)V and 6.4 mV. In this section, the results of compression tests performed on concrete specimens having diameter equal to 100 mm, and slenderness \(\uplambda\) equal to 1.0 and 2.0 are presented (Figure 1). Generally speaking, in a frictionless condition, for a specimen with slenderness \(\uplambda\leq 1.0\) a failure mainly governed by crushing is expected, while for a specimen with slenderness \(\uplambda\geq 2.0\) a failure governed by splitting is taken into account. For intermediate values of slenderness, shear failure is characterized by the formation of inclined slip bands in the bulk of the specimen. For all cases, the cumulated number of AE is strictly correlated to global or local mechanical instabilities in the post-peak regime of the loading process. The loading process was controlled by the specimen circumferential expansion, measured by means of a linked chain placed around the sample mid-height (Figure 1b). Such a control permitted to fully detect the load-displacement curve, even in case of snap-back phenomena [25; 26]. The specimen with a slenderness \(\uplambda\) equal to 1.0 was equipped with a thin layer of Teflon in contact between the platen of the testing machine and the specimen ends, to avoid the friction on the bases and facilitate the transversal deformation. During the compression tests, Acoustic Emission technique was adopted to monitor each specimen. AE signals generating from the fracture phenomena were detected by attaching a piezoelectric sensor to the surface of the specimens with a silicon glue. As stated, the sensor is sensitive in the frequency range (50-800 kHz), and it is placed on the external surface of the monitored samples (Figure 1b). The AE acquisition threshold level was set up to 2 mV, with an amplification gain of the AE signals equal to 60 dB. The recorded waveform sampling frequency was 1 Msample/s. The results obtained from the compression test of specimen with \(\uplambda=1.0\) are represented in Figures 2-4. The signal frequency refers to the average values, the average frequency (AF), which is calculated as the ratio between the AE ring-down counts and the signal duration time, while the energy of each AE signal is calculated according to RILEM [27]. \(1/f^{\gamma}\) distribution of AE signals is divided into two time windows: before the achievement of the peak load, and after the peak load (Figure 2). In Figure 3, the log-log plot of AE power spectrum versus frequency for signals detected before the peak load is represented.