Vibration Isolation Glossary

From natural frequency to transmissibility: here we explain the most important technical terms relating to anti-vibration mats, resilient mounting and structure-borne noise – briefly, practically and with links to further pages. Numerical examples are simplified guide values; the binding design is always project-specific.

Jump to: A · C · D · E · F · I · L · M · N · R · S · T

A

Airborne noise

Airborne noise is sound that travels through the air and is heard directly. An anti-vibration mat does not reduce it directly, only the structure-borne noise component. Enclosures or acoustic hoods help against directly radiated airborne noise.

C

Compression set

Compression set describes what proportion of a compression remains permanently after unloading. It is tested to ISO 815 under specified deformation, temperature and duration and expressed as a percentage. A low value indicates good recovery and favourable long-term behaviour. See also creep and Standards & guidelines.

Creep

Creep is the slow increase in deformation of a material under constant load. With elastomers, a large part of it occurs in the initial period after loading; the result is settlement of the machine. The creep curve of the material concerned in the datasheet is decisive. See also compression set and Materials & technical data.

D

Damping

Damping is the conversion of vibration energy into heat – in elastomers through internal friction in the material. It limits the amplification of vibration at resonance, for example during machine run-up. In the isolation range, higher damping slightly reduces the isolation effect. The usual parameter is the loss factor; not to be confused with vibration isolation, which interrupts the transmission path.

Dynamic stiffness

Dynamic stiffness is the stiffness of a resilient element under oscillating load. For elastomers, it is generally higher than the static stiffness and depends on frequency, amplitude and preload. That is why the actual natural frequency is usually slightly higher than calculated from the static deflection. It is measured, for example, to ISO 10846.

E

Excitation frequency

The excitation frequency is the frequency of the force that a machine introduces into its support. For rotating machines, in simplified form: speed in rpm ÷ 60 = frequency in Hz (e.g. 1,500 rpm → 25 Hz). Transformers hum mainly at twice the mains frequency, i.e. 100 Hz on a 50 Hz network. Together with the natural frequency, it gives the frequency ratio.

F

Acoustic bridge

An acoustic bridge is a rigid connection that bypasses the resilient mounting and “short-circuits” it – for example rigid pipework, a cable tray, a bolt without a resilient sleeve or mortar at the edge of the mat. Through it, structure-borne noise passes the bearing and enters the structure. Even a single acoustic bridge can significantly impair the effect of a mat.

Frequency ratio

The frequency ratio η is the quotient of excitation frequency and natural frequency (η = f_exc / f₀). Isolation only occurs from η > √2 ≈ 1.41; below that, vibration is transmitted or even amplified. In practice, a ratio of at least about 3 is the aim.

I

Inertia base

An inertia base is a heavy block or frame (usually concrete or steel) between the machine and the resilient mounting. It increases the supported mass, lowers the centre of gravity, distributes the load evenly and reduces machine movement. It is used, for example, for pumps, compressors and machines with a high centre of gravity to limit rocking modes. See Pumps & compressors.

Insertion loss

Insertion loss states in decibels how much a resilient element reduces the transmitted vibration compared with rigid installation. In simplified form: dB = 20 · log₁₀(1/T), with the transmissibility T. An isolation efficiency of 90 % thus corresponds to about 20 dB, 99 % to about 40 dB.

Isolation efficiency

Isolation efficiency indicates what proportion of the vibration excitation is not transmitted into the substrate. Without damping, in simplified form: isolation efficiency = 1 − 1/(η² − 1), with the frequency ratio η. At η = 3 this gives around 88 % (simplified example, guide value). It equals 1 minus the transmissibility.

L

Load class

Load classes rank resilient materials by their permissible static continuous load. Our rubber granulate mats are divided into classes G1 (up to 0.02 N/mm²) to G8 (up to 1.50 N/mm²); PUR elastomers cover approx. 0.002 to 2.5 N/mm². The right class follows from the bearing pressure. All classes can be found under Materials & technical data.

Load-deflection curve

The load-deflection curve shows the relationship between load and deflection of a bearing. With a linear curve, deflection increases in proportion to the load; with a progressive curve, the bearing becomes stiffer as the load increases. A progressive curve, as with our rubber granulate mats, keeps the natural frequency almost constant over a wide load range. Curves can be found in the datasheet of the relevant product type.

Loss factor

The loss factor (often given as d or tan δ) is a measure of the damping of a material: the ratio of the energy converted into heat per cycle to the stored elastic energy. Elastomers have a much higher loss factor than steel springs. The value depends on material, frequency and temperature and – if stated – is given in the datasheet.

M

Mass-spring system

A mass-spring system is the simplest model of a resilient mounting: a mass (machine or foundation) rests on a spring (mat, pad or spring element). The natural frequency follows from mass and stiffness. In railway construction, the term also refers to a track form in which a heavy concrete trough or slab is resiliently supported – see Railway & transport.

N

Natural frequency

The natural frequency f₀ is the frequency at which a resiliently mounted system vibrates freely after an impulse. It depends on the mass and the stiffness of the mounting and can be estimated in simplified form from the static deflection: f₀ ≈ 15.76 / √(deflection in mm) Hz. For effective isolation, it must be well below the excitation frequency. You can estimate it with our isolation calculator.

R

Receiver isolation

With receiver isolation (also called passive isolation), a sensitive instrument is protected from vibration in its surroundings, for example a microscope, a balance or a measuring machine. The counterpart is source isolation. The physical principles are the same, but here the interference frequencies come from the building, traffic or neighbouring machines. See Laboratories & research.

Resonance

Resonance occurs when the excitation frequency coincides with the natural frequency of the mounting (frequency ratio η ≈ 1). The vibration is then strongly amplified – limited only by damping. Machines that pass through the resonance range during run-up and run-down should do so quickly.

Rocking mode

Rocking modes are rotational or wobbling movements of a resiliently mounted machine about a horizontal axis. They occur mainly when the centre of gravity is not central above the bearings or is very high. The remedy is a bearing arrangement matched to the centre of gravity and, if necessary, an inertia base. More in our guide Why an anti-vibration mat is not working.

S

Shore hardness

Shore hardness (for elastomers usually Shore A) describes the resistance to penetration by a standardised indenter to ISO 7619. It is a useful material parameter but is not sufficient for design. What matters is the load-deflection curve of the bearing, which also depends on shape, thickness and load. More under Materials & technical data.

Source isolation

With source isolation (also called active isolation), the source of vibration – such as a pump, compressor or press – is resiliently mounted so that as little vibration as possible enters the foundation and building. The counterpart is receiver isolation. EN 1299 describes the information needed for the design (see Standards & guidelines).

Static deflection

Static deflection is the compression of a bearing under the static load of the machine. The greater the deflection, the lower the natural frequency of the mounting. It depends on bearing pressure, material, thickness and number of layers and is read from the load-deflection curve in the datasheet.

Structure-borne noise

Structure-borne noise is sound that travels through solid bodies – foundations, floors, walls or pipework. It is often radiated again elsewhere as audible airborne noise, for example as a hum in neighbouring rooms. Anti-vibration mats interrupt the structure-borne noise path between machine and structure.

Bearing pressure

Bearing pressure is the load per loaded area: weight [kg] × 9.81 ÷ area [mm²] = bearing pressure [N/mm²] (1 N/mm² ≈ 145 psi). It determines which load class is suitable. If it is too low, the mat remains too stiff; if it is too high, there is a risk of excessive deflection and settlement. You can calculate it with our isolation calculator.

T

Transmissibility

Transmissibility T is the ratio of the transmitted to the input vibration amplitude. Without damping, in simplified form, T = 1/(η² − 1) for η > √2. T > 1 means amplification, T < 1 isolation. The isolation efficiency is 1 − T; expressed in decibels, this gives the insertion loss.

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