The upper half is considered the positive cycle, whereas the lower portion is the negative portion. The usual consumer electrical power is 120 Volts A.C. Poorly filtered or defective audio equipment on A.C. power mains will have a typical 60 cycle A.C. hum, and fluorescent lights can be seen to flicker from the alternating of their power source. This is absent in incandescent lights due to the slower reaction to change of their filaments. Sub ELF is within the inaudible to human range. The audible to human range is usually from 10 to 25,000 Hz.
Some animals can hear considerably higher.
Dogs hear supersonic whistles that are seemingly silent. Radio waves overlap into frequencies in the audible range, starting as low as 5,000 Hz. Hz are being touted for therapy. Ultrasound instruments use this to vibrate their sound heads. The resulting output is mechanical, but not electrical, so that a nonelectrical conductive substance can be employed.