# Testing the Frequency Response Control Function of Level-Dependent Earplugs
## 4 Discussion
In the case of measurements for an electrical signal, this attenuation reached about 40-60 dB. In addition, due to the addition of a speaker and microphone into the earplug, the acoustic signal could be slightly distorted. Additionally, measurements were carried out using an acoustic test fixture equipped with ear simulators, which caused the acoustic signal to be boosted at some frequencies, which also affected the operation ofthe filters. It was also observed (Figure 5) that changing the filter attenuation also caused linear changes in the sound pressure level under the earplug in most of the analyzed cases. In the case of filters with center frequencies of 125 Hz, 250 Hz, 500 Hz, 1000 Hz and 2000 Hz, changing the attenuation from 0 dB to 30 dB in 10 dB steps caused the sound pressure level for a given frequency to also decrease by 10 dB with an assumed tolerance of 1 dB. In the case of filters with center frequencies of 4000 Hz and 8000 Hz, the range of the linear attenuation change was 20 dB. The linear operating range was found to be sufficient for practical use in level-dependent earplugs. A limitation of this work is that the filter tests were performed after the test circuit with the sound processor was connected with a universal (pre-molded) earplug, and not in the target earplugs that will be developed in the future. This is related to the second limitation, i.e., that the location of the microphone included in the developed test circuit was not completely consistent with the target location because the form of the final earplugs is not yet known. For the same reason, it was not possible to test the proposed solution in studies involving people. This study did not include user trials or feedback. User satisfaction will be tested in the next stage of work, where the target version of the electronic system will be developed and implemented in custom-molded earplugs. In addition, before the product is offered to customers, it will be necessary to carry out tests for its safety of use, including with the participation of people, in accordance with the conformity assessment requirements for personal protective equipment specified in the EU Regulation [24]. In real-world scenarios, noise levels and signal frequencies can fluctuate rapidly, which might affect the performance of the filters. Signal distortion or interference may also occur. However, verification of whether the presence of the designed system will bring benefits in situations where useful sounds will occur against the background noise will be possible only after the development of the final earplugs. It should be mentioned that the presented results included the issue of signal filtering in frequency bands, but did not include the influence of the signal limiter, which will affect the final signal reaching the user. For example, with a high signal gain set in individual frequency bands and, at the same time, a relatively high sound pressure level of the signal outside the hearing protectors, the limiter will reduce the signal reproduced by the loudspeaker, and in an extreme situation, the loudspeaker will not reproduce the acoustic signal at all. However, conducting tests of the correct operation of acoustic filters required independence from the influence of the signal limiter operation. As a practical application of the developed electronic system, its use is planned in newly developed custom-molded earplugs, i.e., not in earplugs with a universal shape, but in earplugs adapted to the shape of the employee's external ear canal. This solution will ensure a tight fit of the earplugs and minimize the effects of incorrect placement of earplugs in the ear canal by the employee. At the same time, it will be possible to use electronic functions in earplugs of this type. Earplugs with the developed electronic system can be particularly widely used in industry, where a significant advantage will be the possibility of correcting the frequency response of the acoustic signal emitting under the earplugs.