THE original explosion was heard more than 200 miles away in the Netherlands because atmospheric conditions allowed the sound wave from the blast to propagate over longer distances than is normally possible.

While it is unusual for even very loud noises to remain audible at such distances, Sunday’s weather meant that a faint sound reached sharp-eared listeners in continental Europe.

Acoustics experts and meteorologists attributed the effect to a strong “temperature inversion” at the time of the blast, in which warm air at altitude traps cooler air closer to the ground. This created a ceiling against which the sound wave from the explosion would have bounced, channelling it towards the ground. The effect is similar to beeping a car horn in a tunnel, experts said.

Trevor Cox, professor of acoustic engineering at the University of Salford, said: “It is incredibly unusual for sounds generated on the ground to be heard too far away, but it is not unknown. If there is a temperature inversion, sound waves that would have gone up into the atmosphere get redirected towards the ground.”

David Kemp, professor of auditory biophysics at University College London, said: “Typically, when sound travels out and up from a single source, the volume level would drop by about 84 decibels (dB) by the time you got 60 miles (97km) away, but if the sound travelling horizontally was trapped between temperature levels then the decibel reduction can be much less.”

The acoustic effect, he said, was similar to whales’ singing underwater, which can be heard over thousands of miles because of the temperature differences in the oceans.

“It will depend on the sheer volume of such a blast and there has to be a temperature change such that the sound effectively gets channelled close to the ground. But it’s not impossible that even in Holland someone could hear it,” Professor Kemp said.

Sound levels of around 200dB can be generated near large explosions. Anything above 85dB is considered harmful, while 120dB is unsafe and 150dB can cause damage to the human body. Windows break at about 163dB.

Jet airplanes cause levels of about 133dB at a distance of 30m (98ft). Eardrums can rupture at 190dB to 198dB. Shock waves and sonic booms cause levels of about 200dB at 330m. The space shuttle generates levels of around 215dB. Even louder are nuclear bombs, earthquakes, tornadoes, hurricanes and volcanoes, all capable of exceeding 240dB.

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