A tiny chip made of a smart material called Terfenol-D expands and contracts thousands of times a second inside the Soundbug, a gadget aimed at youngsters with MP3 players that will turn windows or other flat surfaces into loudspeaker diaphragms.
"We could have put the chip into a box with the shape and size of a matchbox, and then designed the product around that," says Brian Smith, managing director of FeOnic, the UK company behind the Soundbug and other products working on the same basic principle. "Or we could say, let's not commit to that, but let's define what we want, and establish the engine shape that fits into it."
FeOnic took the latter course.
As such, the development of the Soundbug illustrates a big change that has overtaken the product design market in the past five to 10 years. Unless they are forced to do so - normally for economic reasons - designers are increasingly reluctant to let the shape of an internal component dictate the overall design and look of the product.
Volume production of devices such as mobile phones, and the economic clout that comes with it, gives designers the ability to take this approach. "The designer who has the vision is not the executer," says Mr Smith. "You have to communicate the vision but what you end up with depends on how significant the device is, in terms of volume sales."
At the volumes in which Apple is selling the iPod - more than 20m have been sold since the music player was launched in 2001 - the US company could redefine any part within the product, irrespective of where it comes from, and buy to its specification, he says.
The refusal of designers to be constrained by the dimensions or other parameters of components, unless there is no option, reflects their perception of design's increasing importance as a differentiator when it comes to customers' buying decisions - because there is often so little to choose between products on criteria such as performance and reliability.
It may not have been the case 10 years ago, but design is a fundamental differentiator in all kinds of products, says Mike Paton, leader of the design group in PA Consulting's product and process engineering practice.
This, he says, reflects a fashion element - consumers are often not interested in what is inside the product, but in what the product says about themselves and how it enhances their lifestyle. There are still examples, he says, of fairly large companies being seduced by technology to the extent that they put all their efforts into improving performance.
But even here things are changing: an example is power tools, says Mr Paton, where not much attention was paid to design until recently.
Inevitably, there will always be some limiting factors for designers, however much effort goes into technology innovation. In mobile phones, batteries are volume-based, and can be made whichever way the manufacturer wants, says Mr Smith, but this is not the case with displays.
These are normally made on glass cut in regular squares or rectangles - to cut costs - that sit on a "zebra conductor" block with hundreds of connections to the chipset. This creates four geometric points that designers can do very little about, he says.
"There are still one or two constraints that are fundamental," says Mr Smith. "A clever designer will solve the problem by not creating the feature in such a way that the feature is an issue. Most designers will let the limiting factor determine the form of the product."
A further limiting factor occurs at the start of a product's development, where even the largest manufacturer may have to work with standard components as it has no idea what the eventual volumes will be. FeOnic avoided that constraint in the early stages of developing the Soundbug because the "magnetostriction" motors that the product uses cannot simply be bought off the shelf.
However, this was not the case with the batteries for the device: as a small company with a new product - Soundbug was launched in 2002 - FeOnic was hardly in a position to dictate the size and shape of the power source to battery companies.
Mobile phone batteries may come in a wide variety of size, shape and weight, but they also illustrate two other constraints on designers, says Mr Paton at PA.
The first is an ergonomic point: designers may feel that the overall product will be better if the battery is reduced in size by a third, but this will not have any effect unless the whole format of the device is changed. This is because the keypad still has to be a certain size for users to feel comfortable with it.
Occasionally the urge to exploit ever-smaller components can be taken to absurd levels, leaving users struggling to work the device.
Paul Pankhurst, chairman of PDD, the UK product design consultancy, recalls the early days of electronic calculators. Soon after Sir Clive Sinclair's pioneering launches of the early 1970s, calculators came down so much in size that they could be fitted onto a watchstrap, but at a cost in terms of usability.
"Consumers had to use a ballpoint pen, and (these models) have all disappeared," says Mr Pankhurst. Now some calculators have gone the other way, he notes: "Quite a lot are really quite clunky now, bigger than most people would need."
Reducing the size of a component simply because it is possible to do so, when other components or the whole device cannot shrink for technical or ergonomic reasons, can also increase the amount of "free air" in a device, says Mr Smith.
That reduction in perceived solidity or rigidity could reduce the attractiveness of the device for consumers, he warns. "A good design team will create a product only with the necessary volume, and not more."
Reliability is the second constraining issue for designers that is highlighted by battery trends and identified by Mr Paton.
Mobile phone designers may be tempted to use ultra-modern flexible or paper batteries, but manufacturers need to use technologies with maturity and robustness, qualities that can take time to develop in new technologies.
"On the one hand there could be a technology that would revolutionise a product, but on the other hand is the need to maintain reliability," he says.
