A finite element model to improve noise reduction based attenuation measurement of earmuffs in a directional sound field

• Published in: Applied acoustics Vol. 119; pp. 66 - 77
• Main Authors: Gaudreau, Marc-André, Sgard, Franck, Laville, Frédéric, Nélisse, Hugues
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-04-2017
• Subjects: Continuous F-MIRE; Finite element model; Hearing protection; HPD field attenuation; Perfectly matched layer; Finite element model; Perfectly matched layer; Hearing protection; Continuous F-MIRE; HPD field attenuation
• ISSN: 0003-682X; 1872-910X
• DOI: 10.1016/j.apacoust.2016.12.003

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•Continuous F-MIRE measurements method for earmuffs need to be improved.•The measured noise reduction (NR∗) varies as a function of sound incidence.•A finite element model of an earmuff is developed to investigate this variation.•The model is evaluated experimentally (SPL external mic, NR∗, NR∗ variation).•The model is used to choose the optimal position for F-MIRE external microphone.
The real attenuation of hearing protection devices (HPD) can be assessed in the field using a method based on continuous field microphone-in-real-ear (F-MIRE) measurements. The two-microphone method provides an indicator called the measured noise reduction (NR∗), defined as the difference between the measured exterior (outside the protector) and interior (under the protector) sound pressure levels (SPL). The HPD’s attenuation expressed in terms of the more common insertion loss (IL) can then be obtained from NR∗ using compensation factors. For earmuffs, NR∗ has been shown to vary of up to 20dB depending on the angle of incidence of the sound source. Therefore, there is a need to use sound incidence dependent compensation factors to relate NR∗ and IL. To evaluate these factors and more generally to improve the continuous F-MIRE method, a finite-element (FE) model of an earmuff on an ATF (acoustic test fixture) exposed to a directional sound field has been developed and its predictions compared with lab measurements for several incidence angles. Regarding the external microphone SPL and the NR∗, in one-third of octave bands, the model correlates very well with measurements for frequencies below 1250Hz whatever the sound incidence. Above 1250Hz, the FE model captures the trends, as a function of the incidence angle, but the agreement generally decreases with increasing frequency. A better correlation between the FE model and the experimental data is achieved for the variation of NR∗ (ΔNR∗) as a function of the sound incidence. Actions, such as (i) accounting for the headband in the model, (ii) refining the modeling of the sound source, (iii) improving the cushion modeling and (iv) better describing the backplate/cushion coupling conditions, are suggested to improve the model accuracy. To illustrate the potential of the modeling to improve the continuous F-MIRE measurement method, the FE model is used to determine an optimal position of the external microphone and to obtain estimates of exposure levels using the left and right ear exterior microphones.