Climb 257 steps within the great dome of St Paul’s, and you’ve found yourself at perhaps the greatest ‘accidental’ man-made tourist attraction in London, if not the world.
The Whispering Gallery, as it’s become known, sits 30m above the crossing of the nave. It’s a circular walkway which hugs the base of the dome structure, offering a vertiginous view of the cathedral floor far below.
Architect Sir Christopher Wren hadn’t designed this walkway with acoustics in mind, but because it became such a fashionable meeting spot after the consecration of St Paul’s in 1708, keen-eared visitors were soon picking up on an unintentional sonic quirk you can still witness today.
Whisper along the curving wall, and — provided there’s not too much background noise — someone positioned anywhere along that same wall will be able to hear you. Even if they’re on the other side of the circular walkway, more than 33m away, and far beyond the reach of a regular whisper.
Just imagine how many sweet nothings, political secrets, and "can you hear me"s that curving wall has heard over the past three centuries. But (whisper it): not many people know how this curious and charming phenomenon actually works.
A whisper causes a sound wave, just like a shout does. You might think that a shout creates a more powerful sound wave than a whisper, and stands a better chance of being heard a long way away. In fact, the qualities of a whisper are better able to trigger this particular quirk.
A whisper has what’s known as low ‘intensity’, meaning there’s less interference from echoes and other distortions.
Bear with us here. Think of the interior of the St Paul’s dome structure as resembling a thimble (the dome itself) on top of a wedding ring (the walkway, AKA the Whispering Gallery). We’re mostly interested in the wedding ring here. The Whispering Gallery works because of how sound waves bounce around the inside of this curving section of hard wall.
An old theory suggested the phenomenon came about because sound was bouncing off the interior of the 'thimble'. This idea would only work if the whisper was perceptible for a listener stood on the exact opposite side of the gallery to the whisperer.
It was the suggestion of Lord Rayleigh — who experimented here in the late 1870s — that sound waves actually "creep" horizontally along the inside of the 'wedding ring' by a process of reflection.
The circular shape allows sound waves to bounce round and round multiple times because the angles involved are so slight. That’s how someone on the other side of the walkway can hear you so clearly.
Try it out: you’ll get a much stronger effect if you whisper along the wall, rather than at the wall — and the more marginal the angle, and the clearer the whisper, the better.
Before you whisper to us, those holes in the wall have nothing to do with it. But what does help is that the walls of the wedding ring are slightly inclined inwards at the top. That, in combination with the floor of the Whispering Gallery, simply helps to contain the sound waves.
Thanks, then, to St Paul’s — which provided us the original Whispering Gallery. Whether accidental or intentional, there are now many others round the world, from India’s Gol Gumbaz to New York’s Grand Central Terminal.
The climb to the exterior of the St Paul’s dome is — unusually — open on Sundays for May 2016. That’ll give you a break from all the science, not to mention some terrific views.