Many times people ask me the question, "Yes, there were 10 Gurus, but who was Guru Nanak’s Guru?" I always answer that our Guru is exactly the same as Guru Nanak Dev Ji’s Guru. It is the Shabd Guru, which exists beyond time and space and is beyond all physical forms. The Siri Guru Granth Sahib "captures" the Naad (soundcurrent) of the Shabd Guru through the songs of the Bhagats and the Gurus. Gurucharan Singh is the director of the Kundalini Research Institute (KRI). In this video he talks about the technology of sound in the Shabd Guru and takes a scientific approach to answer the question, "Why does listening to or singing the Guru’s Shabd elevate our consciousness?" There are so many ways to answer this question, but if you want to hear a scientific explanation, listen to this six minute video.
If you wish to learn more, you can read the original book/paper titled "Basis, Use and Impact of the Quantum Technology of the Shabd Guru".