Background music may seem harmless, but it can have a powerful effect on those who hear it. Recorded background music first found its way into factories, shops and restaurants in the US. But it is soon spread to other parts of the world. Now it is becoming increasingly difficult to go shopping or eat a meal without listening to music. To begin with, 'Muzak'(音乐广播网) was intended simply to create a soothing(起镇静作用 ) atmosphere. Recently, however, it's become big business—because in part of recent research. Dr. Ronald Milliman, an American marketing expert, has shown that music can promote sales or increase factory production by as much as a third. But, it has to be light music. A fast one has no effect at all on sales. Slow music can increase receipts by 38%. This is probably because shoppers slow down and have more opportunity to search for goods they like to buy. Yet, slow music isn't always the answer. Dr. Milliman found, for example, that in restaurants slow music meant customers took longer to eat their meals, which reduced overall sales. So restaurant owners might be well advised to play up tempo music to keep the customers moving—unless, of course, the resulting indigestion(消化不良) leads to complaints! According to the passage, background music is so widespread because it can ______.