Testing the Frequency Response Control Function of Level-Dependent Earplugs

Abstract

Noise and the use of hearing protectors can impair the perception of useful sound at work. One solution to this problem is the use of hearing protectors with electronic systems. The aim of this article was to present the results of tests that verified the ability of the filters implemented in earplugs to control the frequency response. Tests were conducted to verify the implementation of digital filters in the sound processor. The filtration method proposed in octave bands with center frequencies of 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and 8000 Hz was fully sufficient. A high degree of compliance of the characteristics of the implemented solution with the standardized characteristics was observed. Measurements using an acoustic signal also confirmed the filters were correctly implemented in the designed earplugs. The filters attenuated the sound to about 30–40 dB. The obtained results allowed us to conclude that the developed earplugs can be used in the future to protect employees against noise, and, at the same time, effectively deliver useful signals to their user.

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].

The ambiguous conclusions presented in the studies regarding the effectiveness of hearing protectors with electronic systems to improve the reception of useful signals may have resulted from the fact that most currently existing hearing protector solutions have predetermined characteristics for sound transmission frequency, with no possibility of the user influencing their shape. At the Central Institute for Labour Protection–National Research Institute (Poland), work is being carried out to develop earplugs that have, among other things, a function that allows for shaping the frequency response in octave bands of the acoustic signal emitting under the earplugs. This function implemented in the electronic system could significantly improve the reception of useful signals. The aim of this article was to present the results of research that verified the performance of filters implemented in earplugs in controlling the frequency response.

2. Materials and Methods

2.1. Scope of Research Conducted

The essence of the operation of the designed earplugs was to filter the acoustic signal transmitted to the user and to properly protect the employee’s hearing. This study analyzed the issue of the functioning of filters that allowed for the correction of the signal’s frequency response. It should be noted that the final solution of the earplugs must also include a limiter that will reduce the signal amplification to a level that is safe for the user, regardless of the settings of the individual filters. This allows for the protection of the hearing of the user of the earplugs in the event of an increase in the noise level.

In order to verify the operation of the developed filters, two stages of research were carried out, i.e., electrical measurements, and then measurements using acoustic signals. The first stage of research allowed us to check whether the implementation of filters in the sound processor was correct. In the next stage, the developed test circuit with a sound processor that contained developed filters was connected with standard pre-molded earplugs. It allowed us to assess how the acoustic signal was shaped after passing through the complete signal-processing path.

2.2. Implementation of Filters in the Sound Processor

The developed earplugs consisted of a sound processor, a BAR (Balanced Armature Receiver) loudspeaker and a MEMS (microelectromechanical system) microphone. In order to implement the frequency response control function in the sound processor, digital bandpass filters with center frequencies of 125, 250, 500, 1000, 2000, 4000 and 8000 Hz were implemented. These frequencies were selected due to the fact that hearing protectors were selected using the octave band method using data in the octave frequency bands with the center frequencies given above [4]. This range completely covers the speech range of 170–4000 Hz [17]; therefore, switching on all filters will eliminate the effects of attenuation, allowing speech to be heard in its entire range. In addition, the proposed filtration range also includes the range in which auditory danger signals are designed. In accordance with the requirements of the EN ISO 7731 standard [18], warning signal frequencies should be such that spectral components are present in the range of 500–2500 Hz, and it is recommended that at least two dominant spectral components are present in the range of 500–1500 Hz. In practice, there are also signaling devices that emit a signal with frequencies above the specified range, but not higher than 4 kHz [19].

When designing the filters, the guidelines presented in the EN 61260-1 standard were used [20] concerning the properties of octave and partial-octave bandpass filters. The design was also based on experience in designing filters for the level-dependent system proposed for use in earmuffs [21]. This article describes the process of designing the octave filters, including determining, among other things, the exact lower and upper octave band frequencies of individual digital filters and the minimum and maximum values of the frequency characteristics of the class 2 octave filter. The rest of this article presents the work conducted to verify the correct operation of digital filters and to check the operation of the frequency response control function of level-dependent earplugs.

2.3. Electrical Signal Measurements

The filters’ performance was verified by examining their frequency characteristics and then comparing them to the requirements for octave-band filters described in the EN 61260-1 standard [20].

To check the digital filters were operating correctly, the PULSE measurement input module Brüel&Kjær (3052-A-30) (Hottinger Brüel & Kjær GmbH, Darmstadt, Germany) and the Brüel&Kjær 1049 signal generator (Hottinger Brüel & Kjær GmbH, Darmstadt, Germany) were used. This generator was used to generate white noise, which was fed in the form of an electrical signal to the input of an EVAL-ADAU1787Z evaluation board (Analog Devices, Wilmington, MA, USA), which had an ADAU1787 processor (Analog Devices, Wilmington, MA, USA) with implemented digital filters. The evaluation board was connected, via a programmer, to a computer with the Sigma Studio 4.7 programming environment (Analog Devices, Wilmington, MA, USA) installed, which allowed for switching the filters with different center frequencies on and off. The electrical signal coming out of the evaluation board was recorded using the measurement input module. The recorded signals were analyzed using Brüel&Kjær Connect 2019 software (Hottinger Brüel & Kjær GmbH, Darmstadt, Germany). Figure 1 shows the measurement system used to check the correct operation of filters programmed in the sound processor.

2.4. Acoustic Signal Measurements

The frequency response control function of the earplugs was also tested using an acoustic signal. The measured A-weighted equivalent sound pressure level of the broadband noise emitted under the earplugs was 90 dB. These tests were performed using a test circuit equipped with an ADAU1787 sound processor, which was developed based on the EVAL-ADAU1787Z evaluation board, to which a ED-23147 BAR loudspeaker (Knowles Electronics, Itasca, IL, USA) was connected, where it was placed in a standard pre-molded earplug characterized by a hole located along the vertical axis of the earplug and a DMM-3526-2-B MEMS microphone (PUI Audio, Fairborn, OH, USA). The pre-molded earplug was placed in the RA0045-S8 ear simulator of the GRAS 45CB acoustic test fixture (GRAS Sound & Vibration, Holte, Denmark) and the microphone was placed near this earplug using a stand. The acoustic test fixture used was a standardized device dedicated to testing the properties of hearing protectors [22]. The method of mounting the elements of the entire level-dependent earplug system is shown in Figure 2.

The acoustic signal was recorded using the Brüel & Kjær PULSE system measurement input module (3052-A-30) which was connected to the GRAS 45CB acoustic test fixture via the GRAS 12AA module (GRAS Sound & Vibration, Holte, Denmark), which powered the microphones and amplified the measurement signal. The test signal was generated by a JBL SR4722A loudspeaker set (HARMAN International, Stamford, CT, USA), a Brüel&Kjær 1049 signal generator, a Crown MacroTech 2400 amplifier (HARMAN International, Stamford, CT, USA), a YAMAHA YDG 2030 graphic equalizer (YAMAHA Corporation, Shizuoka, Japan) and a JBL DSC 260 limiter (HARMAN International, Stamford, CT, USA). The measurements were carried out when the filters with individual center frequencies were switched on. In addition, the linearity measurements of the filter attenuation changes from 0 to 40 dB were carried out in 10 dB steps. The filter attenuation values were activated and changed using a smartphone application written for this purpose.

3. Results

The results of the filter characteristics at center frequencies of 125 Hz to 8000 Hz obtained using an electrical signal are shown in Figure 3. In addition to the measured filter characteristics, the graphs also present the minimum and maximum values of the frequency characteristics of the class 2 octave filters with nominal center frequencies of 125 Hz to 8000 Hz, which were determined on the basis of EN 61260-1 [20]. The presented results of the measured filter characteristics with individual center frequencies are between the minimum and maximum values in most of the frequency range.

The results of the filter characteristics with center frequencies of 125 Hz to 8000 Hz obtained using an acoustic signal are presented in Figure 4. The graphs show the sound pressure level values in one-third octave bands measured under the earplugs when the filters with individual center frequencies were switched on. The sound pressure level values in one-third octave bands are also shown when all the filters were switched on and off. In addition, the filter characteristics in these graphs were normalized to the sound pressure level presented in Figure 3. The presented results of the filter characteristics measured using an acoustic signal were similar in a certain frequency and level range to the characteristics obtained using an electrical signal.

The measurement results of the filter characteristics with center frequencies of 125 Hz to 8000 Hz at different filter attenuation settings obtained using an acoustic signal are presented in Figure 5. The values of the sound pressure level in the one-third octave bands are also presented for when all the filters were switched on and off. It can be seen from these graphs that increasing the filter attenuation also caused a decrease in the sound pressure level under the earplugs.

4. Discussion

Hearing protectors with electronic circuits can enhance the hearing of people wearing them in noisy environments [9,10,11]. Current existing solutions for improving the reception of sounds using electronic systems have predetermined characteristics of sound transmission frequencies, with no possibility of the user influencing their shape. The possibility of selecting the attenuation characteristics of the hearing protector would allow the user, through successive trials, to adapt the function of the hearing protector to their needs. For example, they could increase the amplification in the bands related to verbal communication or strengthen the band responsible for receiving warning signals. This type of functionality would therefore provide the possibility of influencing the operation of the electronic system to a much greater extent than is currently the case with level-dependent hearing protectors. Furthermore, it would be possible to configure the electronic system in such a way that the frequency characteristic of sound transmission under the hearing protector is as flat as possible, i.e., the effect of the smallest possible impact on the spectrum of the signal reaching this protector will be obtained. This is important for people who, on the one hand, have to protect their hearing, and on the other hand, whose quality of work depends on the impression related to the received sounds. The results of this study show that it is possible to design earplugs with the frequency control response function implemented using the ADAU1787 sound processor. The sound processor can be used to process signals representing sound for various purposes. The solution discussed in this article filters the signal delivered to the user of the hearing protector according to the preferences of that user, and at the same time, ensures hearing protection. The use of sound processors in hearing protection solutions has already been demonstrated, although not directly in a hearing protector but in a device designed for hearing ambient sounds, which simultaneously simulates hearing loss and includes a tinnitus simulator [23]. This is evidenced by the research results presented in Figure 3, Figure 4 and Figure 5. The characteristics of filters with center frequencies of 125 Hz to 8000 Hz obtained using the electrical signal provided in Figure 3 show a high degree of compliance with the frequency characteristics of the class 2 octave filters according to EN 61260-1 [20]. However, filters intended for use in level-dependent earplugs do not constitute measuring equipment, and therefore, do not have to meet any of the filter class requirements. Therefore, despite the filter characteristics slightly exceeding the range of values defined by the standard at some points, the solution can be considered fully satisfactory and one that will fully realize its task. The ripples observed in Figure 3, which slightly exceeded the limits specified in the standard, occurred at a much lower level than the maximum characteristic and were at least 40 dB away from the value at the peak of the characteristic. In practice, due to such a large distance of the deviations from the maximum characteristic value, the measured deviations will not have a noticeable effect on the sound reaching the user. The results of the filter characteristics obtained using an acoustic signal are also satisfactory. Despite the fact that additional elements appeared in the measurement path, the characteristics of the filters measured using an acoustic signal were similar in a certain frequency and level range to the characteristics obtained using an electrical signal. Several factors will lead to certain differences between these characteristics. For example, due to the properties of level-dependent earplugs with limited passive attenuation, an external signal travels through the earplugs, bypassing the electronic sound transmission path, causing the filters to attenuate sound to about 30–40 dB in real conditions. 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 of the 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. This will improve the perception of sound signals that are important from the employee’s safety perspective. The second significant advantage of such earplugs will be the possibility of adjusting their frequency characteristic of attenuation to the noise present at a specific workstation. This is important when the employee works at different workplaces at different times. Adjusting the earplugs to new conditions will be possible by changing the filter settings using an application installed on a smartphone, without the need to equip the employee with other earplugs, i.e., earplugs with different attenuation parameters.

5. Conclusions

The filtration method proposed in this paper for use in earplugs, i.e., in octave bands with center frequencies of 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and 8000 Hz, was fully sufficient. The range of frequencies used in the filters completely covered the speech and frequency ranges in which auditory danger signals are designed. Testing the characteristics of digital filters implemented using a sound processor allowed us to determine a high degree of compliance between these characteristics and those specified in the standard. In addition, measurements using an acoustic signal confirmed the correctness of the implementation of the filters in the designed system extended with a loudspeaker and microphone. The results obtained during the work carried out allow us to state that the developed earplugs, in which digital band filters were used and implemented using a sound processor, can be used in the future to protect employees from noise, and, at the same time, will be able to effectively provide their users with useful signals.