## 2 Materials and Methods
### Ambient Vibration Tests  
This non-destructive test, used to determine the stiffness of a structure in situ, measures the movement or vibration generated by excitation sources external to the structure. All systems are subjected to a series of ambient loads (e.g., wind, traffic, human activity) that generate a response expressed as an oscillation or vibration. The consequence of these external loads is called ambient vibration. Ascertaining the structure's response enables its fundamental period/frequency to be estimated accurately, which in turn means the structure's stiffness and consequently bracing can be determined.  
In this manner, the outcome of the test makes it possible to estimate the overall stiffness of a structure (Equation (1)), providing an empirical value to assess its state of preservation. The use of this technique in buildings stems from the need to empirically obtain some of their dynamic characteristics in order to work out their behaviour in response to loads such as earthquakes or wind. The energy used to determine these properties can be provided by induced vibration techniques or from existing ambient vibration where the building is located. Compared to the use of forced or induced vibration, cultural noise has significant advantages, such as efficiency, time and economy. Besides, the method avoids the use of any type of stress on a structure that may be in poor condition, thus, eliminating a risk both for the people performing the test and for the structure itself. The outcome of various experiments [32; 33; 34; 35] reveals that the use of ambient vibration is sufficiently precise to obtain the fundamental period of a structure and other dynamic characteristics.  
The tests are performed using a single accelerometer placed on the top floor of the structure, capable of measuring small oscillations, especially those not perceptible by humans. The data registered by the accelerometer are filtered through an analogue-digital converter and the signal is then analysed using appropriate software. Readings were taken with a uniaxial high sensitivity Inonomic KB12 VD (10,000 mV/g) accelerometer, capable of measuring a frequency range between 0.08 and 260 Hz. The accelerometer was connected by a uniaxial transducer connection signal cable to the digital data capturing system, whereupon the filtered signal was analysed and recorded throughout the test using specific software installed on a laptop. Testing points in the different towers are shown in Figures 9-11, which also display the general dimensions of the towers, as well as the dimensions corresponding to the inner levels and their orientations from North. Except for the Campanillas, Tramores and Sal towers, all tests were carried out on the upper level of the towers. Two readings--parallel and perpendicular to the wall--were taken at each point.  
Figure 9: Points where ambient vibration testing was carried out. Towers are sorted by the provinces where they are located—Granada, Almeria and Malaga,—and by municipality.  
Figure 10: Points where ambient vibration testing was carried out. Towers are sorted by the provinces where they are located—Granada, Almeria and Malaga,—and by municipality.  
Figure 12 shows the equipment used to take measurements in the Duque (Marbella) and Torremolinos (Malaga) towers: a laptop computer, accelerometer and analogical-digital converter.  
The set of measurements obtained at the different points in each of the towers were processed analytically to obtain the fundamental vibration frequency of the masonry structure.