Volume 109, Issue 1, January 2001
Index of content:
- NOISE: ITS EFFECTS AND CONTROL 
Isolating the auditory system from acoustic noise during functional magnetic resonance imaging: Examination of noise conduction through the ear canal, head, and body109(2001); http://dx.doi.org/10.1121/1.1326083View Description Hide Description
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 protectiondevices (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 (⩽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.
109(2001); http://dx.doi.org/10.1121/1.1326950View Description Hide Description
The zone of local control around a “virtual energy density sensor” is compared with that offered by an actual energy density sensor, a single microphone, and a virtual microphone. Intended as an introduction to the concept of forward difference prediction and a precursor to evaluating the virtual sensorcontrol algorithms in damped enclosures, this paper investigates an idealized scenario of a single primary sound source in a free-field environment. An analytical model is used to predict the performance of the virtual error sensors and compare their control performance with their physical counterparts. The model is then experimentally validated. The model shows that in general the virtual energy density sensor outperforms the actual energy density sensor, the actual microphone, and the virtual microphone in terms of centering a practically sized zone of local control around an observer who is remotely located from any physical sensors. However, in practice, the virtual sensor algorithms are shown to be sensitive (by varying degrees) to short wavelength spatial pressure variations of the primary and secondary sound fields.
109(2001); http://dx.doi.org/10.1121/1.1327576View Description Hide Description
A laboratory study was designed in which the annoyance was investigated for 14 different impulse sound types produced by various firearms ranging in caliber from 7.62 to 155 mm. Sixteen subjects rated the annoyance for the simulated conditions of (1) being outdoors, and (2) being indoors with the windows closed. In the latter case, a representative outdoor-to-indoor reduction in sound level was applied. It was anticipated that the presumed additional annoyance caused by the “heaviness” of the impulse sounds might be predicted from the difference between the C-weighted sound exposure level (CSEL; and the A-weighted sound exposure level (ASEL; In the outdoor rating conditions, the annoyance was almost entirely determined by ASEL. The explained variance, in the mean ratings by ASEL was 0.95. In the indoor rating conditions, however, the explained variance in the annoyance ratings by (outdoor) ASEL was significantly increased from to by adding the product as a second variable. In combination with a 12-dB adjustment for small firearms, the present results showed that for the entire set of impulse sounds rated indoors with windows closed, the rating sound level, is given by with and For the outdoor rating condition, the optimal parameter values were equal to and, again, In validation studies, in which the effects of the present rating procedure will be compared to field data, it has to be determined to what extent the constants α and β have to be adjusted.