We constructed an effective vibration isolation system that employed an active vibration isolation system on top of a thick base-slab foundation and evaluated the performance by comparing the resultant vibration levels with the most stringent criterion of architectural standards (VC-E). The effect of the thick base slab on reducing vibrations was systematically investigated by comparing vibration levels at the bedrock with those at the base slab. The vibration spectra measured on the active vibration isolation table achieved an overall 1/100–1/200 of the VC-E in the range of 1–100 Hz, except at 5–25 Hz in the vertical direction. In addition, these vibration spectra were compared with those from another vibration isolation system based on a separate foundation. These comparative studies of vibration isolation, including details of the entire systems, can provide useful information for designing an effective vibration isolation system for various experiments that are highly sensitive to low-frequency vibration noise, such as scanning probe microscopy, electron microscopy, and ultrafine patterning.

The dorsal aspect target strength functions of ten species of fish collected by bottom trawl were measured at two frequencies. The average TS were calculated with respect to fish tilt angle distribution, which is assumed to have a mean value of -5 degrees and a stan dard deviation of 15 degrees. Both maximum TS and average TS were normalized with the fish length (L[cm]) squared and the 2/3 power of body weight (W[g]), namely, TS=20logL+A, TS=20/3log W+B.The results obtained were as follows:(1) The target strength is generally higher at low frequency (25kHz) than at high fre quency (100kHz).But for two species (Chub mackerel and Kandari), neither maximum nor average TS differ between low and high frequencies.(2)The target strength of Silver pomfret, a bladderless fish, is clearly lower than that of bladder fish at two frequencies.The normalized TS (A or B) were calculated for the maximum TS (Amax or Bmax) and average TS (Aavg or Bavg). The results are: Amax = -63 --59dB, Aavg = -68 --64dB at low frequency; Amax = -65 --59dB, Aavg = -72 --65dB at high frequency, and Bmax = -52 --45dB , Bavg = -57 --50dB at low frequency; Bmax = -54 --45dB, Bavg = -60 --52 dB at high frequency.

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published ** 20**, current version ** 20**. (Dates will be inserted by IEEE; "published" is the date the accepted preprint is posted on IEEE Xplore ® ; "current version" is the date the typeset version is posted on Xplore ® ).

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