Ringing news – Sound level

After writing April’s article, it occurred to me that not only may you be reaching for your umbrellas as you walk into church during ringing, but that you may like to know some real facts about bell sounds.

Sound is a form of energy, like heat or electricity, but many people are surprised just how tiny are the power levels involved. You have probably heard people talk about decibels, (db). T hey are just a way of comparing two power levels. Ten decibels means 10 times, 20 decibels means 100 times, 30 means 1000 times and so on. By convention, sounds are always compared to a power level of a millionth of a watt per square metre. That is the quietest thing our ears can detect and is equivalent to the power of a torch bulb spread over the British Isles.

The street noise outside is about 100,000 times louder than that, ie 50dB. By the church gate, the bells are about 80dB. The total sound power coming from the bells in the tower is about equal to that from our torch bulb. The reason why such tiny power levels sound so loud to us is that our ears are extremely sensitive.

Six years ago, we installed sound control with shutters so that we could let more sound out for service ringing and much less when practising. This has been very successful. When we measured it, we found that closing the shutters cut the sound down by nearly 20dB. That means only about a hundredth of the power level.

At the time, some people were concerned that ‘bottling the sound up’ in the belfry rather than letting it out may damage the tower. From what I’ve just described about power levels, you can see the fears were groundless, but I did some calculations to put things into perspective. For instance, as the bells swing, they impose sideways forces of up to a few tons on the tower. In a 70mph wind, the total force on the tower could be ten times higher than that, though spread over the whole tower.

So why does the tower not fall I down before this onslaught? It isn’t the cement that holds it up; that is relatively weak. No, our tower, like all masonry structures, is held in place by its weight, which is around 1000 tons. It does sway, though, but that’s another story.

John Harrison (May 1988)