It’s not exactly like a noise cancelling headphones because your hearing aids doesn’t really cancel any outside noise like a pair of noise cancelling headphones does. The external sounds still can go through the vent(s) in your dome and reach your ears and these external sounds don’t get cancelled out like they do in a pair of noise cancelling headphones. So if you still have good hearing in certain frequency range (like maybe the lower frequencies), you can still hear all the external sounds in tcoil mode just as loud as they are naturally.
In most HAs, you can choose to amplify on the tcoil content and not have the HA mics amplify the external sounds, or you can choose to amplify both the tcoil content AND still have the HA mics amplify the external sounds just as well so you can hear both at the same time. You can also adjust to have the HA mics amplify the external sounds but not as loud as normal so you’re aware of your surrounding but still be able to focus on the tcoil content more. But as mentioned above, even if you choose to not amplify the external sounds via the HA mics at all, the sound can still naturally enter into the vents. So it’s not like a pair of noise cancelling headphones unless you have a completely closed-off set of custom molds with no vent at all, and turn off the HA mics amplification.
Your tcoil can pick up not just content from a hearing loop, it can also pick up content from magnetic speakers as well when the speakers are applied next to your HAs, like the one in your telephone landline handset, or from a pair of over-the-ear or on-the-ear headphones (that has larger size magnetic speakers in them).
Not all HAs have tcoils in them. If you really want to have HAs with tcoil, you need to make sure that you choose a model that has this feature built-in right up front. It’s not an add-on that you can enable later. Sometimes the HA model with tcoil built-in may cost a little more than the same HA model without the tcoil option, but not always. So it’s not necessarily “free” to get. The tcoil model will usually be larger/bulkier than the equivalent non-tcoil model so there’s a trade-off, if not only in price, then also in size.
