Page 62 - Summer 2015

TECHNICAL COMMITTEE REPORT
Noise
Continued from page 58
The topic of soundscapes is another area of considerable interest in the TCN. A soundscape is defined as an acous- tic environment as perceived and/or experienced and/or understood by people in context. Soundscape design is be- coming a powerful new tool for the measurement, analysis, and design of environmental and community noise that ap- plies the knowledge of both science and community experts. This approach recognizes that metrics alone do not lead to proper design of acoustic spaces because human expecta- tions are modified by the context in which sound is encoun- tered. Thus the soundscape approach combines acoustical measurements such as sound pressure levels with subjective evaluation obtained from talking with individuals that in- teract within the acoustic space. Because of the context in which the noise is encountered, in some cases sound levels that might normally be considered too high are willingly ac- cepted by those individuals.
Active noise control (ANC) continues to be an area of inter- est within the TCN. Simplistically, ANC can be described as the superposition of two pressure waves that leads to at- tenuation of the combined pressure wave. Although this is true, a simple destructive wave interference approach will generally lead to a localized control solution where a “zone of silence” can be created, but the global energy in the field will actually increase. It has been found that the zone of si- lence created is typically about 0.1 wavelengths in diameter, so if one considers a frequency of a few hundred hertz in air, this corresponds to a diameter of around 0.1 m. This may be perfectly satisfactory in some cases, but there are numer- ous applications where one would like to have a larger, even global, region of attenuation.
ANC solutions that provide localized control at discrete fre- quencies are now relatively straightforward to accomplish. However, achieving more global control is more challeng- ing and requires one to be able to achieve mutual coupling or modal coupling between the unwanted noise source and the control source(s) being used to attenuate the sound field. Although these approaches are generally more difficult to implement, they have the prospect of achieving overall energy reduction, leading to attenuation of the field every- where or nearly everywhere. For applications where global attenuation is desired, one would ideally minimize the radi- ated sound power. However, this is not possible to accom- plish in real time. Thus these applications generally focus on looking at a model of the radiated sound power and then developing a control implementation that is strongly corre- lated with minimizing the sound power. Such applications
60 | Acoustics Today | Summer 2015
include free-field radiation from fans and vibrating struc- tures as well as attenuating the sound field inside the cabins of vehicles, from automobiles to heavy equipment to interior aircraft noise.
High-amplitude military jet noise is another topic being in- vestigated by members of the TCN. Sound pressure levels radiated from military jets can be very high, with the overall levels at maintainer locations (to the side of the aircraft) ex- ceeding 150 dB at times. The spending on hearing loss com- pensation for the United States Department of Defense has grown enormously in recent years, with military jet noise being one of the contributors to this trend. Work continues within the TCN to better understand the mechanisms with the generation and propagation of military jet noise. Histor- ically, it was suggested that shocks are radiated from the jet plume. However, recent work has looked at the skewness of the radiated pressure waveforms, and it has been shown that the skewness increases with shock formation, indicating that the nonlinear propagation effects continue to be important even after the shocks form. Neglecting these effects leads to poor modeling of the radiated sound field at large distances.
Recent work with military jet noise has also applied near- field acoustic holography to measured jet noise near the plume of the jet. This technique has been used to success- fully map out the two-dimensional radiated sound field as a function of frequency. Such work has been used to help identify effective source regions in the plume where the jet noise is radiating from and has been used to confirm earlier suggestions indicating that for higher frequencies, the effec- tive source region moves closer to the jet nozzle, becomes more contracted, and shifts forward in the directionality of the main lobe of sound radiation.
Military noise is a primary cause of hearing loss among ser- vice members. Consequently, efforts to understand the risks of noise exposure and to protect against noise have seen new advances. Impulse noise exposures present an increased risk of hearing loss compared with continuous noise. Occupa- tional noise-damage risk criteria have been developed as- suming continuous noises that lack impulsive components. In April 2015, the Department of Defense updated the MIL- STD 1474E Design Criteria Standard Noise Limits (http:// goo.gl/Tx5W8Z). MILSTD 1474E incorporates both an equivalent energy method and a model-based method to as- sess the risk of a particular exposure. Although these meth- ods were originally designed for estimating the risk of dam- age to the unprotected ear, both approaches allow for the inclusion of a hearing protection device (HPD) to estimate