The featureless soundscape known as white noise is effectively as neutral as music can get for humans—too many frequencies are combined in one waveform and the human ear just isn't able to pick any specific detail out. The same thing happens with white light, which contains all the colours of the visual spectrum. Now it seems that it's possible to create "white smells"—that is, smells that don't smell of anything specific.

White noise isn't just a hum in the background—it specifically refers to a kind of frequency spectrum that's completely flat (and Wired.co.uk has previously explored the different "colors" of sound). Every frequency has the same energy, so the noise just kind of melts into an overall hum of nothingness. White smell works on the same principle. Neurobiologists from the Weizmann Institute of Science, led by Tali Weiss, managed to create something they call "Laurax." And there are several different versions of Laurax, each chemically different, but they each smell "white."

Weiss and her team first derived a range of 86 "monomolecular odorants" which covered the effective spectrum of smell. Each one was diluted to the same odor intensity (so no one smell would overpower any other), and then mixed together into a range of batches containing either "one, four, ten, 15, 20, 30, 40, or 43 components." An algorithm worked out how to make sure that the molecules that were mixed into each batch were consistently spread far apart from each other across the smell spectrum so as to ensure there wasn't smell overlap.

That left them with 191 individual batches for their 56 study participants to have a sniff at. Unsurprisingly, when a batch with only one component, or four components, was compared to a batch with a similar small number of components, the differences were easy to detect. But, importantly, as the number of components in the batch increased, it became harder and harder for the participants to tell the difference between the smells of each batch. And, when comparing batches with 30 or more components, the difference was impossible to discern for most participants.

Even more intriguingly, those batches didn't share common components, yet still smelled the same. It appeared that those batches had gained some quality of whiteness simply from having lots of ingredients. However, Weiss writes: "This trend implies that if more and more non-overlapping components are added to each of two mixtures, these two mixtures eventually should smell the same, despite having no components in common. Indeed, given a sufﬁcient number of equal-intensity spanned components, this trend implies that eventually all mixtures should smell the same. We call this predicted ultimate point of perceptual convergence 'olfactory white.'"

They double-checked their results using the Laurax name—they created four new mixtures with 40 components each, each labelled "Laurax" (a meaningless word designed to "avoid any cognitive influences of the word 'white'"). They asked the participants to get used to the smell of their Laurax over three days, and on the fourth day they were given a new mixture and asked whether it was also a batch of Laurax. As expected, the more components there were in the new mixture, the more likely the participants were to label it Laurax, even if it had none of the same ingredients.

So what does Laurax, or "olfactory white," smell like? Weiss describes it as "intermediately pleasant and edible," which is perhaps almost as vague a description as can be imagined. The terms the participants used to describe Laurax were all similarly vague—the study (published in The Proceedings of the National Academy of the United States of America) includes the suggestions from the participants, like "mouse-like," "eggy," "fruity," "spicy," "medicinal," and even "sweet." It seems to smell like pretty much everything, and that's why it doesn't smell like anything at all—our nasal sense is overwhelmed across such a wide spectrum that it can't pick out one individual note of smell.

Smells like Laurax don't exist in nature, Weiss explains, even though lots of smells (like coffee, or dirty socks) are made up of lots of components. It's just that those components are usually either very similar, instead of spread out across the smell spectrum, or there's one smell that overpowers the rest.

Nevertheless, olfactory white has some potential practical use—it can, Weiss says, be used to cover up other smells. Strong smells—like rose—which were mixed in with Laurax, just smelled to a small group of participants like... Laurax. That could be good news for people who hate the smell of disinfectant, or for manufacturers of smell-covering products.