Sharan Banagiri, Corey Austin, Philippe Nguyen, Anamaria Effler, Robert Schofield
Summary: For impulse injections from multiple locations, signals from accelerometers mounted on the HAM5/6 septum are the best match to DARM in arrival time, have the most consistent amplitude ratios with DARM, and have the most similar frequency structure to the effect of the impulse in DARM. Together with shaking data, this suggests that the septum is the dominant vibration coupling site in the LVEA at both sites. These vacuum enclosure impulse techniques promise to be a useful new tool for diagnosing scattering noise.
Shaking injections made throughout the corner station have shown, in recent PEM injections, as well as for some time before that, that above a few Hz, motion of the vacuum enclosure couples most strongly to DARM in the HAM5/6 area of both LHO and LLO. We have had trouble narrowing down the coupling site further using the shaking amplitude technique because the HAM5/6 region is relatively small and interconnected (if you shake one side of HAM5, the opposite side of HAM6 moves almost as much).
Impulse injection delays
To provide extra information, we investigated the use of impulse propagation delays to help identify coupling sites. In the past, we have narrowed down coupling sites by looking for the microphone that detects an acoustic impulse at about the same time that the signal appears in DARM. And, of course, we have tapped on the vacuum enclosure. But we hadn’t tried using propagation delays from impacts on the vacuum enclosure to accelerometers mounted on the enclosure. While the vacuum enclosure is made of steel, the propagation velocity of waves on the steel membrane and structure is much lower than the velocity for bulk steel, resulting in tens of millisecond delays for propagation between HAM5 and 6.
Figures 1 and 2 show, for LHO and LLO respectively, examples of impulse injection data. These and other injections indicate that vacuum enclosure impulses show up in DARM about the same time as it shows up on the septum accelerometer.
The down side to the impulse timing technique is that higher frequencies are needed in order to discriminate arrival times (here we have used a 70-200 Hz band). Thus, there is the danger that a coupling site that dominates at low frequencies is not the dominant site at high frequencies. However, we have checked bands at lower frequencies and not seen obvious differences and results from two other impulse-based techniques are consistent.
Impulse injection amplitudes
In addition to arrival timing, we also used the amplitude of the prompt impulse vs. the amplitude of the prompt signal in DARM to discriminate coupling locations. The examples in Figures 1 and 2 show that the amplitude in DARM is most consistent with the amplitude of the signal on the septum accelerometer. An advantage of the impulse technique over our usual shaking is that there appears to be a greater difference in amplitude between accelerometers on HAM5 and those on HAM6 for an impulse injection than for a steady state injection. This may be because, for the steady state, the many late reflections have built up the amplitude to nearly the same at all locations in the region (in equilibrium, the energy gets distributed more evenly).
Impulse injection frequency content
A third impulse technique that points towards the septum at both sites, is a comparison of the frequency structure of the impulse signal in DARM to the frequency structure of vacuum enclosure resonances at the various accelerometers. Figure 3 shows that, for both sites, the frequencies that appear strongest in DARM are also the frequencies with the greatest motion in the signals from the beam-axis accelerometers on each site’s septum. That is, the septum resonance pattern appears to best match the frequency pattern produced by the impulse in DARM.
While results from these three techniques are not individually overwhelming evidence that the dominant coupling site is the septum, together they build a strong case.
We started out using soft hammers to impact the vacuum enclosure, but this evolved into using a long rod, with an inch-thick soft rubber pad mounted at the end, that could reach the tops of enclosures and keep the user further away from sensitive regions. In addition, the extended impact associated with the pad and the flexing of the long rod helped emphasize lower frequencies. A photograph is shown in Figure 4. For this study, all impulse injections were analyzed by hand. It may be worth beginning to automate these techniques.
In addition to the HAM 5/6 area, we also made impulse injections at end stations and other LVEA locations during the PEM injection program, which will be discussed in a future log (preliminary: at LLO EY impulse injections are consistent with coupling at the manifold).
