Clapper height tonal variation

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Re: Clapper height tonal variation

Postby CarlSZimmerman on Wed Oct 16, 2013 4:07 pm

As a meteorologist and physical scientist, I can give a partial answer to the question Frances raised about the effects of temperature and humidity. However, since I am not an acoustician, I'll have to express it mostly in qualitative rather than quantitative terms.

Changes in ambient air temperature will lead to changes in temperature of anything exposed to the weather, such as tower bells and their fittings. How fast that happens depends on the thermal conductivity of the material. Most metals tend to gain or lose heat rather rapidly in comparison to other materials. The effect of such a change in the temperature of any ordinary metal is primarily one of expansion & contraction, not any variation in other properties. That is, after all, why such materials are "ordinary" - they behave "nicely" in the range of temperatures within which humans live. So I would not expect atmospheric temperature changes to have an easily measurable effect on bell clappers. Similarly, changes in atmospheric humidity should have no easily measurable effect on either clappers or bells, since common metals are effectively impermeable to moisture.

Far more significant, in my opinion, are the effects of atmospheric temperature and humidity changes on the propagation of sound from the bell to the listener. Changes in temperature affect air density directly - warmer air is less dense, and therefore transmits sound less effectively. This is why bells can be heard farther in winter, even though the velocity of sound in cold air is less than in hot air. But normal daily variations in temperature cause density variations of less than one percent - effectively undetectable by the human ear.

Air density also varies with atmospheric pressure, which can change 2 to 4 percent between the center of a high pressure system and the center of an ordinary mid-latitude storm system, or as much as 6 percent to the center of a tropical hurricane. But the really big changes, and the only ones that the human ear are likely to be able to detect, are altitudinal. The concert-pitch grand carillon at Denver University, a mile above sea level, sounds noticeably less impressive to the listener on the ground than it would in a similar tower in the Netherlands, because the air density at that altitude is 15% less than at sea level.

The effects of changes in humidity seem to be mainly in the attenuation of sound as it travels. They are not linear, so even a qualitative description is difficult. Fortunately, the effects seem to be insignificantly small for the frequencies and listening distances that are pertinent to the carillon.
Carl Scott Zimmerman
Saint Louis, Missouri, USA - home of at least 36 bell foundries or bell sellers, 1821-1961.
CarlSZimmerman
 
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