Showing papers on "Sound transmission class published in 1975"

Ecological Sources of Selection on Avian Sounds

Abstract: This study describes selection derived from habitat acoustics on the physical structure of avian sounds. Sound propagation tests were made in forest, edge, and grassland habitats in Panama to quantify pure tone and random noise band sound transmission levels. The sounds of bird species in each habitat were analyzed to determine the emphasized frequency, frequency range, and sound type (whether pure tonelike or highly modulated). Forest habitats differ from grass and edge in that a narrow range of frequencies (1,585-2,500 Hz) has lower sound attenuation than lower or higher frequencies. Attenuation increases rapidly above 2,500 Hz. Bird sounds from species occurring at the lower forest levels were found to be predominantly pure tonelike with a frequency emphasized averaging 2,200 Hz, conforming to the predictions based on sound propagation tests. The edge habitat is characterized by a wide range of frequencies having a generally similar attenuation rate. Pure tone and random noise band sounds did not diffe...

Acoustics of Aircraft Engine-Duct Systems

Abstract: Noise generated in aircraft engines is usually suppressed by acoustically treating the engine ducts. The optimization of this treatment requires an understanding of the transmission and attenuation of the acoustic waves. A critical review is presented of the state of the art regarding methods of determining the transmission and attenuation parameters and the effect on these parameters of (1) acoustic properties of liners, (2) the mean velocity, including uniform and shear profiles and nonparallel flow, (3) axial and transverse temperature gradients, (4) slowly and abruptly varying cross sections, and (5) finite-amplitude waves and nonlinear duct liners.

Sound attenuation material

Abstract: The sound attenuation material disclosed in this application comprises, in its basic aspects, an outer layer, a sound absorption layer, and a sound barrier layer disposed between the sound absorption layer and outer layer and bonding them together. Over the years there have been developed a number of compositions, and laminated and coated articles, for use as vibration dampening and sound attenuation means. One such a sound and shock absorbing means is in the form of an adhesive tape and is shown in U.S. Pat. No. 3,217,832. The tape is manufactured by coating a woven webbing of plastic material such as nylon with a viscoelastic, tacky, filled composition comprising, inter alia, polybutene, petroleum oil, and barytes. The density of the composition is increased if vibration dampening is of prime importance and decreased for better sound absorption. In U.S. Pat. No. 3,265,154 there is disclosed an acoustical panel comprising a first and second layer of sound absorbing material of different densities, each comprising glass fibers bonded together. The outer layer is provided with dimples and serves also as a decorative layer in certain applications. A septum layer, e.g., of metal foil or plastic screen, is disclosed to be provided between the sound absorbing layers where it is desired to provide even greater sound absorption qualities to the sound absorption layers. An acoustic foam panel is disclosed in U.S. Pat. No. 3,196,975 comprising a flexible polyurethane foam having both open and closed cells. In some instances, as disclosed, a preformed plastic facing sheet is laminated to the acoustic foam layer. The invention disclosed in U.S. Pat. No. 3,298,457 is an acoustical barrier treatment. In accordance with that invention, a structural sheet metal member, which is subject to high intensity acoustic vibration from a source located to one side of the member, has applied thereto an adherent first layer of relatively stiff viscoelastic material having a Young's modulus of elasticity that is substantially less than that of the metal sheet. To the outer surface of the first layer is adhered a second layer of a relatively soft, compliant material having a modulus of elasticity that is substantially less than that of the first layer and having dispersed or suspended therein particulate material of high density. In U.S. Pat. No. 3,489,242 there is disclosed an acoustical panel comprising viscoelastic material filled with dense particulate material. The viscoelastic material can be any viscoelastic polymeric material capable of being compounded with the particulate solid as defined. There are a number of polymers disclosed including natural or synthetic rubbers such as nitrile rubber, vinyl or vinylidene chloride copolymers, and polyurethane elastomers, and EPDM polymers such as ethylene/propylene/nonconjugated diene in which the diene is 1.4 hexadiene, dicylopentadiene, 5 methylene-2-norbornene or 5-ethylidene-2-norbornene. The filled viscoelastic material is applied to a substrate to be damped and functions not only to dampen vibrations but also as a sound transmission barrier. The invention in U.S. Pat. No. 3,424,270 involves a viscoelastic sound-blocking material with filler of high density particles. The viscoelastic material can be, for example, a polyvinyl chloride plastisol. Among the fillers disclosed is barium sulfate (barytes). Various laminated products can be made wherein a dense viscoelastic cores is backed on one side with, e.g., mineral fibers or particle board, and on the other side with a thin facing of, e.g., vinyl film or decorative wood veneer. The structures provided are characterized, according to the disclosure, by excellent sound transmission loss. In U.S. Pat. No. 3,909,488, which issued on Sept. 30, 1975 and is assigned to Tillotson Corporation, the assignee of the present invention, there is disclosed a highly filled vinyl plastisol composition which, among other uses, can be employed to form tile such as wall tile used for noise abatement. The disclosure of this patent is herein incorporated by reference. While many of the sound attenuating materials disclosed heretofore, as exemplified by the above patents, provide satisfactory performance in certain applications, none of them, of course, meet the requirements of vibration dampening and sound attenuation in all applications. This is because the requirements differ depending upon the particular environment, e.g., airplane, factory building, office, car, truck, etc. In more recent years, protection against the effects of occupational noise exposure has become of even more critical concern. Rules and regulations on permissible noise exposure have been established by the Occupation Safety and Health Act of 1970 (OSHA). Accordingly, the demand for sound attentuation means to reduce noise levels in various environments to an acceptable level has increased since these standards were established. SUMMARY OF THE INVENTION The present invention is a sound attenuation material which comprises, in its basic aspects, an outer layer, a sound absorption layer, and a sound barrier layer disposed between the sound absorption layer and outer layer and bonding them together. The outer layer in a more preferred embodiment is provided with reinforcing material so as to make the outer layer more wear resistant for some applications. It can also be provided with decorative features, if desired. The sound attenuation material of this invention will be found useful in a variety of applications; for example, in trucks or other vehicles in the floor, and side or roof panels to decrease noise levels in the cab, and as a pipe wrap. The sound attenuating material in some applications can, if desired, be provided with suitable pressure-sensitive adhesive on the exposed, or free side, of the sound absorption layer so that the material can be applied to, and will remain in relatively permanent contact with, some desired surface. Quite advantageously, the invention provides a three prong approach to control of unwanted sound or noise: first, random incident sound is absorbed by the sound absorption layer; secondly, unwanted sound is blocked by the sound barrier layer and results in reduced sound transmission; and thirdly, vibration in surfaces to which the material is applied is reduced thereby tending to further help to lower sound levels in the environment in which it is used. A further advantage in the use of sound attenuating material according to the invention is that the outer layer thereof permits the exposed surface of the material to be washed clean. Moreover, it permits the material to be used in relatively dusty areas in that it aids to prevent, as does the sound barrier layer, dust from the sound absorption layer which over a period of time could adversely affect its sound attenuation qualities.

Calculation of the effect of internal waves on oceanic sound transmission

Abstract: The signal received by a hydrophone in the ocean many kilometers from a steady sound source fluctuates dramatically due to variations of the speed of sound in sea water. By inserting an empirical model of internal‐wave‐generated sound‐speed variations into an acoustic‐transmission computer code, we have shown that internal waves cause significant variations in sound transmission at 100 Hz, comparable in size and frequency to the variations observed in field experiments. We have also studied the usefulness of vertical hydrophone arrays.Subject Classification: 30.25, 30.82; 28.60.

Disposable noise reducing hearing aid attachment

Abstract: A disposable noise reducing hearing attachment provided for covering the sound inlet of a hearing aid to reduce the amplitude of random noises such as wind noises and to prevent dirt and moisture from entering the sound inlet. The attachment includes a flexible open cellular noise filter that is covered by a thin sheet of tightly woven material to form a wind and dust screen on the top surface of the noise filter. A stretchable bottom ring layer of nonporous sheet material is attached to the bottom surface of the noise filter in which the bottom ring layer has an aperture that is greater than the cross section of the sound inlet. The top wind screen layer and the bottom ring layer are secured to opposite sides of the noise filter layer by rings of adhesive that have diameters greater than the cross-sectional diameter of the sound inlet of the hearing aid so that adhesive does not interfere with the sound transmission through the attachment and into the hearing aid through the sound inlet.

Sampled−data representation of a nonuniform lossless tube of continuously variable length

Abstract: The sound transmission through a nonuniform tube (e.g., the human vocal tract) is digitally simulated using a tube consisting of uniform segments, the length of which is determined by the sampling frequency. Then the problem arises of how continuous changes of the tube length can be represented. It is solved by inserting a segment of variable length, which causes fractional powers of z to appear in the z−transform expressions describing the transmission behavior. These are made rational by first− and third−order expansion in (z−1 − 1). For a uniform tube, the resulting error in formant bandwidth is calculated.Subject Classification: 70.50.

The development of experimental techniques for the study of helicopter rotor noise

Abstract: The features of existing wind tunnels involved in noise studies are discussed. The acoustic characteristics of the MIT low noise open jet wind tunnel are obtained by employing calibration techniques: one technique is to measure the decay of sound pressure with distance in the far field; the other technique is to utilize a speaker, which was calibrated, as a sound source. The sound pressure level versus frequency was obtained in the wind tunnel chamber and compared with the corresponding calibrated values. Fiberglas board-block units were installed on the chamber interior. The free field was increased significantly after this treatment and the chamber cut-off frequency was reduced to 160 Hz from the original designed 250 Hz. The flow field characteristics of the rotor-tunnel configuration were studied by using flow visualization techniques. The influence of open-jet shear layer on the sound transmission was studied by using an Aeolian tone as the sound source. A dynamometer system was designed to measure the steady and low harmonics of the rotor thrust. A theoretical Mach number scaling formula was developed to scale the rotational noise and blade slap noise data of model rotors to full scale helicopter rotors.

Sofar Channel Axial Sound Speed and Depth in the Atlantic Ocean

Abstract: : Axial sound speed and depths of the SOFAR channels in the North and South Atlantic have been estimated and the seasonal variations that affect SOFAR propagation examined. The estimates are derived from applying averaging techniques to archived oceanographic data. The computed values appear to agree within one part in a 1000 with the measured data from long-range propagation experiments. A more accurate sound speed mapping method is recommended, as is the incorporation of measured values of speed and depth in any SOFAR computer program. (Author)

Normal mode FORTRAN programs for calculating sound propagation in the ocean

Abstract: : Two related normal mode computer programs which can be used to calculate the sound field produced in the ocean by a point source are described. The programs are written in FORTRAN for use with the Control Data Corporation 3800 computer system. The ocean environment is modeled as a constant depth layer of water (first layer) bounded above by a pressure release surface and below by a bottom consisting of a constant depth fluid layer (second layer) and a semi-infinite layer (third layer). The two programs differ in that the third layer is taken to be a fluid in one (PROGRAM FLUID) and a solid in the other (PROGRAM SOLID). The sound speed profiles in the first and second layers can be arbitrary functions of depth, however, they and the other environmental parameters must be independent of range. The depth-dependent differential equation which results from the separation of the wave equation is solved numerically for the eigenfunctions, which are the normal modes of the system. Bottom attenuation and surface and bottom scattering loss are included as perturbations. The programs also provide, as a separate option, the calculation of the group velocities of the modes.

Cabinet for improving the sound output of an amplifier

Abstract: A cabinet for improving the sound output of an amplifier including an upright housing having a horizontal partition dividing the interior of the housing into an upper portion and a lower portion, a speaker mounted in the horizontal partition and oriented for directing sound into the upper portion of the housing, a reflective sounding board positioned within the upper portion of the housing inclined at an angle relative to the horizontal partition, the sounding board being inclined from the lower back to the upper front of the housing dividing the upper portion into a forward and rearward resonance chamber, the sounding board reflecting sound outwardly from the cabinet, the sounding board having an opening therein of an area of one-half to seven-eighths of the vibrational area of the speaker, the opening being covered by a metal plate permitting fast sound transmission between the forward and rearward resonance chambers and a front panel covering the housing upper and lower portion, the upper portion of the front panel having a plurality of openings therein through which both direct and reflected sound emanates from the cabinet.

Acoustic coupling structure for microphone

Abstract: The microphone of an electronic device is mounted within the housing of the device, and may be a microphone cartridge having a sound opening exposed to a sound chamber within the housing. The device may be a portable radio transmitter and receiver wherein the housing has a wall with a grille screen for providing sound transmission between the inside and the outside of the housing. A cavity is provided inside the housing adjacent the screen for receiving sound to be applied to the microphone. A sound passage connects the cavity to the sound chamber which acts as a reinforcing chamber to apply sound to the microphone. In a radio transmitter and receiver, a loudspeaker may be positioned to project sound through the grille screen, and the cavity between the loudspeaker cone or diaphragm and the screen forms the cavity for collecting the sound. The passage for applying sound to the sound reinforcing chamber may be provided by an opening in the screen and an opening in a felt disc covering the screen which cooperate with a recess in the housing. The acoustic coupling structure can be provided by a sound collecting cavity and passage which are independent of a loudspeaker.

Sound reproduction system - uses headset with earphones and vibrators for transmitting sound direct to bones

Abstract: It comprises a set of headphones with earpieces for sound retransmission through air, and with special vibrators for sound transmission through bones; the sound source has a first sound track, in which the sounds transmitted through air are recorded, and a second sound track, in which sounds transmitted through bones are recorded; the headphone set has volume controls for each earpiece and for each vibrator for bone conduction; they are individually adjustable, so that different preferences at different times can be satisfied. There is a control box to house the set of volume controls, with two separate leads to each earphone and bone conduction unit. The bone conduction units may be enclosed in the earphone housings.

Sound damping system for windows - heavy baffle plate inside chamber above horizontal window frame member

Abstract: A window frame member built into a wall of a building containing may windows in a row, which tends to amplify sound transmission through the construction, has a baffle system built in above it. This consists of a hollow rectangular casing with an elongated rib extending inwards from each side to engage a resilient sealing strip which has an upper portion slotted to receive these ribs and a lower portion to grip a heavy steel plate. Thus, this steel plate is housed in a chamber greater than the width of the frame member.

Deep-ocean dynamics for environmental-acoustics models

Abstract: A hydrodynamic model for flows in the deep ocean is developed in order to determine the velocity field and sound‐speed distribution for use in acoustic transmission problems. A scaling of the governing equations is constructed that explicitly includes sound speed. A subsequent perturbation expansion yields a set of approximate equations for motions nearly in geostrophic and hydrostatic balance, such as large‐scale, quasisteady currents and Rossby waves. The quasigeostrophic potential vorticity equation or a simpler limiting case of this equation arises from the perturbation scheme to govern higher‐order dynamics of the stream function for these flows. The results of the analysis are used to obtain a significant simplification of the ray equations of geometrical acoustics for moving media. For the particular class of flows considered here, the model equations are applicable if the ocean depth is about 1 km or greater and if the spatial and temporal scales of variation of the motions are of the order of 100...

Burner with noise suppressor

Abstract: A combination oil and gas burner is shown with the noise attendant upon the delivery of the primary and secondary air for combustion and the noise of the combustion muffled or suppressed by sound absorbing material disposed to reduce sound transmission to a low level.

Perturbation method for determining acoustic rays in a two-dimensional sound-speed medium

Abstract: Sound speed is assumed to be a function of one horizontal coordinate and depth, and the medium is taken to be time independent during a sound transmission. Rays are assumed to lie in a vertical plane and a differential equation is given for their solution. A perturbation technique is presented for determining the rays, and is applied when the horizontal sound‐speed variation is slow and much less than the vertical variation. Under appropriate assumptions, ray arcs above the SOFAR axis are found to be approximately elliptical, and a modified Snell’s law is given. The sound speed is taken to be bilinear with depth, with the upper portion pivoting with range, thereby approximating the effect of a single‐frequency internal wave. Numerical examples of SOFAR ray geometry are presented and interpreted for various internal‐wave phase angles and wavelengths. The sound speed near the source has a major effect on geometry, even at long range. However, variations in geometry along the ray suggest the importance of ho...

Sound insulated block molding machine

Abstract: Apparatus for forming concrete building blocks which is constructed and arranged (1) to minimize sound emissions to the environment of a magnitude which might cause injury to the hearing of the operators, (2) to provide shielding around the major movable components of the machine so as to substantially eliminate potential causes of bodily injury to the operators, and (3) to improve the performance and cooperation of the major components for making concrete blocks. To provide shielding against noise and bodily injury, the frame of the apparatus is constructed in conjunction with sound insulation panels to provide an enclosure around the major movable components of the apparatus, and the frame also supports these components on internal surfaces thereof so as to minimize sound transmission openings to the environment. The various components, such as the feed drawer assembly, sizing and compression head assembly, strip mechanism, pallet eject and feed assembly, pallet magazine and the like, are improved and arranged to provide optimum performance not merely from sound and safety standpoint, but also to assure most favorable operation from the standpoint of cycle-time, trouble-free operation, ease of servicing and the like.

The transmission of sound in nonuniform ducts

Abstract: The method of weighted residuals in the form of a modified Galerkin method with boundary residuals was developed for the study of the transmission of sound in nonuniform ducts carrying a steady, compressible flow. In this development, the steady flow was modeled as essentially one dimensional but with a kinematic modification to force tangency of the flow at the duct walls. Three forms of the computational scheme were developed using for basis functions (1) the no-flow uniform duct modes, (2) positive running uniform duct modes, with flow, and (3) positive and negative running uniform duct modes, with flow. The formulation using the no-flow modes was the most highly developed, and has advantages primarily due to relative computational simplicity. Results using the three methods are shown to converge to known solutions for several special cases, and the most significant check case is against low frequency, one dimensional results over the complete subsonic Mach number range. Development of the method is continuing, with emphasis on assessing the relative accuracy and efficiency of the three implementations.

A primer for structural response to random pressure fluctuations

Abstract: A review was made of power spectral methods for determining linear response of structures to random pressure fluctuations. Various simplifying assumptions are made for the purpose of obtaining useful formula for structural response. The transmission of sound through a flexible structure into an interior cavity was also treated.

A Computer Code to Calculate the Effect of Internal Waves on Acoustic Propagation

Abstract: : The signal received by a hydrophone in the ocean many kilometers from a steady sound source fluctuates dramatically due to variations of the speed of sound in sea-water. A computer code that simulates the passage of acoustic CW signals through an ocean with internal waves is written. The code generates a sound-speed field from an internal-wave spectrum that has been derived from oceanographic measurements. The acoustic propagation is achieved through the use of the parabolic-equation approximation. The code is limited fundamentally to acoustic frequencies above approximately 10 Hz, and by practical computer-time constraints to frequencies below approximately 400 Hz. The code allows the calculation of the amplitude and phase of the full acoustic field as a function of depth and range at any desired time.