These rest on the ear but form no seal. The conventional telephone earpiece is a common example, and so are most headphones used with portable sound equipment. The spacing is controlled by an open-cell foam pad which is transparent to sound from the headphone transducer and also to sound from outside. Sound from outside the headphone is attenuated slightly by the bulk of the transducer assembly and much less by the foam which surrounds it. Hearing aids commonly use these.
Some intraaural headphones plug the ear canal, while others leave it partially open to the outside.
Even a small opening will let most sound from outside pass. A few headphone models electronically cancel out some of the outside noise, mostly in the low-frequency range. These headphones are expensive and uncommon, and they all have a switch to turn off the noise cancellation. Some of these laws permit headphones which cover one ear. The idea behind these laws is that the other ear will then be able to receive sounds from outside.