[Bell Historians] Sound Lanterns

[Bickerton, Roderic (SELEX GALILEO, UK)]

There are some basics about sound lanterns.
Sound travels in strait lines.
If a sound lantern is visible from a distant point where the normal
sound louvers are not, the sound at that point will be from the lantern.

The amount exiting from the lantern depends on the amount and direction
of the sound inside the lantern and the size and spacing of the exit
louvers, as well as there angle.

If a sound lantern openings are below tower battlement height the sound
from them will reflect off the roof and the inside of the battlements
and be mostly redirected up and be lost.

If the lantern openings can be seen from the churchyard or the immediate
environs of the church, the bells will be noisy in these places.

To get most sound out of a lantern the inside of its roof needs to be a
hard smooth surface, which will reflect rather than disperse or absorb
sound.

If the lantern is large, say 2m+ across, and squat, more sound would
exit if the inside roof were shaped like an inverted roof, so that sound
coming up from the bells is reflected out from the lantern roof, rather
than back down.

This is not so for a small lantern, particularly if its height and width
are similar, because the inner roof surface is not large enough to
direct sound, but will disperse it in all directions.

Its about the wavelength of sound, but as a rule of thumb, if a flat
surface is comparable with or larger than the diameter of the largest
bell, it will act as a sound mirror and direct sound. If it is small, in
comparison sound hitting it will be reflected but will go in all
directions, which is dispersion.

Sound wave length, meters is approximately 343/Frequency.
A flat square surface begins to reflect rather than disperse sound when
it exceeds 1/2 a wavelength wide.
It really only acts like a sound reflector, with noticeable directive
properties when it is over 2 wavelengths wide.

Dove, pNBR will give the frequency, or it can be looked up from the bell
note.


 

________________________________

From: bellhistorians at yahoogroups.com
[mailto:bellhistorians at yahoogroups.com] On Behalf Of dcwillispiano
Sent: 24 October 2008 13:31
To: bellhistorians at yahoogroups.com
Subject: [Bell Historians] Sound Lanterns


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Have any of you had direct experience of fitting one and did it
do all you expected of it ?

Here is what you might think is the silly part of the question .
If the louvres are sealed up to make the process work natural light
is cut off to the bell chamber . Is there a variation of the sealed
double glazing unit which does that job , lets in light and for
the benefit of those interested in conservation etc - is quite
invisible from ground level ?

David Willis



 

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