Measurement Of Mechanical Impedance Research Articles

Experimental Evaluation of Moving Sound Diffusers for Reverberation Rooms

The acoustic power flowing from a monopole source is proportional to the radiation resistance presented to the source. This value is determined by the modal structure and the acoustic impedance of the walls of the chamber. The rotating sound diffuser modulates the modal structure and with it, the radiation resistance. Comparison of the time average of the radiation resistance as the monopole source location is changed with and without the diffuser rotating indicates the effectiveness of a given diffuser design in removing the dependence of acoustic power output on the source position. The rotating sound diffuser evaluated was a symmetrical biconical surface of revolution rotating at speeds up to 30 rpm. Fifty percent of the surface area of the diffuser was covered with ribbed fiberglass, the remainder was left uncovered. In order to measure the radiation resistance an acoustic impedance head was fabricated from a modified loud speaker. Displacement of the diaphragm was sensed using a noncontacting fiber-optic sensor. The resulting pressure generated in front of the diaphragm was measured by a condenser microphone. Instrumentation normally used for the measurement of complex mechanical impedance was adapted to record the real part of the radiation impedance.

The nature of the mechanical impedance to seedlings by soil surface seals

The nature of seedling emergence through soil surface seals under a variety of field and model situations is described. In this process the importance of the shear and compressive strengths of the seal has been demonstrated. Resistance to emergence has been shown to be also due to gravity, soil cohesion, and friction between structural parts. The mechanics of seedling emergence is too variable and complex to be expressed by a single index, such as the tensile strength of free samples, as in the modulus of rupture determinations of Richards (1953). Direct measurement of mechanical impedance is preferable to refining the use of such indices, and provides a basis for a better understanding of the relation between seedlings and soil properties influencing emergence.

Effect of Contact Area on Mechanical-Impedance Measurements

The weighted average normal mechanical impedance of a circular area on an elastic half-space does not depend markedly on the assumed pressure distribution. Its absolute value is almost independent of frequency at least until the wavelength m equal to the circumference of the circular contact area, but the loss tangent varies almost linearly with frequency up to a value of about 1 at this cutoff. These conclusions are valid only for structures large as compared with a wavelength.

Synthesis of Reactive Mechanical Circuits for Impact Loading

High-speed operation of modern machines has emphasized the effects of high loading rates and impacts. With wide-frequency-band excitation, it becomes necessary to consider an extended range of structural resonances to determine dynamic response and to improve the effectiveness of structural damping. In this paper, synthesis techniques are applied to the design of simple undamped (i.e., reactive) structural elements. Synthesis of mechanical circuits is complicated by the characteristically large numbers of degrees of freedom, and by difficulties of response measurement over a wide frequency range. However, Cauer's method of ladder network synthesis is readily applied for more than 20 alternating zeros and poles arrayed in a wide variety of frequency patterns. Of particular interest for dissipative purposes is the equal frequency spacing of zeros and poles for the reactive structure. Examples are given of circuit synthesis for this zero-pole spacing in the derivation of dimensions for simple circuits, including 15 degree-of-freedom ladders, cantilever beams, and circular disks. Computed circuit characteristics are then verified by mechanical driving-point impedance measurements over a frequency range extending to 50 kc/sec.

Measurement of Mechanical Impedance in Arm

Measurement of Mechanical Impedance in Arm

Performance Characteristics of a Wide-Range Mechanical Impedance Pickup

In recent years there has been increased interest in measuring the mechanical impedance at a point on a structure. This paper describes the results of tests conducted during the calibration and evaluation of a mechanical impedance pickup of new design. The pickup weighs 12 lb and includes three force gauge elements and three accelerometers. The calibration results indicate the impedance pickup is suitable for use in the frequency range from ten to several thousand cycles per second. The effective end-mass of the pickup is less than two-tenths pounds and its stiffness greatly exceeds 10 million lb/in. These and other important characteristics indicate the pickup may be used in a wide range of applications requiring mechanical impedance measurements. The test results were obtained on sinusoidal calibration equipment and on a new type shock motion calibrator.

Noise Sources and Transmission Paths in Refrigerating Compressors

Quieting reciprocating compressors for refrigeration involves all the classical techniques and devices used in high-speed electromechanical machinery—vibration isolation, balancing, mufflers, flexible connectors, etc.—and an important number of special techniques, involving thermodynamics with temperature changes of 200 to 300°F, pressure changes of the order 20 times atmosphere, high flow velocities, and media where only metallic components with very little inherent damping can be used. The working medium must be carried in pipes under pressure and the flow is modulated by complex pulses due to the high-speed pumping and fast valves, which act like fast switches in a transmission line, generating noise. A study of acoustic transients (pulses) in pipes has been made on an analog scale model (linear acoustical pressures up to 100 dyne/cm2 114 db), then on an actual compressor (high nonlinear pressures up to 107 dyne/cm2) with different acoustical components (inductance, capacitance, resistance) in the line, and different input and termination impedances of the line. The influence of these components on pressure pulsation and outside noise, and how the analog helps to identify noise sources, which have same frequency and amplitude noise output (rpm, intake, exhaust) will be described along with structural noise transmission, through flexible “corrugated metallic hose” under different pressures used in refrigerating compressors. Mechanical impedance measurements carried out on compressor shells to find points with high mechanical impedance for optimum locations of exhaust lines will be shown, with vibration modes and resonant amplification of a typical compressor shell.

Measurement of Mechanical Impedance and Its Applications

The impedance concept for mechanical elements is briefly developed through the analogy to electric circuits. This concept then makes the whole field of circuit analysis with its highly developed set of rules, theorems, and mathematics available for the solution of mechanical problems. Many devices are available for the application of forces and the measurement of motion and force. Some of these are combined in several devices designed specifically to measure the driving-point impedance of soils. An analog computer is presented that takes force and velocity information from the impedance devices, computes their complex ratio and plots impedance as the measurement is being made. Examples of the impedance of soils, sponge rubber, and a simple beam are presented. Impedance information is used to predict the motion of the bar to a sinusoidal driving force and Norton's equivalent circuit is used to predict the motion of a mass attached to the bar. Finally, the transient response of a mass dropped on sponge rubber is predicted from the equivalent circuit as determined from impedance measurements. Each of these cases is verified experimentally.

Experimental Measurement of Mechanical Impedance or Mobility

Abstract A device for the experimental measurement of mechanical impedance as a function of frequency is described. The measured values for two systems are shown. It is also shown that the performance of a modified system may be predicted in some respects. These measurements show that this function is extremely complex and that the systems are very limber at audio frequencies.

The Measurement of the Lateral Mechanical Impedance of Phonograph Pick-ups

Instrumentation and techniques for the measurement of the lateral mechanical impedance at the stylus tip of phonograph pick-ups are described and typical results are presented. Results are given as resistance and reactance in mechanical ohms throughout the frequency range from 30 to 10 000 cps. The measured impedance characteristics show the lateral and torsional pick-up-arm resonances and antiresonances and the stylus-arm resonance, as well as various other features attributable to the mechanical system of the pick-up. Response-frequency characteristics are shown for each pick-up, also. In many cases the correlation between irregularities in response and impedance is incomplete. This lack of correspondence can be explained in terms of the analogous circuit of the mechanical system. Measured impedance characteristics for a number of pick-ups, typical of those used in home phonograph systems during the period from 1901 to 1951, show quantitatively the improvement made in pick-ups in this period.

The Measurement of the Lateral Mechanical Impedance of Phonograph Pick-Ups

A device is described for the measurement of the lateral complex mechanical impedance of phonograph pick-ups in the frequency range between 30 and 10 000 cps. The device employs four electromagnetically driven, calibrated vibrators resonating different frequencies. The theory of operation is outlined, and results obtained for a typical phonograph pick-up are presented.

Resonance in the External Auditory Meatus

IN investigations on the extent to which objective tests can be used to obtain information on the magnitude of the effective amplification hearing aids give to a deaf user, a knowledge of the extent to which resonance occurs in the auditory meatus is of value. In normal human beings such a resonance occurs in the region of 2,000–3,000 cycles per second and to some extent accounts for the shape of the minimum threshold hearing curve as the frequency is varied. It is possible to calculate the magnitude of the resonance from results of mechanical impedance measurements at the opening of the meatus and to demonstrate its existence by means of a probing pressure microphone which indicates increase of pressure as the probe approaches the tympanic membrane. The method has limits, however, and in addition to being inconvenient can cause damage if extreme care is not taken.