Tag Archives: noise

Zen and entropy

Picture1Last weekend I went to a performance of Handel’s Messiah in our local cathedral.  The atmosphere in the vast cathedral was wonderful and for part of the performance I was transformed into a zen-like state by the music.

However, there were quite of lot of disturbances during the performance including some that went beyond the usual coughing and sneezing.  It is interesting that a sneeze in the quiet environment of a cathedral or library causes a large disturbance while the same sneeze in a busy street goes unnoticed.  Of course, it is about the change in the noise level, and as a percentage, the added noise of a sneeze is much greater in the quiet library than the busy street.  Noise is a form of energy that becomes dispersed and dissipated as it propagates and so it is easy to equate it to heat which exhibits the same behaviour.  Heat transfers from hot to cold places while noise propagates from loud to quiet places, and neither does the reverse, which was Clausius’ observation that lead to the Second Law of Thermodynamics.  Clausius also defined change in entropy as the heat transfered divided by the temperature at which it occurs.  So the same heat transfer creates more entropy at low than at high temperatures, just as a sneeze causes more disorder/disruption in a quiet than a loud environment.  We can equate entropy to the level of disorder present in any system or environment.

And the second law of thermodynamics states that the entropy of an isolated system will always increase until it reaches a maximum at equilibrium.

Sonic screwdrivers

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No relevance except for the tranquility or absence of noise.

In a recent post on Noise Transfer [27th March, 2013] I highlighted the parallels between energy transfer by heat and noise.  In many cases, the heat and, or noise transfer is by-product of a process through which energy is dispersed to satisfy the requirements of the second law of thermodynamics, that entropy must increase as a product of all real processes.  Entropy, can be interpreted as a measure of dispersion, or the lack of availability to do anything useful and this applies to most heat and noise that we encounter in everyday life.

We can use concentrated sources of heat to produce useful work such as the furnace in a power station, but the second law of thermodynamics demands that we waste a substantial proportion of it through the creation of entropy.  It is also possible to use concentrated sources of noise, such as ultrasonic transducer to perform useful work for us, such as in surgery and the manufacture of composite materials [see Professional Engineering, http://profeng.com/features/good-vibrations ]; although an all-purpose sonic screw-driver of the kind used by Dr Who is not possible, yet.

Noise transfer

This is not the author's house!

This is not the author’s house!

We are privileged to have magnificent views of the river and mountains beyond from our city centre house.  However, the house was built before the motor car was invented when the loudest event outside might have been rowdy party-goers heading for home.  We still have some party-goers walking home under our bedroom window at night but most of them travel by noisy taxis.  I look forward to when the price of fossil fuels, or legislation will force taxis to become electric-powered.  In the meantime, we have been designing secondary glazing that will offer a high resistance to noise transmission and be in keeping with the early 19th century windows.  Noise is a form of energy transfer by vibrations, acoustic energy would be an alternative term for it, and so the combined resistance of the outside wall of my bedroom can be calculated using Kirchhoff’s law, as discussed for heat transfer in my last post [Born in a barn, 20th March, 2013].  In this case, the thin and badly-fitting but antique glass is the dominant component of both the heat and noise resistance.  We were happy to deal with the poor resistance to heat transfer by using plenty of bedclothes, i.e. adding a large resistance in series, but the same approach does not work with noise because earplugs are uncomfortable, fall out in your sleep and have a low resistance at the frequency of taxi-generated noise.  So, the solution is secondary glazing and the best performance is achieved using an acoustic laminate consisting of a polymer sandwiched between two sheets of glass which should be different thickness to avoid resonant effects.  Of course this will also improve the resistance to heat transfer which will be advantageous in winter, but perhaps not in summer…