The subject population
In this experimental study, 15 male students of Hamadan University of Medical Sciences with an age range of 18-30 years were randomly selected. Pure tone audiometry was performed for selecting subjects with normal hearing. As shown in Fig. 1, three common commercial earmuffs and three earplug models (one foam formable and two 3-flange pre-molded) with technical specifications from reliable international manufacturers used in the Iranian’s work environments were examined. A prototype of the proposed molded earplug designed based on subjects’ ear shape and size was also tested. After careful examination of the ear canal and tympanic membrane of each participant by the study partner audiologist, the initial ear mold was made using soft material. Next, the molded earplug was made from it using silicone materials in the lab. A ceramic filter was placed inside the molded earplug so that it can produce a special channel to allow transmit normal conversation. The inclusion criteria for participating in the study included having normal hearing and vision along with Persian native language.
Experiment procedure
In each experiment session, based on REAT method, the hearing threshold of subjects was measured by a reference noise with and without a protective earphone for 40 minutes. Next, the speech discrimination score (SDS) of subjects with and without a protective earphone was measured at two signal to noise (S/N) ratios for 30 minutes. As mentioned, based on the REAT method, pure tone audiometery was performed to measure a person's hearing threshold with and without a protective earphone [15]. In this way, based on the insertion loss of HPDs in one-octave band frequency spectrum, the actual values of the noise reduction rating were calculated [16].
The values of sound attenuation used for calculation of the NRR were determined according to ANSI S3.19-1974 [17, 18]. In the next step, the ambient background noise was fixed at 70 dB, using speakers (Pejvak Ava CO). Two signal to noise ratios (0 and +5) were considered to relatively resemble noise emission in office work environment. The speech levels of the Persian world were adjusted based on these S/N ratios. Participants sat in a chair two meters away from the noise emission speakers and avoided any movement during the measurement, as well as talking and making noise.
Speech intelligibility was measured based on the speech discrimination score and using a real two-channel audiometer (Piano model; Inventis CO). According to ISO 8253-3 standard, using a reliable and accurate list of one-syllable Persian words (25 words), the subjects are asked to repeat the words played from the speaker in the room environment. Then, the percentage of correctly repeated words is determined [19]. For speech audiometry, the speech of the Persian word from a suitable speaker with normal and clear speech with no particular accent was recorded. The speaker maintained the clarity, and natural speed of sound, and avoided emphasizing words during the record of speech [20].
Statistical analysis
Data were analyzed using SPSS software (ver.22, Chicago, IL, USA). The normality of the data was tested using the Kolmogorov-Smirnov test. When data were normally distributed, they were analyzed using paired sample and student T-tests. Wilcoxon’s tests were also used when data were not normally distributed. The significant level for all tests was set at 5%.