<P> Both circumaural and supra - aural headphones can be further differentiated by the type of earcups: </P> <P> Open - back headphones have the back of the earcups open . This leaks more sound out of the headphone and also lets more ambient sounds into the headphone, but gives a more natural or speaker - like sound, due to including sounds from the environment . </P> <P> Closed - back (or sealed) styles have the back of the earcups closed . They usually block some of the ambient noise . Closed - back headphones usually can produce stronger low frequencies than open - back headphones . </P> <P> Semi-open headphones, have a design that can be considered as a compromise between open - back headphones and closed - back headphones . Some believe the term "semi-open" is purely there for marketing purposes . There is no exact definition for the term semi-open headphone . Where the open - back approach has hardly any measure to block sound at the outer side of the diaphragm and the closed - back approach really has a closed chamber at the outer side of the diaphragm, a semi-open headphone can have a chamber to partially block sound while letting some sound through via openings or vents . </P>

When did the first wireless headphones come out