David K. Holger

Experiments on the Fluid Mechanics of Whistling

Experiments to investigate the fluid mechanics of whistling are reported. A model, consisting of a cylindrical cavity with rounded holes at each end, is used to simulate human whistling. It is found that the frequency is very near the Helmholtz resonator frequency, and that the resonator can be excited by flow through the smooth‐edged orifices bounding the resonant cavity. Furthermore, it is found that the flow velocity of the jet which excites the resonator must lie between limits that are proportional to frequency and that increase with both diameter and thickness of the orifice. It is concluded that whistling can be included in the same class of sound sources as the Rayleigh bird call and the Pfeifentone, since the essential mechanism for exciting them depends on the instability of a jet to the formation of vortex rings and the interaction of the rings with a rigid boundary in the flow.

Fluid mechanics of the edgetone

A model is proposed to explain the means by which an edgetone transforms the energy of a fully developed plane jet into energy which is radiated as sound. The edgetone configuration considered consists of a flat plate located in the medial plane of a fully developed two‐dimensional jet. The flow is modeled as follows. A periodic disturbance at the jet origin leads to the formation of an asymmetric vortex street which propagates downstream with a fixed convection velocity and wavelength. The vortex strength, convection velocity, and wavelength are determined as functions of the Strouhal number by applying conservation laws and kinematic relationships. A potential flow analysis of the interaction of the vortices with the edge is used to estimate the nearfield oscillating flow at the jet exit which, in turn, is used to calculate the phase of the feedback mechanism. The phase then determines the operating frequency as a function of jet velocity and edge stand‐off distance. It is shown that the proposed model is capable of predicting the major observed features of edgetone operation. The frequency predictions of the theory are compared with experiments for a wide range of jet parameters in both air and water. The comparison indicates that the frequency predictions are as good or better than previous empirical or semiempirical formulas.

The amplitude of edgetone sound

A model is proposed to predict the amplitude of sound generated by an edgetone. The edgetone configuration under consideration consists of a flat plate located in the medial plane of a fully developed two‐dimensional jet. The flow is modeled as follows. A periodic disturbance at the jet origin leads to the formation of an asymmetric vortex street which propagates downstream with a fixed convection velocity and wavelength. The vortex strength, convection velocity, and wavelength are determined as functions of the Strouhal number by applying conservation laws and kinematic relationships. The farfield acoustic pressure is calculated from a potential flow estimate of the periodic force on the edge. Predicted sound pressures are compared with experimental results and found to be in reasonable agreement.

The Inertance of a Smooth‐Edged Orifice

Three analytical models for the potential flow through a smooth‐edged orifice are discussed and the prediction of the acoustical inertance obtained from these models is compared with experimentally determined values of orifice inertance as a function of the ratio of wall thickness to orifice diameter. It is shown that the simple hyperbolic wall model predicts corrections for wall thickness which are of the wrong order of magnitude for thin‐walled orifices and are inaccurate for thick‐walled orifices. An improved model, consisting of the super‐position of a hyperbolic flow field and a ring vortex, is used to represent an orifice boundary with finite curvature at the throat and finite thickness. By matching either the curvature or the wall thickness of an orifice, upper and lower bounds for the inertance are obtained. By adding another singularity in the potential flow analysis, both the curvature at the throat and the wall thickness of the actual orifice can be matched, and the values predicted by this mod...

North Central Collaboration for Education in NDE/NDT

The North Central Collaboration for Education in NDE/NDT began efforts to improve NDE education in October 1996. The program is funded for three years with a grant from the National Science Foundation. The program is aimed at enhancing education in nondestructive evaluation and improving articulation between community college technician training programs and university technical degree programs. The main participants in the collaboration are: Cowley County Community College in Arkansas City, Kansas; Northeast Iowa Community College in Peosta, Iowa; Ridgewater College in Hutchinson, Minnesota; Southeast Community College in Milford, Nebraska; and Iowa State University in Ames, Iowa. The effort is also supported by industry, a professional society, and regional high school partners via an advisory board. A model of operation is presented in Figure 1.

Determination of effective secondary source locations for active noise control

A numerical method for determining effective secondary source locations for active control of interior sound fields has been investigated. The method uses intermediate results from an indirect boundary element simulation of a sound field to determine effective boundary locations for secondary sources. In the indirect boundary element method (IBEM), an interior sound field is simulated by replacing the physical boundaries with a fictitious source distribution that is determined from the geometry, the properties of the physical boundaries, and the primary source location(s). Locations of high fictitious source strength, as determined by the IBEM, are found to be particularly effective locations for secondary sources that are components in three dimensional active noise control systems. Numerical results for simple geometries are in agreement with previous experimental results [Elliott et al., J. Sound Vib. 117, 35–58 (1987)], and numerical predictions of active noise control using the proposed method for lo...

Vibrations of damaged trapezoidal plates

The modal frequencies and shapes of damaged edge‐clamped trapezoidal plates have been investigated for the first five transverse modes. Damage considered consisted of narrow, rectangular, centered slots. An impulse hammer was used to excite transverse plate vibration, and plate response was measured with a nearfield microphone. The frequency response of the plate was investigated for all combinations of three slot lengths and three slot orientations. The first five modal frequencies and shapes were estimated from ensemble‐averaged, frequency‐response functions. Observed changes in modal frequencies for the damaged plates were in good agreement with predictions of a finite element model. The results suggest that slot presence can be detected from changes in plate modal frequencies, that slot length and orientation have significant effect on the modal frequencies, that a nearfield microphone is an adequate response transducer, and that an appropriate finite element model and the frequency response measureme...

Graduate education in acoustics at Iowa State University

Research and graduate education in a number of areas of acoustics and noise control are carried out in the Department of Engineering Science and Mechanics at Iowa State University. Graduate level courses in theoretical acoustics, signal processing, ultrasonic nondestructive measurements, and mechanical vibrations are offered by the department. Facilities for graduate instruction and research include an anechoic chamber with computer‐controlled microphone scanning system, three two‐channel FIT analyzers, an ultrasonic nondestructive testing laboratory in the Non‐Destructive Evaluation Center, and several thousand square feet of laboratory space. Eight faculty members and more than 20 graduate students are conducting research in spatial transformation of acoustic fields, boundary integral equation methods in acoustics, acoustic intensity methods, ultrasonic transducer characterization, inverse problems in acoustic scattering, and ultrasonic nondestructive evaluation. Descriptions of some typical graduate st...

Acoustic wave propagation in a radial duct

The results of an experimental investigation of acoustic wave propagation through a radial duct are reported. The investigation was limited to large wavenumbers in the absence of flow, and experimental results are compared to a correspondng analytical model. The apparatus consisted of an anechoic termination, one rigid duct wall, and one variable impedance duct wall. Preliminary experiments revealed that the measurement techniques were appropriate and that the apparatus was behaving as expected. Experimental plots of pressure versus radial position were found to be in agreement with analytical predictions for cases involving two rigid duct walls. A variable backing depth Helmholtz resonator array was substituted for one of the rigid duct walls, and measurements of pressure versus radial position were made for three backing depths. Values of radial attenuation were estimated from the pressure measurements using a large wavenumber approximation. Experimental radial attenuation estimates are in good qualitat...

A comparison of the noise environment in open‐plan and traditional single family dwellings

The noise environment within two single family dwellings was investigated using a sound power source and sound level meter. The dwellings have three levels and are roughly the same size. One of the dwellings is an open‐plan design, both within and between levels, and the other is a traditional multilevel design. Both dwellings are representative of common residential construction and neither incorporate special noise control features. Octave band pressure levels from 125 Hz to 8 kHz were measured at eight locations in each dwelling for each of three source locations. Average NC values in living areas of the traditional dwelling were found to be 7 dB less than in the open‐plan dwelling. In bedroom areas, the average NC values were more than 20 dB lower in the conventional dwelling. These results are in good agreement with those of a previous study [D. K. Holger and M. S. Pickett, J. Acoust. Soc. Am. Suppl. 1 72, S91 (1982)] which concluded that boundary absorption is very important in open‐plan residential...