Intercity 125 trains have an engine at each end

Download train125.ddw

This week, just to show you don't have to prove some obscure theory using data-logging, a little environmental investigation. How much noise do the trains at the bottom of the garden really make?

The recording was made on Tuesday. 4 May 1993 at 07:49:24. The sound level sensor from Philip Harris reads down to 50 dB so it might have been even quieter than this with no train.

The event was the passing of an "InterCity 125 train" and the sensor was about 6 m from the track.

It seems that the leading engine was working harder (or at least making more noise about it). If we knew the distance between engines we could calculate the average speed. Can you see evidence that some bogies in the train are noisier than others?

These are one of those logarithmic units that worry some people. They tend to be used for things we can sense ourselves (light and sound) and are to all intents and purposes essential where the range of the measured parameter is very large. This recording covers a range of sound power levels in the ratio 1 : 10 000. The thing to remember about logarithmic units is that each equal division on the graph represents the same ratio of "amount" we are measuring. For the fussy, the actual figure is that each 10 dB change represents a 10 fold increase in sound level.

The other thing that confuses some about sound measurement is the crucial influence of distance. Sound levels fall off roughly by an inverse square law (exactly in ideal conditions). Short-circuiting the maths again, this means that at 3 m from the track we might have expected to find four times the sound power or 6 dB higher peak readings - practically 100 dB. Halve the distance again and you get 106 dB and so on. Those diesel units really are deafeningly loud close up!

This means that anyone can top the sound sensor scale with their own voice if they hold it close up. This confuses some users but its just the Physics again and if somebody else shouts or even talks loudly right in your ear for long periods its can be just as damaging as more generally accepted noise hazards.