Showing papers on "Sound power published in 1971"
TL;DR: A theory of combustion noise is developed in this paper, which follows rigorously from the principles of fluid mechanics, and the sound radiated from open, turbulent flames is found to depend strongly upon the structure of such flames; at present their structure is not well known.
Abstract: Upon review of past experimental results and theoretical efforts it is apparent that the mechanism by which combustion noise is generated is not well understood. A theory of combustion noise is developed in this paper which follows rigorously from the principles of fluid mechanics. Lighthill's approach, used in his studies of aerodynamic noise, is closely followed in the present work. The sound radiated from open, turbulent flames is found to depend strongly upon the structure of such flames; at present their structure is not well known. However, meaningful bounds and scaling rules for the sound power output and spectral content are derived based upon the present limited knowledge. A framework is developed which explains past experimental work and the origin of combustion noise.
244 citations
TL;DR: In this paper, the effects of axial and swirling flow on the radiated sound power, for different types of source contained in the duct, were analyzed and suggested for analyzing the generation of sound by sources at a mean-flow discontinuity.
241 citations
TL;DR: In this article, a simple analytic expression is proposed for the calculation of the excess attenuation due to diffraction around a barrier of sound rays emanating from a point source; this analysis takes into account oblique sound incidence at the barrier and also includes the transition region from the bright zone to the shadow zone behind the barrier.
164 citations
TL;DR: In this article, the authors reviewed the physical mechanisms involved in each of these processes, including the generating of unsteady lift by turbulence, and compared the sound power estimates, where possible, with experiment results.
Abstract: Experiments and theory relating to fan noise sources are reviewed with emphasis on axial flow machines At supersonic rotor speeds, the steady shock pattern attached to a rotor is an efficient radiator of sound In most practical cases of subsonic rotor operation, however, direct radiation from the rotor‐locked pressure field is negligible compared with the indirect radiation, or scattering, caused by circumferential distortions in the steady flow field surrounding the rotor Random timewise modulation of the distortion changes the scattered spectrum from discrete to continuous, with a gradual progression from narrow‐band tones to broad‐band noise as the modulation bandwidth is increased Similar scattering occurs when a non‐uniform unsteady flow impinges on stator vanes, but here the radiated frequency is that of the impinging flow Finally, for blades operating in flows free from circumferential distortions, self‐generated turbulence becomes an important source of noise The paper describes the physical mechanisms involved in each of these processes, including the generating of unsteady lift by turbulence Order‐of‐magnitude sound power estimates are compared, where possible, with experiment
91 citations
TL;DR: In this article, the problem of sound generation due to free stream turbulence incident on a rotor or stator row is studied analytically, and a linearized analysis is carried out with the blade rows being modeled as a cascade of flat plates.
78 citations
TL;DR: In this paper, a theoretical study of sound radiation from a time-varying point force in accelerative motion is presented, where the acceleration arises from steady rotation in a circle.
68 citations
TL;DR: The point-to-point correlation of the sound pressures has received little attention except for an elegant paper by Cook et al. published in 1955 as mentioned in this paper, which showed that under certain conditions it can asymptotically approach Cook's formula.
37 citations
TL;DR: In this paper, two methods for generating sound at microwave frequencies with a Q-switched ruby laser were presented, based on the modulation of the laser due to its mode structure and on the nonlinear properties of the solid.
Abstract: Two methods are presented for generating sound at microwave frequencies with a Q‐switched ruby laser. They are based on the modulation of the laser due to its mode structure and on the nonlinear properties of the solid. Sound is created at the laser modulation frequency. (1) Electrostrictive mixing in a LiNbO3 crystal, using a forward Brillouin scattering geometry, produces sound pulses at 800 MHz inside the crystal. Acoustic power densities were around 0.1 mW/mm2. (2) We then used surface heating of a metal film deposited on the surface of a dielectric crystal. At microwave frequencies, good efficiencies can be achieved; at 840 MHz, we generated in a gold film nearly 10 mW/mm2 of sound in 30‐nsec pulses.
31 citations
25 citations
TL;DR: In this paper, it is shown that the rotating diffuser is effectively a parametric modulator which introduces new frequencies in the room and the remarkable performance of rotating diffusers is described in terms of their improvement in the statistical accuracy of the measured mean.
22 citations
TL;DR: In this article, a new concept for characterizing microphone response to wind flow fluctuation is presented, defined by the difference between the magnitude of the pressure fluctuation of turbulence and the output level of the microphone produced by the turbulencefluctuated pre...
Abstract: Investigations were made to find a physical concept for evaluating the pressure level of sound that exists in an airflow by means of a wind screen or probe microphone. In order to provide a sound pressure in the airflow, sound of a known pressure level, which is assumed to be of higher level than the aerodynamically generated noise, was radiated into a duct from an external sound source. The output level of the microphone was measured for various differences between the sound‐pressure level produced by the external sound source and the pressure fluctuation level of turbulence in the airflow. Results obtained agree with the superposed level of output by sound pressure and output produced by pressure fluctuation of turbulence. A new concept for characterizing microphone response to wind‐flow fluctuation is presented. This characteristic is defined by the difference between the magnitude of the pressure fluctuation of turbulence and the output level of the microphone produced by the turbulence‐fluctuated pre...
Patent•
26 Apr 1971TL;DR: In this article, a method and apparatus for reducing the audible or annoying sound emanating from a gas turbine engine is described, where the sound energy is modulated by imposing on it a higher frequency sound source, which may be of either higher or lower intensity.
Abstract: Method and apparatus for reducing the audible or annoying sound i.e., the noise emanating from a gas turbine engine are described. The sound energy is modulated by imposing on it a higher frequency sound source, which may be of either higher or lower intensity. In one form, the modulation is achieved by producing a higher frequency sound source with an ultrasonic siren. The higher frequency sound energy and the sound energy generated by the gas turbine engine interact so that the original tones disappear and two new tones appear, the new tones being at frequencies which occur at the sum and at the difference of the frequencies of the original tones.
TL;DR: In this paper, the authors used the moving-source approach used by Morfey and Tanna for broadband sound radiation from a point force in circular motion to evaluate the sound radiated in the far field due to point sources of random time variation rotating uniformly in a circle at subsonic speed.
TL;DR: In this article, a rotating sound diffuser was used to measure the radiation resistance of a monopole source, which was a symmetrical biconical surface of revolution rotating at speeds up to 30 rev/min.
TL;DR: In this paper, the sound radiated when inflow turbulence is present in axial flow fans has been investigated, and two noise radiating mechanisms can be identified: (i) interaction of turbulence with the rotor potential field results in a quadrupole-type volume source distribution, producing flow-interaction noise; (ii) impingement of turbulence on the blades results in dipole type (fluctuating force) surface source distribution.
TL;DR: In this paper, an expression relating the normalized impact sound level (Ln) and the sound transmission loss (TL) of a floor is derived, taking into account both the resonant and the forced response of the floor, thus extending the utility of the formulas derived by Heckl and Rathe into the frequency range below the critical frequency.
Abstract: An expression relating the normalized impact sound level (Ln) and the sound transmission loss (TL) of a floor is derived. It takes into account both the resonant and the forced response of the floor, thus extending the utility of the formulas derived previously by Heckl and Rathe into the frequency range below the critical frequency. This relation between normalized impact sound level and sound transmission loss permits an experimental evaluation of the potential TL of a floor partition in the presence of certain types of flanking.
TL;DR: In this article, a solution for the acoustic power extracted from the near potential field of a subsonic rotor, by interaction with a nonuniform inlet flow, was found.
Abstract: A solution has been found for the acoustic power extracted from the near potential field of a subsonic rotor, by interaction with a nonuniform inlet flow The analysis is based on Lighthill's aerodynamic sound equation, modified to allow for a uniform mean flow, and is restricted to low‐solidity rotors (lifting line approximation) Within this approximation, the power at blade‐passing frequency is compared with that radiated from unsteady blade forces arising from the same nonuniform inlet flow The comparison shows that more power may be radiated by the potential field interaction at high subsonic Math numbers, if the rotor disk loading is high
TL;DR: In this article, a rotating blade is replaced by a lineal progression of accelerating "torpedoes" representing the rotor tips, and the sound of lift and thickness effects is radiated strongly forward.
Abstract: This work shows theoretically that airfoils accelerating at a speed near to, but less than, the speed of sound can be powerful emitters of sound The sound generation is due to both lift and thickness effects The airfoil is replaced by volume velocity and body force distributions, and the solutions are obtained by standard methods A rotating blade is replaced by a lineal progression of accelerating “torpedoes” representing the rotor tips The sound of lift and thickness effects is radiated strongly forward Sound power calculations made for thickness radiation alone for a series of airfoil shapes show some reduction of radiated sound with tip shape, and a promising correlation with experimental results
Abstract: The propagation of random sound in a cylindrical duct has been treated theoretically. The conditions under which the simple wave equation applies to the transmission of sound generated by industrial air‐moving devices is made explicit. The acoustic forcing function and termination conditions are discussed extensively as well as the structure of the sound field in the duct. Equations for the power spectral density and pressure cross‐spectral density are derived in order to define the problems associated with deducing sound power, given sound pressure at two points. The results suggest that an anechoic termination is almost mandatory and that the modal spectra must be known, if accurate measurements are to be made. A number of simplifications to the theory were tested experimentally and found to apply. Modal spectra were measured and were used to compute sound power and predict pressure measurements. The predicted and observed were found to be within 1 dB, suggesting that a practical means for measuring duc...
Dissertation•
01 Jan 1971
TL;DR: Thesis (M.S.) as mentioned in this paper, Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1971, was the first paper to address the problem of self-confidence in human decision-making.
Abstract: Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1971.
TL;DR: In this article, the theoretical values of mechanical displacement, potential, and velocity of surface waves propagating in the [001] direction on [110] cut Bi12GeO20 are reported.
Abstract: Theoretical values of mechanical displacement, potential, and velocity of surface waves propagating in the [001] direction on [110]‐cut Bi12GeO20 are reported. The results of an experimental investigation performed at 50 MHz using interdigital transducers and optical probing are presented. The velocity measured is in excellent agreement with the theoretical value and the attenuation is very low, 0. 1 dB/μsec being an estimated upper limit. Second‐harmonic generation which was measured by optical probing had an intensity proportional to the square of both the interaction length and the fundamental acoustic power, indicating nonlinear effects in the propagating medium at an acoustic input power of 28 dBm.
TL;DR: In this article, the ASHRAE standard test procedure for measuring the sound power output of mechanical equipment in reverberation room is described and discussed, with particular attention given to the qualification test required of a facility for the measurement of pure tones.
TL;DR: In this paper, it was shown that the field distribution due to a point source of sound within an absorbent-lined enclosure can be calculated using a geometrical ray treatment and the appropriate plane-wave pressure-reflection coefficient.
TL;DR: In this paper, the authors show that the space-averaged value of W, i.e., the average for all source positions in the enclosure, does not equal the free-field value W0, but depends on the modal damping.
Abstract: When a point source radiates steadily in a rectangular enclosure and excites one mode only, its power output W depends strongly on its position. Distribution functions for W are found when the source is positioned randomly, and an axial, tangential, or oblique mode is excited. The functions for these three cases form a consistent family of curves, whose shapes are controlled by the mode shapes. The space‐averaged value of W, i.e., the average for all source positions in the enclosure, does not equal the free‐field value W0, but depends on the modal damping. The value of W0 can be derived from sound‐pressure measurements, however. The results are derived theoretically, and checked by a Monte Carlo procedure. Some experimental data which confirm the theoretical results are given for a small source driving an axial mode in a reverberation chamber. The work has application to standard methods of measuring the sound power output of a source.
TL;DR: A critical evaluation of the state of the art relating to the measurement and characterisation of the acoustic performance of fluid system components, in particular of pumps and acoustic filters for liquid piping systems leads to the conclusion that the difficulties therein have only begun to be recognised, and are far from solved.
TL;DR: In this paper, the authors show that sound pressure in water under a point source overhead in air propagates as from a virtual source situated (c1/c2) times the actual source height above the water and emitting unit-distance sound pressure that is (2c 1/c 2 ), where c1 and c2 are the speeds of sound in air and water, respectively.
Abstract: Sound pressure in the water under a point source overhead in air is shown by theory and experiment to propagate as from a virtual source situated (c1/c2) times the actual source height above the water and emitting unit‐distance sound pressure that is (2c1/c2) times the actual unit‐distance sound pressure, where c1 and c2 are the speeds of sound in air and water, respectively.
TL;DR: A critical evaluation of the state of the art relating to the measurement and characterisation of the acoustic performance of fluid system components, in particular of pumps and acoustic filters for liquid piping systems, is presented in this paper.
Patent•
04 Aug 1971
TL;DR: In this paper, a ramp-voltage generator is triggered at the acoustic pulse transmission instant and a gate is opened by the corresponding sea bottom echo to sample the ramp voltage to reduce disturbing effects of reverberation when an echo-sounder is used in shallow water.
Abstract: 1,241,663. Echo-sounding. MARCONI SOUNDING DEVICE CO. Ltd. 13 Nov., 1968 [30 Jan., 1968], No. 4743/68. Heading H4D. To reduce disturbing effects of reverberation when an echo-sounder is used in shallow water the transmitted acoustic power is controlled automatically in dependence on the water depth so that the power is lower for shallow soundings than for deep soundings. In a preferred embodiment a ramp-voltage generator is triggered at the acoustic pulse transmission instant and a gate is opened by the corresponding sea bottom echo to sample the ramp voltage. The sample voltage is applied via an amplifying and integrating stage to control the gain of a power amplifier which provides drive pulses for a transmitting/ receiving transducer. Echo signals are fed to a chart recorder which may also be used to trigger the ramp-voltage generator. The transmitted power may be controlled in dependence on the strength of received bottom echoes instead of on their occurrence time, and the power variation may be stepped instead of continuous.
TL;DR: The results of recent fundamental work, both theoretical and experimental, give clear indications of the accuracy that can be achieved in these measurements of equipment sound power in reverberation rooms.