沉积物中声速和衰减系数的宽带测量方法

海底沉积物作为海洋波导声传播的下边界普遍存在于大洋中,获知其特性对于准确的声传播和混响建模是十分必要的。为了能够快速而准确地测量沉积物中的声速和衰减系数,提出一种基于脉冲压缩技术的测量方法,对接收信号进行压缩来提取透射波,根据不同厚度样品的透射波来计算沉积物中的声速和衰减系数。该方法不仅可以克服实验过程中经常遇到的多途干扰,而且测量过程简单,可以同时获得测量频带内所有频点的声速和衰减系数,即实现了对声速和衰减系数的宽带测量。在实验室环境条件下,90~170kHz的测量频带内,测得沙样品中的声速为1710~1713m/s,衰减系数在56~70dB/m之间。通过窄带和宽带测量结果的比较可以看出,声速的宽带测量结果与窄带测量结果吻合得较好,而衰减系数在频带后半部分存在较大的起伏。      

Design parameters of stainless steel plates for maximizing high frequency ultrasound wave transmission

This work validated, in a higher frequency range, the theoretical predictions made by Boyle around 1930, which state that the optimal transmission of sound pressure through a metal plate occurs when the plate thickness equals a multiple of half the wavelength of the sound wave. Several reactor design parameters influencing the transmission of high frequency ultrasonic waves through a stainless steel plate were examined. The transmission properties of steel plates of various thicknesses (1–7 mm) were studied for frequencies ranging from 400 kHz to 2 MHz and at different distances between plates and transducers. It was shown that transmission of sound pressure through a steel plate showed high dependence of the thickness of the plate to the frequency of the sound wave (thickness ratio). Maximum sound pressure transmission of ∼60% of the incident pressure was observed when the ratio of the plate thickness to the applied frequency was a multiple of a half wavelength (2 MHz, 6 mm stainless steel plate). In contrast, minimal sound pressure transmission (∼10–20%) was measured for thickness ratios that were not a multiple of a half wavelength. Furthermore, the attenuation of the sound pressure in the transmission region was also investigated. As expected, it was confirmed that higher frequencies have more pronounced sound pressure attenuation than lower frequencies. The spatial distribution of the sound pressure transmitted through the plate characterized by sonochemiluminescence measurements using luminol emission, supports the validity of the pressure measurements in this study.     

Ultrasonic shear wave properties of soft tissues and tissuelike materials

Determinations of shear wave speeds of sound and attenuation coefficients are reported for soft tissues, a silicone rubber reference material, and a gel used in manufacturing ultrasonically tissue-mimicking materials. Fresh bovine tissues were investigated, including calfskin, liver, cardiac muscle, and striated muscle. Because of the very large shear wave attenuation coefficients, reasonably accurate determinations of shear wave properties are difficult to make. The quantity measured directly was the complex reflection coefficient for shear waves at a planar interface between the sample and fused silica. Measurements were made at frequencies spanning the range 2-14 MHz. The shear wave attenuation coefficients increase with frequency and are of the order of 10(4) times the longitudinal wave attenuation coefficients. The shear wave speeds of sound also increase with frequency but are only a few percent of the longitudinal wave speeds of sound. The results are accurate enough to allow frequency dependencies to be proposed.     

Long-range sound propagation in the shallow-water part of the Sea of Okhotsk

Experimental data on the long-range propagation of explosion-generated sound signals in the shallow-water northern part of the Sea of Okhotsk are analyzed. The propagation conditions in this region are characterized by a fully-developed underwater sound channel that captures the rays crossing the channel axis at angles lower than 3°. The experimental data reveal a small increase in the duration of the sound signal in proportion to the range with the proportionality factor lower than 0.00025 s/km. The frequency dependence of attenuation exhibits a pronounced minimum whose position on the frequency axis is close to the critical frequency of the first “water” mode (about 160 Hz). The increase in the attenuation coefficient at lower frequencies is confirmed by the field calculations performed with the wave-field computer code and is explained by the sound energy loss in the bottom sediments. At frequencies higher than 200 Hz, as in the Baltic Sea, the most probable reason for the attenuation to exceed the absorption in sea water is sound scattering by internal waves.     

水中气泡之间的声相互作用

单个气泡的声衰减的实验结果可以用热传导粘滞和辐射损失来解释,但对于气泡层中(这里有许多气泡)的实验来说,上述机理却不能解释。因为根据这些机理算得的理论值比实验值小很多,这一问题三十多年来尚未解决。本文考虑气泡之间的声相互作用,算得的相互作用场和原始入射场有90°的相位差,这等效于在每个气泡上附加了一项阻力,从而使其阻尼增大很多。应用本文理论与上述实验结果比较,两者符合得很好。本文理论也指出,声相互作用并不改变气泡的共振频率,只是使其共振曲线展宽。当声波频率低于共振频率时,气泡的伴振质量增大,劲度减小。当频率高于共振频率时,情况则相反。 关键词：     

Comparison of the noise attenuation of three audiometric earphones, with additional data on masking near threshold

The noise-excluding properties of a standard supra-aural audiometric earphone, a widely used circumaural-supra-aural combination, and an insert earphone sealed to the ear with a vinyl foam eartip were measured in a diffuse-field room complying with ANSI S12.6-1984. Data on attenuation were obtained monaurally with the nontest ear plugged and muffed. Results for the supra-aural earphones generally agreed well with previously reported measurements. A broadband masking noise was used to directly test the ANSI S3.1-1977 permissible background noise levels for measuring to audiometric zero using standard audiometric earphones. This "ANSI noise" raised the average thresholds of 15 normal-hearing test subjects by 3 to 5 dB at the octave frequencies from 500 to 4000 Hz. With a noise conforming to the less stringent OSHA-1983 regulation, average thresholds were elevated 9 to 17 dB. An "ENT office noise" with an overall sound level of 54 dBA raised average thresholds even further, by as much as 29 dB at 500 Hz. Use of the circumaural system in the office noise limited the threshold elevation to 11, 5, 2, and 0 dB at the four octave frequencies tested. With the fully ("deeply") inserted foam eartips, the threshold elevation in the simulated office noise was 2 dB or less at all test frequencies. Actual threshold elevations agreed closely with predictions based on a critical ratio calculation utilizing measured sound field noise levels and measured earphone attenuation values.     

Vibration characteristics of bone conducted sound in vitro

A dry skull added with damping material was used to investigate the vibratory pattern of bone conducted sound. Three orthogonal vibration responses of the cochleae were measured, by means of miniature accelerometers, in the frequency range 0.1-10 kHz. The exciter was attached to the temporal, parietal, and frontal bones, one at the time. In the transmission response to the ipsilateral cochlea, a profound low frequency antiresonance (attenuation) was found, verified psycho-acoustically, and shown to yield a distinct lateralization effect. It was also shown that, for the ipsilateral side, the direction of excitation coincides with that of maximum response. At the contralateral cochlea, no such dominating response direction was found for frequencies above the first skull resonance. An overall higher response level was achieved, for the total energy transmission in general and specifically for the direction of excitation, at the ipsilateral cochlea when the transducer was attached to the excitation point closest to the cochlea. The transranial attenuation was found to be frequency dependent, with values from -5 to 10 dB for the energy transmission and -30 to 40 dB for measurements in a single direction, with a tendency toward higher attenuation at the higher frequencies.     

A theoretical study of passive control of duct noise using panels of varying compliance.

It is theoretically demonstrated that, in a duct, a substantial amount of sound energy can be transferred to flexural waves on a finite wall panel when the upstream portion of the panel is made to couple strongly with sound. The flexural wave then loses its energy either through radiating reflection sound waves or by internal friction. The effectiveness of the energy transfer and damping is greatly enhanced if the panel has a gradually decreasing in vacuo wave speed, which, in this study, is achieved by using a tapered membrane under tension. A high noise attenuation rate is possible with the usual viscoelastic materials such as rubber. The transmission loss has a broadband spectrum, and it offers an alternative to conventional duct lining where a smooth air passage is desired and nonacoustical considerations, such as chemical contamination or cost of operation maintenance, are important. Another advantage of the tapered panel is that, at very low frequencies, typically 5% of the first cut-on frequency of the duct, sound reflection occurs over the entire panel length. This supplements the inevitable drop in sound absorption coefficient, and a high transmission loss may still be obtained at very low frequencies.     

Using impedance measurements to detect and quantify the effect of air leaks on the attenuation of earplugs

The impedance of a simple artificial ear occluded with an earplug and bypassed with narrow air leaks was measured along with the attenuation of sound through the air leaks. A lumped element model is suggested for the simple occluded artificial ear with an air leak. The suggested model was adapted to the impedance measurements and the attenuation was predicted from the model. The attenuation predictions were compared to the attenuation measurements and were found to be within [-3.5,+3] dB of the measured attenuation over the frequency range of 50-1000 Hz and an attenuation range of -2-38 dB. The average difference between the measured and predicted attenuation for four different leaks in the frequency range of 50-1000 Hz was -0.7 dB, indicating a very slight underestimation of the attenuation.     

Measurement of sound insulation of acoustic louvres by an impulse method

This paper presents results of a study of the sound attenuation of acoustic louvres. At the core of the study is an alternative method of measuring sound insulation, impulse response analysis, which circumvents the limitations imposed by standard and proposed standard methods. Using the impulse method, the sound transmission coefficient is measured at different angles of incidence and the angular dependency of transmission loss obtained. In the low frequency range, the transmission is governed by a mass layer effect. The value of transmission loss is independent of angle of incidence. For the mid and high frequencies, diffraction, interference and absorption determine louvre performance and an angular dependency is observed. The transmission at the angle of incidence, corresponding to a line-of-sight through the louvre blades, is the dominant contribution to the angle average value and a single measurement at the pitch of the louvre approximates the overall transmission loss. For the case considered, the geometry of the blades has little influence on the transmission at low frequencies and the mass of the blades has little influence at higher frequencies. In a companion paper, the impulse data are used to predict the insertion loss provided by the louvre when installed in a plant room.     

Isolating the auditory system from acoustic noise during functional magnetic resonance imaging: examination of noise conduction through the ear canal, head, and body

Approaches were examined for reducing acoustic noise levels heard by subjects during functional magnetic resonance imaging (fMRI), a technique for localizing brain activation in humans. Specifically, it was examined whether a device for isolating the head and ear canal from sound (a "helmet") could add to the isolation provided by conventional hearing protection devices (i.e., earmuffs and earplugs). Both subjective attenuation (the difference in hearing threshold with versus without isolation devices in place) and objective attenuation (difference in ear-canal sound pressure) were measured. In the frequency range of the most intense fMRI noise (1-1.4 kHz), a helmet, earmuffs, and earplugs used together attenuated perceived sound by 55-63 dB, whereas the attenuation provided by the conventional devices alone was substantially less: 30-37 dB for earmuffs, 25-28 dB for earplugs, and 39-41 dB for earmuffs and earplugs used together. The data enabled the clarification of the relative importance of ear canal, head, and body conduction routes to the cochlea under different conditions: At low frequencies (< or =500 Hz), the ear canal was the dominant route of sound conduction to the cochlea for all of the device combinations considered. At higher frequencies (>500 Hz), the ear canal was the dominant route when either earmuffs or earplugs were worn. However, the dominant route of sound conduction was through the head when both earmuffs and earplugs were worn, through both ear canal and body when a helmet and earmuffs were worn, and through the body when a helmet, earmuffs, and earplugs were worn. It is estimated that a helmet, earmuffs, and earplugs together will reduce the most intense fMRI noise levels experienced by a subject to 60-65 dB SPL. Even greater reductions in noise should be achievable by isolating the body from the surrounding noise field.     

Studies of sound transmission in various types of stored grain for acoustic detection of insects

In developing passive acoustic systems for detecting insect infestations in bulk-stored grain, it is advantageous to understand the transmission of sound in the grain between the insects and the sensors. In the work presented here grain is shown to be a strong acoustical absorber, the attenuation coefficient increasing roughly as the square root of the frequency. Tests with soft wheat immersed in three different gases: air, argon and carbon dioxide, support an earlier conclusion that sound is transmitted principally through the gas in the passageways between the grain kernels. The speed of sound and the attenuation coefficient were measured as a function of frequency for six different types of grain: hard and soft wheat, brown rice, soybeans, corn and sorghum. It was determined that sound is transmitted over longer distances in grains with a larger inter-kernel spacing, such as corn and soybeans. Grain depth, up to several meters, appears to have little effect on sound transmission.     

An investigation of the measurement of broadband ultrasonic attenuation in trabecular bone

Commercial devices for the ultrasonic characterisation of bone normally report the broadband ultrasonic attenuation (BUA). This is the slope of the attenuation against frequency in some part of the frequency range 200–1000 kHz. The assumption is that the relationship is linear and hence independent of the frequency range selected. In this study the ultrasonic attenuation in the frequency range 200 to 800 kHz was measured with a variety of transducers in ten trabecular heel bone samples from elderly cadavers, assumed to be osteoporotic.

The results indicate that the attenuation fits better to a second order polynomial function of frequency, than to the linear fit. The use of a straight line fit is only satisfactory in the higher frequency ranges (above 400 kHz). The use of lower frequencies results in a significant measurement error caused by the combination of a poor signal to noise ratio and the departure from linearity and this is greatest for samples with low attenuation. In the worst cases this can amount to a 30% discrepancy between the BUA values measured over different frequency ranges.     

Space-time structure of the low-frequency acoustic field in an underwater sound channel

Experimental data are presented on the fine structure of the sound field in an underwater sound channel for low and infralow sound frequencies. The experiments are performed in the Black Sea, on a 600-km-long path, with explosive sound sources. The intensity, space-time, and frequency characteristics of the sound field are analyzed. The geometric dispersion of the first normal wave is experimentally studied. The role of the channel inhomogeneities in the violation of the sound field coherence is determined for different frequency bands. On the basis of the experimental data, the vertical distribution of the critical frequencies of the waveguide is obtained, and the validity limits are established for the wave and ray calculation methods. The applicability of the phase methods for calculating the sound fields in waveguides with dispersion is discussed. The frequency-angular dependence of the effective sound attenuation coefficient in an underwater waveguide is revealed and explained.     

一种低声速沉积层海底参数声学反演方法

软泥底环境下沉积层参数的声学反演是国际水声领域的一个研究热点.浅海中,当高声速基底和海水之间存在一层低声速(小于海水声速)的沉积层时,小掠射角情况下不同频率声传播损失会出现周期性增大现象.基于此现象,提出一种适用于低声速沉积层的海底参数声学反演方法.首先,推导给出小掠射角情况下传播损失周期增大的频率间隔与沉积层声速、厚度及近海底海水声速之间的解析表达式;其次,利用一次黄海实验中软泥底环境下的宽带声传播信号,提取了小掠射角下传播损失增大的频率周期;再次,把该解析表达式作为约束条件,结合Hamilton密度与声速的经验公式,采用匹配场处理反演给出沉积层的声速、密度、厚度及基底的声速、密度;然后,利用声传播损失数据反演得到泥底环境下不同频率的声衰减系数,通过拟合发现泥底声衰减系数随频率近似呈线性关系;最后,给出了双层海底模型和半无限大海底模型等效性的讨论.反演结果为低声速沉积层海底声传播规律研究与应用提供了海底声学参数.     

Roughness of organ pipe sound due to frequency comb

In the sound spectrum of flue organ pipes in addition to the usual harmonic partials, sometimes a series of equidistant but not harmonic lines can be found. This phenomenon has been observed in the recorded sound of pipes from different pipe ranks. The second set of spectral lines is similar to "frequency combs" used in optics for accurate measurement of optical frequencies. Analysis of measured sound spectra with and without frequency comb and simulations are presented and discussed in the paper. The appearance of frequency combs in the sound spectrum is explained by a model that assumes the presence of a mouth tone in addition to the pipe sound. Mouth tone bursts are generated when the oscillating air jet passes the upper lip. The burst repetition frequency is locked to the fundamental frequency of the pipe and the bursts are coherent with a pulse-to-pulse phase shift. The phase shift explains the observed frequency offset of the frequency comb to the harmonic frequencies. The simulations also show that weak and fluctuating mouth tones cannot generate frequency comb due to a lack of coherence.     

声波在含气泡液体中的线性传播

为了探讨含气泡液体对声波传播的影响, 研究了声波在含气泡液体中的线性传播. 在建立含气泡液体的声学模型时引入气泡含量的影响,建立气泡模型时引用 Keller的气泡振动模型并同时考虑气泡间的声相互作用,得到了经过修正的气泡振动方程. 通过对含气泡液体的声传播方程和气泡振动方程联立并线性化求解,在满足 (ω R₀)/c << 1 的前提下,得到了描述含气泡液体对声波传播的衰减系数和传播速度. 通过数值分析发现,在驱动声场频率一定的情况下,气泡含量的增加及气泡的变小均会导致衰减系数增加和声速减小;气泡的体积分数和大小一定时, 驱动声场频率在远小于气泡谐振频率的情况下,声速会随驱动频率的增加而减小; 气泡间的声相互作用对声波传播速度及含气泡液体衰减系数的影响不明显.最终认为气泡的大小、 数量和驱动声场频率是影响声波在含气泡液体中线性传播的主要因素. 关键词： 含气泡液体 线性声波 声衰减系数 声速     

Underwater sound pressure variation and bottlenose dolphin (Tursiops truncatus) hearing thresholds in a small pool

Studies of underwater hearing are often hampered by the behavior of sound waves in small experimental tanks. At lower frequencies, tank dimensions are often not sufficient for free field conditions, resulting in large spatial variations of sound pressure. These effects may be mitigated somewhat by increasing the frequency bandwidth of the sound stimulus, so effects of multipath interference average out over many frequencies. In this study, acoustic fields and bottlenose dolphin (Tursiops truncatus) hearing thresholds were compared for pure tone and frequency modulated signals. Experiments were conducted in a vinyl-walled, seawater-filled pool approximately 3.7 x 6 x 1.5 m. Acoustic signals were pure tone and linear and sinusoidal frequency modulated tones with bandwidths/modulation depths of 1%, 2%, 5%, 10%, and 20%. Thirteen center frequencies were tested between 1 and 100 kHz. Acoustic fields were measured (without the dolphin present) at three water depths over a 60 x 65 cm grid with a 5-cm spacing. Hearing thresholds were measured using a behavioral response paradigm and up/down staircase technique. The use of FM signals significantly improved the sound field without substantially affecting the measured hearing thresholds.     

热粘性对发动机排气管道中噪声传播的影响

声波在气体中传播时,气体的热粘性效应会使声波产生一定程度的衰减,且气体的声吸收系数随温度的升高而增大。由于发动机的排气温度较高,热粘性效应引起的排气管道中的噪声衰减应加以考虑。基于准平面波理论,首次计算了考虑热粘性效应时不同温度、流速和管道尺寸下排气管道中的传递损失,分析了各参数对管道中噪声衰减的影响。结果表明,随着温度和频率的升高热粘性声衰减增强,而气流流速和管道直径的增加会降低直管中的热粘性声衰减。对于简单膨胀腔,传递损失的预测结果表明,热粘性效应使通过频率处的声衰减有所改善。     

Bubble dynamics in a standing sound field: the bubble habitat

Bubble dynamics is investigated numerically with special emphasis on the static pressure and the positional stability of the bubble in a standing sound field. The bubble habitat, made up of not dissolving, positionally and spherically stable bubbles, is calculated in the parameter space of the bubble radius at rest and sound pressure amplitude for different sound field frequencies, static pressures, and gas concentrations of the liquid. The bubble habitat grows with static pressure and shrinks with sound field frequency. The range of diffusionally stable bubble oscillations, found at positive slopes of the habitat-diffusion border, can be increased substantially with static pressure.