Corinne M. Darvennes

Effects of absorption on the nonlinear interaction of sound beams

Nonlinearity in the propagation and interaction of sound beams in real fluids is considered, with special emphasis on the effects of absorption. Asymptotic formulas are derived for the sum and difference frequency sound generated by two harmonic sound beams. The amplitude and phase conditions of the sources are arbitrary, within the limits imposed by the parabolic approximation. A distinction is made between two main contributions to the nonlinearly generated sound in the far field, the continuously pumped sound and the scattered sound. The relative amplitudes of the pumped and scattered waves are shown to depend critically on the frequency dependence of the absorption coefficients. Numerical results are presented for the noncollinear interaction of Gaussian primary beams, and also for second harmonic generation in the field of a circular uniform piston source.

Boundary effect of a viscous fluid on a longitudinally vibrating bar: Theory and application

A theoretical model has been developed to represent the interaction of a bar in longitudinal vibration with its surrounding viscous fluid. This model was generated to better predict the length of timber piles used in bridge foundation. Experiments were performed with sand and with several water–glycerin mixtures surrounding an aluminum bar, showing a significant effect of the bounding material on the vibration characteristics of the bar. The viscous fluid was observed to lower the measured frequencies, indicating system mass loading, while the presence of the sand caused an upward shift in frequencies, indicating increased system stiffness. Hamilton’s principle was used to model the longitudinal bar motion with the fluid loading due to the viscous boundary, following the procedure of a known soil–bar interaction model. It was found that the fluid model gives an excellent prediction of the natural frequencies of the bar. The modal mass and damping were not predicted quite as accurately, but the fluid model...

Help! There are 60 screaming kids in my lab!: Outreach activities for 5th graders (L)

In this letter a day of hands-on activities for 60 5th graders is described in the Acoustics and Vibrations Laboratory at TTU. This includes the logistics of having 60 kids in the lab and keeping their attention, a description of each activity, and the equipment used. The purpose of this publication is to encourage my colleagues to reach out to the next generation of scientists and engineers, by showing them that a few well-targeted activities do not take a gargantuan effort but have a big impact.

Improving noise source location using the unwrapped phase method

Time Delay of Arrival (TDOA) is the key to using a microphone array for sound source location. The Unwrapped Phase (UP) Method has been used to estimate the TDOA, which is proportional to the slope of the phase of the cross‐power spectrum plotted versus frequency. However, reflecting surfaces cause ‘‘jumps’’ in the phase plot. A Zero‐Magnitude of Transfer Function (ZMTF) criterion is proposed to automatically pick up the ‘‘jumps’’ to better estimate the TDOA. To verify this, tests were performed in three rooms with different acoustical properties. The sound source consisted of a speaker broadcasting a computer‐generated broadband random signal. ZMTF works well in an ordinary room with small reverberation and yields accurate TDOA results. An average error of 0.2% is obtained. In a medium‐sized live room, ZMTF identifies ‘‘jumps,’’ and an average error of 1.6% is obtained. In a large reverberation room, however, ZMTF only marginally improved the UP method. The norm of residues and Delay Error were found hel...

Acoustics in the public school classroom

As mentioned in a DAGA 07 paper 1, physical science preparation in American schools often leaves something to be desired. In recent years the ASA Committee on Education in Acoustics has made an effort to participate in finding relief for that problem. Three approaches have been of some influence. 1. Teacher workshops; 2. Hands‐on student sessions. 3. Secondary school curriculum input. Teacher workshops have emphasized music as a vehicle to introduce science in the elementary classroom. Hands‐on student sessions have included about 20 acoustics experiments of varying degrees of sophistication for students both in high school physics classes and in elementary general science classes. Secondary curriculum input has included both, development of laboratory experiments in acoustics, and exposure to relatively low cost educational versions of computational software. Examples of teacher workshop content, a number of hands‐on experiments, and some finite element calculations will be discussed. 1Musik: Zugang zur ...

Help! There are 60 screaming kids in my lab!

What do you do when you have a large noise control facility available and you want to introduce local children to science and engineering? You invite them in! This presentation will describe a day of hands‐on activities for 60 fifth graders in the Acoustics and Vibrations Laboratory at TTU. This includes the logistics of having 60 kids in the lab and keeping their attention, a description of each activity, and the equipment used. Activities included learning about hearing and hearing loss, propagating and standing waves, ultrasound, vibrations, and resonance. Students used slinkies, sound level meters, a strobe light, Vernier LabPro data collection systems and sensors (ultrasonic motion detectors and microphones). They popped balloons in the reverberation chamber to listen to the sound decay. Their most memorable experience, however, was the screaming contest in the anechoic room!

Feasibility of using imperfect microphone arrays in noise source location

Microphone arrays are being used in many applications such as noise source location, sound intensity measurement, or acoustic holography. For the purpose of source location using the unwrapped phase method, it is important to use phase‐matched microphones. The paper studies the feasibility of replacing phase‐matched microphones with an array of unmatched microphones and of finding faulty microphones within the array. For this purpose, two sets of tests were performed. With the first set, the microphone characteristics were measured. An electronic circuit provided time delays, i.e., phase information, between the microphones, and narrow and wideband source signals supplied frequency responses for all the microphones. With the second set of tests, optimization techniques and neural network methods were used to locate a noise source with (1) a matched microphone array, (2) an unmatched array, and (3) an array containing faulty microphones. Comparison of the data from the various microphone arrays is presente...

Acoustic source location using a neural network

This paper presents an artificial neural network (ANN) approach to locate an acoustic source and determine its distance from a microphone. First, two sets of experiments were performed to locate a rattling bolt on a plate using the unwrapped phase method, by comparing the acceleration signal from the vibrating plate to the acoustic signal received by an array of four microphones. The first test used a wide frequency bandwidth random signal and the second test used a short frequency bandwidth signal. Then the neural network was trained using this experimental data. The effectiveness of the ANN at locating a source using four microphones was evaluated, in light of experimental problems associated with frequency resolution and effects of reflecting surfaces. The ANN was also used to locate the same source using only three microphones. The expectation is that fewer microphones will be needed if an ANN is used and still provide the same level of accuracy on source location. This would result in better accuracy...

Fatigue monitoring of a thin laminated composite using nonlinearity parameter and ultrasonic C‐scan

Previous research [X. Hou and C. Darvennes, J. Acoust. Soc. Am. 100, 2568(A) (1996)] has shown that fatigue cycles can affect the nonlinearity parameter of laminated composites. In this research a thin laminated composite plate is put under fatigue until it fails. The fatigue test is stopped at regular intervals, when a C‐scan picture is taken and the material nonlinearity is measured. To determine the nonlinearity parameter, the growth of a second harmonic is measured in the material. A monochromatic ultrasonic signal is sent into the sample via a contact transducer placed on the top surface. The growth of the second harmonic is recorded, with a second transducer placed on the bottom face, as the amplitude of the input signal is gradually increased. The nonlinearity parameter is then plotted as a function of the number of fatigue cycles. The C‐scan images provide a visual clue about changes in the material structure that can be correlated to changes in nonlinearity. Hopefully this method will provide a m...

Comparative measurement of second‐harmonic generation in various materials

Second‐harmonic generation was measured in several man‐made materials for possible application of nonlinear properties to nondestructive testing. Samples included several thicknesses of two types of polymer matrix composites, three types of concretes, and plywood. Steel and aluminum specimens were used as references and one of the composite samples was evaluated before and after fatigue cycles. A monochromatic ultrasonic signal was sent into each sample via a contact transducer placed on its top surface. The growth of the second harmonic was recorded, with a second transducer placed on the bottom face, as the amplitude of the input signal was gradually increased and for several values of the input frequency. Nonlinearity parameters could not be measured, due to the limitations of our equipment. Nonetheless, some interesting observations were made: (1) the two composites were much more nonlinear than the metals; (2) the concretes and the wood were extremely absorptive and an output signal was observed only at the lowest input frequency; and (3) fatigue cycles significantly increased the second harmonic, even though no damage was observed by C‐scanning. [Work supported by NSF, TTU Manufacturing Center, and the FRG Program.]