Advertisers can be sneaky bastards. There’s a commercial for Asda at the moment that positions WalMart’s British wing as being a place for shoppers to save money, and specifically cheaper than Morrisons. Some pseudo-facts are marshalled in support of this thesis, but the really interesting bit is the music used. It’s a boppy update on the classic Dad’s Army theme. And with only minor prompting, you’ll remember the words to that song: ‘Who do you think you are kidding Mr Hitler?’. With moving arrows echoing the title sequence of the sitcom, one set for Asda and the other for Morrisons, there is no doubt at all that the viewer is meant to be left with the impression that Morrisons are the Hitler in this version of events. Nice.

Despicable as that example is (but what were you expecting from Asda?), it points to the power of what happens when image and sound are fused. In fact, things go even further than that: the way the brain is wired, there’s a 40% overlap between the neurology that processes sound, and that which deals with feelings. Hence the ability of particular pieces of music to make you feel a certain way: songs that are special to you and a lover, tunes that take you back to certain holidays, and so on.

Filmmakers have been exploiting these connections for a long time now. Sometimes it’s music that does the job, either original scores or appropriations of existing themes. Scorsese and Tarantino have a genius for using popular music in their films, and there are plenty who try to emulate their easy way with marrying image to songs. And it’s good when people get it right: Donnie Darko’s version of Mad World is amazingly well suited to the film, and part of the power of There Will Be Blood is Jonny Greenwood’s majestic orchestral score.

More interesting to me is the use of sound in film. Much has been made of No Country For Old Men’s absence of music, which is true enough, but overlooks the importance of sound within the story. The tension created by Javier Bardem’s use of a simple device that beeps when it nears the case of money liberated by Josh Brolin is palpable. Who needs orchestration when you’ve got a machine that goes ping?

Natural sound is key to creating atmosphere in horror films and thrillers in particular. Creaking floorboards, footsteps in leaves, wind in trees: much of the effect of the excellent French thriller Ils came through well-judged sound design. And bear in mind that what’s heard isn’t necessarily as straightforward as it appears…one interview I read described how subtle audio touches were added to increase the impact of key scenes, culminating in the mixing of a gunshot into the sound of a slammed door to reinforce the notion of finality and closure reached at that point in the story.

Filmmaking is a kind of alchemy, in which constituent parts become more than they appear through the way they’re combined with other elements, figure and ground shifting throughout the progress of the narrative. Sound is a critical part of that mix, and easily overlooked when writers and directors think of it in purely naturalistic ways. And if Asda can play powerful tricks with audio in a 40 second commercial, think of what you can do with 90 minutes to play with, and a purpose more worthwhile than rubbishing a rival supermarket.