For a comparison, the length fluctuation of Signal-Recycled ITMX (SRX) and ITMY (SRY) have been measured.
Roughly speaking the length motion of SRX and SRY are as loud as that of PRCL.
Some details about the measurement and data analysis can be found in the past elog entry (#5582).
In the process of converting the raw spectra to the calibrated displacements the SRM actuator was assumed to have a resonance at 1Hz with Q = 5.
(Notes on SRX/Y locking)
The MICH and PRCL motions have been measured in some different configurations.
+ PRCL is always noisier than MICH.