Tower and bell acoustics

Bill Hibbert bill at h...
Mon Apr 5 10:24:24 BST 2004

One or two people have asked about tower acoustics, resonant 
frequencies of bell-chambers, etc. It's difficult to explain without 
getting technical, here's an attempt to put it in straightforward 
terms. I have done an awful lot of work on this over the past couple 
of years - enough to understand the effects, but not enough (yet) to 
be able to say what makes a tower 'good' or 'bad' acoustically. 
Little of the stuff below is on my website, yet.

Towers don't just resonate at a single frequency. Their response to a 
bell vibrating inside them has a characteristic at every frequency. 
This characteristic accentuates some frequencies and attenuates 
others. The acoustic effect of the tower is very important indeed to 
the sound of a bell. My extended experiments in simulating bell 
sounds have proved that unless tower acoustics are taken into 
account, it is almost impossible to create a realistic bell sound; 
with tower acoustics, it is quite easy.

I did some experiments a year ago, repeatedly striking a bell with a 
machine I built giving a clapper blow of known strength. I discovered 
that the acoustic characteristic of the bell chamber varied 
considerably from one place to another - some partials were stronger, 
some weaker, depending where the microphone was within the bell 
chamber. One would expect this from the basic physics. Obviously once 
the point of listening moves outside the bell chamber, into the 
ringing room or the street, these differences get greater.

This sounds quite complicated, but it turns out that all the 
charateristic for a particular sound path from bell to listener can 
be captured by recording the response to a sudden sound (the impulse 
response, in technical jargon). I use a handclap. It is then possible 
to apply this impulse response to a bell sound (the technical term is 
convolution) to 'add in' the acoustic of the tower. The results are 
very convincing in practice.

The reverse process (deconvolution) is also mathematically possible, 
so in principle it is possible to take away the effect of one tower 
from a bell recording before adding in that of another. If this 
worked, it would be very exciting, but unfortunately, for deep 
technical reasons, the result does not sound good enough in practice 
to be usable.

As I said, the tower acoustics make an enormous difference to the 
intensities of the various partials which can clearly be heard as a 
difference in the sound. However, I still believe that features of 
the bell itself also make a difference (e.g. the example Andrew 
Higson recently gave of thick-shouldered bells having a weak prime). 
There are differences between recent bells from Taylors and 
Whitechapel which I have heard in more than one tower which must be 
due to something fundamental in the bell design.

Enough! If anyone wants any more, contact me privately.

Bill H

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