Today we performed an Acoustic Injection using loudspeakers placed on the terrace near DET tower, as done in the past.
This time we used sweep sine instead of white noise. Results are interesting (see attached plot).
First of all we unlocked immediatly th ITF even using a "normal" value (e.g. <1Pascal seen in ENV_CEB_MIC): it seems Susp controls of all towers were saturated. Why? To be checked.
Then we used a very low acoustic level (e.g. 0.1 Pascal) sending a sine sweep from 30 to 1000Hz (logaritmic).
We can see it clearly in Dark Fringe. This deserve more tests.
The sweep amplitude detected by the hall microphone was of about 0.05 Pa/sqrtHz, that is about a factor 100 above standard noise, all the way between 30Hz and almost 1000Hz. The sensed noise amplitude is not so uniform (Figure 1 - Hrec spectrograms) and indeed we expect it to depend on location since sound wave reflections in the hall cause reinforcement and cancellations. Figure 2 shows the spectral excess noise (peak amplitude versus quiet noise) in the microphone and Hrec. Figure 3 shows the spectral noise ratio: peak sweep amplitude / quiet noise for the microphone and Hrec.
Hrec was clearly excited in a non-homogeneous way. The excess noise was larger at some single frequencies. These frequencies are: 256 Hz, 265Hz, 378Hz, 410Hz, 442Hz, 491Hz, 557Hz. They all (except 557Hz) associate to pre-existing peaks. Frequencies 256, 265, 410 and 442 were also noted to excite in previous injections.
To be noted that some noise increase is sensed also by MICH, PRCL and B4_DC (... which would suggests scattering at PR) ... BUT their excitation is more uniform and it does not concentrate at the single frequencies listed above (except maybe for the 378Hz) ...this fact would suggests for them a different noise path. Figures 4,5,6,7 show the spectral noise ratio between the peak sweep excitation and the quiet condition for B4_DC, MICH, PRCL and B4_GHOST accelerometer, respectively.
As expected, acoustic noise caused a significant vibration of all CEB towers an vacuum pipes, we observe a noise increase of a factor between 10 and 100 in all CEB accelerometers. Acoustic injections are good to show up weaknesses but do not help to localize couplings.
Using the sweep data we measured a transfer function (spectral noise ratio) between Hrec and the CEB microphone (Figure 1) and used it to project the standard hall acoustic noise: Figure 2 (red marked regions correspond to an excess spectral noise of at least a factor 2 in both microphone and hrec). Asdiscussed in the previous comment to this entry, we notice that the noise is close to limit at a few peaks between 250-260Hz and between 410 and 490Hz: see also Figure 3 (zoom-in of Fig.3).
This TF will be remeasured after the baffle installation in PR.
