56 Please remember to mention OVL when responding to adverts Stelios Mores In 1908 the meteor that exploded above the Podkamennaya Tunguska River in Siberia in Russia is estimated to have reached a noise level of 315dB and is the loudest sound ever known. Although the meteor exploded between 5 to 10km above the ground, the noise created an earthquake equivalent to five on the Richter scale, and the explosion was heard as far away as Britain. Leonid Kulik was the Russian mineralogist who led the initial expedition to Tunguska to investigate this event. Sound is a physical phenomenon which has been with us forever. In fact it is the sounds we exchange that give us language and make us into a species that is uniquely conversant, and uniquely human. The first experimental observations relating to sound date back to the Greek philosopher and mathematician Pythagoras, who during the sixth century BC linked the pitch of a note to the length of a vibrating string and allowed him to create the musical scales from a simple mathematical relationship. During this time only seven heavenly bodies were known, and as music was considered to be a link between the physical and spiritual worlds it was thought that they represented the seven basic musical notes, which were created as they vibrated while moving through the heavens. So Pythagoras established the basic notes (diatonic scale) which are used in Western music to this day. During the two millennia that followed, much of the studies relating to sound were in fact related to music. The Roman and Byzantine empires influenced early European music with the Church playing a major role during Medieval times until the Renaissance period. Yet despite all these endeavours the physics behind sound was poorly understood as it was for the most part an art form, relying on the skills of the craftsmen who used intuition to improve the sounds emanating from their various musical instruments. During the first century, the Roman thinker Seneca proposed that sound might be some sort of wave; however it was not until the early 1500s that the first scientific attempts at describing how sound travels were made by Leonardo DaVinci in Italy. Using his understanding of anatomy and an experimental approach he inferred the existence of pressure waves, concluding that these were responsible for transferring sound through air. In essence he attributed sound to differences in pressure within the air which are created by repeated movement at the source (such as a vibrating string). These waves are now referred to as compression waves and act along the direction of travel (i.e. longitudinally) and their loudness is directly related to the differences between the highest and lowest pressures. This is also known as the amplitude. A century later the British scientist Robert Boyle showed that sound required a medium to travel. He did this using a bell in a jar from which he removed the air until the sound could no longer be heard. Around the same time in Italy, Galileo Galilei showed that pitch is directly related to the frequency of the sound wave. This laid the foundations for Isaac Newton to make one of the first attempts at establishing the speed of sound in air in the late 1670s. Instead of measuring it directly however, he derived a mathematical relationship which allowed it to be estimated from the pressure and the density of air (√=P/p). Unfortunately his estimate of 298 m/s was 15% too low, and his contemporaries Marin Mersenne and his compatriot Pierre Gassendi came closer to the real value by measuring it directly as 420 m/s and 449 m/s respectively. A hundred years later the famous French mathematician Pierre- Simon Laplace corrected Newton’s relationship by adding the effects of temperature on air to it. This gave a value of 350 m/s which is out by 2.5% when compared to the modern day estimate of 331.2 m/s. Science Silently Loud