published_or_final_version / Speech and Hearing Sciences / Master / Master of Science in Audiology

Thesis (M. Sc.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 85-89).

Noise Induced Hearing Loss (NIHL) is a sensorineural hearing loss resulting from excessive noise exposure (von Geirke, 1975). In New Zealand the Department of Labour stipulates that employees must not be exposed to more than an eight-hour equivalent continuous A-weighted sound pressure level (LAeq,8h) of 85 dB(A) or a peak sound pressure level (Lpeak) of 140 dB(A). Sandblasting is a process, whereby an abrasive is mixed with air at high pressures and exposes the operators to excessive noise (Blair, 1975). The protective equipment currently worn is a closed respirator system consisting of a helmet and cape, overseas research indicates noise levels inside the helmet are above the allowable levels (Blair, 1975; Environmental Medicine Unit, 1998; Irving, 1995; Patel and Irvings, 1999; Price and Whitaker, 1986 and Sussel 1992). There is disagreement in the literature as to the contribution of the noise created by the respirator system to the total noise exposure (Blair, 1975; Environmental Medicine Unit report, 1998; Irving, 1995 and Price and Whitaker, 1986). In some blasting sites additional hearing protection such as foam insert earplugs and/or earmuffs are worn in conjunction with the helmet. No research could be found investigating the resulting attenuation these combinations provide. It is well reported in the literature that the combined attenuation is more complex than the simple sum of the two components attenuation (Abel & Armstrong, 1991; Abel & Odell, 2006; Behar, 1990; Berger, 1983; Damongeot, Lataye & Kusy, 1989). The aims of the current study were to investigate noise levels during blasting inside booths and the operators’ helmets at two sites in New Zealand. The attenuation provided by the helmet alone and in combination with earmuffs was investigated in the laboratory. The contribution of the respirator airflow to the total noise exposure was also investigated. Finally the measured combined attenuation was compared to that calculated using three methods from the literature. Findings indicate that noise levels in blasting booths and inside operators’ helmets are above allowable levels. Laboratory findings indicate that the use of earmuffs in conjunction with the helmet increases the attenuation provided, therefore decreasing the risk of NIHL and that noise from the respirator airflow does not contribute to the total noise inside the helmet. The combined attenuation calculated from two methods in the literature was inaccurate while one was close. The findings, recommendations, limitations of the study and areas for further research are discussed.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2006. / Härtill 4 uppsatser.

Thesis (M. Sc.) -- University College London, 1998. / A project submitted (to the Institute of Laryngology and Otology) as a requirement for the degree of Master of Science in Audiological Medicine, University College London. Bibliography: leaves 83-101.

Thesis (M. Sc.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 32-36).

Thesis (M. Sc.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 65-73).

Thesis (M.Communication Pathology)--Universiteit van Pretoria, 2002. / Summary in English and Afrikaans. Includes bibliographical references.

Thesis (M. Communication Pathology)--University of Pretoria, 2005. / Includes bibliographical references.

The term ‘tipping event’ is used to describe a certain class of phenomena as observed in many different fields of science. It refers to an event where a gradual change of external forcing causes a sudden, large, often unwanted, transition to the state of the system. Some examples of known tipping events in science include: Arctic sea ice melting (climate), epileptic seizures (biology), collapse of ecosystems and populations (ecology) and market crashes (finance). Three mathematical mechanisms for tipping events have been proposed in the literature: bifurcation-, noise- or rate-induced tipping. Recent research has focused on developing early-warning indicators to potentially offer forewarning, which can extract from output time series whether the external forcing approaches a critical level at which tipping occurs. Two commonly used early-warning indicators are an increase of autocorrelation and variance in the time series data for the system’s output. The theory behind the presence of these indicators is the loss of stability of the system’s current state known as ‘critical slowing down’ for the approach of a bifurcation-induced tipping. Rate-induced tipping occurs when the external forcing reaches a critical rate instead of level. For rate-induced tipping there is no loss of stability of the system’s current state and therefore it is not clear if the early-warning indicators should exist. In this thesis we investigate the presence of early-warning indicators for models that show rate-induced tipping with additive noise. We also explore a technique for determining the most likely time of tipping using optimal paths for escape. Research has mainly focussed on testing the early-warning indicators for examples of known tipping events in the past. The ultimate aim of early-warning indicators would be to have the ability to predict future tipping events. Using the early-warning indicators in isolation is susceptible to incurring false alarms and missed alarms. We present a method for approximating the probability of experiencing rate-induced tipping with noise for slow to moderate drift speeds.