I am a big fan of sounds from nature and sometimes listen to an MP3 of waves crashing on a beach (with seagulls circling overhead), to provide a degree of isolation at work when I need to concentrate.
For me 'natural sounds' seem to facilitate concentration in a non-distracting way, unlike songs (classical or otherwise). Even something as relaxing and placid as Nick Drake will lead my mind to float off on to contemplation of his voice, guitar tuning, technique etc...
To this end The Guardian featured an article on a three year program of research by the University of Surrey in to the effects of birdsong on our creativity and on our sense of wellbeing,
- "The study will examine the psychological impact of being exposed to birdsong, including whether it helps us relax, can assist our ability to complete tasks and even think creatively.
Eleanor Ratcliffe, the researcher undertaking the study said while there was a growing body of environmental psychology looking at how the natural world affects people, there was still a lot to understand about the power of specific natural sounds."