J. Adin Mann

Instantaneous and time‐averaged energy transfer in acoustic fields

The fundamentals of energy transfer in an acoustic field are addressed and it is shown that describing the flux of energy in an acoustic field with the active intensity alone is inaccurate. A single active intensity vector describes only the time‐average energy flux at a point in space, but not where the energy came from nor where it is going. Consequently, the instantaneous intensity must be used to properly describe energy flux as a time‐dependent process. The phenomenon of the acoustic vortex is examined and, from the perspective of active intensity, it is seen to represent a resultant wave rotating around a zero pressure line or point at which the pressure phase is discontinuous. It is shown that this resultant wave travels with a phase speed cp, which is generally different than the plane‐wave phase speed c. The instantaneous intensity, however, shows that energy is flowing through the vortex and not with the resultant waves. Although the complex intensity vector is normally separated into the active...

Propagation Characteristics of Capillary Ripples. I. The Theory of Velocity Dispersion and Amplitude Attenuation of Plane Capillary Waves on Viscoelastic Films

The boundary value problem for velocity of propagation and attenuation of capillary ripples over viscoelastic surface films is set up and solved from the viewpoint of continuum hydrodynamics. Both soluble and insoluble films are treated. Successive approximation methods are used to derive explicit formulas for velocity of propagation and amplitude attenuation; formulas for these quantities, accurate to second order, are given for slightly viscoelastic and highly viscoelastic films, and formulas, accurate to first order, are given for films of intermediate viscoelasticity.

Propagation characteristics of capillary ripples. II. Instrumentation for measurement of ripple velocity and amplitude

Abstract An instrument for measuring the propagation velocity of capillary ripples to within 0.3% and for measuring the amplitude of such ripples to within 1% to 10% (depending upon amplitude) was designed and constructed. The principal components of the instrument were a Langmuir trough of Teflon (with provision for automatic recording of compression data); generating and receiving probes consisting of a thin rod and a razor blade respectively mounted so that the rod and razor edge were parallel and accurately placed in the surface to be studied. The generator probe was driven sinusoidally by a highly modified audio “tweeter” speaker and the receiving probe was coupled to a crystal phonograph cartridge mounted on a micromanipulator. The standing wave pattern generated between the probes caused a sinusoidal output from the crystal cartridge which was amplified; the signal amplitude was proportional to the amplitude of the ripples, and the phases of generator and receiver oscillations could be compared. Propagation velocity and damping coefficient dispersion could be readily calculated from this information collected as a function of frequency.

Measuring the structural intensity and force distribution in plates

Identifying the sources of vibration in a structure and the energy exchange between structure elements is an important issue in vibration and noise control. Several formulations for the structural intensity have been presented in the past, but little is said about implementing the formulations using measured data. In this paper, two structural intensity formulas based on different assumptions are compared, and practical issues of implementing the calculations of the structural intensity using k‐space analysis are discussed. A force distribution, calculated from Mindlin’s plate motion equations, is shown to be an additional tool to locate vibration sources. Since the calculation of both the structural intensity and the force distribution require the use of fast Fourier transforms (FFT), numerical difficulties arise that are solved by signal processing techniques such as windowing and filtering. The influences of the windowing and the filtering on the calculation of the structural intensity and the force di...

Propagation characteristics of capillary ripples, III. Capillary ripple velocity and attenuation dispersion on clean water surfaces and on various monolayers

Abstract The theory and instrumentation previously described by us were applied to the systems monolayer-free distilled water; stearic acid, mixed hexadecanol and octadecanol, and several long-chain fatty ester monolayers; stearic acid-polyvinyl acetate mixed films and egg albumen spread films; heptanoic acid adsorbed monolayers. Experimentally determined velocities of propagation and amplitude attenuations agreed with theory to within 1% and 7%, respectively, with distilled water and the insoluble monolayers (using limiting formulas for highly viscoelastic films). Data for heptanoic acid soluble films were considerably less reproducible, with frequency dependence of velocity of propagation appearing to deviate somewhat from theoretical expectation.

Examples of using structural intensity and the force distribution to study vibrating plates

The influence of attachments and substructure on the vibration of a plate is a very important problem in noise and vibration control. This paper presents several examples of using an energy viewpoint with experimental data. The normal velocity of each plate, measured with a laser vibrometer or in some cases obtained from analytic calculation, is then used to calculate the structural intensity and force distribution. The structural intensity and force distribution are then used to locate single and double forces applied to a plate with some limitations. In addition, a rib attached to a plate and a constrained layer damping patch attached to a plate is studied. The energy absorbed by the rib and the force exerted by the rib in response to the waves incident on the rib are discussed. Results also show that a constrained layer damping patch is more effective in absorbing shear and bending vibrational energy than in absorbing twisting energy and that the damping patch does not apply a normal reaction force to ...

Wave‐number domain separation of the incident and scattered sound field in Cartesian and cylindrical coordinates

Two‐surface Cartesian coordinate system and cylindrical coordinate system measurement techniques are applied to obtain the scattered sound fields of a general shape. This decomposition method is based on the principle that any waveform can be decomposed using a two‐dimensional spatial Fourier transform into wave components that propagate in a known manner. The Cartesian method was developed by Tamara [J. Acoust. Soc. Am. 88, 2259–2264 (1990)] to measure the reflection coefficients of waves incident at oblique angles onto planar surfaces, so our paper focuses on the optimization of this method when applied to scattering investigations. A two‐surface technique to separate the incident and scattered field is also developed in cylindrical coordinates. The separation process is carried out in the wave‐number domain in a manner similar to what has been developed for Cartesian coordinates. However, because the incident and scattered fields are not directly separable in cylindrical coordinates, knowing general pr...

Placing small constrained layer damping patches on a plate to attain global or local velocity changes

Experimental results show how structural intensity can predict where to locate small constrained layer damping patches to attain either local or global velocity changes on a plate. If damping is applied to a region of low shearing reactive structural intensity magnitude, a local velocity change is seen. If damping is applied to a region of high shearing reactive structural intensity magnitude, a global velocity change is seen. In addition, the researchers noted that large damping patches produce global velocity changes. The filtering process needed to calculate the structural intensity was partially automated to reduce computational time.

Acoustic intensity analysis: Distinguishing energy propagation and wave‐front propagation

Three arguments are presented to demonstrate the distinction between energy propagation and wave‐front propagation in an acoustic field where the reactive intensity is nonzero. This distinction is especially useful for understanding the acoustic near field. Physical interpretations of the active and reactive intensity vectors and their contributions to the energy propagation described by the instantaneous intensity are summarized. Also, two differential equations are developed that show the interdependence of the active and reactive intensity vectors. Other important results show the contribution of the reactive intensity to the wave fronts propagating at speeds other than the speed of sound, and also show that energy associated with the reactive intensity does propagate to the far field.

Adaptive filtering of sound pressure signals for monitoring machinery in noisy environments

Abstract An experimental evaluation of the ability of sound pressure microphones to diagnose different machinery conditions in noisy environments was performed. An adaptive filtering (ANC) routine was incorporated to reduce the noise. The detection process utilized frequency spectra of the data, along with cepstrum and kurtosis methods of analysis. Two different machine components were monitored: ball bearings in a ball bearings test stand and milling bits in a milling machine. The effect of the placement of the microphones on the ANC routine to reduce the background noise in the signal was investigated and found to influence the results. The results show that the sound pressure microphones could not reliably diagnose ball bearing condition but could diagnose the milling machine bit condition.