## 1 Introduction  
The presence of power laws in critical phenomena [1,2] seems to indicate that something deep appears in the spectral density of the so-called flicker noise, that is actually rather variable. It runs like \(1/f^{\gamma}\), where \(0.5\leq\gamma\leq 1.5\), over different frequency magnitude orders. In particular, \(1/f\) noise is associated to a signal whose spectrum is linear in a logarithmic scale. A color name, \"pink noise\", is given by analogy with pink light, which has such a linear power spectrum over the visible range [1]. More generally, different colors are associated to noise with spectral profiles other than pink noise (\(\gamma=1\)), in reference to the light with analogous spectra [3], e.g., the spectral density of white noise is flat (\(\gamma=0\)), while red (or Brownian) noise has \(\gamma=2\). Power laws or \(1/f\) spectra were found very unexpectedly in different phenomena, and several authors [1,4,5] are convinced that there had to be an ubiquitous reason for this kind of power-law noises, linked to the universality of critical phenomena exponents. Curiously enough, \(1/f\) noise is present in nature in very unexpected places, i.e., the flow of the river Nile, the luminosity of stars, the loudness of a piece of classical music versus time, the flow of sand in an hourglass, and many others [1]. [PERSON] and [PERSON] [6] found that both voice and music broadcasts have \(1/f\) spectra, and they proposed an algorithm to compose \"fractal music\". A crucial feature of \(1/f\) noise is its scale invariance for any frequency or time range, and for this reason it has been considered as a wide manifestation of the fractal character of several natural phenomena. Moreover, also nonlinear processes can be often considered as sources of \(1/f\) noise [1,4,5]. After [PERSON]'s work [2], 1/_f_ noise has been linked to fractal phenomena and power laws. The concept of scaling, which leads to power laws, is present also in the physics of earthquakes [7; 8; 9]. This field has its cornerstone in the Gutenberg-Richter (GR) law [10], i.e., \(Log_{10}N(\geq m)=a-bm\), or \(N(\geq m)=10^{a-bm}\). This equation expresses the relationship between magnitude and total number of earthquakes with the same or a higher magnitude, in any given region and time period, and it is the most widely used statistical relation to describe the scaling properties of seismicity. \(N\) is the cumulative number of earthquakes with magnitude \(\geq m\) in a given area and within a specific time range, whilst \(a\) and \(b\) are positive constants varying from a region to another and from a time interval to another. [PERSON]'s law has been successfully employed also within the Acoustic Emission (AE) technique to study the scaling laws of signal amplitude distribution [7; 8; 9]. According to [PERSON]'s law, the \(b\)-value systematically changes at different times during the damage process. Therefore, it can be used to estimate the modalities of fracture evolution [11]. To establish a relationship between the size \(L\) of the defect associated to an AE event, and its magnitude \(m\), the [PERSON]'s law can be rewritten: \(N(\geq L)=cL^{-2b}\), where \(2b=D\) is the fractal dimension of the damage domain, \(c\) is a constant of proportionality, \(N\) is the cumulative number of AE events generated by source defects with a characteristic linear dimension \(\geq L\). The implications of the assumption \(2b=D\) has been pointed out by [PERSON] [12], [PERSON] [13; 14; 15], and [PERSON] [16; 17], assuming that \(2.0\leq D\leq 3.0\), i.e., the cracks are distributed in a fractal domain comprised between a fracture surface and the volume of the structural element [18; 19; 20; 21]. In the present work, the authors have observed that even in quasi-brittle materials, like rocks, concrete, and masonry, there is a source of 1/_f_ noise in AE from fracture phenomena. The results of laboratory tests on specimens of rocks and concrete, and of in-situ monitoring of Italian historical buildings are considered.