I'm not sure  ENV_NOISE_MAG_WEB was connected to DAC ch7

I think it was ch 6 instead - To Be Check

Today we injected again the same acoustic noise.

Timing are:

1429969272  29 Apr 2025 13:40:54 UTC  (29 Apr 2025 15:40:54 CEST)
Quiet time
1429969581  29 Apr 2025 13:46:03 UTC  (29 Apr 2025 15:46:03 CEST)

1429969648  29 Apr 2025 13:47:10 UTC  (29 Apr 2025 15:47:10 CEST)
injection
1429970509  29 Apr 2025 14:01:31 UTC  (29 Apr 2025 16:01:31 CEST)

Analisys will follow.

We also reconnected the IFAE Smart Plug at about
1429970703  29 Apr 2025 14:04:45 UTC  (29 Apr 2025 16:04:45 CEST)

A first look at the data shows:

• overall, no anomalous behaviour of Hrec or something that might be correlated to the present excess noises of hrec.
• hrec noise increases at 10Hz with a tail reaching 20Hz (to be checked if this used to occurr also before) (Figures 1 and 2) 
• there is a hint of (small) noise increase around 105Hz (Figures 3 and 4)

Results of acoustic injections, comparison before and after the WE mirror and baffle replacement

On April 1st the coupling was mostly evident in a pronounced peak at 105Hz.  April 29th injections proofs that the coupling has reduced. However, when excited with the same acoustic levels (and loudspeakers position) a small peak in hrec at 105 Hz seems still excited.

• Figures 1 and 2 are the ASD (mic and hrec) during the two injections, April 1st and April 29 resp.
• Figures 3 and 4 are the two Projections, April 1st and April 29 resp.   
• Figures 5 and 6 are the coupling functions (measured=red dots, and Upper limit=blue dots) for resp. April 29 and April 1st.

Figure 7 shows a comparison of the CF before and after.

Figure 8 shows the hrec spectrogram during the April 29th injection: it seems to notice a faint peak excited around 105 Hz.

Here the plot comparing the projected noise, before and after.

For this injection (April 1), we injected filtered white noise (8–1000 Hz) into the WEB using two loudspeakers. Although the loudspeakers were unable to generate acoustic waves below approximately 50 Hz, Hrec nonetheless exhibited an increase in noise between roughly 10 and 50 Hz.

Simultaneously, the "in-loop" optical lever signal (Sc_WE_MAR_PSDT2_Y2) was heavily disturbed by the acoustic injection. This disturbance may explain the rise in low-frequency noise observed in Hrec (to be confirmed).

On July 21, we repeated the same injection using only one loudspeaker, halving the acoustic power. Under these conditions, Hrec showed a much smaller response (colour scale are unchanged).

We plan to upgrade the acoustic injection system next months. The enhanced setup will allow us to vary sound levels across the terminal buildings to determine whether the Hrec low-frequency noise persists at higher amplitudes and whether its behavior is linear or shows a threshold effect. It remains to understand and correct the behaviour of this optical lever system, to check if it is really the source of the Hrec noise.

A question raised during a discussion with Paolo Ruggi was whether the coupling observed on the optical lever originates from the magnetic field.
This scenario appears unlikely: the standard Metronix magnetometers (middle row in the attached spectrogram) do record a field increase, but they sit immediately adjacent to the loudspeaker cables (see first photo).
By contrast, the triaxial fluxgate (bottom row) located near the tower—and thus close to the optical lever—shows no measurable change in magnetic field.