Figure 7. Spectrogram of all six horizontal channels at station CBC, the Broad Center, February 2003 through November 2003. One-hour spectrogram windows, no scaling, linear color bar. Sharp red horizontal lines are due to machine noise in the building. Natural frequencies are represented by the broad peaks, near 2.6 Hz, 3.0 Hz, and 3.6 Hz, for the east–west channels, and near 2.4 Hz, 2.8 Hz, and 3.6 Hz for the north–south channels. Notice the 7-day noise cycle with relative quiet on the weekends.