Adaptive noise reduction and speech intelligibility using shape memory metamaterial-based hearing protection earmuffs

Conventional passive earmuffs, which have a single number rating (SNR) of 28–32 dB, can effectively protect users' hearing but block useful lower-intensity sound waves, such as speech, causing communication difficulties. This leads employees to remove their earmuffs to comprehend important information or to avoid miscommunication while wearing earmuffs, which might lead to accidents. This problem can be solved using pressure level-dependent passive earmuffs with selective frequency and intensity filtration to allow lower intensity waves (such as speech) to pass through while blocking high-intensity waves (hazardous noise). This study evaluates speech intelligibility and hearing protection of an in-house developed pressure level-dependent passive earmuff prototype made of shape memory metamaterial inserts. Hearing protection and speech intelligibility were evaluated in a controlled environment. Numerical simulations of earcups and inserts were performed to optimize their design and obtain the required noise reduction at various intensities. The prototype can ensure an in-ear sound pressure level of less than 82 dB (across frequencies) up to 110 dB pink noise. The prototype exhibits fair speech intelligibility in both quiet and noisy environments. User-experience feedback indicated that 85% of the participants preferred the shape memory metamaterial earmuffs over commercially available earmuffs, for use in diverse working environments.