Eric Ungar’s Acoustics from A to Z: C – Criteria

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 class rooms 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.