On a recent trip to Cape Elizabeth, Maine, I came across this fog signal:

photo of Cape Elizabeth fog signal

It appears to have an electromagnetic diaphragm in the middle, surrounded by concrete and some very curiously shaped steel pieces. Why is the steel surrounding the horn shaped the way it is?

I've found some material on fog signal design, but nothing looks remotely close to what I see here.


  • $\begingroup$ +1 for genuine steam punk aesthetic. I think they will be asking the same question a thousand years from now. 1 - Maybe easiest to just leave the concrete forms on site. 2 - To discourage people climbing the fence to play squash. 3 - Secretly a temple to Saint Medardus disguised as a fog signal. 4 - prototype Bose Acoustic Wave Machine. $\endgroup$
    – Phil Sweet
    Sep 13, 2019 at 3:07

1 Answer 1


The steel structure will be designed as a diffraction grating to create a directional sound beam.

To make a directional sound source, the area covered by the source needs to have linear dimensions several times bigger than the wavelength of the sound, and at low frequencies that means tens of meters. It is impractical to make the actual sound producing device as large as that, but the steel structure "guides" the sound to make the complete structure equivalent to a single loudspeaker cone the same size as the whole frontal area of the device.

The same principle is used on a smaller scale when creating loudspeaker "stacks" at rock concerts, etc. A tall narrow "stack" of speakers generates a sound field which covers a wide area horizontally (i.e. a wide enough angle to cover the whole of the audience) but a focussed beam in the vertical direction, so you are not wasting energy on sound radiating up into the air, or down into the ground close to the speaker location, but radiating all the sound "horizontally" into the audience area.

Incidentally this is also why high-frequency loudspeakers are small (only a few mm in diameter,) so that they do not produce a directional beam of sound that can only be heard if you are positioned directly in front of the speaker.


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