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    • The acoustically opaque hood for the

    2018-10-24

    The acoustically opaque hood for the MF (Fig. 3) was made in the shape of the dolphin\'s rostrum and was snugly fit over it. The length of the hood was about 15cm. It was made from a 5 mm-thick sheet of neoprene foam with closed pores. This material has a high strength and high water and oil resistance. Due to these properties it can retain its sound-shielding properties provided by gas bubbles within its pores for a long time. The efficiency of this sound-shielding material was measured prior to the experiments. The attenuation of the peak sound pressure for wideband short pulses with herpes simplex virus maxima at frequencies of 10, 55 and 170kHz as the pulses were shielded by one layer of this material for normally incident sound reached 28, 32 and 36dB, respectively. We should note that the wavelengths corresponding to the frequencies of the energy peaks of the stimuli used in the experiment lie in the range from 1.5 to 20cm, so the linear dimensions of the shield at low acoustic frequencies become smaller than the stimuli wavelengths. Because of this, in order to improve the shielding efficiency, the hood had the appropriate shape, covering both the upper and the lower jaws (see Fig. 3). If the hood were made to match only the shape of the outer surface shape of the lower jaw, the effectiveness of such a shield would be lower even at a frequency of 100kHz (1.5cm wavelength) due to diffraction. Shielding by such a device would not occur at all at frequencies well below 8kHz (wavelength of more than 19cm), since the distance from the edge of the hood to the MF would be less than 2cm.
    Experimental results In this paper, we measured the auditory thresholds at which the dolphin can detect short broadband acoustic pulses with energy peaks at frequencies of 8, 16, 30, 60 and 100kHz, as well as the detection thresholds for the same pulses under acoustic shielding of the MF. The measurement results (Fig. 4) are represented as a function showing the relative deterioration of the auditory thresholds for detecting the stimuli, caused by the acoustic shielding of the MF. Taking into account that the stimuli were broadband (see Fig. 2), shielding efficiency was high over the entire frequency range examined (6–160kHz), and increased with frequency from 30 to 50dB. The measurement results for the absolute values of the stimuli detection thresholds in this experiment are consistent with the audiogram of the bottlenose dolphin [15] taking into account the phenomenon of energy summation [16–18]. This further proves that the dolphin studied in our experiment had normal hearing. The specifics of the dolphin\'s mandibular foramina, namely their dimensions, shape and architecture are governed by acoustic feasibility [10,12,13]. This natural conclusion, based on the results of the morphological study and the results of modeling the mechanisms of sound reception and conduction through the mental foramina to the middle ear of the dolphin, is confirmed to a large extent by the experimental results obtained (see Fig. 4). Under MF shielding, the mean values of the auditory thresholds for detecting pulses with energy peaks at frequencies of 8, 16, 30, 60 and 100kHz grow by 30, 34, 40, 46 and 50dB, respectively. This means that such shielding significantly impairs the sensitivity of the dolphin\'s hearing in the 6–160kHz frequency range (taking into to account that the stimuli are broadband), i.e., over the whole frequency band of the dolphin\'s hearing. Therefore, the mental foramina are involved in receiving and conducting sounds to the mandibular fat, and are the only sound-conducting channel for the sounds of all frequencies used in the experiment. At the same time, a decrease in the MF shielding efficiency with a decrease in stimuli frequency reaching 20dB can be attributed to the increasing penetration of sound into the hood with an increase in the stimuli wavelength as a result of diffraction. The sound wavelength increases significantly with a decrease in frequency (see curve 2 in Fig. 4). Moreover, while the dimensions of the acoustic shielding at frequencies of about 100kHz exceed the wavelength of the stimulus sound by 10 times, the linear dimensions of the shield become comparable to the wavelength at frequencies around 16kHz, and are even smaller than the wavelength of sound at a frequency of about 8kHz. It follows from the theory and the results obtained that shielding efficiency is the highest at a maximum ratio of the shield dimensions to the wavelength (the value at 100kHz in Fig. 4). Additionally, for a hood with these dimensions, the sound-shielding efficiency becomes even higher than the shielding efficiency of the material from which the hood is made. This is due to the fact that sounds are transmitted the mandible (and hence to the hood) almost tangentially and travel a path in the material that is substantially longer than its thickness.