[2] The tracks were all recorded and released during Younha's stint as host for MBC radio program Starry Night.

It has been stated by the artist as a departure from her early career as a mainstream radio mainstay, and thematically reflects a relief from the pressure of maintaining popularity while also pursuing other genres.

[3] The album's title and subsequent songs make references to subsonic phenomena, thematically alluding to concepts of speed, movement, and time, especially in contrast to Younha's previous album Supersonic, which generally contains more guitar-driven, faster tempo music than Subsonic.

[5] A clip featuring behind-the-scenes footage of the filming of the music videos were also released as a preview.

13 for physical album sales for the month with 9,705 copies sold on the Gaon Music Chart for December 2013.