This small dongle doesn't fall right into any available category, but being a passive element connected directly between headphones and a music source, it is probably best described as a "cable dongle".
Information from the creator:
Out of your ears and into your head!
The Bard’s Headspace simply plugs in between your music source like your phone, portable music player, tablet or computer and your headphones. Using the standard 3.5mm (1/8″) Jack connection the cable provided is connected between the Headspace unit and the music player’s headphone socket and your headphones plugged into the other end. The unit is non-directional so it does not matter which end. That’s it! press play and and enjoy a more natural sound.
The Bard’s Headspace makes your headphones sound like real loudspeakers. The main difference between using headphones and listening to loudspeakers is the physical space or air that is between you and the music coming from them. The Bard’s Headspace adds back most of what is lost in this space to give a more real, natural sound.
Most, if not all recorded sound is produced to be heard through stereo loudspeakers. The use of headphones is rarely considered in the recording process as the main way of listening and hence listening through them doesn’t give you the optimal experience. The Bard’s Headspace fixes that.