Sound Propagation Path Research Articles

Correlation of underwater acoustic signals simultaneously received by omnidirectional and directional means

A comparison of experimental data on the spatial correlation between acoustic signals simultaneously received by an omnidirectional hydrophone and a directional vertical array is carried out. The spatial correlation was measured between the signals received at different distances in a deep ocean. The points of reception were positioned in two convergence zones along the path of sound propagation with a point-to-point distance of about 64 km. Pseudonoise signals were emitted in the frequency range (0.8–2.0) kHz and received by a vertical array, whose beam had a width of ∼2°. Concurrently, multipath signals received with the central hydrophone of the array were recorded. Signals in the first and second convergence zones were received at different times. Nevertheless, in the case of the directional reception, the coefficients of spatial correlation between such signals appeared to be as high as 0.64–0.74 even under the conditions of incomplete resolution of signals in the angle of arrival in the vertical plane. At the same time, in the case of omnidirectional reception, the coefficients of spatial correlation were below 0.32.

The acoustical effect of reveal blocks, from measuring method to prediction

The analysis of the efforts to create high quality building from the point of view of energy‐efficiency, and the resulting variety of new products, building constructions and systems leads to the conclusion: there are characteristic deficiencies in the field of measuring and prediction methods related to sound insulation, and there is a need to stop the gap. The example on which the problem is shown is the acoustical effects of reveal blocks, used around window openings, to reduce heat propagation and increase energy efficiency. The reveal blocks include more sound propagation paths, the sound insulation of which can not be determined using the existing measuring standards. The paper describes new measuring concept and method to characterise the sound insulation characteristics of reveal blocks in their built‐in form. The methods are fitted to the standardised measuring methods of sound insulation. Proposal is presented for the modification in the prediction procedure of sound insulation of façade constructions, utilising the results of the new measuring method.

Modeling acoustic propagation of airgun array pulses recorded on tagged sperm whales (Physeter macrocephalus)

In 2002 and 2003, tagged sperm whales (Physeter macrocephalus) were experimentally exposed to airgun pulses in the Gulf of Mexico, with the tags providing acoustic recordings at measured ranges and depths. Ray trace and parabolic equation (PE) models provided information about sound propagation paths and accurately predicted time of arrival differences between multipath arrivals. With adequate environmental information, a broadband acoustic PE model predicted the relative levels of multipath arrivals recorded on the tagged whales. However, lack of array source signature data limited modeling of absolute received levels. Airguns produce energy primarily below 250 Hz, with spectrum levels about 20-40 dB lower at 1 kHz. Some arrivals recorded near the surface in 2002 had energy predominantly above 500 Hz; a surface duct in the 2002 sound speed profile helps explain this effect, and the beampattern of the source array also indicates an increased proportion of high-frequency sound at near-horizontal launch angles. These findings indicate that airguns sometimes expose animals to measurable sound energy above 250 Hz, and demonstrate the influences of source and environmental parameters on characteristics of received airgun pulses. The study also illustrates that on-axis source levels and simple geometric spreading inadequately describe airgun pulse propagation and the extent of exposure zones.

The coherence function of a sound field propagating in a turbulent atmosphere

The coherence function of a sound field propagating through an atmosphere with temperature and wind velocity fluctuations is important for many practical applications involving microphone arrays. In the literature, the transverse coherence function has been studied in detail, when acoustic sensors are located in a plane perpendicular to the sound propagation path. In this paper, for the case of line‐of‐sight sound propagation, a closed‐form expression for the coherence function of a plane wave is derived for arbitrary location of acoustic sensors. Using this expression, for some limiting cases simple formulas for the coherence function are obtained which are valid for arbitrary spectra of turbulence. Furthermore, the coherence function is calculated and analyzed in detail for the Gaussian and von Kármán spectra of temperature and wind velocity fluctuations. In particular, the longitudinal coherence radius, when acoustic sensors are located along the sound propagation path, is calculated. It is also shown that contours of the coherence radius are elongated perpendicular to this path for relatively small propagation distances, and along it for large distances.

Low-frequency sound scattering by internal waves in the ocean

In this paper, long-range propagation of low-frequency sound through an ocean waveguide with random inhomogeneities in the sound speed is studied. Closed equations for the mean field and correlation function of the sound field are derived using the Chernov method. These equations can be considered as a generalization of equations derived by Dozier and Tappert [J. Acoust. Soc. Am. 63, 353–365 (1978)], which accounts for 3D effects and cross-modal correlations. The equations derived in this paper in a general form are similar to the equations obtained by many other authors. However, without simplifications these equations are difficult to solve even numerically due to high dimension of the matrices appearing in the equations. Some simplifications of the equations for the mean field and correlation function are suggested that account for narrowness of the angular spectrum of the scattered acoustic field and which make these equations amenable for numerical implementation. To study solutions of the simplified equations, they are additionally averaged along the sound propagation path. This allows us to obtain some analytical results. Using the theory developed, the horizontal coherence length of the sound field is estimated for the GM spectrum of internal waves.

A Simple Technique for Realizing an Ultrasonic Anemometer Using a Loudspeaker

We describe a simple technique for the realization of a two-dimensional ultrasonic anemometer using a loudspeaker. The ultrasonic anemometer was realized in order to reduce the number of acoustic transducers. Two sound propagation paths were configured by the loudspeaker, a microphone and an acoustic reflector. The number of loudspeakers was reduced to one by using a reflected sound from the acoustic reflector. The propagation path of the reflected sound (reflected path) was tilted at an angle 45 deg from the propagation path of the direct sound (direct path). The reflected path was received at the microphone after reflection at the acoustic reflector. Experiments were performed under winds of 4.0 m/s and 5.5 m/s at the center of the measurement area. The lengths of the direct and the reflected paths were 0.5 m and 0.71 m. The wind velocities were 2.8 m/s and 3.0 m/s, respectively. Results showed that the ultrasonic anemometer measured mean wind velocities in the measurement area.

The use of wideband signals in the acoustic monitoring of hydrobionts

A method is presented for the coherent detection of the signal scattered by a hydrobiont moving across the path of sound propagation from a wideband source against the background of the sum of an intense insonification signal and additive noise. The method consists of the calculation of a cross-correlation function of the signal at the receiver input and the scattered signal model constructed from the received signal by introducing the square-law time delay. It is shown theoretically and with numerical modeling that such a processing ensures a gain in the signal-to-noise ratio that is proportional to the square root of the bandwidth multiplied by the observation time. The efficiency of the method is demonstrated by the results of an experiment.

적응 삼각형 빔 방법에 의한 실내음장 해석

In this study, the adaptive triangular beam method(ATBM) considering different sound reflection coefficients and angles of a triangular beam on two or more planes as well as diffraction effect is suggested. The ATBM, subdividing a tracing triangular beam into multiple triangular beams on reflection planes, gives reliable and convergent sound-field analysis results without the dependancy on the number of initial triangular beam segmentation to search sound propagation paths from source to receiver. The validity of the method is verified by the comparison of numerical and experimental results for energy decay curve and steady-state sound pressure level of rooms having direct, reflective and diffractive sound paths.

Position guiding method and system using sound changes

A system guides a user (or a vehicle 100 ) in reference to a target position via sound changes. The system (i) virtually disposes a sound source at the target position defined in a geographical data field covering the positional arrangement of the user and the target object, and (ii) actually outputs simulated sounds to the user as if the target position emitted sounds and the sounds were propagated from the target position to the user at the current position. The sounds change as the user moves in the geographical data field. For example, sound propagation paths, through which sound waves from the sound source are propagated to the user while being influenced by other objects or obstacles, are calculated based on positional data from a position detection system 10 . Simulated sounds expressed by combining directly transmitted waves, diffraction waves, and reflection waves, which arrive at the user through the sound propagation paths, are generated, and the generated sounds are outputted from multiple speakers 34 a to 34 d in such a way that the three-dimensional position of the sound source can be identified.

The ultrasonic receiving system of a Pacific white-sided dolphin: Part 2. Multi-input/two-output system

According to anatomical investigations, measurements of the receiving sensitivity distribution and investigation of ultrasonic sound wave propagation paths, each ear may be constructed as a multi-input/single-output system in the head. These two multi-input/single-output systems may be considered to act together as one multi-input/two-output system. Here, the input points are highly sensitive areas lying just inside of the lower mandibula. The two output points are the tympanic bones in each mandibula. All input points are connected acoustically to both tympanic bones by the the digastrici, mylohyoidei and sternohyoid muscles. Mathematical simulations were carried out using this multi-input/single-output system, applying impulse response functions previously measured from the right tympanic bone to each of the measuring points on the dolphin’s head surface. From the results of these simulations, it is considered that the highly sensitive areas located in lines along both mandibula act as line arrays, so the beamwidth of the receiving system can be narrow. Furthermore, the spectrum of the echo signal is changed by the target direction, and human beings can recognize this difference when frequency components of the echo signal are shifted down to the range of human hearing.

A PRACTICAL PROBABILITY EXPRESSION FOR TRANSMITTED SOUND POWER OF A SINGLE WALL IN UNDERWATER TO NON-STATIONARY GAUSSIAN RANDOM NOISE

For the purpose of designing the underwater transmission system using audible sound directly projected by underwater loudspeaker to prevent a diving accident and/or to give a working instruction, it is important to estimate the transmission loss from a wall not only for pure tones but also for wideband signals such as voice and noise. On the other hand, the sound pressure waves of voice signal and noise usually exhibit the non-stationary property with temporal changes of various statistical moments. Furthermore, the sound propagation environment between the sound source and the observation point shows non-stationarity, caused by temporal changes of the sound propagation path, etc. From the above practical viewpoints, in this paper, an approximate expression for the probability density function of transmitted sound power fluctuation is theoretically derived, in the case when a Gaussian-type non-stationary random sound pressure wave with fluctuation of variance is passed through a single wall in underwater. The validity and the usefulness of the theoretical method is confirmed by experiments conducted in a water tank.

Acoustic Measurement of Temperature Distribution in a Room Using a Small Number of Transducers

We propose a method for noncontact measurement of room temperature distribution using a small number of acoustic transducers. The sound propagation velocity is dependent on the air temperature, and it is acquired from the time of flight of sound. The mean temperature parallel to the sound probe is acquired using an acoustic delay line consisting of a pair of transducers. The measured object is a room of size 4,000 mm (D)× 4,200 mm (W) partitioned into a 3×3 grid, with 9 unit cells. By arranging a certain geometrical property of the sound propagation path, we can formulate an expression for the temperature distribution as a matrix function of the sound velocity. Twelve transducers are installed in contact with the four walls. We create a temperature gradient throughout the room using an electrical heat source. Experimental reconstruction results of the room temperature distribution were in agreement with the distributions estimated from the temperature profiles of the heat sources. This technique has the advantages of noncontact sensing, quicker response time and simpler calculation of room temperature distribution, in comparison with conventional temperature measurement systems.

Fundamental aspects of pulse phase-locked loop technology-based methods for measurement of ultrasonic velocity.

A new instrument based on a constant frequency pulse phase-locked loop concept has been developed to accurately measure the ultrasonic phase velocity in condensed matter. Measurements of the sound velocity in ultrapure water are reported in which both damped and undamped transducers are used with the instrument together with reflectors of various thicknesses placed in the sound propagation path. An analysis of measurements made with the new instrument and similar measurements, taken under identical experimental conditions, using a popular variable frequency pulsed-phase-locked loop instrument is reported. Uncertainties in both measurement systems are analyzed and discussed. A method for measuring inherent phase shifts, not addressed by previous investigators, within the variable frequency pulsed phase-locked loop system and a derivation of the equations that govern the overall use of variable frequency systems using phase-sensitive comparisons are presented. The effects of a finite pulse length on the measurements of phase velocity in dispersive media are addressed in detail.

Pulse phase-locked loop technology-based methods for measurement of ultrasonic velocity.

A new instrument based on a constant frequency pulse phase-locked loop concept has been developed to accurately measure the ultrasonic phase velocity in condensed matter. Measurements of the sound velocity in ultrapure water are reported in which both damped and undamped transducers are used with the instrument together with reflectors of various thicknesses placed in the sound propagation path. An analysis of measurements made with the new instrument and similar measurements, taken under identical experimental conditions, using a popular variable frequency pulsed-phase-locked loop instrument is reported. Uncertainties in both measurement systems are analyzed and discussed. A method for measuring inherent phase shifts, not addressed by previous investigators, within the variable frequency pulsed phase-locked loop system and a derivation of the equations that govern its use as a bolt-tension measuring device are presented. The effects of a finite pulse length on the measurements of phase velocity in dispersive media are addressed.

Low‐frequency long‐range sound propagation over impedance discontinuities with the parabolic approximation

Previous studies have shown that the parabolic approximation method, widely used in ocean acoustics, can be successfully applied to low‐frequency long‐range atmospheric sound propagation over a locally reacting boundary, an important problem with many applications. This work models cw acoustic signals as they propagate over horizontal ground surfaces with impedance discontinuities. An implicit finite‐difference implementation of the parabolic approximation sound propagation model incorporating locally reacting, variable‐impedance ground surfaces is employed to compute estimates of the sound field excess attentuation for a variety of flow resistivities and source frequencies. Both windy and stationary atmospheres are considered. An accuracy benchmark and several example problems in which the sound propagation path includes an idealized lake will be discussed. [Work supported by NASA.]

Coupled ocean-acoustic model studies of sound propagation through a front

A three-dimensional (3-D) numerical ocean model has been used to study sound propagation through an ocean front. The model has been used to provide environmental data for input to a range-dependent acoustic model to study the effect of eddies that form at the front on sound propagation characteristics. The model was set up in an idealized ocean domain but with the model physics and the temperature contrast across the front configured so as to represent the polar front east of Iceland. Acoustic ray tracing was carried out to illustrate the effect of frontal eddy features on sound propagation paths, and propagation loss calculations were performed to quantify their effect acoustically. It was found that dependent upon sound source/receiver depth combinations, the effect of the front and the eddies was to increase propagation loss by as much as 10–20 dB. This is comparable with the magnitude of the frontal effect that is seen in studies using analytical models of ocean fronts and with acoustic calculations that are based on measured environmental data. However, the results of this study have also shown that the acoustic predictions may be sensitive to the choice of ocean model parameter, in particular the horizontal eddy viscosity coefficient.

Measurement of down-slope sound propagation from a shallow source to a deep ocean receiver

An experiment has been performed to investigate the coupling of surface–ship noise to the deep ocean sound channel. These calibrated measurements of sound propagation from the 18-m, 135-Hz source were obtained at a deep ocean receiver while the source tow proceeded from deep water up the Sable Island Bank at ranges between 730 and 910 km. The sound propagation path was from the cold slope waters over the bank through the Gulf Stream and to the edge of the Sargasso Sea. The mean value of the transmission loss was 110 dB with a down-slope enhancement estimated to be 4 dB resulting from the combined effects of trapping in the strong shallow sound channel and reflections from the slope. Comparisons with calculated results using the parabolic equation method were good and demonstrate coupling to the deep ocean sound channel. The acoustic field, sampled by 16 transverse hydrophone groups with a 4.75-m center spacing over six consecutive (12.5 s, 0.08 Hz) samples, yielded a mean standard deviation of 1.7 dB (6≤S/N≤20 dB) corresponding to a coefficient of variation of 0.6. Coherent summation of slope-reflected and deep-refracted hydrophone signals yielded estimated mean values of the spatial coherence of 0.89 and an estimated spatial coherence length of 460 m, when multipath effects were not dominant; however, these estimates were found to range as low as 0.63 and 150 m. These results facilitate the interpretation of the spatial coherence of the ship-induced, ‘‘slope-enhanced’’ contribution to deep ocean ambient noise.

Sound propagation from a coastal source to a deep receiver

An experiment has been performed to investigate the coupling of surface ship noise to the deep ocean sound channel. These calibrated measurements of sound propagation from an 18-m, 135-Hz source were obtained at a deep ocean receiver from a source tow proceeding from deep water up the Sable Island Bank at ranges between 730 to 910 km. The sound propagation path was from the edge of the Sargasso Sea, through the Gulf stream, and into the cold slope waters over the bank. The mean value of the transmission loss was 110 dB with a slope enhancement estimated to be 4 dB resulting from the combined effects of trapping in the strong shallow sound channel and reflections from the slope. Comparisons with PE calculated results were good and indicate a strong coupling to the deep ocean sound channel. The acoustic field, sampled by 16 transverse hydrophones over six consecutive (12.5-s, 0.08-Hz) samples, yielded a standard deviation of 1.7 dB (6⩽S/N⩽20 dB), consistent with theoretical expectations of a well behaved multipath field. Coherent summation of slope reflected and deep refracted hydrophone signals yielded estimated mean values of the spatial coherence of 0.89 and a spatial coherence length of 460 m, when multipath effects were not dominant; however, these estimates were found to range as low as 0.63 dB and 150 m. These results will facilitate interpretation of the spatial coherence of the ship-induced, “slope-enhanced” contribution to deep ocean noise.

Field investigations of the sound insulation in school buildings

Sound insulation deserves special attention when school buildings are being designed. The sound insulation between classrooms depends partly upon the direct path of sound propagation, which depends on the properties and dimensions of the partition between classrooms, and partly upon the flanking paths which involve the properties of the two façades and their thickness as well as on the specification and relative area of windows. Field investigations on the sound insulation between classrooms in different school buildings were carried out. Comparing the results of the airborne sound insulation with the recommended value, the minimum thickness of partitions and façades and the corresponding maximum percentage area of windows to the facade area were deduced.

A technique for measuring the complex acoustic ground impedance

The goal of the study described in this paper was to demonstrate the use of an impulsive source and a Fast Fourier Transform (FFT) processor in an efficient and precise measurement of the complex ground impedance. In essence, this technique measures the impulse response of a sound propagation path that includes a reflection from the ground surface of interest. By simultaneously measuring both the input and output of the system (i.e., the “direct” wave and “reflected” wave) and Fourier transforming them, the Transfer Function can be evaluated. This Transfer Function is identically equal to the complex reflection factor, from which the complex impedance can be derived. Because of the difficulties reported in the past in obtaining the phase of the ground impedance, particular attention was paid to obtaining valid complex results. [This work supported by NASA Langley Research Center.]