Vibration Isolation Principles

Vibration isolation principles is a short article outlining some of the initial points that need to be considered for optimum performance systems.

Techniques to obtain optimum results

Structure-borne noise is that which is transmitted through the structures of a building from a machine installation.

This radiation noise becomes airborne noise.

Low noise frequencies are those that are usually less damped in the air and are therefore better transmitted through structures. The range of low frequencies is between 20 and 500 Hz.

Natural frequency of the Akustik+Sylomer mounts

The akustik+sylomer® ceiling mounts can obtain very low natural frequencies of down to 7Hz at the optimal loading point.

At this loading point the decoupling frequency of the akustik+sylomer® mounts is 9.9Hz.

Such a low natural frequency is optimal for the false ceilings of soundproofed premises. This type of
suspensions are also particularly interesting for the isolation of machines or vibrating elements that work at more than 600 rpm.

Examples are:
• Ducts / pipelines:
– Of cooling liquids from refrigerating compressors, and are ideal for use in supermarkets, the frozen food section.
– Air conditioning.
– Pumping of water
– From fume exhausts.
• Suspension of air conditioning machinery.
• Suspension of vibrating elements in general.

Behaviour of the akustik+sylomer mounts at low frequencies in soundproofed premises.

The range of audible frequencies in the human being may vary according to age and to other factors
although in general it is between 20Hz and 20kHz. By way of example the notes produced by
a guitar have a frequency range from 82 to 698 Hz.

Considering that the most unfavourable excitation frequency, i.e. 20 Hz, the isolation degree of structure-borne noise produced by an akustik+sylomer® suspension would be close to 90%. (*)
(*) Installation of the optimal loading point of the akustik + sylomer for a theoretical single mass spring system.

Behaviour of the akustik+sylomer mounts at medium and high frequencies.

Sound waves are not comprised of just one frequency, but rather of a set of frequencies superimposed without any order, which is the main reason why noise is unpleasant. Therefore, the ideal vibration isolation mount must be able to isolate the widest possible range of frequencies.

Behaviour of a metal spring:

These mounts are often recommended for the elastic suspension of false ceilings. It is important to know that this type of mount is suitable for the damping of low frequencies, whereas the high frequencies are propagated through the coils of the spring.

To filter this type of frequencies the springs must be combined with a stage of visco-elastic material under the spring to stop the propagation of this type of vibration.

Behaviour of the akustik+ Sylomer:

Thanks to the viscoelastic properties of the Sylomer, the akustik+Sylomer has a behaviour similar
to the spring at low frequencies and at the same time not only prevents the high frequencies as occurs in the spring via its coils, but also considerably improves the behaviour of the rubber at high frequencies.

Creeping and long term behaviour

Static loads produce a certain degree of creeping. This phenomenon can be observed in all
elastomers.

Creeping is the increase in deformation under consistent loading.

Within the field recommended for the application of continuous loads, the additional deflection of Sylomer remains under 50% of the initial deflection even after an extended period of 10 years.

The dynamic stiffness of ceiling/floor mounts must increase as little as possible over time.

Static and Dynamic stiffness

The stiffness of a rubber anti-vibration mount changes when a dynamic force is applied to it. This parameter depends on architecture, the compound used and even the frequency of excitation.

Generally speaking, dynamic stiffness is always greater than static stiffness, so calculations based on static
stiffness may lead to wrong conclusions.

In some cases it is possible to reach limits of dynamic
stiffness which are two and even three times greater than the static stiffnesses.

To Summarise:

Anti-vibration mounts/hangers generally all look very similar.

However, cheap black rubber type units may end up being just that…cheap.

They may well deteriorate over time, may not have any technical information or data (as outlined in this article), so thinking, “well hey it’s rubber isn’t it” may not quite be the end of the story.

When designing for high load acoustic flooring it is essential to model both static and dynamic information because, for example, a recording studio may have equipment at one end that weighs considerably more than the settee at the other and equipment/personnel movement on one side may be significantly more frequent than the other.

If this is not taken into account and cheap ‘all rounder’ pads are used the chances are that some will be overloaded whilst others do not receive enough deflection to work properly to the designed isolation frequency.

If you found vibration isolation principles useful we have a few other anti-vibration articles here:

What is Vibration Natural Frequency