Tony F.W. Embleton
80 Sheardown Drive
Nobleton, Ontario L0G 1N0
Tel/Fax 905-859-1136
Contact During ASA Meeting:
Marina Marriott Hotel
Ft. Lauderdale
Tel 954-463-4000
Popular version of paper 1eID
Presented Monday evening 7:00 - 9:00 pm, December 3, 2001
142nd ASA Meeting, Fort Lauderdale, FL
WHY IS UNDERSTANDING OUTDOOR NOISE IMPORTANT?
1) Social Factors: most human activities and transportation
make noise. Rules are needed to regulate what is acceptable if people
are to live in habitable communities with an optimum quality of life.
Rules should be technically reasonable and equitable for both
the noisemaker and the listener; otherwise they are likely to be ignored.
2) Economic factors: tens of thousands to several billion dollars are involved in each project of highway barrier construction, and highway, airport or community location. Environmental impact decisions should be based on the best technical knowledge available in order to be cost-effective.
3) Military factors: detection and location of the enemy
at greater distances by acoustical means are important.
Listening can hear sources behind obstacles where radar is ineffective.
Listening is passive, whereas radar and other techniques are usually active
and divulge one's own location. Reducing the detectability of one's own
forces is equally necessary.
CHANGE FROM AN EMPIRICAL PAST TO A QUANTITATIVE FUTURE
Many widely used prediction schemes use a simple set of features to predict
the sound levels at a distance from a source of sound. Sound is assumed
to spread according to simple geometrical laws over a flat ground. At
large distances, corrections are made to allow for extra absorption of sound
at high frequencies. (This is a well-known effect and explains why distant
thunder "rumbles" while nearby thunder "cracks.") Sounds levels at moderate
to large distances are usually a bit lower than expected so a few decibels are
sometimes subtracted, based on experience, to allow for an unknown ground effect
and improve the predictions. The rationale for this approach is that individual
situations are all different but most details cannot be quantified accurately,
so the best course is to assume average conditions.
Within the past year or two, authorities in the European Union have concluded that their existing prediction schemes should be revised to include much of the new quantitative knowledge now available from research studies of the past 25 years. In both Denmark and Germany, urban planners and traffic engineers are studying changes in standards for measuring vehicle sound levels and in methods to provide more accurate predictions of sound levels at building facades.
WHAT IS THE NEW KNOWLEDGE?
The individual features of most situations are not average, and can cause sound
levels to be very different from an average value. Recent work in several
countries has largely unraveled the wave-propagation, topographical, and meteorological
factors that can have major effects on the levels of sound at the listener.
Different factors are important at different distances, different heights above
the ground, over different types of ground, between night and day, and in different
weather conditions. The core of the new knowledge is the simple realization
of two obvious factors. Firstly most sound sources and listeners are relatively
close to the ground compared with the horizontal distance between them.
Secondly, most ground surfaces, such as grass or snow are acoustically quite
soft, and few are very hard. Sounds reflected from the ground at small
grazing angles vary greatly depending on the nature of the surface.
Almost every wave propagation mechanism can be involved. Geometrical spreading and molecular absorption are always present, so are scattering and turbulence. Other mechanisms occur only for waves near the ground. Concrete, grass and snow are very different surfaces, and they have very different effects on the sound field above. The type of ground determines the reflections from the ground surface, and some surfaces can give rise to a special type of wave.
The behavior of the atmosphere can either increase or decrease sound levels significantly, and the effects of refraction caused by wind and temperature gradients must be taken into account. Sound levels near the ground are usually higher at night in a temperature inversion than during the day when there is often a temperature lapse. The shape of the ground - its hills and valleys - can also increase or decrease sound levels and must be considered.
This tutorial talk reviews these various effects by considering measurements from as close as ten inches to about three miles, and comparing these to theoretical predictions that quantify these several mechanisms. Many different types of ground surface and weather conditions are studied. The talk concludes by applying these factors to three very common practical situations, namely highway noise barriers at medium distances, and the prediction of sound levels at distances of a mile or more when the ground is not flat.
SOME SELECTED HISTORICAL REGULATIONS
About 4000 BC, Epic of Gilgamesh; in one version of the text the great flood was the punishment of the people for annoying the gods by making too much noise (first community noise problem)
About 600 BC, Sybaris; to keep the city quiet, potters, tinsmiths and other noisy trades were required to work outside the city walls (first industrial noise regulation)
BC to AD, Rome; chariots and delivery vehicles were banned from the forum in the day because of the noise and congestion they caused (first traffic noise regulation)
1930, New York City; first full-scale city noise survey, entitled "City Noise"; with the exception of aircraft, air-conditioners and the increased use of powered tools, the significant sources of 70 years ago are still the same today
1934, Great Britain; proposed the use of sound level meters to measure and control motor vehicle noise; there were no regulations until 1968 and no enforcement until 1970
1952 to 1955; first quantitative noise ordinances in USA with limits that could be measured; Seattle and Cincinnati - control of vehicle noise as operated: Chicago - control zoning in the city on the basis of noise performance
1960s and 1970s; commercial jet aircraft came into service; this was
followed by a great proliferation of federal, state and local noise ordinances.
SOME SELECTED HISTORICAL MEASUREMENTS
1636, Mersenne measured the speed of sound by timing the echo of a gunshot
1738, Academy of Paris sponsored evaluation of the precise speed of sound, 332 m/s
1816, Laplace calculated the speed of sound, 331.5 m/s
1860 to 1870, Henry and Tyndall noticed that fog absorbed and scattered sound
About 1870, Stokes found that temperature inhomogeneities caused scattering
About 1880, Stokes found that horizontal temperature layers caused refraction
1916, French and British armies used microphones and parabolic reflectors to locate enemy artillery