Sound Transmission Characteristics Research Articles

Measurements of the Sound Transmission Characteristics of Model Aircraft Fuselage Using a Reciprocity Technique

Measurements of the Sound Transmission Characteristics of Model Aircraft Fuselage Using a Reciprocity Technique

Characteristics of sound pulse transmission in shallow sea and correlation detection

Statistical characteristics of sound pulses and some typical marine noises in a shallow sea were analysed. With special requirements for underwater telemetric and telecontrolled devices in mind, a specific signal processing scheme (digital correlation accumulation) is put forward. Theoretical analyses on some underwater acoustic instruments showed this signal processing scheme is applicable to a shallow acoustic channel with variable spatiotemporal characteristics, multipath pulses and high noise level, and may be of general significance for underwater acoustic telemetry and telecontrol.

Spectral characteristics of sound transmission in the human respiratory system

The amplitude of sound transmission from the mouth to a site overlying the extrathoracic trachea and two sites on the right posterior chest wall over the 100-600 Hz frequency range was measured in eight healthy adult subjects. An acoustic driver and a rigid tube were employed to introduce sound into the mouths of the subjects at resting lung volume, and the transmission measurements were performed using lightweight accelerometers. Similar spectral characteristics of acceleration were observed in all of the subjects showing peaks in the transmission. These characteristics included 1) two regions of increased transmission over the frequency range of the measurements, 2) a decrease in the magnitude of acceleration of the chest wall as compared to the tracheal site of roughly 20 dB at lower frequencies, 3) a strong trend of decreasing acceleration of the chest wall with increasing frequency. These spectra agreed favorably with the predictions of a theoretical model of the acoustical properties of the respiratory system. The model suggests the primary structural determinants of a number of the observed characteristics including the importance of the lung parenchyma in sound attenuation.

Sound transmission characteristics of a liquid state electromagnetic transducer

Sound transmission characteristics of a liquid state electromagnetic transducer

Sound transmission through layered cylindrical shells with applied damping treatment

Sound transmission through closed circular cylindrical shells with unconstrained damping treatment and sandwich shells with constrained damping treatment is considered in an acoustoelastic formulation in which the full interaction between the structural vibration and the internal cavity pressure field are taken into account. Only the axisymmetric modes of vibration of the shell are considered in an initial formulation. The structural response and the sound transmission characteristics for an external random pressure field are computed through an efficient matrix inversion procedure. Results indicate insensitivity of noise reduction and structural response to variations in core parameters of the sandwich shell. The sandwich shell has better noise transmission characteristics in the higher frequency ranges as compared to an equivalent layered shell with unconstrained damping treatment.

Sound transmission through a damped sandwich panel

The sound transmission through an infinite sandwich panel with a constrained viscoelastic damping layer is analyzed. Expressions for the sound transmission loss (TL) and the coincidence frequency are derived in terms of the core parameters and incidence pressure angle. Results show that the sandwich panel has better sound transmission characteristics than a homogeneous elastic panel of equivalent weight.

Vibration damping material as a means to reduce steel noise barrier cost

A preliminary analysis has been made of the use of vibration damping material as a means to reduce the cost of steel noise barriers in primarily highway applications. For cost-effective barriers, the sound transmission through, as well as over, a noise barrier must be considered. The through-barrier sound transmission characteristics of sample panels from a Toronto noise barrier were measured with and without damping material. It was found that a given through-barrier sound transmission performance could be achieved at less cost with the damping material than without it. Further study is recommended.

Hearing differences among Hawaiian squirrelfish (family Holocentridae) related to differences in the peripheral auditory system

1. Auditory sensitivity as a function of frequency has been behaviorally determined for two species of fish from the teleost family Holocentridae which is characterized by marked variation in peripheral auditory structures. 2. Best sensitivity measured forMyripristis kuntee was -50 dB re: 1 dyne/cm2 for frequencies between 300 and 2,000 Hz, while best sensitivity measured forAdioryx xantherythrus was -28 dB at 500 Hz (Figs. 1 and 2). 3. Both species can detect sounds at 100 Hz while the high frequency end of the auditory range extends up to 3,000 Hz forM. kuntee and to 800 Hz forA. xantherythrus (Figs. 1 and 2). 4. It is hypothesized that these differences in auditory capabilities are related to differences in sound transmission characteristics of the peripheral auditory system.

Acoustics of an East Australian current anticyclonic eddy

The sound transmission characteristics of a typical eddy associated with the East Australian current during the Southern Hemisphere summer have been measured for the frequencies 25–2000 Hz. The dominant acoustic features of this warm core eddy were a 36-m surface duct and a deeper 175-m thick eddy duct. The eddy duct behaved as a classical acoustic wave guide with an optimum transmission frequency of 100 Hz. Strong acoustic coupling was observed between the ducts with rays up to 20 deg from the source in the surface duct being trapped in eddy duct. The coupling was independent of range and weakest for the frequency range 300–1000 Hz. Below 300 Hz coupling steadily increased due to diffracted energy. Above 1000 Hz coupling also increased due to scattered energy. [Work Partially Sponsored by Office of Naval Research.]

Analytical and experimental studies of folded cavity duct acoustic liners

Low-frequency noise suppression in flow ducts can be achieved without excessive acoustic liner backing depth if the backing depth if the backing volume is formed as a long shallow cavity, folded so as to lie along the duct axis. Initial studies of such configurations revealed that sound transmission characteristics could not be accurately predicted when locally reacting impedance models were used. Consequently, an extended reaction model was developed for folded cavity liners by use of eigenfunction expansion techniques. Accuracy of the model has been verified by comparing analytical and experimental results for liners mounted in a 6-in.-square flow duct test facility. Several face sheet impedances and cavity geometries were tested at Mach numbers of 0.0 and 0.2 and frequencies from 500 to 1000 Hz. Good agreement was obtained between theoretical and experimental values of panel insertion loss as well as between analytical and measured duct centerline axial standing wave patterns.

A study of sound transmission in curved duct bends by the Galerkin method

A computational procedure based on the Galerkin method is developed to study the sound transmission and reflection characteristics of circularly curved bends of rectangular ducts. This procedure is adaptable to routine computer calculation. An important component of the computer program consists of a QR iterative algorithm which solves the matrix eigenvalue problem generated by the Galerkin method. The present procedure produced very satisfactory convergent results when applied even to cases where the incident wave has high frequency and high wave mode numbers. Some properties of the Galerkin solution are investigated. Its relationship to the classical solution by the method of separation of variables is discussed.

Systematic method to determine the acoustical characteristics of series‐parallel duct configurations using transmission matrices

The design of duct systems to minimize their transmitted sound requires a calculation procedure which can handle various duct geometries and liner configurations. Drawing upon the experience of electrical and mechanical engineers in applying matrix techniques to the analysis of electrical circuits and vibrating systems, a systematic method is developed to determine the acoustical characteristics of complicated duct arrays (“acoustical circuits”). By defining impedance, admittance, and transmission matrices for each component of the duct system, the sound transmission characteristics for the complete system can be obtained by the appropriate combination of component matrices. System optimization is facilitated because elements of the component matrices appear explicitly in the resultant matrices characterizing the overall duct system. The rules governing the combination of component matrices of lined duct sections joined in series and/or in parallel are set forth. Example problems are presented in which the transmission loss is determined for N ducts aligned in parallel and for a series‐parallel lined duct combination. [This work was supported by the U.S. Department of Transportation under Grant Agreement DOT‐OS‐3011.]

Sound transmission through a double wall with studs

A common structural configuration consists of two parallel walls, or partitions, structurally connected to each other via a periodic array of studs. The sound transmission characteristics of such a configuration are analyzed. In particular, the relative strengths of the parallel transmission paths, viz, the airborne path through the cavity space and the structure-borne path through the studs, are compared for common configurations. The mathematical model that is used consists of two plates infinite in extent, and governed by the classical plate equation, The cavity space between the plates as well as the surrounding spaces satisfy the wave equation. The studs are assumed to be infinitely periodic with translational impedances corresponding to those of line masses.

Sound transmission in the thorax

The use of an intracardiac sound source to determine transfer functions and propagation paths and modes for the thorax using digitial data reduction techniques is discussed. It is important to know the sound transmission characteristics of the thorax for diagnostic purposes. Studies made using naturally occurring heart sounds have not satisfactorily defined either the origin or the transmission modes and paths for the signals. An intracardiac sound source inserted by standard cardiovascular catherization techniques [“Artificial Sound Production in the Cardiovascular System,” H. C. Merchant et al., J. Assoc. Advan. Med. Inst. 7, No. 4 (Sept. 1973)] was used as the sound source. The sound source and external receiver were located in three dimensions using x-ray techniques [“Three Dimensional Location Using Planar X-ray Systems,” R. D. Golden et al., J. Assoc. Advan. Med. Inst. 6, No. 3 (May 1972)]. Preliminary experiments were performed using experimental animals. The analog data were digitated and reduced on a digital computer using correlation and spectrum analysis techniques.

Simple sound transmission loss measurements using a modified impedance tube technique

A method of comparing the sound transmission characteristics of various materials, and combinations of materials, is presented, using a modified impedance tube technique. The procedure proved to be relatively quick and inexpensive in comparison with standard reverberation suite tests, and is therefore particularly useful for the qualitative ranking of multiple samples. The limitations of the technique are discussed in some detail, and particular emphasis is given to the problems of small sample size and method of mounting in the apparatus.

Spectral characteristics of sound transmission through the rough sea surface

The predicted dependence of sound transmission on the statistics of the randomly rough interface between dissimilar fluids has been studied by use of the Helmholtz integral [J. Acoust. Soc. Am. 51, 1083–1090 (1972)]. The predictions have been verified for a plane wave passing from air through an aperture into water (laboratory model) and for radiation from a point source in air (laboratory and ocean experiments) for a wide range of surface acoustical roughnesses, R = k22σ2[(c2/c1)cos θ1−cos θ2]2. The roughness parameters σ,k ,c , and θ are the rms height of the surface, propagation constant, speed of propagation, and angle with the normal, respectively; subscript 1 refers to air and subscript 2 to water. When the surface is mirror smooth, the transmitted sound pressure is due to the change in divergence and change in impedance at the interface. For low roughness, R < 1, the mean-square transmitted pressure becomes decreasingly coherent with increasing frequency; its magnitude is decreased by the factor e−R compared to the perfectly smooth surface. For R ⩾ 1 the incoherent component dominates and the transmitted pressure depends also on the correlation length of the surface displacements.

Detection of Noise Level Generated by a Grille in an Airflow

Measurements were made in order to evaluate the pressure level of sound generated by a grille in an airflow. A streamlined probe microphone, which is covered by a solidified glass fiber of high density, was used for detection of noise existing in the airflow. Distributions in a flow duct of the turbulence-fluctuated pressure of the airflow and of the output level of the microphone were measured and then compared with sound transmission characteristics specified in the duct. It was found that the distribution of the microphone output agrees with the sound transmission characteristics, but not with the fluctuating pressure of the airflow. As a result, the levels of the sound pressure generated by a grille were about 50 dB (re 2 × 10−4 μbar) at frequencies greater than 200 Hz, with a maximum value of 65 dB at 1250 Hz, for a flow velocity of 8.4 m/sec. It was also found that the output of the probe microphone produced by the turbulence-fluctuated pressure of the airflow can be reduced to 40 dB below the turbulent pressure level by a covering of this type.

Study of Speech Privacy

Speech privacy—freedom from annoyance owing to intrusion of speech sounds—poses an important acoustical problem in the design of private offices, conference rooms, hotel rooms, and similar spaces. The provision of adequate speech privacy can be more difficult than reverberation control or the reduction of noise, especially in building construction of low cost and light weight. A study of factors relevant to speech privacy is reported in this paper. Psychophysical tests were conducted in which subjects performed simulated tasks in a private office in the presence of controlled background noise and of speech signals that were generated in an adjacent office. The subjects were instructed to respond when “… the speech reaches a level that you consider bothersome….” The experimental variables were: (1) signal-to-noise ratio as a function of frequency; (2) the frequency characteristic of the sound isolation between the rooms; and (3) the type of task. The subjective responses correlated closely with the calculated speech articulation index. Results of the study, including physical data on sound transmission and absorption characteristics of building components, form the basis of a self-contained procedure for designing speech privacy. [Written for use by architects and building designers, this procedure is being published by Owens-Corning Fiberglas Corporation (who sponsored this study) in cooperation with other interested manufacturers.]

Sound Transmission Characteristics of a Layered Wall Construction

It is well known that the transmission loss of most solid, isotropic walls is seriously limited by the high dynamic stiffness and the correspondingly fast speed of bending waves in a stiff wall. On the other hand, while a relatively limp wall will have a significantly higher transmission loss due to the lower bending wave speed, it may not be acceptable to the architect because of the low static rigidity. A new construction has been found which has high static stiffness but drastically lowered dynamic stiffness, thus permitting the simultaneous satisfaction of both acoustical and architectural requirements. The change in stiffness is effected by establishing a “shear-wave” motion of the core of a layered construction. A shear wave has a speed which is independent of frequency. Thus, the speed of transverse waves is constant over the frequency range in which this type of motion predominates. If the shear wave speed is chosen to be suitably low, the wall impedance is essentially massive in this frequency range. This construction is expected to have important applications especially in the field of architectural acoustics. Measured data on some particular embodiments of the idea will be presented.

Effect of the Splayed Walls of a Room on the Steady-State Sound Transmission Characteristics

The steady-state sound transmission characteristics in model rooms of various shapes were measured. The shape of the model was varied from rectangle to trapezoid and to quadrangle with nonparallel walls, while the interior volume of the room was kept constant. In the results, it was observed that the trapezoidal rooms in which the ratio of l to a is about 10%, where a is the mean length of the parallel walls, and l is the difference between them, are effective in decreasing frequency irregularity in the transmission characteristics. On the other hand, the sound characteristics of a fan-shaped room, adopted as an example of splayed rooms, were analyzed mathematically. The fan-shaped room with ρ0≃1.2∼1.4 has good characteristics in low-frequency region, where ρ0 is the ratio of the outer and inner radius of the room. Such a fan-shaped room corresponds to a splayed room with l/a≃0.09∼0.17. The results are in good agreement with the experimental results and our experience.