Showing papers on "Sound power published in 1975"

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.

Thermoelastic waves generated by laser beams of low power

Abstract: A modulated or chopped beam from an argon ion laser, with CW power at the 1−W level, is used to generate thermoelastic waves in several liquids. The ceramic cylinder surrounding the liquids is the transducer for the acoustic waves. Output signal is measured as a function of frequency, with different laser powers and different amounts of absorption in the liquids. The analysis makes use of thermoelastic potentials in the liquid and in the ceramic cylinder, with appropriate continuity conditions between the two regions, and predicts results in good agreement with those measured. Conversion efficiency from laser power to acoustic power is extremely low at this power level, but increases directly with the laser power. Thus it is concluded that the acoustic wave generated by a modulated laser beam at the 1−W level propagating through sea water would only be detectable near the beam, but for a pulsed beam at the megawatt level, acoustic power might be detectable for several kilometers.Subject Classification: 35.65.

Edge Scattering of Aerodynamic Sound by a Lightly Loaded Elastic Half-Plane

Abstract: We investigate the acoustic field radiated when a compact turbulent eddy lies close to a semi-infinite wave-bearing surface, but many acoustic wavelengths from its edge. The Lighthill acoustic analogy is used to reduce the problem to the simpler one of the diffraction of a plane sound wave by a semi-infinite elastic plate. This is solved exactly in terms of a contour integral by means of the Wiener-Hopf technique. An explicit solution for the far sound field is then developed for the limiting case when a non-dimensional fluid loading parameter $\epsilon $ tends to zero. Particular attention is paid to the subsonic surface wave modes which couple the fluid and plate vibrations. At low frequencies the bulk of the elastic wave energy induced in the plate by the adjacent turbulent eddy propagates without attenuation in a subsonic surface wave mode. Interaction with the edge of the plate induces an indirect scattered field whose intensity exhibits the dependence on cos$^{2}\frac{1}{2}\theta $ (the line $\theta $ = 0 defines the semi-infinite surface) typical of the acoustic field of an edge scattering half-plane. The acoustic power of the eddy is increased over its free space value by a factor $\epsilon ^{2}(Ma\_{\text{s}})^{-1}$, where $Ma\_{s}=c_{\text{p}}/c$ represents the ratio of the natural speed of plate waves in vacuo to the speed of sound. In typical aeronautical applications e can be as large as 18 dB at characteristic eddy frequencies of 100 Hz.

Comparison of an acousto‐optic and a radiation force method of measuring ultrasonic power

Abstract: During the interim period while nationally acceptable methods of measurement of referential standards for ultrasonic power are being developed, comparisons are needed among measurment techniques currently used. This is particularly necessary for determining the acoustic power from ultrasonic medical devices. Such a comparison was made between an optical and a radiation force technique to measure the ultrasonic power output of a 1‐MHz 1‐in.‐diameter PZT crystal from 70 mW to 2.3 W total acoustic power. The optical technique follows the theory of Raman and Nath, assaying the diffraction of monochromatic light by ultrasound. In addition, a correction factor is introduced into the calculation of the phase retardation parameter and eliminates the need to determine the pathlength of the light through the sound. The radiation force technique relates the change of position of an air‐backed, self‐centering, reflecting float to the ultrasonic power. A linear regression analysis between the square of the voltage app...

Modes of Engine Structure Vibration as a Source of Noise

Abstract: The overall modal characteristics of a simplified engine structure have been analysed theoretically using finite element programs developed at I.S.V.R. and a comparison is made with experimental results obtained on an equivalent physical model cast in one piece to minimise damping. The mechanism whereby the modes of vibration of a typical engine structure are excited by the forces in each cylinder is analysed in conjunction with experimental results obtained by a novel static deflection technique. Also presented are the results of a study into the optimum way of isolating a light panel from blcok induced vibration. The acoustic power radiated by representative modes of engine vibration through the acoustically important frequency range is discussed and some results of directivity tests are also given.

Evaluation of the intensity method of sound power determination

Abstract: If the sound power of a source is obtained by forming the surface integral of the normal component of the sound intensity vector, the shape of the surface of measurement is unimportant and projection error only occurs as a result of nonideal polar response of the composite microphone. Furthermore, the sound power of the source may be determined selectively in the presence of background noise originating outside the closed surface. If a sufficient number of points of measurement are taken, the error depends mainly on the signal−to−noise ratio at each point and on the accuracy of the measuring system.Subject Classification: 50.25, 50.85.

Excess noise from gas turbine exhausts

Abstract: There is evidence to show that the exhaust noise from gas turbines contains components which exceed the jet mixing noise at low jet velocities. This paper describes a theory developed to calculate the acoustic power produced by temperature fluctuations from the combustor entering the turbine. Using the turbine Mach numbers and flow directions at blade mid-height, and taking a typical value for the fluctuation in temperature, it has been possible to predict the acoustic power due to this mechanism for three different engines. In all three cases the agreement with measurements of acoustic power at low jet velocities is very good. Using a measured spectrum of the temperature fluctuation the prediction of the acoustic power spectrum agrees quite well with that measured.

Measurements of farfield sound generation from a flow-excited cavity

Abstract: Results of 1/3-octave-band spectral measurements of internal pressures and the external acoustic field of a tangentially blown rectangular cavity are compared. Proposed mechanisms for sound generation are reviewed, and spectra and directivity plots of cavity noise are presented. Directivity plots show a slightly modified monopole pattern. Frequencies of cavity response are calculated using existing predictions and are compared with those obtained experimentally. The effect of modifying the upstream boundary layer on the noise was investigated, and its effectiveness was found to be a function of cavity geometry and flow velocity.

Broadband acoustooptic deflectors: new results.

Abstract: A new broadband acoustooptic (AO) Bragg deflector using acoustic beam steering to track the Bragg angle is described. It features, instead of the conventional stepped array acoustic grating to produce the steering sound column, a flat transducer grating made from a single piezoelectric platelet bonded to the AO medium, with the spatially periodic phase variations created by interdigitating the electrode configuration. The advantage over the conventional stepped array, for deflectors operating at frequencies above 100 MHz, is the ease of fabrication of the device, an advantage that far outweighs its somewhat reduced acoustic power available for light diffraction. An analysis is presented to show that the achievable half-power bandwidth from beam steering deflectors under optimum design considerations is over four times the bandwidth of fixed beam deflectors having the same transducer dimensions. Experimental results on lead molybdate deflectors confirm the predicted fourfold improvement.

Minimization of jet and core noise of a turbojet engine by swirling the exhaust flow

Abstract: A noise abating improvement for jet engines has been investigated and demonstrated using a full scale turbojet engine which provided the means for significant reductions in jet and core noise with minimal thrust loss. This was accomplished by controlled solid body rotation (swirling) of the flow in the nozzle and jet exhaust. Relatively moderate angles of solid body rotation in the presence of temperature, density, pressure, and velocity gradients were utilized. Of great interest was the finding that significant reductions of sound pressure levels and sound power were obtained with only a small percentage of the total primary mass flow swirling at the exit of the nozzle. Therefore, tradeoffs between noise reduction and engine performance can be optimized to satisfy aircraft performance and federal noise standards with minimal wasted rotational kinetic energy.

Sound absorption with variable acoustic resistance means by oscillatory air pressure signal

Abstract: A system for varying the acoustic resistance of an acoustical lining disposed in a duct of an air propulsor comprises a nonlinear sound suppression liner having a porous facing sheet overlying a plurality of cells and means for impinging a predetermined oscillatory air pressure signal of 100-160 dB at an inaudible frequency on the facing sheet to vary the acoustic resistance of the facing sheet to make it optimum for a selected sound level and airflow condition in the duct.

The convergence of theory and experiment in direct combustion generated noise

Abstract: Current theories of combustion generated noise are reviewed with regard to their ability to predict the sound power output and spectral characteristics of noise generated by several flame types. New experimental information on open turbulent flames and on gas turbine combustor cans is presented. Available information on gas phase diffusion flames is reviewed. It is concluded that if some of the gross turbulence features of the flame are known and if the acoustical behavior of any flame enclosure is known, then scaling rules for behavior of the sound power output and spectral content may be quite accurately produced by theory. On the other hand, the theory is not sufficiently advanced to make absolute predictions; such predictions must await more detailed knowledge of turbulent flame structure.

Study of noise sources in a subsonic fan using measured blade pressures and acoustic theory

Abstract: Sources of noise in a 1.4 m (4.6 ft) diameter subsonic tip speed propulsive fan running statically outdoors are studied using a combination of techniques. Signals measured with pressure transducers on a rotor blade are plotted in a format showing the space-time history of inlet distortion. Study of these plots visually and with statistical correlation analysis confirms that the inlet flow contains long, thin eddies of turbulence. Turbulence generated in the boundary layer of the shroud upstream of the rotor tips was not found to be an important noise source. Fan noise is diagnosed by computing narrowband spectra of rotor and stator sound power and comparing these with measured sound power spectra. Rotor noise is computed from spectra of the measured blade pressures and stator noise is computed using the author's stator noise theory. It is concluded that the rotor and stator sources contribute about equally at frequencies in the vicinity of the first three harmonics of blade passing frequency. At higher frequencies, the stator contribution diminishes rapidly and the rotor/inlet turbulence mechanism dominates. Two parametric studies are performed by using the rotor noise calculation procedure which was correlated with test. In the first study, the effects on noise spectrum and directivity are calculated for changes in turbulence properties, rotational Mach number, number of blades, and stagger angle. In the second study the influences of design tip speed and blade number on noise are evaluated.

Experimental demonstration of the absorption of sound by sound in water

Abstract: Intense low‐frequency sound can severely attenuate sound of higher frequencies through nonlinear interaction. On the other hand, intense high‐frequency sound does not appreciably alter a lower‐frequency sound field. An experiment is described wherein a 65‐kHz underwater sound beam was made to propagate collinearly with a 240‐kHz beam, demonstrating the absorption of sound by sound. [Work supported by NAVSEA.]

Far Noise Field of a Two-Dimensional Subsonic Jet

Abstract: The acoustical radiation characteristics of a high aspect ratio, cold subsonic, model slot jet have been investigated experimentally. Acoustical measurements were made in both a free-field environment and a reverberation field environment. Mean velocity distribution of the slot jet was determined to provide information on the mixing characteristics of the flow. Free-field results obtained reflected a characteristic asymmetrical directivity pattern of the slot jet noise. The total radiated sound power yielded a typical U to the 7-th power dependence on mean jet velocity. The experimental findings are compared with data from similar studies and also with radiation characteristics of circular cold subsonic jets.

Real-time wavefront correction through Bragg diffraction of light by sound waves.

Abstract: The direction of propagation and phase of a light wave diffracted successively by two sound waves of the same frequency but different wavelengths can be controlled by varying the acoustic frequency and the relative phase of the sound waves. Because the Doppler frequency shifts produced by the two sound waves are of equal magnitude but opposite signs, they cancel each other, with the consequence that the diffracted-diffracted wave is coherent with the incident (or undiffracted) light. If a wavefront distorted by atmospheric turbulence is divided into subwavefronts, each subwavefront being planar, it can be corrected in real time if it is allowed to pass through an array of Bragg cells. Each cell carries two pairs of orthogonal sound waves to correct for tilts and phase errors of a corresponding subwavefront.

Acoustic signal reproduction device

Abstract: PURPOSE:To make possible the realization of distance feel within a small distance range by generating exactly the same complex sound pressure ratio as that when a single sound pressure exists in a certain direction at a certain distance, using two acousting converter such as speakers and the like.

Sound power level determination for large steam turbine generators

Abstract: A technique is presented for calculating the approximate total sound power level (PWL) of large steam turbine-generators as well as the PWL of individual turbine-generator components. A near field approximation is used which is similar in structure to the new IEEE test code 85. Modifications are introduced in the test code procedure to reduce the effect of component flanking noise, and noise contributed by extraneous sources. The overall turbine room PWL is also calculated by a reverberant field technique and compared to PWL as calculated by free field methods. Measurements were made around ten large steam turbine generator units ranging in name plate rating from 390 to 1050 MVA. PWL's of turbine-generators were found to increase by approximately 3.5db for each doubling of unit rated MVA. PWL's calculated from indoor near field approximation show a 3 dbA increase over PWL's of outdoor units of equivalent rating. This difference is an indication of the need for further development of methods for determining sound power level.

Measurement of the sound speed in bottom layers

Abstract: In many applications of underwater acoustics, the ocean bottom is adequately characterized by a critical angle and a density. An estimate of the sound speed in the bottom is obtained by seismic refraction measurements (i.e., measurements of arrival time) or bottom−reflection measurements. In this paper, the theoretical basis is presented for an alternate technique that, it is hoped, will simplify the determination of the critical angle(s) associated with high−speed bottom layers. The technique requires the measurement of the normal component of the specific acoustic impedance at the ocean−bottom interface. As a function of incidence angle, this impedance function displays a strong peak near the critical angle. The precise location depends in a predictable manner on the location of the source, the source frequency, and the sound speed in the bottom. This technique exploits this dependence to obtain an estimate of the sound speed in the bottom.Subject Classification: 30.25, 30.20.

Noise-reducing partition or surface-cladding unit - with panels at acute angle reflecting sound and prolonging sound path length

Abstract: The system is designed for the deflection, attenuation or extinction of noise, using single or combined panels or other surfaces, with acoustic characteristics as required, such as units assembled scale-fashion. These are set up parallel or at angle between 1 deg. and 89 deg. to the direction of the sound, or to walls, ceiling or floor. Sound is deflected by reflection, and reduced by elongation of the sound path, interference and attenuation. The panels may be perforated to allow part of the sound waves to pass through, or may have special dimensions at a set ratio to the sound wavelengths. Sound-insulating material, or a combination of sound-insulating materials, may be used, or set up at an interval from the panels or other surfaces.

Parametric receiving array and the scattering of sound by sound

Abstract: The scattering of interaction‐frequency components from two sound waves interacting at a nonzero angle has been the subject of controversy for the last two decades. This paper attempts to provide better understanding of the problem by discussing the relationship found between the scattering of sound by sound and the parametric receiving array. First, the solution for a high‐frequency plane wave interacting with a low‐frequency plane wave over a half space is discussed. The high‐frequency sound wave is assumed to originate at a planar boundary and to propagate away from it. Then the interaction‐frequency components at some distance from this planar boundary are found. When this solution is related to a solution for a parametric receiving array with a low‐frequency plane signal wave and a high‐frequency narrowbeam pump wave, the two solutions are found to have very similar properties. Some experiments, as well as solutions, for various configurations of the parametric receiving array are discussed in light ...

Sound wave converter unit

Abstract: PURPOSE: In a sound wave conveter unit, to enable easy adjusting of the oscillator circuit and resonance room to specified frequency by making the sound source frequency to be adjustable. COPYRIGHT: (C)1977,JPO&Japio