High Clapper

Introduction

We have all heard of the "sweet spot" - a location on the sound bow of the bell which the bell tuner has identified as the best place to strike the bell.  And, fortunately for those who follow, marked it on the bell.  But what happens if we don't hit the sweet spot?  Is the difference in sound quality really subtle, something perhaps only bell tuners and a few other gifted individuals might detect?  Or is it totally in-your-face, something only the cloth-eared could miss?  Or somewhere in-between?  How can we find out?

Quite easily, as it turns out, all you need is friends in high places.  One such high place is the bell tower at Culver Academies, where our friend is John Gouwens, organist and carillonist.  But why Culver, you ask, why not any carillon?  Because Culver has been fitted with clappers that can be rotated on their shaft.  This enables a fresh face to be presented to the bell whenever that's needed to deal with wear.  That same feature also enables the clapper to be "wound up" the shaft so that it hits the bell higher than intended.  And so that's what we asked John to do.  Sneaky, eh?

We can see the general arrangement here in this image kindly supplied by John.  The clapper takes the form of a ball missing its top and bottom.  The clapper shaft is threaded, and a large nut and washer above and below the ball clamp the ball in place.  When the tiny wear spot visible now on the face of the clapper becomes large enough to make the bell sound clangy, just loosen a nut, rotate the ball a smidge, and tighten up again.  No more grinding (with the implicit risk to the bell if you slip!)  No more clapper weight loss.  Easy to adjust clapper height.  Nifty, eh?

The plan

So what's the plan?  First, we asked John to rotate the clapper a bit to present a fresh face to the bell, just so we start with a clean slate.  And record a clapper strike.  Then we asked him to wind the clapper up the shaft a bit, and record a strike there.  John emails me the sounds and I do the analysis.  Not quite as much fun as being there....

The note's nominal pitch is F#4.  John tells me the clapper had been at 11 5/16 of an inch (287mm) down from the inside of the bell head.  When wound up as high as possible, that distance reduced to 10 9/16" (268mm).  So we're talking about differences in height of 3/4" or 19mm, or less than 7% in clapper shaft length.  Is that enough to make a difference?

John also tried to wind the clapper down, to give us a "clapper too low" sample, but found that, because of the curves of clapper and sound bow, it didn't actually change the strike point on the bell, simply the strike point on the clapper.  It's possible that introduction of some "scrape angle" by this means would produce an audible effect, but that's not what we're testing here.

Enough talk!

OK, enough with the talking, let's hear something.  Three strikes on the sweet spot with a fresh clapper, and then three strikes with the same clapper wound up by 3/4" (19mm).  It will take a few seconds to download...

Fresh & High clapper.mp3

OK, I'd call that a significant difference in tone, but not one that might bring local residents running, flaming torches in hand, threatening to burn down the building if John doesn't get his clapper height right.  Momentarily disregarding the well-founded advice that "talking about music is like dancing about architecture", I'd perhaps characterise the second trio as being more "steely".  I'd also suggest that the second group are comprised of two subtly different sounds.  Not so much Bong, as Bo-ong.

(If you're having trouble hearing the difference, make sure you have the level reasonably high, and ideally use a device with reasonable speakers.  Play the recording several times.)

So, we can hear the difference.  Can we explain it?  I'll make use of Bill Hibbert's WavaAnal software in decay mode, and my subsequent number crunching to establish overall bell sound decay curves.  (See Internal versus External Clappers for an explanation of how that works.)

Fresh Face

Let's start with the sound of a fresh clapper face presented to the bell at the correct height.  As we would expect, we see a text-book response:

WaveAnal detected only 4 significant partials, whose names and deviations from the Prime are given in the legend at top right of graph.  The deviations are in the format "semitones.cents", so we can see the Hum is 12 semitones (an octave) and 4 cents flat of the Prime, while the Nominal is 4 cents less than a full octave above it.  Wouldn't worry about any of that!

We can also see that the full bell decay and the full bell without Hum decay (both in dotted lines) follow the text-book decay form.  All lovely.

High Clapper

Now let's look at what happens when we wind the clapper up the shaft by 19mm (3/4"). 

Woah, big changes!  Firstly, WaveAnal has now identified 7 partials of significant size, three more than we had been dealing with.

To make the relationship clearer, I've highlighted the new additions in the table below:

WaveAnal wasn't prepared to name the partial at 12 semitones and 52 cents, so I've given it a tentative title of "False Nominal?".  Note that it is 56 cents (a quarter-tone) sharp of the real Nominal.  Fortunately, it's level is fairly low (brown trace), otherwise it might really play havoc.  I ran an FFT analysis of the two sounds just to confirm.  There is absolutely no sign of this partial when the bell is struck at the right spot; it does appear and only appears in the high clapper sound.

Perhaps the most notable difference is the Tierce partial.  In the ideal strike point, this is one of the major partials; in the Too High strike point situation, the Tierce is suppressed at first, before fighting its way back.  We saw in the Internal versus External Clappers article that the Tierce was a major difference there too.

Indeed, is this an explanation of my feeling that the High Clapper sound comprised two subtly different sounds in series?  Is the early sound being dominated by the Nominal, until joined by the Tierce from around 0.3 seconds in?
 

Overall tone development

So, what's the overall impact of striking the bell too high?  As well as the tonal impacts implied by the rearrangement of partials, there's also an overall volume impact.  When we take my computed overall-sound and overall-sound-minus-Hum for both bells, and plot them on the same scale, this becomes quite clear:

In summary, we can say the "too high" clapper strike is failing to energise the bell's most efficient modes, but is energising some less attractive ones.
 

Conclusion

Quite encouraging isn't it.  We normal humans can hear the difference between a bell struck at the right spot and the same bell struck a little higher.  (Don't give up the day job to seek out a new career as a bell tuner just yet; I suspect there's a little bit more to it than that!) 

Further, we can delve into those sounds to find out why they are different, and, when we do, we find that the software can also discern those differences.  That means we can probably use it to investigate more subtle differences than this one.  I look forward to that.

Acknowledgements

My thanks to:

• John Gouwens for being prepared to tweak his clappers and record the examples

• Bill Hibbert for making available his WaveAnal software

Further discussion

To discuss issues raised (or that should have been raised), go to the GCNA forum on technical discussions.
 

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Created: 12 April 2013