Whispering Gallery at St. Paul church

[Greg Bernhardt]

My GF visited St. Pauls church in London recently and it had a whispering gallery. basically it's constructed so that in several parts of the church you can whisper at a wall and hundreds of feet away a person can hear you. Sounds crazy! She wanted to know how it works. Let's hear it :)

 

[TVP45]

Focus on the problem. Focus...

 

[f95toli]

It is due to the presence of whispering gallery modes (which actually got their name from the gallery in St. Pauls). As far as I understand it is just an ordinay resonant phenomena, but "along" the walls instead of "between" them. This happens because or nearly total internal reflection.

Whispering gallery resonators are used in e.g.. very precise microwave frequency standards, the modes can be extremely sharp and stable.

 

[turbo]

Greg, think of mirrors that can concentrate light on a distant focal point. The same thing can happen with domes, vaults, and walls that a spherical or ellipsoid in shape, except they are reflecting pressure waves in the air.

 

[Greg Bernhardt]

ah-ha! That sounds easy enough! Thanks all!

 

[arildno]

My GF visited St. Pauls church in London recently and it had a whispering gallery. basically it's constructed so that in several parts of the church you can whisper at a wall and hundreds of feet away a person can hear you. Sounds crazy! She wanted to know how it works. Let's hear it :)

A subway station here in Oslo also has a whispering gallery.

 

[rcgldr]

A local theater mult-plex has a domed ceiling over a fairly large snack area. People on opposite sides just under the dome could hear each other speaking quite lowly even though the crowd in between was fairly loud. There's nothing in the theater that mentions this effect. I first noticed that I could clearly hear a couple talking at a table on the other side, so I then had my wife stand under one side, then I walked to the other side and we could hear each other quite clearly using lowered voiced. Without the noisy crowd, whispering would have worked. In this case the dome didn't resonate, but it did reflect all of the sound from one focal point to another.

Another experience was a low volume of music appearing to eminate from my cubicle wall at work, but just one specific song, somewhat randomly. Turned out it was coming from headphones resting on the desk of a cubicle diagonally across from me, and it just happened that that one song was recorded louder than the others. The far corner walls of both cubicles were hard and reflective of sound.

 

[robphy]

http://www.falstad.com/ripple/
choose "Setup: Ellipse" (from the first menu... about midway down that menu's list).

You might have to drag the point around in an attempt to locate a focus.
At times, you might need to "Clear Waves" to remove any residual signals.

 

[rbj]

My GF visited St. Pauls church in London recently and it had a whispering gallery. basically it's constructed so that in several parts of the church you can whisper at a wall and hundreds of feet away a person can hear you. Sounds crazy! She wanted to know how it works. Let's hear it :)

does it work for only two people standing at opposite ends on spots marked "X"? if so, i'll bet you're in an ellipsoid. if it works for every location, then i dunno.

 

[robphy]

http://www.iop.org/EJ/abstract/0959-5309/50/2/315

Note on the whispering gallery of St Paul's Cathedral, London
A E Bate 1938 Proc. Phys. Soc. 50 293-297 doi:10.1088/0959-5309/50/2/31

A E Bate
Northern Polytechnic

Abstract. The apparent disagreement between Rayleigh's deduction and those of Raman concerning the whispering-gallery phenomenon is shown to depend on the fact that Raman used a sustained source which gave rise to interference, whereas Rayleigh dealt with a whisper projected in one direction. Further, it is shown that, whereas Rayleigh attributed the success with a whisper to the fact that it is composed of high frequencies, low frequencies are projected equally well, and that actually the success attained with a whisper is due to its low intensity, which does not give rise to audible echoes. Explanations are offered of the circumferential and radial alternations of sound mentioned by Raman and Sutherland.


[ "[URL [/URL]has a small blurb on this whispering gallery.]

 

[FredGarvin]

My high school's lunch room was a large dome. It too had the same effect. You could hear someone as if they were sitting right next to you.

 

[Danger]

I can't remember what it was, whether an opera house or a Shakesperean theatre, but I do remember from my childhood that there was some place that prided itself that a whisper on stage could be heard clearly from the balcony. The entire room was designed from the acoustical aspect.

 

[TVP45]

Tour guides at the Mormon Tabernacle in Salt Lake do a demonstration where they drop a pin to the floor up by the pulpit and you can hear that anywhere in the room.

 

[robphy]

Tour guides at the Mormon Tabernacle in Salt Lake do a demonstration where they drop a pin to the floor up by the pulpit and you can hear that anywhere in the room.

(it's difficult to hear the demonstrator speaking)

 

[brewnog]

It's not a church; it's a cathedral!

 

[Danger]

It's not a church; it's a cathedral!

It's not a coffee shop; it's a Starbucks! Sheep in wolf's clothing.

 

[mgb_phys]

The correct term would be "cathedral church of St Paul's"
cathedral simply means the seat of the bishop so cathedral church is like "head office".

 

[Greg Bernhardt]

Are there any other ways to explain this to a layman?

 

[robphy]

"constructive interference"

 

[Danger]

Greg, think about having a long hose a couple of inches in diameter. If you speak into one end, someone a mile away could hear you through the other end. The hose acts as a waveguide to channel the sound. Now think of the same thing without the hose. The structure of the building also acts as a waveguide.

 

[Loren Booda]

Ever hear folks whispering on the other side of a planetarium?

 

[Danger]

Nope, but that's because I've never been in a planetarium.

 

[CaptainQuasar]

I saw an exhibit like this in a science museum once. It involved two fairly massive, 4ft deep and 8ft diameter freestanding stone or concrete cylinders. The cylinders were laying on their sides (so that they would've rolled around if not firmly braced). One end of each cylinder had a parabolic depression in it and the parabolic depressions were facing each other across a large room full of exhibits (and screaming children, at the time.) They worked as promised.

It's like tin cans with a string between them, but the advanced wireless version.

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[CaptainQuasar]

Are there any other ways to explain this to a layman?

In the case of the museum exhibit I mentioned and any whispering galleries that work the same way, it works like a parabolic dish or lens works for focusing light or radio waves, but in reverse - instead of taking light beams that are moving parallel and focusing them down to a single point, it takes the vibrations from a point source of sound, lines them up parallel for transmission across the room, and the 2^nd surface re-focuses them back down to an intense point on the other side of the room so it's loud enough for the listener.

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