Imagine Sony "balls" with skull-crushing techno instead of a tender acoustic track, or the quirky strings on Volkswagen's Polo spot "guardian angels" being replaced by thrashing guitars. Through music or visual effects, sound can be more subtle than visuals, gently coaxing the audience to feel a certain way.
Paul Brazier, Abbott Mead Vickers BBDO's executive creative director, says: "Changing the sound drastically changes what we 'see' and, therefore, what we think."
Andy Gulliman, Saatchi & Saatchi's head of TV, thinks sound contributes up to 80 per cent of an ad. "Sound communicates emotion, whether it's humour or intensity," he muses.
A former Bartle Bogle Hegarty man, Gulliman learned about the power of sound while working on Levi's ads. "Levi's 'twist' could have been conceived as grotesque imagery, but the track (a jaunty remix of Before You Leave by the Finnish big beat trio Pepe Deluxe) made the ad viewable. With 'odyssey', an emotional tension was communicated through using classical music (Handel's Sarabande) to address a youth audience," he says.
Brazier makes the point that what you hear does not necessarily have to match what you see. In fact, the most striking effects can be achieved via contrast. "Stanley Kubrick understood this when he used The Blue Danube over footage of the spaceship in 2001: A Space Odyssey," he says.
The Fallon creative partner Andy McLeod agrees sound can often counterpoint the action in an ad. "A lot of people use that technique to achieve a different effect," he says.
The climax of a horror film, for instance, can be creepier when a soundtrack suggesting innocence, instead of the default high strings, creates tension.
Ben Leeves, a sound designer at Grand Central, says road-safety ads are fond of this tactic. He says: "Gentle, lyrical music will always draw you in and will almost heighten the fact that you're seeing graphic images." Think of Mungo Jerry's In the Summertime or Crash by The Primitives.
Leeves makes the point that one reason why these ads opt for cheery soundtracks is to make them more palatable for viewers: insensitive sound design can stop an ad being approved by the Broadcast Advertising Clearance Centre.
Nigel Crowley, the managing director and chief engineer at 750mph, learned this first-hand while working on a car ad where the vehicle wins a race against a train. Instead of hinting at risky, boy-racer driving skills, Crowley kept the BACC at bay by making the car sound like "breaths of wind - wispy and light - while the train was like a thing from Hell - distorted, ugly, loud and dangerous. Above all, the train sounded like it was really struggling to keep up."
Crowley also proved how effective juxtaposition can be when he worked on Rexona's "Stunt City". He created the effects of stunt men hurling themselves through panes of glass and leaping on to moving vehicles and layered them on top of a 50s mambo track. This gave the ad a real lightness of touch and helped to convey the humour of the creative idea.
Ask London's finest to cite examples of inspiring sound design and they are unanimous in their praise for the Honda "choir" spot, an ad Crowley describes as "one of the best pieces of pure sound design".
Ben Walker, a copywriter at Wieden & Kennedy, reveals a score was composed for a 60-strong choir so they could imitate the sounds of the Honda Civic.
"We had to enhance the reality," Walker says. "Like 'cog', if we were going to bother doing the idea, we wanted to do it for real."
W&K collaborated with Owen Griffiths, a sound designer and director at Jungle Studios. Griffiths says: "Honda 'choir' is people doing sound effects, so it is both music and sound design. This particular business is interesting because you are not just creating whooshes and bangs; instead, you find yourself making weird sounds, such as a goat entering a wormhole."
Ian Chattam, a sound designer at Blue, recently worked on a Publicis ad for The Children's Society. The main character, a little girl who has run away from home, does not actually appear until the ad's final frames, yet Chattam builds her personality through her shallow breaths and wary footsteps. The visuals are shot from her point of view as she encounters either sympathetic or scary adults. Again, sound helped build tension.
"We twisted up the adults' sounds to make them unintelligible, confusing and frightening," Chattam says.
One of Chattam's all-time favourite examples of great sound design is the Audi "bull" spot. "It does a brilliant job of building tension and goes perfectly with the visuals," he enthuses.
Audi "bull" was sound engineered by Johnnie Burn, who, with Warren Hamilton, founded Wave Studios. "Sound is such an emotive, subjective thing," Hamilton observes. "People tend to have much stronger opinions on visuals than on sounds because they work more through their eyes than their ears and because visuals are easier to understand. Sound design as an art form is relatively new, but people are paying more attention to it. With a car ad, for instance, you could not put the car on a cliff edge and use a rock track like you could in 1983."
What is more, audio-visual equipment is much more sophisticated in 2006 than it was in the 80s, with digital technology and surround sound increasingly becoming standard in the average living room. On the one hand, this is good news for ads, but on the other it means agencies and studios have to resist the urge to over-design sound. "When you get too enthusiastic and chuck in loads of sounds, it just doesn't work. One or two well-chosen sound effects will do the job much better," Chattam says.
His advice? "Less is definitely more."