摘要:

Fans used in the cooling of mechanical and electrical equipment are often the major sources of noise. This paper reviews the past and current noise control activities in the fan industry and related trade and professional organizations. Technology trends are discussed. Research needs and priorities are identified. The impact of fan design (blade, housing, and motor) and fan manufacturing (materials and processes) on fan noise and system acoustical quality are discussed. This paper illustrates with examples that fan noise control is more a challenge for design, application, and manufacturing than an investigation of fundamental aeroacoustic mechanisms. The approach is multi‐disciplinary and comprehensive in its treatment of each integration level—from bearings and windings to motor, housing, and end‐use equipment, thereby integrating noise control into fan design and manufacturing to achieve total quality for tomorrow's market.

ISSN:0001-4966    

DOI:10.1121/1.2026857

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Measurement of fan noise requires extremely careful control of a multiple of variables—including fan operating point and test system configuration—to obtain useful results. Frequently, fan noise measurements are contaminated by unrecognized fan or test system changes and spurious noise sources. A new fan test system was designed to eliminate extraneous noise sources and to provide good control of the fan and the test system. This system uses round sheet metal ducts to connect the test fan to inlet and discharge anechoic terminations. Rotating microphone probes, one in the inlet and one in the discharge, are used to measure induct SPL while the fan's air performance is being determined. Two test systems, one with a mean diameters of 8 in. and the other of 16 in., have been built. Measurements have been made on both axial and centrifugal fans of sizes from 6 to 20 in. Representative results of these measurements and data from recent studies of microphone probe design and surface microphone systems use for in‐duct noise measurements will be discussed. Data will also be presented on the influence of tip clearance on axial fan noise.

ISSN:0001-4966    

DOI:10.1121/1.2026901

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Cylindrical shells, such as industrial piping system components, are efficiently excited by broadband internal noise at discrete frequencies below the ring frequency. Two types of excitation are recognized: finite length pipe resonances that are visible in short pipes, and coincident excitation that has been studied for long or anechoically terminated pipes. Both mechanisms occur to varying degrees in pipes of any length, and both require that the acoustic and structural wavenumbers be closely (or exactly) matched. Because ducts possess an infinite number of potential coincidence frequencies, coincidence transmission (i.e., precisely matched wavenumbers) is dominant in pipes. In the limit for long shells, both mechanisms are damping controlled and approach the same levels. Experimental data for short and intermediate shells show that coincidence transmission (“infinite shell”) can be the sole cause of vibration over wide frequency bands where the density of resonant modes is low. A simple theory is used to predict the frequency and amplitude of response peaks caused by both mechanisms. [Work supported by NSF.]

ISSN:0001-4966    

DOI:10.1121/1.2026954

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Periodic wavetrains propagating in fluids whose specific heats are large compared to the molal gas constant are examined. In the dense gas regime, the fundamental nonlinearity parameter 1 +B/2Aof these fluids may become negative. The present study examines the nonclassical properties of the evolution of a sinusoidal wavetrain including the formation and propagation of expansion shocks and sonic shocks. Analytical solutions are presented for the inviscid problem and are compared to numerical solutions describing the dissipative evolution.

ISSN:0001-4966    

DOI:10.1121/1.2027001

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

An experimental apparatus has been developed to determine thresholds for cavitation produced in a fluid by short tone bursts of ultrasound at 0.76, 0.99, and 2.30 MHz [A. Atchleyet al., Ultrasonics26, 280–285 (1988)]. A fluid jet was used to convect potential cavitation nuclei, such as 1‐μm polystyrene spheres, echo contrast spheres, and whole blood constituents, through the focal region of the insonifying transducer. Cavitation thresholds measured with this system in water and in a fluid with ten times the viscosity of water will be presented. Cavitation was detected by a passive acoustical technique that is sensitive to sound scattered from cavitation bubbles. Results from these experiments that permit the control of nuclei and host fluid properties will be compared to an approximate theory that predicts the onset of cavitation [C. K. Holland and R. E. Apfel, Trans. IEEEUFFC‐36(2) (1989)]. [Work supported by NIH, grant number 1RO1‐CA‐39374.]

ISSN:0001-4966    

DOI:10.1121/1.2027062

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The behavior of emulsions composed of immiscible liquids has been the object of many theoretical and experimental studies [for review, see Chenet al., AIChE J.30, 622–630 (1984)]. The coalescence of a drop separated by a film from its homophase has been studied to explain the stability of emulsions; there have not been experiments, however, on free drops contacting other free drops over long periods of time due to the difficulty encountered in containing the drops. Acoustical radiation pressure provides a means of controlling free fluid drops and was considered for application to the study of coalescence. Hexane drops were trapped by a 45‐kHz standing wave in a resonant water column. The time two drops remained in contact before coalescing was measured for both pure and surfactant stabilized systems. This time was then compared to theoretical predictions and nonacoustical coalescence experiments. [Work supported by the Heyl Foundation.]

ISSN:0001-4966    

DOI:10.1121/1.2027105

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

In the interaction of the quasistatic electric field with deformable insulators, the condition of rotational invariance causes a combination of the electric field and the deformation gradients to occur in the constitutive equations along with the finite strain. Since the resulting system is intrinsically nonlinear, the linear dynamic equations are more general than those of linear piezoelectricity when a bias is present and reduce to them only in the absence of a bias. Even in the simplest case of stress‐free thermal deformation, which is just about always present, the more general equations arise when the fixed reference coordinates at the reference temperature are employed. The advantage of the use of reference coordinates, which cannot be employed within the usual linear theory, in the accurate calculation of the temperature sensitivity of high precision contoured quartz resonators is shown. In the treatment, the equation for the perturbation in eigenfrequency of the piezoelectric solution due to a bias, which is obtained from the more general linear equations, is employed. However, both the biasing state and the vibrational solution are obtained by solving systems of unbiased linear equations. The change in frequency resulting from any bias may readily be calculated from the perturbation equation when the linear piezoelectric solution and biasing state are known. The importance of the phenomenon of energy trapping in crystal resonators is discussed and means of controlling it are noted.

ISSN:0001-4966    

DOI:10.1121/1.2027155

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

This paper deals with the theory of dielectric waveguides in a graded‐index media. The theory is first developed in terms of electrical and magnetic potentials and a gauge condition which is a generalization of the Lorentz gauge used in classical electromagnetic theory for the isotropic, homogeneous medium. The existence of these potentials under the generalized Lorentz gauge in the context of a generalized Helmholtz theorem are then proven. Even though this generalized gauge uncouples the governing equations, it does not lead to partial differential equations in thenormalform. To alleviate this problem, superpotentials of the Hertzian variety are introduced and it is shown how equations governing the vector superpotential can be reduced to a form which is amenable to standard analysis. Then several uniqueness theorems associated with this superpotential are proven. As an illustration of the use of the superpotential and its relative merits, it is applied to certain waveguide problems in an inhomogeneous medium with variable permittivity.

ISSN:0001-4966    

DOI:10.1121/1.2027199

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The top and back of a violin are among the most important components of the instrument, both acoustically and structurally. Centuries ago luthiers identified spruce as the ideal wood of the top and maple as the nearly unanimous choice for the back. This situation holds true today. Why is this so? The answers rests in the mechanical properties of these woods, particularly the properties that characterize the behavior of the top and back in flexural vibration. Five fundamental mechanical properties that hold most of the keys to the acoustical and structural qualities of spruce and maple are (1) density, (2) Young's modulus and (3) damping of waves traveling along the grain, and (4) Young's modulus and (5) damping of waves traveling across the grain. Shear distortion also affects the behavior of the vibrating spruce top and maple back even at low frequencies well within the audio range. Wood selection criteria and the influence of mechanical properties on the quality of the instrument will be discussed.

ISSN:0001-4966    

DOI:10.1121/1.2027251

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Sound intensity measurements can now be used to determine accurately thein situsound power of machinery noise sources in the presence of high levels of background noise. Meetings of working group ISO/TC43/SC1/WG25 began in 1982 and a first draft standard was produced in late 1985. Meetings of working group ANSI S12‐21 began in 1983 and produced first draft of a standard in 1984. The ANSI draft standard was circulated for balloting as a standard in 1987, and the ISO third draft was circulated in 1988. Although the ISO and ANSI drafts are similar, there are some differences. The ISO draft allows precision, engineering, and survey grade determinations and requires four indicators to be measured to decide the grade achieved. The ANSI draft only allows engineering determinations of sound power. The ISO draft only allows fixed point measurements, while the ANSI draft allows either fixed point or scanning measurements of intensity on a surface enclosing the source. Last year, 17 laboratories in North America took part in a round robin using the ANSI draft.

ISSN:0001-4966    

DOI:10.1121/1.2026637

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP