Shin'ya Kuwahara

On the underwater masking pattern of pure tones.

Many people now enjoy marine sports such as skin diving and scuba diving in sallow water area. Acoustic signals through an underwater loudspeaker can be used as a simple and effective way of preventing diving accident. At this time, it is fundamentally important to research in advance the masking of objective signals by ambient noises. In our research, measurements of masking effects of auditory ambient noise provide data from which the excitation pattern of the masking stimulus is derived in air and under water, using a pure tone masker and a band-limited noise masker centered at 1200 Hz and 2500 Hz. The present study casts further light on underwater masking results by comparing masking effects in air. The underwater results of masking measured are almost the same as the results in air. Therefore, it can be considered that under water masking effects are obtained on referring to usual experimental masking results in air.

Ability of underwater sound localization in man and its mechanism.

水中のダイバーに対する音声伝送の研究の一環として, 水中で発せられた音がどの方向から伝搬してきたか, どの程度まで聴き分けられるかという, ヒトの水中における方向弁別能力をプールでの聴取実験を行うことによって調べた。その結果, 水中においてヒトは, 予想以上に方向弁別の能力のあることが明らかとなった。また, 最小可聴角度 (Minimum Audible Angle : MAA) の値は, 大気中に比べて1桁程度増大するものの, 方位角に対する全体の傾向は大気中の場合と同様であるのが分かった。次に, 水中での聴覚が主に頭蓋骨などの骨伝導によることから, 簡単な音の伝搬モデルを仮定し, 水中におけるヒトの方向弁別メカニズムについて考察した。その結果, 水中での時間差弁別閾Δtthは, 可聴周波数の範囲にわたってほぼ一定の値を示し, しかもその値は大気中の1300Hz以下の場合と同程度になることが分かった。従って, 水中での方向弁別は可聴周波数の範囲にわたって両耳間に生じる音の時間差・位相差を手がかりに行われていると考えられる。

Effects of diving hood and face mask for underwater auditory sensitivity.

To fabricate an underwater information system using audible sound projected by underwater loudspeaker, it is important to estimate the effects of diving hood and face mask for underwater auditory sensitivity. Measuring the underwater sound pressure levels between inside and outside neoprene hoods, it is clear that the hood material becomes a more effective attenuator as frequency is incresed. The attenuations of the signal show a good agreement with threshold performance increases under the experimental condition touching only forehead to water. It appears from the another experiment, in high frequency region above 2 kHz, that the hearing sensitivity of divers is reduced when they wear a neoprene hood and a diving face mask. On the other hand, in low frequency region below 1 kHz, it can be noted that the hearing sensitivity in a case wearing the hood does not result in any functional change.

An experimental consideration on the propagation mechanism for underwater auditory sensation.

To clarify the propagation mechanism of underwater hearing, threshold levels were carefully measured as a function of submerged skull and ear canals by means of a tank with low background noise level. From several experimental results, it is concluded that the mechanism of underwater hearing is primarily due to bone conduction but the external ear canals also play an important role in total sensation as listed below. (1) At frequencies above about 1 kHz, underwater hearing is primarily bone conduction via skull and more sensitive around the ear than at top of head. (2) Water in the ear canal acts as a loss to bone conduction. (3) At frequencies below about 1 kHz, the ear canal conduction gradually makes greater contribution to the auditory sensation and total characteristics slightly vary depending on the ear canal conditions.

A statistical method of the practical probability expression for nonstationary ship noise fluctuations and its experimental confirmation

Abstract Many people now enjoy marine sports such as skin diving and scuba diving in shallow water areas. Acoustic signals through an underwater loudspeaker can be used as a simple and effective way of preventing diving accidents. The use of audio signals is very important since divers usually have no communication apparatus. It is necessary and timely to investigate the state of the shallow water acoustical environment for the audio frequency bandwidth. Theoretical approximate probability expression for the sound power fluctuations from a moving ship (an important sound source) is first derived in the form of an infinite expansion series, by paying special attention to the standard shape of the sound pressure spectrum. Successively, for the sake of real applications, a practical probability expression in the form of a finite weighted sum of Gaussian distribution functions is proposed. The validity and the usefulness of the theoretical method have been experimentally confirmed using a digital simulation technique and has also been applied to actually observed ship noise data. These experimental results are in good agreement with the theory.

The sound insulation effect of a single wall in underwater to the Gaussian random noise with arbitrary power spectrum.

Divers, who engage in search and/or rescue in a sunken or capsized ship, usually have no communication apparatus because of the complexity of the hull construction. For such divers, the most simple and effective way to provide some information is to transmit an acoustic signal directly by an underwater loudspeaker. Though the outer wall of the ship functions as a sound insulation wall, there is no appropriate evaluation method for the insulation effect of a wall to random sound pressure wave such as voice signal or noise.In this paper, a statistical evaluation problem concerning sound insulation effect of a wall in underwater is considered based on the idea of statistics such as Lα noise level, defined as the (100-α) percentile point of the noise level probability distribution. Concretely, a single wall is employed as the most basic model of the hull and a new statistical treatment is first proposed for the transmitted sound pressure wave in the case where a Gaussian type random sound pressure wave with an arbitrary power spectral density is passed through the wall, and next, an explicit expression for the sound insulation effect, defined as the difference between the Lα of the incident sound intensity and that of transmitted sound intensity, is derived as a function of the system parameters.Finally, the validity of the theoretical evaluation method is confirmed by an analog simulation and a water tank experiment using several kinds of single walls.

A prediction method of the response probability distribution for arbitrary sound insulation systems

Abstract This paper describes a practical evaluation method of the response probability distribution for an arbitrary sound insulation system with a random input noise of arbitrary distribution. Here, the response probability distribution is derived in a fairly simple form using the statistical Hermite expansion-type series expression. The effect of the sound insulation system and the input fluctuation on the resultant probability distribution form is hierarchically reflected in various types of statistics such as the mean, variance and each expansion coefficient. Finally, the proposed evaluation method is experimentally confirmed by applying it to the observed data for typical examples of single- and double-wall insulation system, with white noise and music excitations.

New functional evaluation methods of sound insulation system using the SEA method and an Lx evaluation criterion

Abstract Recently, for the purpose of reducing residential environmental noise, many sound insulation systems have often been improved acoustically by changing their geometrical scales and/or acoustical characteristics. In this paper, new functional evaluation and probabilistic prediction methods for these improvements are theoretically and experimentally proposed in practical expression forms by introducing a few functional parameters. These functional parameters introduced only for the prediction are supported by many physical structural factors closely related to the well-known statistical energy analysis method, and are easily estimated in a preliminary experiment. The estimation procedures developed are based on two error criteria using the actual overall frequency band data. The used least-squares error criterion is the most fundamental method and the L x evaluation criterion matches the actual situation of estimating the representative evaluation indices. Finally, by using musical sound as an input noise, the effectiveness of the proposed method is experimentally confirmed by applying it to some actual problems.