Retailers are using a new tool to reach shoppers and encourage them to spend more: music. Apparently. ''There's quite a lot of evidence that music can have different kinds of impacts on various kinds of customer behavior,'' said Adrian North, a psychology professor at the University of Leicester in England and one of only a few music psychologists studying the effect of music in a range of everyday situations. Music can influence the speed with which people shop, their willingness to spend, their perceptions of value, and more, he said. So retailers are taking it seriously and using increasingly sophisticated systems for delivering music to stores. ''Retailers are clearly getting more intelligent in the ways they're using music,'' he said. ``And obviously, they're getting much more professional about the way they deliver music to the store.'' North said people complain about the music they hear while shopping because they tend to notice it most when they don't like what they hear. One reader complained intensely about a late-night visit to Kinko's, where country music was blasting. That's the typical ''negative stereotype'' situation, North said, most often found in smaller, independent retail shops where employees can put on their own tunes as loud as they want. The most sophisticated retailers pick their music by balancing two main variables: the demographics of the shoppers and what kind of store it is. For example, urban club music is more appropriate for a store with a youthful customer base, while a high-end, gourmet food store might choose classical music to help strengthen its classy image. But retailers also consider other variables, such as the mood of shoppers and the time of day. ''If you want your shoppers to move quickly -- spend less time in a restaurant, say -- if you want to do that you've got to play fast music, because customers are going around the store more quickly,'' North said. ``If you want to slow them down, put in slow music. They'll shop more, and put more in their basket.''