Question Active noise cancellation - is white noise / hum normal what its on?

[The Electro Machine]

I would like to know if it is an immanent for all ANC headphones to generate some noise / hum when this feature is turned on? Is this just how this technology is suppose to work?

And if yes- are there big differences between models / manufacturers - i.e. do some headphones generate noticeably less inside noise when cancelling the outside noise? [And if yes - does then less inside noise equal to more outside noise on the account of not filtering it enough?]

 

[Math Geek]

noise cancelling is not really "filtering" out the outside sound but rather adding in a second sound wave which counteracts the outside noise thus "cancelling" it.

that hum or white noise you hear is normal and what is supposed to happen as the headphones do their thing

 

[The Electro Machine]

So far I have manage to test out Bose QuietComfort 35 II Wireless Headphones and Sennheiser PXC 550-II

And both of them have this hum / noise, although different sounding from each other