flutter echo – Sound Tourism

Mapparium

A spherical room that allows you to whisper sweet nothings in your ear!


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The Mapparium is a giant hollow globe of the world, with the seas and continents vividly drawn on stained glass. It was built in 1935 following a suggestion by architect Chester Lindsay Churchill. It took eight
months to paint and bake all 608 glass panels, which are mounted on a spherical bronze frame.

You traverse a walkway cutting through the center of the Earth linking up two opposite points on the equator. Three hundred lightbulbs illuminate the globe from the outside. Looking at the world from the inside out is an odd experience, but what also strikes visitors are the strange acoustics, which were an accidental by-product of the geometry.

One of the effects you’ll hear is false localisation:

“Suppose you are on the Mapparium bridge facing South America. There is a source of noise to your right, but you discover that you hear the noise coming from your left!”
William Hartmann, Michigan State University

This happens because the reflections from the globe are focussed and very loud. Your brain thinks the sound is coming from the reflection direction and not direct from the source. This focussing also allows you to do things like whisper in your own ear!

As you approach the exact center of the Mapparium sphere you suddenly become aware of strong reflections of your own voice . . . If you sway to the left, you hear yourself in your right ear. If you sway to the right, you hear yourself in your left ear.
William Hartmann, Michigan State University

Location

Marky Baker Eddy Library in Washington. They have regular tours. At the end of my tour, they allowed us to linger a little to play with the acoustics.

Credits

Photo: Smart Destinations

Tilted Spheres

Richard Serra’s giant concave artworks can produce an astonishing diversity of sounds.


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I stumbled across this Richard Serra sculpture when flying back from Tononto Airport. Clapping is one way of exploring the space, it’s like having a giant sound-effects unit to play with. Get the right place, and the focused reflections from the arcs follow repetative patterns creating Gatling gun echoes.

The start of this video nicely shows off the effects of clapping

It’s also fun to listen out for people with noisy roller suitcases walking down the middle. What’s nice about this sculpture is being outside a gallery means the public can play with the sound. Here is a spot of impromptu singing exploiting the acoustic.

Location

Pearson Airport. Terminal 1, after security, International departures, hammerhead F.

Credits

Photo: By Ken Mist from Brampton, Canada – Tilted Spheres, CC BY-SA 2.0

Train booking hall

An accidental sound sculpture created by a wierd flutter echo.


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This doesn’t appear in travel guides to Oslo, but every sound tourist should look it out if they’re in town. Here is me describing the strange acoustic, and as I walk into the middle of the ticket hall you can hear how my voice takes on a strange warbling. (The end of the audio when I walk out from the centre is the most dramatic).

A description of the sound, listen to the warble on my voice in the middle. No electronic effects applied!

Flutter echos are caused by repeated reflections arriving at regular intervals. In the graph below, you can see them as a set of clearly spaced spikes in the response to the room to a balloon burst. A warble is heard when the reflection pattern has some subtle irregularity about it. There also has to be a delay of more than about twenty-milliseconds for the brain to sense a temporal fluttering.

A balloon burst in the railway booking hall.

A balloon burst in the booking hall.

The architect Arne Eggen kindly sent me an article about the station that includes a plan for the ticket hall. The red lines are added by me to show that the inside space isn’t a simple cylinder but two half cylinders of different radii joined together. This means focussed reflections from the top half-cylinder arrive quicker than those from the bottom half-cylinder. But it is actually more complicated than that, because the domed roof also plays a role in breaking up the regular reflections.

The other thing I learnt from the architect was it wasn’t deliberate. And the railway company didn’t like the effect when it was first built. However, once musicians and others started visiting the remarkable acoustic as a tourist attraction, the train company grew to love it and put a plaque on the floor that says ‘Akustik Skulptur.’

Location

Map. Henrik Ibsens gate, 0010 Oslo, Norway

Thurgoland railway tunnel

This railway tunnel creates an extraordinary metallic flutter.


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This old railway tunnel near Sheffield creates an extraordinary metallic flutter when you shout or clap your hands. I was alerted to this strange acoustic when I was interviewed for the Channel 5 programme Walking Britain’s Lost Railways. While many tunnels have lots of reverb, it’s unusual to have such a warbling effect.

Listen to the warble in the Thurgoland Tunnel

The warble is a type of flutter echo, caused by repetitive reflection paths. The animation shows sound being modelled as a simple bouncing ball. At each wall the ball follows the law of reflection (angle of incidence = angle of reflection). The source is the black circle and the red circle is where the listener is. You can see that the sound keeps returning to the receiver, but it takes a few reflections before it returns. There is a pattern of regular reflection arrivals, but the bowed shape of the tunnel creates paths that traverse the width more than once before returning to the receiver. There’s a slightly longer explanation on my acoustical engineering blog.

A simple ray tracing in the Thurgoland Tunnel

Logistics

If you want to experience it yourself, it’s open as part of the National Cycle Network. The tunnel is here.

Credits

Photo Dave Pickersgill under this Creative Commons Licence.

El Castillo, Chichen Itza

The echo from the staircase disorts the sound, creating a chirp like a bird, maybe even the sacred quetzal bird.


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Listen to the distorted echoes immediately after each handclap: they sound rather like a chirping bird.

The Kukulkan Pyramid in Chichen-Itza, known as “El Castillo” (the castle), is one of the seven new wonders of the world. It’s arguably the most spectacular and most frequently visited Mayan site in Mexico. If you stand at the bottom of the steps and clap your hands you get this incredible chirping sound.

Whether the pyramid was constructed to deliberately make this noise or it happened by chance is still a matter of debate among archaeologists. Reflections from the treads of the staircase are responsible for the echo being altered. The reason that a chirp like a bird is produced is because of geometry. The time between later reflections is longer than early reflections, causing the frequency of the echo to rapidly drop by about an octave.[2]

There is another sound effect here, but unfortunately as people are no longer allowed to climb the steps it’s hard to experience. Apparently, if you sit at the bottom of the stairs the sound of footsteps from people above you is like raindrops falling into a water-filled bucket rather than footsteps. The effect is caused by sound skimming the surface of the staircase. The sound reflects off the regular pattern of the stairs, creating a very distinctive effect [3].

Other acoustic phenomena

The Great Ballcourt is a huge semi-enclosed areas. If you clap your hands, the sound repeats a dozen times (this is a flutter echo) [3]. Also a whisper carries a huge distance if you speak from one of the raised areas [4].

Location

Wikipedia page.

Credits

Photo: By Mariordo (Mario Roberto Duran Ortiz) – Own work, CC BY-SA 3.0
David Lubman: http://www.ocasa.org/MayanPyramid.htm
The Acoustic Raindrop Effect at Mexican Pyramids: The Architects’ Homage to the Rain God Chac?, Calleja, JAC; Declercq, NF, ACTA ACUSTICA UNITED WITH ACUSTICA, 95(5) 849-56 (2009)
http://www.luckymojo.com/esoteric/interdisciplinary/architecture/ecclesiastical/mayanacoustics.html
Sound (c) robgodd