Hum & Interference   

The bane of the audio artist is the omnipresence of various unwanted low-pitched hums, crackles, and buzzes. 

Usually refers to electronic interference from alternating current electrical supply.  In North America the frequency of the alternating current is 60 Hz - thus most hum sounds a touch flat of Bb. Hum may also include other harmonics of 60 Hz. (120 Hz, 180 Hz, 240 Hz, etc.)

Sources of the problem: Some solutions:
60 cycle AC and its harmonics - secure connections to ground. 
- use shielded, balanced lines. 
- avoid ground loops. this may mean not grounding a particular device or connecting the cable shield at one end only.
radio frequency interference (RF) - avoid lighting dimmers, if they must be used separate the lighting AC from the audio AC . 
- ground any metal audio equipment chassis. 
- avoid long cable runs wherever possible. 
- install small value capacitors to ground out high frequency RF.
interference - power line interference in audio cables.
other equipment - other equipment, esp. lighting dimmer packs.
noise - noisy AC from the wall.
bad connections  - bad connections between equipment. 
- defective equipment (particularly cables and/or connectors).
at the mixer  - 0.01 uF Mylar between mic input hot to ground and between mic input connector shell and ground 
also between pins 1 and 2; and between 1 and 3 for each of the balanced mic inputs at the power amp for each channel use 0.01 to 0.03 uF disk capacitor between one speaker lead and the amplifier chassis