But you will say, "So what if the cable is poorly designed, if it doesn't hum in my system?"
In a typical household, 50 or 60 Hz mains aren't the only source of electromagnetic noise. Electric motors (HVAC, washing machines, dryers, dishwashers, refrigerators), bang-bang controlled
appliances such as microwave ovens, and especially switched mode power supplies (SMPS)
generate massive amounts of electromagnetic noise.
Unlike a fixed frequency of the power noise, these devices spread their emissions over a wide spectrum. You will not hear the interference from them as hum or buzz—but rather as veiling. The signal to noise ratio suffers.
This is why replacing a poorly designed cable with something better often results in "opening" of the sound.