## 3 Results and Discussion
The configuration is the same as shown in Figure 2 schematic. Figure 7: FFT data of cantilever vibration with a smaller peak at 150 Hz due to the phone. Figure 3 shows the measurement configuration used, with the sensing beam placed a few cm away from the free end (case 3). This configuration introduces a higher tensile pre-strain in the FBG, which causes the baseline to shift (Figure 10) to a higher wavelength by approximately 0.02 nm. The two spectra marked A and B denote the wavelength shift measured at positions A and B of the sensing beam, respectively. When an 80 Hz signal was used to test both cases with the speaker, the lower baseline had a relatively lower peak than the higher baseline (Figure 11). This may be because the sensor was effectively moved closer to the acoustic source and received a higher amplitude. The same number of samples was used to calculate the FFT in both cases. Normalizing the two signals with reference to the central wavelength also yielded similar spectra. In this case, there is also no oscillation of the beam and no peak located around 10 Hz. These tests have been successfully repeated in several configurations and with different types of wedges and plates. When the gap is in the range of 10 cm, the sensor can detect frequencies up to 200 Hz. These results prove that the sensor can be attached to vibrating structures with metal conduits to measure the vibrational frequencies. All the beams and plates used were made of aluminum. Beam 1 was also tested as a sensing beam but its stiffness was higher and it was not sensitive to the forced vibrations. Thus, we may conclude that increasing the thickness or reducing the length can contribute to higher stiffness. Furthermore, when beam 2 was completely flat on the optical table and the speaker was placed on top of it at 10 cm from the FBG location, the sensitivity was zero. Thus, it is important to place the sensing beam in a position where it can flex in both directions to allow for the presence of compressive and tensile stresses. Figure 10: Time varying data of central wavelength position of FBG (beam 2) at two different locations along beam 1 due to the 80 Hz tone. The lower signal in the data is measured at position B and upper signal at position A, as shown in the schematic of Figure 3.  
Figure 3:0 FFT data of negligible cantilever vibrations with a dominant peak at 150 Hz due to the phone. Figure 12 shows the FFT data of three tests (case 4). A frequency tone of 150 Hz was used to test the effect of vibrations when the FBG bearing surface was facing upwards with a central wavelength that is near 1549.72 nm. After this, the sensing beam was flipped at the same location and the central wavelength of the grating was shifted to 1549.68 nm. However, the repeated measurements showed that the forced vibrations were detected with a similar magnitude. We outline the possible reasons for the sensor response in the following paragraph. As the length of the beam is significantly greater than the width and thickness, the stiffness of the beam is low. The sensing beam does not behave as a single uniformly strained cantilever but as several sections that behave as smaller cantilever-type sections. When the sensor beam was wedged near the center and was free to vibrate on both ends, dual peaks were obtained that were both under 20 Hz. The sensing capability is possible in this type of wedged design because it creates a pre-strain. This allows vibrations to induce compression and extension on the fiber grating periods. Thus, its sensitivity to vibrations increases, while the FBG is unable to compress or expand freely when it is stuck or glued to metallic surfaces. The flexibility of the beam allows the signals to pick up multiple vibrational  
Figure 3:1 FFT data of 80 Hz tone measured at two different distances from the acoustic source, positions A and B, as shown in schematic of Figure 3:2  
Figure 3:3 FFT data of 150 Hz tone used to test sensing beam at one location, with FBG facing upwards.