I'm doing some research on the 'sacredness' of unaccompanied singing. I'm looking at a number of religious traditions such as Qur'anic singing/call to prayer and Christian chant. My main focus is on the singing traditions of Britain and Ireland though and I was wondering if anybody had any thoughts or pointers to research and writing done on WHY the singing tradition has such a special place for unaccompanied performance or indeed how this evolved. Any similar examples from other cultures would be of interest too. Thanks.