Abstract
When estimating the audible distance of the signal sounds issuing from ships, the items which should be taken into consideration are the divergence decrease due to the spreading of a sound wave from a point source, the attenuation of sound in air due to absorption, the masking effect on the signal sound caused by environment noise and remarkable attenuation due to the shadow zone created by the bending of sound rays caused by atmospheric conditions. When the environment noise of the side where one hears the signal sound from a ship is too loud, the signal sound which has reached the place is masked by the environment noise and it cannot be heard unless it has a sound pressure level of a definite value. We made a study of the relationship among the sound pressure levels, auditory threshold and masking effect mentioned above by performing some experiments. Further, we obtained, by experiments and calculations, the required sound pressure level of the signal sounds with respect to the masking noise of the bridge of a ship, divergence decrease and attenuation of sound waves in air. From the results of the above study, we have found out that ships' signal sounds with a higher fundamental frequency than that of the signal sounds used by ships in the past are necessary for more effective signaling.
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