Internal versus External Clappers




"Love and marriage, love and marriage, 
Go together like a horse and carriage. 
Dad was told by mother - you can't have one 
(You can't have none.) 
You can't have one without the other."

(Frank Sinatra, written Sammy Cahn & Jimmy Van Heusen)

And so it is with bells.  A bell without a clapper is like a fiddle without a bow.  Pretty, but useless.

Further, there are clappers and there are clappers.  Clappers can be too big, too small, too high, too low, too rounded, too flat, too hard, too soft, and these differences in clappers can bring out dramatically different responses in bells.  That's the subject of this article.

There is a secondary purpose to the article, which it to bring an understanding of the ways we can now delve into the sounds to see what's actually going on.  Never was it easier to do that.  But can these technologies help explain the differences in the sounds we hear?

Meet the bell

The bell we will hear comes from the beautiful inland city of Bathurst, NSW, Australia.  Bathurst is an unfinished carillon.  It's 100ft art-deco war memorial tower, set in a magnificent public park, was built by public subscription back in the 1920s and 30s.  A Taylors 3 octave carillon was obtained and installed, but has never had a manual/pedal clavier.  We trust that's all about to change, but that's another story.

The Bathurst War Memorial Carillon

The installation has had, at different times, a pneumatic keyboard, acting on the usual internal clappers that came with the bells, and a MIDI keyboard and controller, acting on electric strikers more recently fitted to the outside of the bells.  The fact that each bell has two clappers makes this comparison easy.  The fact that they bring out such different responses from the bell makes it fascinating.

The top tier of bells at Bathurst

Here's an image of our bell - the second full bell from the right.  (And of me, caught rather by surprise by the camera-lady!) You can see the external electrical striker mechanism on the side of the bell facing me, and can just make out the tail of the original internal clapper dangling behind the power lead of the bell closer to the camera. Note the return springs, closer to me, disconnected, with their tails pointing skyward.  The old mechanical transmission that let the pneumatic keyboard control the original clappers is long gone, replaced by the electrical clappers under MIDI control.  The old clappers hang forlornly, loved only by gravity.

Hear for yourself

Now I mentioned the clappers produce a different sound, but don't take my word for it - have a listen for yourself.  I think you will be amazed at how two clappers on the same bell can sound so different.

The bell was recorded digitally in the uncompressed .wav format using a Rode NT55 studio condenser microphone, a Tascam US-122 USB/MIDI interface and a Toshiba laptop.  The recording you are about to hear has been digitally edited to give three strikes on the internal (original) clapper, followed by a pause, and then three notes on the external (electrical) striker.  I'm presenting it like that to give you time to absorb the details of the sounds.  No changes to tone were made during the editing.  You may wish to play the file a few times to become really familiar with the difference in sound.  Be sure to use a device with decent sounding speakers you may not be able to tell on portable devices with tiny speakers.  And turn the sound up enough to be able to hear clearly. 

Comparison of Internal and External clappers.mp3

I promised in the introduction that new technologies makes it easy for us to delve into the sounds to see why they sound so different.  So, let's delve!

1. Waveform comparison using the Digital Audio Editor. 

Our first tool is the PC-based Digital Audio Editor (DAE), and it's one freely available.  You can think of a DAE as a word processor for sound.  You can delete, edit, drag-&-drop, copy, move, do anything with sound that you can do with words in your word processor.  And add reverb or special effects, speed up or slow down, change the tone, add voiceovers, etc, etc.  You can download Audacity for your own use by clicking on this link

I used Audacity to record the bells on site on the laptop, and used it again to edit the recording to produce the comparison above.  Now I'm using it to compare the waveforms of the two strikes side by side.  It's a wonderful bit of software, especially considering the price (totally free!).  In our case here, we're using it to see how the two soundwaves look on screen.

The external (electrical) clapper is on the left in the image below.  The scale across the top gives the passage of time in seconds.  You will see that the electrical clapper produces more noise at the instant of strike, then dumps all its sound energy in a very quick burst, while the energy from the manual internal clapper (right) decays more slowly and more linearly, and the tail maintains the traditionally expected bell resonance. 

DAE screenshot, showing the different envelope waveforms produced by the External and Internal clappers

So we've already confirmed that the decay of the two bell sounds vary differently with time.

2.  Spectral (FFT) analysis. 

Fast Fourier Transform software allows us to visualise the spectrum or harmonic content of each note.  The remarkable Audacity software includes FFT analysis in its package of features.  Yes, you can do this at home too!  And for free.  There's no excuse really, is there?

I should explain a bit for those not used to analysing sounds. Firstly, an FFT (Fast Fourier Transform) takes a waveform (recorded sound in this case) and converts it into the "frequency domain" rather than the "time domain".  (The result is also sometimes called a "spectrum", because it's like splitting up a light beam into its composite colours.)  So we now see peaks representing the various partials. The Hum frequency is the peak on the left, just below 600Hz. The other partials are the peaks that stretch out from that towards the right. The higher a peak is, the louder it is in the mix of partials that comprise the sound.

Now, let's take the horizontal line at -48dB as an arbitrary but convenient loudness threshold. We'll say that peaks louder than that are significant, while those lower than that are not. On the right hand image (old clapper), we see 5 peaks above that. They are our old familiar friends, hum, nominal, tierce, octave and twelfth. (The quint and several others don't quite make it up to our line.) The bell is a D, so they are D5, D6, F6, D7 and A7, all concordant as Fminor.  Nice.

Now follow the same horizontal line in the left hand spectrum and you'll see the same 5 partials, plus 5 other peaks, so we've doubled the number of partials that exceed our threshold. Further, if we carefully analyse these, we find they are not concordant (the poor old bell tuner is good, but he ain't God!). Some of the new partials immediately obvious are G7, C#8, D#8 and G8. Further, they fall in a part of the overall spectrum where our ears are very sensitive, so they punch well above their weight in terms of sonic impact. That's why the external clapper sound was so bad. We've uncovered the bell-tuner's nightmare notes. It's like a really badly flattened internal clapper would produce, but maybe made worse by poor location and being too light, too hard or both.

Fast Fourier Transforms showing the partials produced by the external clapper (left) and the internal clapper (right).

3. WaveAnal

We can go further, but we need some specialist bell software.  Uh-oh, I imagine you thinking, this is gonna prove expensive!  But nope, again free.  UK bell researcher Bill Hibberts created WaveAnal as part of his research program into bell sounds and generously makes it available for download.  

WaveAnal automates the process of identifying partials.  In the screenshot below, the graph at left shows the partials in the external striker sound in a manner similar to the FFT we saw above.  In the table at the right, you can see what WaveAnal made of them.  It was puzzled by two very low sounds, possibly the vehicle noise you can hear in the background in the recording.  WaveAnal has a facility for deleting such spurii, but I've left them in to illustrate that external noises can try to trick you.

The real data starts at line 3 of the table, where WaveAnal has identified a partial at 584Hz.  It successfully identifies it as the bell's Hum note, tells us that it is 9 cents flat of D2 and therefore 2396.2 cents flat of the Nominal.  2400 cents would be a perfect 2 octaves (24 semitones, 0 cents).  The real data starts at line 3, where WaveAnal has identified a partial at 584Hz. It successfully identifies it as the bell's Hum note, tells us that it is 9 cents flat of D2 and therefore 2396.2 cents flat of the Nominal.  (2400 cents, i.e. 24 semitones, 0 cents, would be a perfect 2 octaves.)















WaveAnal screenshot, showing the table of partials produced by the external striker

As we progress down the table, we see all the usual suspects - prime, tierce, nominal etc - identified and quantified.  WaveAnal isn't so sure about identifying some of the others, but gives us the ratios needed to identify them if we want to.  Note that WaveAnal hasn't found all of the partials we can see in the left hand FFT up in section 2; presumably Bill has set the maximum sensitivity at a point where he figures the partials aren't loud enough to be significant.  Even so, we can see why the bell sounds bad.  How often do you include G, Db and Eb in your Fminor chords?   Especially when the Db and Eb are also extra flat.

4. Decay analysis

But for our purposes, there's a much more exciting facility offered by WaveAnal - Decay Analysis.  It isn't fully resourced within the program, you need to do a bit of the work yourself, using an external spreadsheet and graphing package.  But hey, they can be free too!  If you don't have Microsoft Excel, download Open Office. (I used Microsoft Excel, but Open Office works the same.)

Once you've done some spreadsheet number crunching and graphing, you end up with results like these below.  On the left, we can see the five melodious partials that WaveAnal identified from the Internal clapper sound.  On the right, we see the nine partials, not all harmonious, that WaveAnal identified in the external striker case.  In both, they rise quickly at the instant of impact, left of graph, and decay at various rates as we progress to the right of graph, one second in time later.  (WaveAnal gives us data for more than a second, but I truncated it there so we could zoom in on the interesting stuff.)

Looking at the left image, we can see that the Tierce and Nominal are the dominant partials at the start, until the Hum comes up to vie for supremacy later.  But remember the Hum isn't that audible, so the sound is mostly characterised by Tierce and Nominal.

In the right image, the external clapper brings out mostly the Nominal and its Octave at first, with the Tierce and then the Superquint taking over later.  The Hum is very low in the mix, probably inaudible in any practical sense.  Referring back up to the WaveAnal table, we see that the Nominal Octave is actually 65 cents sharp of the Nominal (Eb -47cents compared to D -13 cents), so we can expect some disagreement there!  These high partials are a bit like hot chilli.  A little can add some interest to the mix, but too much makes it inedible.

5. Resynthesis

Now, it struck me, while working with this WaveAnal decay data, that we can recombine the separate decays to give an overall decay curve.  You do have to be a bit careful here - simply adding the various partials at any instant will give an overinflated total.  The correct procedure is to find the Root Mean Squared value, sometimes abbreviated to RMS value.  You square each of the instantaneous values, sum the squares, and take the square root of the lot.  Tedious by hand, but a snack in a spreadsheet.

Then it struck me further that we can also do the same but leaving out the Hum, on the grounds that the Hum doesn't contribute much if anything to audibility.  So, in the charts above, you'll see two extra curves called Total and Total less Hum (shown dashed).  (A variant - and we might want to discuss this - would be to "weight" the Hum value - and maybe other partials too - so that it contributes only its meaningful share, and not an excessive one.  We might call that Total, weighted.  But we'd probably need to do some experimental work first to establish a weighting scheme that closely modelled reality.)

Immediately on comparing the two decay charts above, we can see that both the Total and the Total Less Hum decays are longer in the case of the internal clapper.  Since we have all this data conveniently in our spreadsheet, it's no great effort to produce a direct comparison chart featuring just the totals:

Two things are evident:

  • the small difference between the two External clapper traces (aqua & yellow) reminds us that the Hum is not excited much by the external striker
  • the difference between the two Total Less Hum curves (aqua and pink) illustrates how much quicker the external clapper energy is dissipated.  If we took the horizontal line at 40 as an indicator, the decay times differ by a factor of 2:1.

It's usually assumed that a bell decays at a rate decided by the bell, and not external factors like clappers.  But this seems to suggest otherwise.  Perhaps an explanation lies in the low level of Hum in the External clapper case.  The Hum mode is really the fundamental mode of vibration in the bell (even though the term Fundamental is sometimes confusingly used for the Prime partial one octave higher).  Should we think of the Hum as the bell's engine room, without which the bell will splutter and die? Or perhaps it's enough to note that the Tierce is the longest lived partial after the Hum and is well audible.  We can see in the two decay graphs further up that, in the Internal case, the Tierce is the major constituent of the sound, whereas in the External case, two other high partials are considerably louder.  Whatever it is about the external clapper, it not only discriminates against the Hum, but also the Tierce, the only pair of partials really capable of sustain.

Wrapping it up

So, we really know much more now about these two situations, the same bell hit by two different clappers:

  • the old internal clapper excites only a few partials
  • all these partials are harmonious
  • of these partials really only two (plus the Hum) are doing the lion's share
  • the newer external electric clappers excites about twice the number of partials
  • the additional partials are inharmonious among themselves and with the original partials
  • the sound of the bell decays much faster when struck by the external clapper
  • we perceive that as more staccato, probably compounding the tonal difference.

Why so different?

So why do the clappers make the bell sound so different?

Firstly, the internal clapper is 75 years old and hasn't been used for maybe half that time.  It is probably encrusted with rust on the old strike surface, and so will probably be more mellow than usual.  A few minutes playing and it would probably brighten up nicely!

The hammers on the electrical strikers look small to me, and I wonder, looking at the image above of the bell in question and the one to its right if there is not a big shift in sizes.  That might be one reason why this particular bell sounds so bad under the electrical hammer - the manufacturers of the strikers make them in a limited range of weights, and only one bell in the tier has anything like a suitable weight hammer?

It's also possible that the electric striker is not ideally placed.  I've also heard it said that an external clapper has to be heavier than an internal clapper to achieve the same results.  I can't comment as I haven't looked into it.  There may well be a reason we have traditionally rung our bells from within.

It might simply be the case that the external striker's contact point has been hammered flat and it desperately needs revoicing.  I didn't check that as my reason for being there wasn't related to the condition of the external clappers.  Needless to say, curiosity now demands I take a peek when there next!

I am a little puzzled by the design of the electrical clapper hammers.  They are essentially cylindrical, with chamfered ends.  A horizontal cylinder will touch a bell shape at only one point, just like a spherical clapper.  But as the cylinder or the bell contact point wears, the horizontal axis of the wear point can be expected to grow quickly.  So why choose that shape?  We should perhaps direct that question to those who know more about electrical strikers than I do.

It can only get worse...

Comparing one note of course tells only part of the story.  Imagine now that we play not one note, but say a four note chord (left and right hands, left and right feet).  As each bell note is really a chord in itself, we now have four chords played at once, with all their complex harmonics fighting among themselves.  This is when it is really important not to excite partials that are discordant with the bell itself, let alone with other bells.  Four chords can be challenging, four corrupted chords constitute a nightmare. 


You might perceive I'm waging a one-man-war against external electrical strikers, but that would be simplistic.  I'm waging war on anything that makes a bell sound less well than it could.  That might be underweight clappers, flattened clappers, improperly placed clappers or a combination of them all. 

I am saying that we need to remember that our bells sound only as well as our clappers allow them.  If your clappers are letting down the side, have them tended to.  If they are doing a good job, thank them.

I'll leave the last word to the MIDI controller, as it belts out its startling arrangement of the well-known "Londonderry Air".  To the acoustic faults we've identified in the article, the controller adds its other remarkable skills, the ability to play in unrelenting time with unyielding lack of emotion.  It's a "tour de force", literally.  Press on the link below - be patient, it'll take a little time to load.  Value those few moments left to you.

And, while it plays, consider my "It can only get worse" comments above.  Would you want to sound like this?


Further discussion

You'll find a lot of the same material and an opportunity to discuss issues raised (or that should have been raised) in the GCNA forum on technical discussions.

For more on the physics of clapper and bell dynamics: Bell clapper impact dynamics and the voicing of a carillon

On to: High Clapper

or Back to McGee-flutes Index page...

Created 6 April 2013