The disclosure generally relates to earmuffs having ear cups adapted to be used as circumaural hearing protectors. More specifically, this disclosure relates to earmuffs comprising dual ear cups with one cup being positioned entirely within and mechanically isolated from the other and ear seals for use with such earmuffs.
Circumaural earmuff devices are commonly used to protect against hearing damage or hearing loss in noisy environments where the unwanted sound levels are higher than safe limits. Compared to other types of hearing protectors, such as earplugs, earmuffs (i.e. circumaural) are usually preferred in single hearing protection applications as the major hearing protection device because they are easy to don and doff, reliable in attenuation, with less dependence on user training, skill and motivation, less sensitive to fitting, and capable of being used as a one-size-fits-all device. Earmuff devices known in the prior art generally include a pair of ear cups made of rigid plastic materials. Each ear cup is typically single walled and lined with an acoustical material in the interior to help dampen and absorb sound energy. The two ear cups are mounted on the ends of a spring-like headband that provides a clamping force to keep the ear cups in contact with the wearer's head. A soft ear cushion (i.e. ear seal) having a concentric ring shape and with a certain thickness is interposed between the ear cup and the wearer's head for comfort purposes as well as to form, ideally, an airtight seal around the ear. Maximum sound attenuating capabilities are constantly pursued by developers to provide optimal protection against noise exposure.
At lower frequencies, such as below 1,000 Hz where the sound wavelength is considerably larger than the dimensions of the earmuff, the ear cup in the earmuff assembly is displaced, by the acoustic energy, as a rigid mass vibrating against the wearer's head through the soft cushion. That is, the ear cup can be considered as a single-degree-of-freedom (SDOF) vibration system at low frequencies where the acoustic pressure underneath the ear cup is proportional to the amplitude of the ear cup displacement. Therefore, at the low frequency range, the sound attenuation performance of the earmuff substantially depends on the ear cup volume, mass, headband clamping force and ear cushion stiffness. Many attempts have been made at increasing the sound attenuation of earmuff devices of the kind described above, i.e. the earmuffs with single-walled cups, by increasing the cup mass/volume and increasing the headband clamping force to tighten the fit of the ear cushions as well as to increase the cushion stiffness.
At higher frequencies, such as above 1,000 Hz, the sound wavelength becomes comparable or even smaller than the earmuff dimensions, and the ear cup flexural modes are excited by the acoustic energy. The sound attenuation performance of the earmuff is then largely determined by the structural rigidity and acoustical resonances inside the cavity enclosed by the ear cup and the wearer's head. Accordingly, many attempts have been made to increase the high frequency sound attenuation performance of earmuff devices by increasing the ear cup's structural stiffness and/or the acoustical damping inside the cup cavity. In addition, many other attempts have also been made at increasing the earmuff comfort and minimizing air leak by various ear seal designs that are for use with, particularly, earmuffs having single-walled cups.
Although these attempts may have resulted in certain increases in attenuation, the resulting increase in attenuation is limited by practical considerations in terms of comfort since adding cup mass, volume and headband clamping force, increasing cushion stiffness and structurally stiffening the cup usually raise the discomfort level. Further, the resulting increase in attenuation is also limited by physical laws associated with earmuff structures with single-walled cups. Sound attenuation provided by earmuffs is generally 10-30 dB for lower frequencies such as below 1,000 Hz, and 25-40 dB for higher frequencies such as above 1,000 Hz. These attenuation levels may not be sufficient for certain environments, such as the vicinity of jet engines of military and commercial aircrafts and extremely loud industrial or manufacturing plants where a very high sound attenuation is desired from earmuff devices.
Therefore, what would be useful would be a new earmuff design that not only is able to provide significantly improved sound attenuation performance compared to other circumaural earmuffs of the similar kind known in prior art, but also is able to offer sufficient comfort for practical use.