Are compact florescent light bulbs know to be strong sources of ultrasounic noise? I've been playing with a "Marksman Ultrasonic Diagnostic Tool", which presents ultrasounds as audible sound. It isn't a scientific instrument, so it doesn't give a decibel reading. It is used to detect leaks in AC systems, automobile engines etc. I find that the primary sources of ultrasonic noise in my house (according to this device) are compact florescent light bulbs. Subjectively speaking, they are extremely loud compared to everything else I've tested such as whistling tea kettle, the refrigerator, computers, exercise machine etc. I can stand on one side of a plaster wall (genuine 1941 era plaster, not simply drywall) and , with the aid of the device, I can hear a lamp with a compact florescent light bulb on the other side. The wall seems to spread the sound over a large area. When I listen in the same room as the lamp, the source of the sound is more localized. I suppose the device could inadvertently pick up EM waves and pesent them as sound, but its behavior in detecting florescent bulbs is consistent with the blubs being strong ultrasound sources. I wonder if cats and dogs can hear them.