The frequency-time-amplitude characteristics of seismic background noise at Chester, New Jersey have been studied for three years with high frequency resolution spectrum analyzers operating continuously in real time. Short-period (0.3 to 3.0 cps) and long-period (0.02 to 0.2 cps) bands have been analyzed simultaneously. In addition to the main microseism peak in the 6- to 8-second period region, secondary peaks are frequently observed at both longer and shorter periods. These usually persist for times on the order of days, often exhibit gradual shifts of center frequency, and show considerable variation in bandwidth; they are presumed to be of natural origin. In addition, several line components are found; these are believed to be of cultural origin, originating from a number of independent machines. Tape recordings from other U. S. seismic observatories have been analyzed, disclosing the presence of machinegenerated noise at several sites; notably, a strong spectral peak at 2 cps at the Wichita Mountains Seismological Observatory has been demonstrated to consist of a group of spectral lines.