Ήχοι απο απορροφητήρα, κλιματιστικά, "χιόνια" στην τιβι, ο ήχος της θάλασσας κλπ... το μυστικό πίσω απ' όλα αυτά λέγεται "λευκός θόρυβος".http://simplynoise.com/--------------------------"The effects of white noise upon cognitive function are mixed. Recently, a small study found that white noise background stimulation improves cognitive functioning among secondary students with Attention deficit hyperactivity disorder (ADHD), while decreasing performance of non-ADHD students.[5][6] Other work indicates it is effective in improving the mood and performance of workers by masking background office noise,[7] but decreases cognitive performance in complex card sorting tasks.http://en.wikipedia.org/wiki/White_noise