Alan O. Sykes

Wave Effects in Isolation Mounts

Both theoretical and experimental studies of wave effects in isolation mounts have been made. From the standpoint of vibration isolation, wave effects are important in the sense that the vibration isolating properties of a mount are changed by their presence. The well‐known “lumped parameter” theory of vibration mounts predicts that the vibration isolation of a mount increases at 12 db per octave for frequencies well above the resonant frequency of the spring‐mass system. This theory holds true only when the wavelength of the elastic wave in the mount is large compared to the dimensions of the mount. Standing waves occur, as would be expected, which in certain frequency ranges decrease the vibration isolation properties of the mount by as much as 20 db. For practical mounts, wave effects are most detrimental in the most audible frequency range (500 to 1000 cps). The theoretical and experimental treatments are in good agreement, and indicate various methods for improving the vibration isolation properties ...

The Reciprocity Calibration of Piezoelectric Accelerometers

Two piezoelectric accelerometers have been calibrated by an absolute reciprocity technique in the frequency range from 100 cps to 10 kc. A theoretical and experimental evaluation of the technique is given.

On the Use of Metallic Noise Isolation Mounts

Transmissibility data are presented for steel and copper manganese helical springs. It is shown that for frequencies 10 to 30 times the resonant frequency of the spring-mass system, wave phenomena are important and that due to the low damping of the metals, the isolation mount may amplify the force felt by the foundation at the standing wave resonances rather than reduce it. It is shown that several modes of vibration are possible and that a one degree of freedom treatment of the problem is inadequate. The effect of coupling between the modes is discussed. Improvement of the isolation properties by coating the springs with highly viscous materials is demonstrated. It is further demonstrated that if sufficient damping is added, the value of critical coupling exceeds the actual coupling between the modes and in this case the system resembles a one degree of freedom system.

Vibration isolation: A retrospective prospective perspective

Over the past three decades, a substantial investment has been made by government and private industry in developing an improved understanding of the vibration isolation problem and new vibration isolation techniques. This paper presents a personal view of what has been accomplished, discusses recent analytical and experimental studies both here and abroad, identifies problem areas requiring further investigation, and looks through a smoky glass toward the future.

On the Multi‐Terminal Extensions of the Thevenin and Norton Theorems and Their Application in Vibration Problems

Generalizations of the Thevenin and Norton theorems are as useful in solving multi‐terminal mechanical network problems as their original forms are for solving lumped and four‐pole problems; however, care must be used in applying them in that different forms of the theorems must be used depending on the nature of the sources being considered. Two forms of both the Thevenin and Norton theorems are presented, and limitations in applying them are discussed.

Intensity Meter for Underwater Use

The mechanical and electronic design and the calibration of two types of intensity meters are discussed and their merits compared. Both are designed for a frequency range of 50–7000 cps. The principles of both types are not new; the paper is concerned mainly with design problems and calibration procedure.

Passive Vibration‐Cancelling Isolation Mounts

The design of passive vibration‐cancelling isolation mounts is discussed. Design criteria for a single‐frequency vibration‐cancelling mount and experimental data for a crude prototype are presented.

Estimating the Effectiveness of Isolation Mounts

Mechanical network theory is applied to the problem of estimating the effectiveness of mounts isolating nonrigid machines from nonrigid foundations. The spring‐dashpot mount is treated first, the continuous mount second. Effectiveness curves are presented for a variety of machine, mount, and foundation conditions, and the application of these curves to practical isolation problems is discussed.

A Useful Theorem for the Testing of Isolation Mounts

In the testing of isolation mounts, or more generally any vibrating system, it is often desired to know the ratio of forces at two points in the system. Since the measurement of dynamic forces is usually more difficult than the measurements of velocities of accelerations, it is useful to have a theorem relating the force ratio at two points in a system to the velocity ratio at the two points. A theorem relating the force ratio to the velocity ratio has been proven quite generally as a consequence of the reciprocity theorem. For lumped systems the theorem is equivalent to the ideas of duality in electrical circuits. The utility of the theorem is illustrated by examples drawn from actual practice.

Wave Effects in Vibration Mounts

Theoretical and experimental studies of wave effects in vibration mounts have been made. From the standpoint of noise isolation, wave effects are important in the sense that the noise isolating properties of a mount are impaired by the presence of wave effects. The well‐known lumped parameter treatment of vibration mounts predicts that the vibration isolation of a mount increases at 12 db per octave for frequencies well above the resonant frequency of the spring‐mass system. This is true only when the wavelength of the elastic wave in the mount is large compared to the dimensions of the mount. Standing waves occur as would be expected and often impair the vibration isolation properties of the mount by as much as 20 db. For practical mounts, wave effects are often most deleterious in the audible frequency range 500 to 2000 cps. The theoretical and experimental treatments give good agreement and indicate various means for improving the vibration isolating properties of the mount.