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Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 1:11 am
by Gideon Bodden
JohnGouwens wrote: So you are stating that a more rounded clapper all things being equal, will in fact produce a louder (and therefore longer) hum note?

I am not saying it always will, but under specific circumstances it would not be impossible.

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 1:33 am
by TerryMcGee
Gideon Bodden wrote:A bell has no memory of what struck it and how. There is nothing a clapper can do to influence the way the bell sound decays. Gentlemen, if you don't make the full distinction between the effects of the bare clapper (hardness, mass, shape, suspension, speed etc.) on the sound of a bell, and the transmission influencing the behavior of the clapper and thereby of the sound of the bell, this whole discussion is totally in vain.


Gideon, I'm not so sure about that. Look at the decay data below, for our much discussed pair of clappers on the same bell. Compare in particular the aqua curve, Nominal. You'll see in the Internal case, it has a clear decay slope which then morphs gently into a curve. But in the External case, there are three distinct slopes.

The yellow Tierce in the internal case also has a linear decay until it starts to flatten out around 1.2 seconds, but that happens earlier (at about 0.5sec) in the external case. Compare the initial slopes and they would, if continued, cross the axis at over 1.3 seconds, and under 1 second.

Image

If you total all the partials at each increment of time and display that, you also see the linear slope in the internal case, and the shorter exponential curve in the external case that we saw in the waveforms.

I think the clapper can and does influence the bell decays. But I think this is relatively minor when compared to the much greater spectral effects it has.

Terry

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 1:50 am
by JohnGouwens
Actually, on a carillon like Canberra, the behavior of the bells in the fourth octave would probably be quite dramatically different with worn clappers as opposed to re-shaped ones. It was on the mostly-identical (at the time) carillon of the University of Kansas that I first learned how to work on clappers, and there was a particular note (d in the top octave) that was especially clunky, and I was able to make it enough better by working on the clapper that Bert Gerken (the University Carillonneur, who taught me all this in the first place) commented on how much better it was.

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 2:39 am
by TerryMcGee
Now, this might help. I summed all the energy in the various partials for both clappers and presented that in the graph below.

Image

You can see at any time from impact onwards, there is more energy in the bell when it had been struck by the internal clapper. Note again we see the exponential decay of the external clapper vs the more linear decay from the internal clapper.

If you pick a threshold, eg the horizontal line marked 40, the external clapper reaches it after 0.3 seconds, while the internal clapper takes 0.9 seconds. On the face of it, you could say the external clapper decays 3 times faster, which I think supports John's instinctive feeling.

Putting a real figure on it is harder. I just summed the partials arithmetically, whereas it would probably be more accurate to use the Root Mean Squared method. And I haven't allowed for the differing audibility of the different partials, which then gets you into discussion of differing perspectives at differing distances! So, I wouldn't quote the numbers, John, but you could say that it appears to confirm that the bell energy produced by a flattened clapper (or a too-light clapper, or a misplaced clapper) appears to decay faster than the sound produced by a well-rounded appropriately sized and placed clapper.

I'm not sure, incidentally, that Gideon and John are talking about the same things exactly. I sense that John is talking about the length of the splash, while Gideon seems to be including the tail. The differentiation seems to be easier to see on the Internal curve, at about 1.3 seconds, whereas the External curve seems to fade into the tail, rather than switch into it. Not sure what that tells us!

Terry

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 2:51 am
by JohnGouwens
I was offering conjecture, basically. The really clunky bells I'm talking about don't have a "tail" to the sound that I ever noticed. They just die very quickly. I was simply trying to follow the explanation in a way that reconciled your findings.

The graphic does indeed show something revealing. But Terry, could the external clapper behavior be influenced by the fact that the clapper is traveling down to strike the bell? I don't care how big a spring you put on it, the mass is moving down, and it is going to take longer for any force to counter the inertia, when compounded by simple gravity. Maybe a very, very stiff spring could almost eliminate that effect, but as a practical matter, external hammers, if operated by the player (referring here to external hammers fitted to swinging bells in particular, so the carillonneur must play on a mechanical action acting upon a spring-returned hammer), it won't be desirable to have the spring very heavy. A clapper that is traveling mostly up to strike the bell would surely change direction more readily, albeit not without inertia of its own.

It would be interesting to see the results of your contact time measurement applied to the external hammer, compared to the internal clapper.

By the way, the external clapper in your sound sample didn't approach the clunkiness I'm talking about. Gideon is no doubt right that the transmission is a factor, but some bells are nearly immune to clunking. (I'm thinking especially of bells by Paccard, Petit & Fritsen, and also Perner.) If the bells are connected to a typical transmission, it's a given that the clapper, no matter how it is shaped, is going to be held to the side of the bell longer than is desirable. The difference I'm talking about is with the same transmission, the only difference being the shape of the strike spot on the clapper.

I am not convinced, by the way, that it is possible to get an identical sound out of a bell with an external or an internal clapper. There is one place I could look at, though. Ball State has external hammers and internal clappers that are identical in dimensions and weight. Of course, you're still dealing with the direction each is traveling, and that a spring is pulling the hammer back off the bell, and gravity is pulling the clapper off the bell (except in the treble range, where return springs play a major role).

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 3:19 am
by JohnGouwens
By the way, in speaking of clunkiness, I mean only in the treble range. Bass or midrange bells just don't do that unless they are really poor bells, poorly tuned.

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 4:02 am
by JohnGouwens
I am reminded of another discussion Gideon and I had years ago. He contended that on a large bell, a counter-weighted action might produce a fuller sonority than a spring- balanced action. At the time, I had the bass range at Culver partially spring-balanced. (It is now entirely spring-balanced, but I was making do with the hardware I had.) He came through on a visit, and we tested it. It wasn't a perfect test, since we hadn't eliminated counter-weights completely, but it was an easy matter to disconnect the spring and put the second counter-weight back on. (That was the arrangement we had when I arrived at Culver.) The sound was clearly more harsh and metallic with the counter-weight. It's obvious that a spring-balanced clapper will bounce off a bell much more quickly than a counter-weighted clapper, since the weights will indeed "want" to keep moving down, only reluctantly changing direction. It may be that the clapper was striking more than once in the process (albeit quickly enough that one couldn't hear a second articulated stroke). After removing the second weight and re-connecting the spring, the sound was again more "round" (mellow). Whether it rang longer I don't know. It's a 4,400-bell so it will never clunk! My point is that in that instance, the arrangement that surely kept the clapper on the bell longer did not produce the mellower sound.

There is still the matter of the difference we can get with a "staccato" touch as opposed to a forward stroke with a more gradual acceleration.

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 10:59 am
by TerryMcGee
I think it's true that a clapper held against the bell by some external force (eg some mischievousness aspect of the transmission, poor adjustment, etc) will produce very different results to a clapper that is spending its ideal time in contact. So perhaps we should be thinking in terms of (at least) three cases:
- a badly flattened clapper that spends too little time in contact with the bell, eliciting the development of a nasty mix of inharmonic partials
- a nicely rounded clapper of appropriate weight and placement that spends just the right amount of time in contact with the bell, producing a nice harmonious blend of well-tuned harmonics and permitting more expressive command of the instrument, and
- a clapper that is artificially kept in contact, or that re-makes contact with the bell because of some maladjustment or dynamic mishap.

Are there other cases we should add to that list?

I think we probably now have the tools to investigate this sort of stuff. Easy digital recording, easy FFT analysis, decay analysis, electronic contact-time measurement etc, high speed cameras, accelerometers, etc etc. But I'm not seeing anyone doing anything. Anyone aware of any research initiatives in this field?

Terry

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 3:18 pm
by JohnGouwens
Not that I know of. Joey Brink (from Yale) was studying other physics issues relating to the carillon. It would be worth recruiting him to this discussion. He's a member here. I just sent him an e-mail, with links, to try to draw him into all this!

There is also the matter of "staccato" and "legato" touch, but you'd need to do that with a carillonneur who really knows how to produce that distinction. (Too bad we don't live closer together!) The spring balancing versus counterweight balancing is also interesting, but only theoretically. I don't think there is anybody advocating counter-weights any more, as the key return is so much better with spring balancing.

Re: Comparison between internal and external clappers

PostPosted: Tue Mar 12, 2013 5:24 pm
by Gideon Bodden
I'm afraid the decay graphs shown here do hardly clear up the situation. There are simply too many factors influencing the sound of the bell and the behavior of the different partials that are not set to be similar in both cases, with the internal clapper and the external striker.
-Some, if not all, partials of this bell are splitted, they have higher and lower components, and the difference frequencies of the two cause the partials to go up and down in volume all the time. For instance this explains the 'crescendo' in the hum note in the first 0.4 seconds, and the great dip in volume reached at about 1.2 sec. But it also explains the uneven decay of the nominal, with the 'three distinct slopes'. Because the clapper hits the bell at a different position than the striker, all partials behave differently, already only because of that.
-add to it the fact that both graphs use different scales on the vertical axis, and it explains wrong interpretations. The three decay slopes in the nominal with the external striker, are also visible in the clapper graph, if only you look carefully.
-I have never denied that the starting balance between the partials is very different for the two different ways of striking the bell. And if the balance between the partials at the start is so radically different, then of course also the behavior of the bell after the stroke will be different. It is a mistake to imagine partials of a bell sound are actually sounding individually. They are always sounding together, a bell is one piece of metal which vibrates as one piece. In general the lowest partials, so the hum note, is a good indicator of the amount of energy stored inside the bell. Is there a lot of energy (in the form of deformation) stored in the bell, you will likely hear the hum note sounding with great volume. The hum note will keep sounding as long as there is energy in the bell, but not only the hum note, also higher partials will tend to ring longer. The bell in this case was fed with a lot more energy by the clapper than by the striker, and this explains the different decay pattern.
-So, different clappers will obviously feed a bell with very different amounts of energy, influencing the decay patterns of partials. In a way, John was correct when he said the clapper might influence the ringing time. But it is important to realize that what is happening is only the generation of a different 'offset' sound, an offset balance of volumes of the partials. The actual sound that we hear, is ONLY produced and organized by the bell itself, it is the bell that decides how long it keeps ringing, not the clapper.

The graph of the total energy of all partials illustrates only that the stroke with the clapper fed the bell with a lot more energy than the external striker did, and logically both curves are very different. The hum note plays a little role only in case of the striker, and it is dominant in case of the clapper. Interesting to see this or not? Anyhow, this does NOT illustrate well what you actually hear. For a bell of this size, and bigger, the screaming high partials are very important for the tonal balance as we hear it, but physically they are only low-energy.