The required instrumentation consisted of four total microphones: an infrasound GRAS 47AC (serial number 468275), an infrasound Bruel&Kjaer 4193 (serial number 2615925) and a standard GRAS 46AZ (serial number 231053) which was then substituted by another infrasound B&K (serial number 3032669) for reasons that will be clear in a moment.
First of all, we executed a cross calibration of the instruments using the HR mic and the B&K 4193 present at the Central Building (ENV_CEB_MIC, ENV_CEB_MIC_HR). Specifically, we placed our GRAS 47AZ and our B&K 4193, adequately protected by windproof balls, in the same direction of the existing mics, as near them as we could, for them to measure (roughly) the same data. This is shown in Figure 1.
The GRAS mic acquisition required a GRAS 12AL CCP supply whilst the B&K needed a NEXUS amplifier.
After an acquisition of roughly 30 minutes, from 11:05 to 11:25 LT, we were able to conclude that our microphones have a similar response to each other both at low and high frequencies and they're quite coherent with the existing mics from 0.1Hz on. For lower frequencies, coherence is lost, as one can see in Figure 2.
Additionally, in Figure 3 we can see the usefulness of the windproof ball, since the coherence between the protected mics and the HR one is higher than the one between the B&K 4193 unprotected one and the HR.
The following step was the installation of the mics at the North End Building. Even though the installation procedure was identical to the one executed at CEB, we had several problems that forced us to substitute one of the GRAS microphones.
We started by positioning the infrasound GRAS and B&K mic, protected by the windproof ball, in line with the tiltmeter present in the building, as one can see in Figure 4. The third microphone, the standard GRAS, was positioned facing the same direction as the exiting one (ENV_NEB_MIC) for similar reasons as before, free of the windproof ball.
While reading the acquisition data, however, we acknowledged that the two GRAS mics were not able to read the same low frequency data of the other B&K microphones. In addition, the frequency spectrum showed an irregular reduction of their response.
Since this problem wasn't faced in the CEB measurements, we reckon that we either had problems with the electronics, or we made mistakes we're currently not aware of.
Nevertheless, since we wanted to measure the difference of pressure between the two faces of the tiltmeter chamber, we decided to substitute the infrasound GRAS with another B&K 4193 (serial number 3032669), connected to the same NEXUS used for the other measurement. This swap should actually improve our measurements, since in the first place the implied microphones were not identical, whereas now they’re both B&K 4193. The other standard GRAS (connected to PCB 482C05, channel 3) remained in its original position but will only be used for acoustic noise monitoring.
After analysing the data coming from this last configuration, shown in Figure 5, we can successfully affirm that the two infrasound B&K placed near the tiltmeter (ENV_NEB_MIC_4193_N and ENV_NEB_MIC_4193_S), are coherent with the already existing B&K mic at the North Building. The standard GRAS response (ENV_NEB_MIC_GRAS_HR) is definitely different and not quite performing at frequencies lower than 1Hz, but it’s not concerning, since that microphone aim is to monitor higher frequencies.
