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What is the significance about the bell shape, when its hit at the rim it rings/produces sound better than other shaped objects? If so could anyone explain a little bit on it.

EDIT: From the suggestions in the comments, clarification for the term "sound better": Sound more effective for the purpose which bells are created for. (Thanks Justin)

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I guess it's traditional, not all bells are bell shaped... –  Sklivvz Jan 3 '11 at 21:03
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What's the close vote for? This is a challenging question and a good one. –  user346 Jan 3 '11 at 22:02
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The reason is that "sound better" is subjective and off topic. What's the formula for good sound in physics? Furthermore, as I said the basic premise is false: it's not true that bells are bell shaped (and tuned bells, like tubular bells, are cylindrical). So in my opinion the question is fundamentally flawed. –  Sklivvz Jan 3 '11 at 23:19
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It does not take too much of a mental strain to change "sound better" to "produce a clearer, more defined, resounding, louder, far-reaching sound with a durable form". "sound better", as in "sound more effective for the purpose which bells are created for" –  Justin L. Jan 4 '11 at 0:24
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Now tubular bells sound best. They are tuned. They are used by symphony orchestras. They are not bell shaped. Otoh church bells sound awful, they are not "tuned" (percussive instruments act like membranes and do not have a fixed pitch). If I have to think about which shape produces an objectively better tone, it would not be the bell shape. I say this out of professional experience as a music producer and sound engineer. –  Sklivvz Jan 4 '11 at 22:05
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5 Answers

up vote 24 down vote accepted

The bell is typically bell-shaped for two reasons, first because the circle is structurally strong and this allows bells to be struck with greater force than if the shape was flat or had sharp edges which would be more prone to cracking, further the circular shape allows a wave to travel around the bells perimeter so that standing waves can develop around the circumference of the bell. It is the resonance from standing waves that is responsible for the sound of the ringing.

And second the bell's shape makes the timbre of the bell more musically pleasing. The reason for the increasing diameter as you go from the top to the bottom of the bell is so that the bell resonates at different frequencies which can be tuned in a large bell so that you have what amounts to a complex musical chord playing when the bell is struck.

For example, a given bell might have a resonance at the fundamental, a subharmonic one octave lower, a minor third above, a fifth above, and a full octave above. The different diameter sections of the bell contribute to these different harmonics.

Bell construction is as much an art as a science. Here is a good online resource that describes the process of creating a large bell:

https://www.msu.edu/~carillon/batmbook/chapter4.htm

also the next chapter which goes deeper into the acoustics of bells:

https://www.msu.edu/~carillon/batmbook/chapter5.htm

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Your answer makes it sounds as if having a circular shape is a prerequisite for having normal modes of oscillation. –  Mark Eichenlaub Jan 3 '11 at 23:54
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Probably not @mark. But it probably makes the analysis and the construction a lot simpler. –  user346 Jan 4 '11 at 0:01
    
Thank you inflector. –  riderchap Jan 4 '11 at 2:50
    
@Mark - Sorry if I implied that. Of course you can have standing waves with normal modes on other shapes of bells, it just makes it harder to tune and build them. For instance, Carillon bells are tuned be removing metal on a large turning lathe, this can't be done if the bell is not round. Lathes don't like ellipses. :) –  inflector Jan 4 '11 at 17:07
    
@riderchap - You are most welcome. –  inflector Jan 4 '11 at 17:16
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A completely missed part of the answers so far has to do with acoustic matching. We hear sounds that are waves in the air; sounds that we hear are created by vibrating systems and materials, and those vibrations must get efficiently transferred to the air for us to hear them. The shape of brass musical instruments like trumpets, trombones and tubas is "bell shaped" so the standing vibrations that are created inside the instrument get efficiently matched to the acoustic impedance of the air. We can create those same sounds in long straight tubes, but they would not get efficiently transferred into the air outside of the tube. A large church bell, when it is struck, has internal vibrations in the bell's materials that initially get transferred to the air inside the bell, and then, because the bell is "bell shaped", get most efficiently transferred out into the surrounding air, making the sound we hear to be "loud".

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Bells have evolved into the "bell" shape because a vibrating bell produces different partial frequencies depending on metallic thickness and shape. My source says that "The shape of the bell has evolved so that the main partials are in roughly the right relationship. Removing small amounts of metal from different parts of the inside of the bell adjusts the different frequencies. A modern bell is cast deliberately thick, and then tuned on a vertical boring machine (a giant lathe). Before this, metal was chipped away with a tuning hammer, a practice that persisted into the early 20th century, despite the introduction of tuning machines from the late 18th century."
In some bells, the tuning is done by stripping inner annular rings from the bell. Obviously this is much easier than trying to remove the proper amt of metal from other shapes. Thanks for an interesting question :)

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Vertical boring machine is wrong, I think You meant a carrousell or vertical lathe. Did You ever try to work on bell bronze? I doubt that, because otherwise You knew that "chipping away with a hammer" is next to impossible. On the other hand, You are right, bells are the end point of more than 2 thousand years of trial and error, not everything with bells is straight rationa "physics". –  Georg Feb 10 '11 at 16:09
    
@Georg Note that the text you quote as mine is in quotation marks. It is from my source. I don't know a vertical boring machine from a toothbrush. –  Gordon Feb 11 '11 at 0:07
    
then throw away that source. –  Georg Feb 11 '11 at 0:16
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The correct answer is the following: bells have the shape they have because they are made to be hung in high places. The shape focuses sound downwards and maximises loudness.

This has a cost. The shape is not favourable to hold a steady pitch, and they sound much worse than tubular bells, which are used in symphony orchestras because of their clear and precise sound, which is obviously a more important requirement in that context.

Japanese bells have cylindrical shapes, too.

Finally, cow bells have yet a different oval-like shape probably because of the ease of production. They don't sound very well at all, they are more percussive than melodic.


As a side note. The argument in another answer about harmony is quite wrong. First of all basically any musical interment produces harmonics. That is the fundamental reason why different instruments have different timbres. So, sure, it would apply to bells too (with the caveat that church bells don't have a constant pitch) but it is not specific at all to bells or to their choice of shape in particular. For example the material with which a bell is wrought is just as important.

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I never said that harmonics were specific to the bell shape. I said that the shape gives a particular mix of harmonics. I also said that the circular shape allows those different resonances to be tuned separately. Follow the links I noted and you will see what I mean about the harmonics and tuning. They even have pictures of a bell foundry tuning on a large lathe. The chapters at the two links also mention that the metallurgy is quite important as well, as you noted. –  inflector Jan 4 '11 at 23:33
    
Tubular bells are shaped for another purpose. They are used to play individual notes, not chords like a carillon bell plays, that is why they are used in orchestras. It is also much easier to tune a tubular bell by changing its length. –  inflector Jan 4 '11 at 23:36
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uhm .. so you can hangup something inside and have it hit the walls? if you want a sound and you hit it from the outside, i doesn't need to be bell shaped. consider a "gong". also bells are / were expensive to manufacture during a certain time and used for religious purposes. it is easier to "cast" a huge sounding object in a bell shape rather then as, a tube or pipe? furthermore also,you can hang a bell in a way, as opposed to a "gong", in a way so it doesn't need an army of men to operate and make much noise. one novice hanging on to the rope will suffice.

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Cow bells (the real ones, not the instruments) are not bell shaped and yet have a clapper. Japanese bells are cylindrical, etc... –  Sklivvz Jan 3 '11 at 22:07
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The shape of a bell certainly affects its sound. Your answer is almost incomprehensible, and appears to have nothing to do with physics. –  Mark Eichenlaub Jan 3 '11 at 22:32
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This answer is total nonsense, sorry. –  Noldorin Jan 3 '11 at 22:52
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The tone of this answer seems a bit condescending. –  Justin L. Jan 4 '11 at 0:26
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Enough with the downvotes already. I'm sure @freeside got the point at -2 or -3! –  user346 Jan 4 '11 at 15:39
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protected by Qmechanic Dec 22 '13 at 20:47

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