This is a simplified guide for those who want to face the problems connected with vibration insulation.

Any reference to formulas has been avoided for better understanding of the text, even for people who are not expert in this field.

A more technical essay is available in our Catalogue, which can be downloaded at

Sound and Noise
Vibrations
Vibrations Effects on the Human Body
Natural Frequency and Damping
Forcing Frequency and Resonance
Vibration Transmission
Vibration Isolation
Antivibration Mountings
Selecting an A.V. Mounting
Applications

Sounds are variations in pressure that propagate in an "elastic" environment, usually air, which can generally be perceived by human ears. A well-known example of variation in pressure is the one that takes place in the atmosphere, but these air changes are too slow to be heard. However, if these changes occurred at least 20 times per second, we would perceive them as sounds. In fact, man can hear any variation in pressure that takes place with a frequency ranging from 20 to 20,000 times per second. These variations per second are called "frequency" and are measured in Hertz (Hz). Variations below the threshold of 20 per second generate infrasound, and variations above 20,000 per second generate ultrasound. A drum and a whistle are examples of low and high frequency. Sounds are also characterized by their "amplitude": the buzz of a mosquito and the shot of a cannon are examples of small and large amplitude.

Noises are unpleasant and undesired sounds. The degree of nuisance is subjective, and can vary in the same person depending on the circumstances. Excessive exposure to noise can cause a series of specific damages to the hearing, as well as more general damage to the entire body.

Sound waves propagate in the medium in which they find themselves, (at 340 m/sec in air, at 1480 m/sec in water, at 4880 m/sec in steel) , through a series of compressions and rarefactions. Sound transmission cannot take place in a vacuum.

The unit of measurement for sound is the decibel (dB), a mathematical unit of measurement which uses a logarithmic scale (20 dB=rustle of leaves, 120 dB =jack hammer noise) to represent the enormous sensitivity of the human ear : the energy released by the loudes withstandable sounds is 10,000 billions greater than that of barely perceptible ones.

Wave length depends on frequency : at 20 Hz it is 17m, at 200 Hz is 1.7m, at 2 KHz is 17cm, at 20 KHz is 1.7cm.

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The propagation of sound and noise transmits energy in the form of pressure waves in the air; in the case of vibrations the energy takes the form of waves that propagate in a solid structure. A body vibrates when it performs an oscillating movement around a position of static equilibrium.

Similarly to sounds, vibrations are characterized by a "frequency" that represents the number of cycles per second, and by an "amplitude" that describes its intensity. These two quantities are inversely proportional: as the frequency increases, the amplitude decreases, and vice-versa.

The tuning fork is an example of oscillation of only one frequency. In reality, many different frequencies are present simultaneously.

Vibrations represent a very particular case in the vast field of oscillating phenomena, which include different events, from the pendulum motion to earthquakes. Particular attention has been given to studies on the fluid dynamics effects on different structures.

An example of the catastrophic consequences that vibrations produced by wind can cause to some structures is the collapse of the Tacoma bridge on November 7, 1940 in Washington State, U.S.A.. The wind velocity was just 72 km / hr, but repeated oscillation imposed on the structure caused it to resonate, enormously increasing the amplitude of the oscillation.

Mechanical vibrations are those particular vibrations caused by mechanical machinery. Common sources of these kinds of vibrations are alternating and rotating machines (unbalanced forces), pressing machines (impact forces), compressors and pumps (forces due to flowing fluids).

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The effects of mechanical vibrations on the human body can be divided into three main groups:

a) Effects of very low frequency vibrations (1-2 Hz) that cause kinetosis, known also as motion sickness, car or sea sickness. Symptoms include malaise, asthenia, dizziness, pallor, cold sweat and nausea. The oscillations stimulate the vestibular receptors and the numerous connections between these and the anatomic reflex centres induces developement of the complex motion sickness symptomatology.

b) Effects of low frequency vibrations (2-20 Hz) caused by surfaces, plants and machinery. These vibrations can cause a general distortion of the normal biological and psycophysiological responses to mechanical stimuli, such as lesions to bones and joints, (e.g. in drivers of heavy vehicles), cardiocirculatory disturbances, and maladies of the digestive track.

c) Effects of medium-high frequency vibrations (20-1000 Hz) caused by electrical or pneumatic machinery. Exposure to this kind of stress affects in particular hands and arms and causes the onset of osteoarticular lesions with arthrosis and angioneurotic damage, such as pin-and-needles and numbness of the fingers.

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Every mechanical system, even an apparently rigid one, has a certain degree of elasticity, and is therefore deformable. The system can be outlined as a mass M and a rigidity K, represented by a spring.

If we move the mass downwards and then we release the entire system, it will start to oscillate ("free" oscillation) with a constant amplitude and a frequency that is directly proportional to stiffness and inversely proportional to mass.

This "natural" frequency is specific for each system, and the greater the mass and lesser the rigidity, the lower the frequency will be.

The natural frequency does not depend on the intensity of the stress, but only on the physical characteristics of the system, mass and rigidity: a violin string always generates the same note, whether grazed, or plucked with force. The amplitude changes, that is the entity of the deformation, but the natural frequency of the string remains the same.

Experience tells us however that oscillation amplitude diminishes in time until the system stops oscillating, for the damping effect due to internal friction which, in various degrees, are present in every body.

More precisely, the diagram that represents a real system should include a mass M, a rigidity K and a damping S. The value of the natural frequency remains pratically unaltered, that is the same as the one of the free system. On the contrary, the amplitude of the oscillation is reduced because of the damping ( "damped" amplitude).

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In the examples so far illustrated, after having been induced to oscillate, the system is let free.

Imagine now an external disturbing force, that varies periodically and is able, once applied to our system, to excite it, altering its equilibrium.

The oscillation of the system (called "forced oscillation") takes places independently of the natural frequency, and depends only on the forcing frequency.

This external force can be produced by any number of factors, such as dynamic effects due to unbalanced rotating parts, misalignement of joints and bearings, eccentricity, interference, defective belt and gear transmission, torque variance hydraulic, aerodynamic, or electromechanical forces, mechanical friction or loosening.

If the forcing frequency is higher than the system's natural frequency, oscillation amplitude remains small, if lower, it will increase.

Resonance occurs when the external forcing frequency coincides with the natural frequency of the system, and they act in phase. In this case the amplitude of oscillation tends to increase indefinitely, and the structure is subjected to increasing deformation which tend to provoke its collapse.

Sometimes resonance occurs only in the transition phases, namely during the starting and stopping phases of the machinery. Resonance should always be avoided as much as possible.

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When dealing with vibration transmissibility, is fundamental to remeber that a high damping value is desirable to reduce the resonance peak but is damaging with regard to isolation. On the other hand, trying to reach a high level of isolation, there is the risk of provoking system instability with excessive oscillation amplitude. In the real world, the issue is to obtain a good compromise between isolation and oscillation amplitude.

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Vibration isolation means containing the propagation of oscillation through the structures.

Machinery with unbalanced forces cannot be equilibrated, but it can isolated from its support to avoid the transmission of vibrations.

Isolation can be obtained when the natural frequency of a system is lower than the external frequency that excites it.

One approach can be to keep the natural frequency constant and increase the forcing frequency as much as possible. The latter, however, is usually constant as for instance the rotation speed of the machinery itself.

A second approach consists of reducing the natural frequency as much as possible : that is what vibration isolators do.

Since the natural frequency depends only on the physical characteristics of the system, namely its mass and rigidity, to cut vibrations one should act on these two variables. The rule says, the greater the mass and lower the rigidity, the lower the natural frequency will be.

It is not always feasible to increase the mass, so it is preferible to reduce the rigidity of the system as much as possible, increasing its elasticity.

The vibration damping effect which results from the deformability of the elastic support, protects the support structures and surroundings rather than the isolated machine : an unbalanced machine will continue to vibrate, but these vibrations will be kept from propagating to the surrounding structures.

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Antivibration mountings can be defined as those particular devices that make a system less rigid increasing its elasticity.

In order to be effective, they must be capable of being deformed by the stress they have to bear: if they were rigid, they would not be able to provide any isolation. On the other hand, they must not be too subsiding, otherwise they could provoke excessive oscillation.

Antivibration mountings transform the energy of the machinery vibrations into heat, through internal dissipation friction. In order to obtain this result, several deformable materials can be used, such as rubber, springs, cork or even air.

Elastomers made of special rubber mixtures achieve the best compromise among the several characteristics which must be fulfilled, such as high efficiency, endurance, resistance to solvents and environmental agents, and low cost.

Antivibration mountings made of elastomers can work by compression, shear and torsion, and with combined stress. For special requirements it is possible to realize elastomers which are able to resist unconventional applications.

Spring mounts generally feature a higher deflection, and a high resistance to extreme temperatures, oils or corrosion.

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One criteria for choosing an antivibration mounting could consider its position on the floor: some antivibration devices need to be secured to the support bearing of the machinery, whereas others can simply rest on it.

Even the connection with the machinery to be suspended may be different, as the latter can be secured to the device or just rest on it.

However, the most important criteria for choosing the device is the weight of the machinery that it has to bear. Specific tables allow to identify the field of application for each antivibration mounting, characterizing their capacity. A better view of the problem can be obtained using the load/deflection diagrams.

Other considerations include the position of the center of gravity of the suspended group, the external forcing frequency, the share of the load and the overall dimensions.

The degree of isolation depends on the ratio between the external forcing frequency and the natural frequency of the system: in order to obtain good isolation, the former must be higher than the latter.

In the presence of a constant external forcing frequency, isolation increases as the natural frequency decreases, corresponding to the increase of deflection under load conditions. However, the stability of the machinery placed on the deformable devices must also be considered.

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Noise and vibration are intrinsic components of several activities in which machinery with moving parts are used.

The positioning of antivibration devices between the machinery and their support structures acts as isolation, without any intervention on the balancing of the machine itself.

Vibrations' damping prevents their propagation through other structures. It is easy to imagine the multitude of fields that benefit from the application of antivibration devices.

Antivibration mountings reduce the vibrating disturbance caused by machinery that have alternating as well as rotating inertial forces, such as engines (with internal combustion, electrical), pumps or compressors.

Other machinery that benefit from the use of antivibration mountings are textile looms, machine-tools and printing presses.

Antivibration mountings are widely used in air conditioning and refrigeration, since the machinery generating the vibrations is usually located in close proximity of homes or offices.

When a complete intervention is desired, pipes, as well as all suspended systems, can also be insulated from the ceiling. The same rule can be applied to speakers.

Other machines that are usually mounted on antivibration mountings are fans, refrigerators and washing machines. In addition, the use of antivibration mountings can reduce the stress due to presses or hammers.

Elastomers are also used for special purposes, such as support for railway tracks, bridge spans, and antiseismic devices for buildings.

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