Ravicz M E, Melcher J R
Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston 02114, USA.
J Acoust Soc Am. 2001 Jan;109(1):216-31. doi: 10.1121/1.1326083.
Approaches were examined for reducing acoustic noise levels heard by subjects during functional magnetic resonance imaging (fMRI), a technique for localizing brain activation in humans. Specifically, it was examined whether a device for isolating the head and ear canal from sound (a "helmet") could add to the isolation provided by conventional hearing protection devices (i.e., earmuffs and earplugs). Both subjective attenuation (the difference in hearing threshold with versus without isolation devices in place) and objective attenuation (difference in ear-canal sound pressure) were measured. In the frequency range of the most intense fMRI noise (1-1.4 kHz), a helmet, earmuffs, and earplugs used together attenuated perceived sound by 55-63 dB, whereas the attenuation provided by the conventional devices alone was substantially less: 30-37 dB for earmuffs, 25-28 dB for earplugs, and 39-41 dB for earmuffs and earplugs used together. The data enabled the clarification of the relative importance of ear canal, head, and body conduction routes to the cochlea under different conditions: At low frequencies (< or =500 Hz), the ear canal was the dominant route of sound conduction to the cochlea for all of the device combinations considered. At higher frequencies (>500 Hz), the ear canal was the dominant route when either earmuffs or earplugs were worn. However, the dominant route of sound conduction was through the head when both earmuffs and earplugs were worn, through both ear canal and body when a helmet and earmuffs were worn, and through the body when a helmet, earmuffs, and earplugs were worn. It is estimated that a helmet, earmuffs, and earplugs together will reduce the most intense fMRI noise levels experienced by a subject to 60-65 dB SPL. Even greater reductions in noise should be achievable by isolating the body from the surrounding noise field.
人们研究了在功能磁共振成像（fMRI，一种用于定位人类大脑激活区域的技术）过程中降低受试者所听到的声学噪声水平的方法。具体而言，研究了一种用于将头部和耳道与声音隔离开的装置（“头盔”）是否能增强传统听力保护装置（即耳罩和耳塞）所提供的隔离效果。同时测量了主观衰减（佩戴与不佩戴隔离装置时听力阈值的差异）和客观衰减（耳道声压的差异）。在功能磁共振成像最强噪声的频率范围（1 - 1.4千赫兹）内，头盔、耳罩和耳塞一起使用时，可将感知到的声音衰减55 - 63分贝，而仅使用传统装置时的衰减则要小得多：耳罩为30 - 37分贝，耳塞为25 - 28分贝，耳罩和耳塞一起使用时为39 - 41分贝。这些数据有助于明确在不同条件下，耳道、头部和身体传导路径对耳蜗的相对重要性：在低频（≤500赫兹）时，对于所考虑的所有装置组合，耳道都是声音传导至耳蜗的主要路径。在较高频率（>500赫兹）时，佩戴耳罩或耳塞时耳道是主要传导路径。然而，同时佩戴耳罩和耳塞时，声音的主要传导路径是通过头部；佩戴头盔和耳罩时，是通过耳道和身体；佩戴头盔、耳罩和耳塞时，是通过身体。据估计，头盔、耳罩和耳塞一起使用可将受试者所经历的最强功能磁共振成像噪声水平降低至60 - 65分贝声压级。通过将身体与周围噪声场隔离开，有望实现更大程度的噪声降低。
