Acoustics from A to Z: C and D
By Eric E. Ungar
Editors Note: We are republishing Eric Ungar’s Acoustics from A to Z and Stig Ingemansson’s Noise Control: Principles and Practice from previous NNI issues as part of our initiative to include more educational articles. Their lessons are just as valid today. Look for more in future issues and on noisenewsinternational.net.
During the March 1999 joint meeting of the Acoustical Society of America with the European Acoustics Association, held at the Technical University of Berlin, I had the opportunity to visit the Institute of Technical Acoustics of that university. Prominently posted on a bulletin board in a corridor, I found an eleven page 1958 article which was coauthored by Lothar Cremer, the late former director of the institute.
This unusual publication, whose title may be translated as “The ABC of the Acoustics of Buildings,” consists of brief verses. There is one verse for each letter of the alphabet; each deals lightheartedly with some aspect of acoustics, each is illustrated by a cartoon structured around the letter, and each is followed by a brief summary of related facts. I was so intrigued with this article’s approach that I resolved to translate it and asked the director of the Institute, Professor Michael Möser, to send me a copy. Soon, after he kindly complied with my request, it became clear that translating the article without losing its basic spirit was beyond me. So, I decided to give up and start from scratch, developing my own verses and discussions, but maintaining the spirit. Here’s the result.
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CRITERIA for floor vibration
Or sound (that is, air oscillation)
Depend on what is really needed; Intended usage must be heeded.
If set too tight, there’s undue cost;
If set too loose, there’s function lost.
In the index of any book on acoustics or on noise and vibration control you will find a profusion of listings under criteria. You will find criteria for environmental noise from aircraft and surface vehicles, for the prevention of hearing damage in industrial settings, for acceptable conditions in dwellings and offices, for efficient speech communication, for good listening conditions in classrooms and auditoriums. Also for acceptable vibration environments in buildings, surface vehicles and ships (sea-sickness effects are considered in standards being developed), for vibrations exerted on workers’ hands and arms by machine tools, and for evaluation of the quality of rotating machinery. I apologize if I have omitted your favorite among the many other criteria that are available.
Many criteria that have been developed on the basis of extensive studies have gained broad acceptance, have been set forth in international and national standards, have been the basis of ordinances and regulations, and have been cited in cases involving litigation. But how firm are these criteria and standards? Standards are meant to represent the consensus of experts, and they do – to the extent that the experts participating in development of the standards can agree. Unfortunately, the number of specialists involved in this development process typically is small, some may have limited ranges of interest and concern, and some may have certain prejudices. Therefore, standards tend to reflect only the small amount of information on which the participants in the development process can agree. And even then, the consensus is not always one hundred percent. Some standards only address how measurements should be made, leaving criteria – the (usually controversial) magnitudes against which to judge the measured values – to appendixes that are not official parts of the standard.
Some criteria, for example those limiting the exposures of sensitive equipment, are set forth by the equipment suppliers. These criteria often are more stringent than necessary, perhaps because the sensitivity of the equipment is not well known or – as a suspicious person may feel – to give the supplier the opportunity to blame the noise and vibration environment for the occasional less than optimal performance of his equipment.
Equipment criteria often are written by non-specialists in acoustics and vibrations. This has led to problems with inappropriately specified noise spectrum weightings, with confusion between vibratory displacements and acceleration, and with omission of measurement duration and bandwidth requirements, among others. I have spent much time explaining to suppliers of optical equipment that relative displacements of the optical components are important, and not the absolute vibratory displacements of the equipment’s support points. And I have often tried to convince clients that one cannot limit the displacement amplitudes of buildings to very small values in a range of frequencies that extends down to zero, relying on the argument that the moon induces tidal motions in the earth much as it does in the oceans and we as yet don’t have the technology to hold the moon still.
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DAMPING is poorly understood.
It doesn’t always do much good. Although it may speed wave decay, It makes few problems go away. Damping does mean dissipation, But may not yield attenuation.
According to my dictionary, the verb “to damp” means “to make damp; moisten” or “to check or retard the energy of” or “to stifle or suffocate, extinguish.” Similarly, the verb “to dampen” is defined as “to make damp, moisten” or “to dull or depress.” In acoustics and vibrations, however, ‘damping’ has nothing to do with moisture. Although ‘damping’ sometimes is used to mean attenuation, in precise technical language its usage preferably should be confined to processes involving energy dissipation.
I have long advocated that ‘dampening’ not be used instead of ‘damping,’ because ‘dampening’ primarily means ‘moistening.’ Nevertheless, the writers who produced my former employer’s annual report a few years ago proudly proclaimed that “we dampen submarines.” I found it difficult to explain how we can ask to be paid for that.
Most vibration textbooks deal only with viscous damping – that is, with damping due to a force that opposes the motion and is proportional to velocity. The primary reason for focusing on this type of damping is not that viscous damping necessarily represents the real world (although it fortunately is a reasonable approximation in many cases), but that the assumption of this sort of damping gives us linear differential equations with constant coefficients, which we know how to solve comparatively easily. So, in fact, we act much like the proverbial drunk w ho lost a silver dollar in the middle of the block, but looks for it near the corner because the street lamp’s light is better there – that is, we solve an easier problem rather than the real one.
Of course, much can be learned from the textbook problems, and generally the answers we get from viscous damping analysis are reasonable as long as the damping is not too large. But, watch out! Most texts and handbooks, as well as much sales literature for vibration isolators, show equations and curves that indicate that the isolation that a spring-damper combination provides above a system’s natural frequency is severely compromised by large damping. This result, derived for viscous damping, tends to overstate markedly this detrimental effect of damping for metal or rubber isolators, in which the damping is not of the viscous type.
Contrary to some misguided commercial claims, damping is not a cure-all. Basically, damping has a significant effect only on motions that are controlled primarily by energy dissipation. These motions include steady responses at and near resonance, freely decaying vibrations, and freely propagating waves; they do not include vibrations due to steady excitation at frequencies that are not near a system’s natural frequency. Space limitations and my desire not to bore those of you who are not particularly interested in this topic keep me from further preaching here. Those of you who are curious to learn more may find my “Structural Damping” chapter1 useful, though less entertaining than this brief discussion.
1. Chapter 12 of Noise and Vibration Control Engineering, edited by Leo Beranek and Istvan Ver, John Wiley & Sons, NY, 1997.
