Chris B. and Ryan F. reported that during the hardware injections in ER9, the H1:CAL-PINJ_TRANSIENT_OUT channel did not fall below 1e-200 counts until about 1 hour after the last BBH injection. He also said this was not observed in O1. One needs to consider the following two points:

First, one cannot directly compare the TRANSIENT_OUT channel from O1 and ER9 because they had fundamentally different units. In O1, the channel had units of strain, in ER9, it had units of counts. In ER9, the strain time series has passed through the inverse actuation filters.

Second, if one instead looks at the HARDWARE_OUT channel in O1 (after the IAF during O1), any impulse response to the transient signal ending would be completely masked by the continuous wave signal that has been added to the HARDWARE_IN time series. Thus it doesn't make sense to have a 1e-200 threshold on the HARDWARE_OUT channel during O1.

To invstigate the ER9 IAF, I have separately computed the impulse response of the different filters. The biggest impulse response comes from the f^2 filter module because it has high gain at high frequencies (see first attachment). This is to be expected.

I also compare the impulse response of the IAF used in O1 and the IAF used during ER9 (second and third figures, respectively). The O1 actually has a bigger impulse response and longer duration. I would surmise from this that any oscillations are actually worse in O1 than they are now.

Conclusion:
From this investigation, there is nothing to suggest that the ER9 IAF are worse than O1. In fact, the new IAF provide improved signal fidelity and the impulse response suggests that the new IAF is actually better than the O1 version.