Passage 3 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 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 atmosphere Recently，however，its become big business - thanks in part to recent research Dr Ronald Milliman，an American marketing expert，has shown that music can boost(促进)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 spot items they like to buy Yet，slow music isnt always answer DrMilliman 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 leads to complaints！ 41 The reason why background music is so popular is that ______ [