KAGRA Logbook

Washimi, TanakaT, Irene, Federico, Eleonora.Polini, Takaki.Yokozawa

On May 17 we returned to Center area and, in addition to the above, we also switched off:
MCE booth air filters
MCF booth air filters
IYC cooler and fan
IXC cooler and fan
Big air compressor

In Figure 1 is a picture of the microphone in center area.

In Figure 2 we show microphone spectra: blue=before switching off devices (typical condition at present), red = after switch off of devices (note this picture is from the yesterday switch off so the here mentioned devices were still on), green = when we inject white noise to the loudspeaker.

We injected white noise to the loudspeaker (DAC level of 10000). And we abruptly interrupt the sound by unplugging the loudspeaker. We repeated the same three times.

Here a rough analysis of the data.

Figure 3 shows the time decay of the sound. Blue is unfiltered, red is applying high-pass filter above 100Hz (see from Figure 2 that below 100Hz we are not able to overcome the background noise, so this part is useless).

Figure 4 shows the "sound pressure Level" in dB which is computed from microphone RMS (Prms) as: L(i)=20*log10(Prms(i)/Pref), Pref = 20 microPa, in steps of 0.02s. Also shown is a linear fit of the sound decay. From the slope of the fit it is computed the "reverberation time" or RT60 that is the time in which the sound pressure level (or rms) reduces by 60dB, according to this reference: https://en.wikipedia.org/wiki/Reverberation

Figure 5 is the similar plot of the second measurement.

Figure 6 is the similar plot of the third measurement. In this case, we attempted independent fit for the first part and the second part of the decay, as it is evident that all cannot be explained with a single exponential decay.

Table with rough results:
measure.n RT60[s] comment
1 1.2
2 1.0 fit looks not so nice
3 0.8, 1.7 average = 1.25

Conclusions:
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A rough measurement of Kagra center hall reverberation above 100Hz is 1.2s.
For comparison RT60 of the Virgo experimental halls is about 4.5 s: see https://tds.virgo-gw.eu/ql/?c=13570) and https://logbook.virgo-gw.eu/virgo/?r=42991
The much larger decay time of Virgo it is likely due to the fact that the walls are made of concrete or clean panels which are very reflective, while Kagra walls have rough surface (that helps a lot!)
The fact that the decay time cannot be explained with a single decay is first because reverberation time typically depends on frequency, and becomes shorter for high frequencies. This effect is clearly measured at Virgo (see same references)
The Kagra measurement can (if needed) be improved by increasing the level of injected noise. This was done at Virgo using the impulsive noise generated by exploding firecrackers.