## 2 Experimental Methods  
A FBG with a central wavelength at 1549 nm, which was fabricated on a single-mode fiber, was used as the sensor. The bare FBG was glued to a thin and light metal beam (aluminum, 470 mm \(\times\) 15 mm \(\times\) 0.3 mm) with a rectangular cross-section. The FBG was located at around 15 cm from one end of the beam. The wavelength shift was recorded with the Micron optics interrogation (MOI) system sm130 (Micron Optics, Atlanta, GA, USA). The sampling frequency of the interrogation system was limited to 500 Hz but the same measurements can be obtained using an edge filter and a photoreceiver or using alternate interrogation systems with higher sampling frequency. Two common acoustic sources were used for the experiments in this study. An LG phone was used in-contact for pulsed vibrations and a Bluetooth speaker (connected to a phone) was used for frequency tones. The wavelength shift was recorded in the time domain and converted to the frequency domain by Fast Fourier Transform (FFT) function in MATLAB (R2014a, MathWorks, Natick, MA, USA). The sensor beam was tested in two different configurations. The first batch of experiments was set up with another beam acting as a wedge (or intermediate medium) that couples vibrations on to the sensing beam. The tested configurations are described as cases 1 to 4. The second setup used a simply supported aluminum plate as the intermediate medium, with the sensing beam placed near one edge on top of the plate (case 5). Finally, the field application test is described in case 6, where the sensing beam was propped against the external casing of an operational three-phase motor. The following is the list of cases tested with the sensing beam to determine whether it is able to detect forced vibration frequencies in different orientations. * First setup: Beam on beam
* (i)  Case 1: Test with different frequencies from a speaker
* (ii)  Case 2: Test with phone vibration simultaneously with beam oscillation
* (iii)  Case 3: Test with the beam moved closer to speaker
* (iv)  Case 4: Test with the FBG bearing surface flipped downwards
* Second setup: Beam on plate
* Case 5: Test with different frequencies and comparison with analytical results
* Third setup: Beam propped against operational motor
* Case 6: Beam in contact with motor housing  
Figure 1 shows one of the configurations used to test the sensor (case 1) for detecting different vibrational frequencies. A metallic block was placed on an optical table. A beam with a length of 15 cm and thickness of 1 mm was placed with one end on the block and the other end free, which is a similar position to cantilevers. Beam 1 was weighed down with an acoustic speaker, which was termed the fixed end. The speaker was placed face down to allow better coupling of the sound to the metal. The sensing beam was placed without fasteners on the free end of beam 1. Several orientations of this configuration were tested with the ends of the beam 2: both taped-free; both touching the table; or one end touching the table and the other end free to oscillate. The FBG was either attached to the wedge point or within 5 cm distance of it. All the tests were repeated several times and the selected cases were analyzed in the next section. Figure 2 shows the schematic for case 2 where a phone is replaced as the acoustic source. The setup is used for testing the vibrations of a phone, which is superposed on the oscillations of beam 2. Figure 3 shows the picture and the schematic used for case 3, where the sensing beam 2 was moved closer to the Bluetooth speaker (from B to A). In this measurement configuration, the sensing beam was placed a few cm away from the free end. Since the sensing beam is pliant and flexible, this configuration introduces a higher tensile pre-strain in the FBG, which causes the baseline to shift to a higher wavelength. In case 4, similar tests were conducted by flipping the sensor aimed to ascertain whether the orientation of the FBG-bearing side changed the response to vibrations.