American studies and on the work of other research workers in this field. Most of the studies that led to this criterion were performed with noise from small arms, but the criterion is general enough to permit assessment of most other types of impulse noise. The variables that must be considered in determining the potential hearing hazard and in the practical application of the criteria are presented, and the parameters that must be measured are defined. The measurement technique and type of transducers to be used are discussed. INTRODUCTION OR steady-state types of noise, the relationships between the intensity, spectral content, duration of exposure, off periods, and the extent of the pure-tone hearing loss that results are fairly well known, and a considerable number of relatively similar damage-risk criteria • (DRC) have been published. 2-0In contrast, * Surgeon Commander R. R. A. Coles, R. N., is also a staff member of the Institute of Sound and Vibration Research, University of Southampton. • The term damage risk criterion, unfortunately, has two meanings in the literature. First, it refers to the individual criteria such as allowable TTS, acceptable peak level and duration, etc. Second, it refers to the total document that results as the sum of the individual criteria. Both meanings of the term are implied in this paper, but it is believed that the context will enable the reader to keep the meanings separated. HAZARDOUS EXPOSURE TO IMPULSE NOISE ducers, apparatus and environment for such measurements are as follows' © A good phase response. © A uniform amplitude response characteristic over a wide frequency range. (It has been found that a bandwidth from 100 Hz to 70 kI-Iz is adequate for most smallarms measurements. However, larger-caliber weapons and such noises as sonic booms with long pressure-wave durations will require an extension of the low-frequency response?) © Less than 1.5 dB ringing and overshoot at the pressure being measured (ringing should be completely damped after 100 t•sec). ß Rise-time capability of 10 usec or less at the pressure being measured. ß Sufficiently robust to withstand acoustic damage from the pressure being measured. ß All apparatus must be mounted so as to eliminate microphonics. ß Measurements should be made of the sound field in the absence of the firer or associated personnel and other relatively large reflecting surfaces, other than the ground or the surfaces of the enclosure in reverberantfield measurements. increments often recommended for short-duration ex-Ground, Md. (1965). 40 R. R. A. Coles and J. J. Knight, unpublished studies on PTS from 7.62-mm rifle noise, R. N. Personnel Res. Committee, London (1962, 1965). 50 M. A. Elwood, P. F. Brasher, and L. M. Croton, "A Pre-

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The recovery from impulse-noise induced temporary threshold shift was systematically traced for individual rhesus monkeys and men. In addition to the well-known logarithmic recovery, three other types of recovery were seen (diphasic, plateau, and rebound). A descriptive model is developed for the classification of these recovery functions. The model postulates the existence of two types of temporary threshold shift, process M and process S, both of which may be seen after impulse-noise exposure.

Because existing research treats residential displacement as a consequence of housing market conditions associated with the urban core, the incidence of displacement is thought to be greater in: (1) metropolitan areas than nonmetropolitan areas, (2) the 'urban heartland' than other regions of the country, (3) central cities than suburbs, and (4) older, inner-city neighborhoods than younger, peripheral ones. Data from the US Annual Housing Survey are analyzed and case studies of individual cities are reviewed to test these four spatial hypotheses. While the strength of the evidence favoring the hypotheses depends in part upon the type of displacement rate employed, spatial contrasts in displacement generally appear to be less marked than the literature suggests. We attribute the lack of convincing support for the hypotheses to housing market pressures operating at the national level that generate involuntary mobility across all geographical sectors of the population.

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