# Testing the Frequency Response Control Function of Level-Dependent Earplugs
## 1 Introduction  
The ability to perceive sounds correctly plays a key role in ensuring the safety of employees in the workplace, especially in environments where they are exposed to noise. Sound is often used to communicate with co-workers in dangerous situations to notify threats, for example, sounds such as machine alarms or verbal warnings from co-workers. Unfortunately, excessive noise can mask useful sounds, making it difficult to perceive them accurately. Another problem resulting from the presence of noise is the use of hearing protection. Although hearing protection reduces the adverse effects of noise on the employee's hearing, it can affect the perception of useful signals. Detecting sound can be difficult because hearing protection can attenuate sound to the level of the employee's hearing thresholds, making sounds inaudible when wearing hearing protection [1]. This is particularly important for workers with significant hearing loss, which occurs most often in the high-frequency range [2]. The sound attenuation of hearing protectors also tends to be the highest in the high-frequency range. This effect is even more pronounced if there is a noise-induced temporary hearing threshold shift, e.g., when hearing protectors are worn occasionally in high-noise environments [3]. In addition, sound detection may be impaired when the noise is predominantly in the low-frequency range when hearing protectors that have lower sound attenuation values in the low-frequency range than in the high-frequency range are used (which is the case with most hearing protectors) [1]. To avoid such a situation, the EN 458 standard [4] recommends the use of hearing protectors with \"flat/uniform\" sound attenuation. The research results presented in the literature are ambiguous. In some research studies, the use of passive hearing protectors has been shown to improve speech intelligibility, while in other situations, they worsen it [5,6]. When it comes to locating and detecting warning signals, the use of passive hearing protectors generally impaired these skills [7,8]. The results of studies on the perception of sound signals when using hearing protectors equipped with electronic systems are less ambiguous. These systems are incorporated into hearing protectors precisely to improve the perception of sound signals. For example, several studies have concluded that the use of level-dependent hearing protectors can lead to better speech intelligibility than that achieved with the use of protectors without electronic systems or when no hearing protectors are used at all [9, 10, 11]. However, other studies [12] have shown that speech intelligibility is only better when using level-dependent hearing protectors in certain situations. In other situations, passive hearing protectors give better results. In addition, speech intelligibility is influenced by the specific hearing protector model [13]. Studies have also been conducted on the perception of warning signals when wearing level-dependent hearing protectors. In one of these studies [14], the ability to localize sounds was compared in two situations: without hearing protection and with hearing protection equipped with electronic systems. The results of the study show that the use of hearing protection with electronic systems did not worsen the ability to localize sounds compared with the situation without hearing protection. Other studies [7,15] have shown that the ability of users to locate reversing alarms was not significantly improved when using level-dependent hearing protectors compared with passive hearing protectors. In turn, in the other work--the aim of which was to examine the distance at which an alarm signal could be detected--this ability was slightly lower for hearing protectors with electronic systems than when hearing protectors were not used, but at the same time, the use of passive hearing protectors gave significantly worse results [16].