Resonance absorber

A resonance absorber for the damping of structure-borne vibrations has a number of freely swinging slats with different resonance frequencies. The slats are arranged on a common base which can be connected with a body to be damped, and are constructed as double slats comprising a damping coating which is in each case squeezed between the two slats elements.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to a device for damping structure-borne noise 
vibrations, comprising a number of free-swinging vibration absorbing 
elements in the form of slats, with different resonance frequencies. The 
slats are arranged on a common base which can be connected with a body to 
be damped. 
Vibration absorbers of the above-mentioned type are known, for example, 
from German Patent Document DE 1 071 364 or German Patent Document DE-OS 2 
163 798. In the case of these vibration absorbers, the resonance frequency 
of the individual slats is coordinated with the vibrations of the body to 
be damped. The slats, which are excited in this manner to carry out 
resonance vibrations, therefore absorb vibration energy from the body to 
be damped which, by an appropriate damping of the slats, will finally be 
converted to heat. 
To increase the vibration-damping effect, it is known from European Patent 
Application EP 0 020 284 B1 to stack plate-shaped slats above one another, 
with layers of a damping material being arranged between the slats. The 
individual slats and the damping material are coordinated with one another 
so that the individual plates vibrate against one another and in the 
process compress and relax the damping material. In such a resonance 
vibration absorber, the intermediate layers made of damping material must 
be relatively thick and soft, in order to avoid excessive coupling between 
the individual slats, which would change the whole vibration behavior. 
It is an object of the present invention to provide a vibration absorber of 
the above-mentioned type which, while the effect is the same, permits a 
more compact construction, can be coordinated with a frequency range which 
is as wide as possible, and requires smaller amounts of damping material 
than previously. 
This object is achieved by the vibration absorber according to the 
invention which on the one hand has freely swinging slats that are 
constructed as double slats but are not vibrationally coupled to adjacent 
double slats and therefore exhibit a defined vibration behavior. On the 
other hand, to increase the damping of one double slat respectively, a 
known technology referred to as a "squeezed coating" is used for the 
damping of bending vibrations of thin metal sheets. In this case, the 
damping layer is deformed by shearing rather than by compressing or 
relaxing, so that the damping layers may be extremely thin. 
The slats may be arranged either side-by-side in a layer, as, for example, 
according to German Patent Document DE 2 163 798, or sandwiched above one 
another, as, for example, corresponding to German Patent Document DE 1 071 
364 or European Patent Document EP 0 020 284 B1. In a particularly compact 
arrangement, several layers having a congruent outer circumference are 
stacked above one another so that a block of freely swinging double slats 
is created, which are arranged in a linear and column-shaped manner. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWING 
The resonance absorber illustrated in the figure comprises seven layers (1 
to 7) of double slats 1.1, 1.2, 1.3 . . . , each layer being made of two 
congruent metal plates of the same thickness. On two opposite ends A and 
B, the plates are notched in the manner of a comb. A base strip 8 of a 
width b extends diagonally over the plate surface, and remains unnotched. 
Congruently extending spacer pieces 10 to 15 inserted between the 
individual layers parallel to the base strip acoustically couple the 
individual plates in the area of the base strip 8. The layers and the 
spacer pieces are held together by means of the base strip 8 and a 
so-called adapter plate 9 by means of tightening screws 9.1 to 9.7. 
Particularly in mass production of the absorbers, the spacer pieces may 
also be integrated directly with the plates, for example, by means of 
corresponding casting molds for the plates or by a stamping or milling of 
flat plates. By way of the base plate 8, the resonance absorber is 
connected with a body to be damped in a force and moment-locking manner. 
Before stacking and cutting, a damping layer for instance type E3512A+B 
from Company Permabond is applied between the two plates of a layer (1 to 
7) so that a sandwich-type structure is created. After the cutting of the 
plates, double slats 1.1, 1.2, . . . 1,7 are created in this manner, each 
comprising two metallic slat elements, for example, 1.11 and 1.12, with a 
coating, such as 1.13, which is sandwiched between them. 
If, as in the illustrated embodiment, the base strip 8 extends 
asymmetrically over the plane of the plate, a plurality (2.times.7) of 
double slats are created, each having a different length and therefore a 
different resonance frequency. 
If, as also shown in the embodiment, the layer thickness of the individual 
plate elements increases from layer to layer (from 1 to 7), the number of 
slats with different resonance frequencies is multiplied by the number of 
layers. 
The following equation may be used to calculate the resonance frequencies 
of the double slats having a constant cross-sectional course: 
##EQU1## 
wherein s.sub.n : natural frequency factor 
l: length of slat 
I: geometrical moment of inertia 
A: cross-sectional surface of slat 
E: modulus of elasticity 
p: density 
The natural frequency factors depend on the manner of the clamping of the 
slats and on the ordinal of the natural vibration. In "Technische Akustik" 
("Technical Acoustics") by IVAR VEIT, Publishers: Vogel Fachbuch, 4th 
Edition, 1988, the following factors are indicated for the fundamental 
oscillation and the first 4 harmonic oscillations for a rod which is 
clamped in on one side and which may be considered to be the equivalent of 
the absorber slat according to the invention: 
s.sub.1 =1.875 (fundamental oscillation) 
s.sub.2 =4.694 (1st harmonic oscillation) 
s.sub.3 =7.855 (2nd harmonic oscillation) 
s.sub.4 =10.996 (3rd harmonic oscillation) 
s.sub.5 =14.137 (4th harmonic oscillation) 
By the connecting of two slat elements to form a double slat with an 
intermediate damping layer, a coupling factor must be taken into account 
with respect to the determination of the resonance frequency of such a 
double slat. Thus, the following will apply: 
EQU f.sub.nD =K.f.sub.n. 
wherein 
f.sub.nD : natural frequency of the double slat 
K: coupling factor. 
According to the material, the coupling factor ranges between the values of 
1 and 2. In the case of a very soft damping mass, the natural frequency of 
the double slat will increase only insignificantly; in the case of a very 
hard damping mass, the coupling factor of almost 2, thus a frequency 
doubling, is obtained. 
A gradation of the slats according to the following rule is particularly 
advantageous: 
##EQU2## 
wherein l.sub.n =length of the nth slat 
l.sub.o =length of the longest slat 
n=course index between 0 and N-1 
N=total number of different slat lengths 
f.sub.1 =first resonance frequency of the longest slat 
f.sub.2 =second resonance frequency of the longest slat. 
The provision of double slats with different damping layers results in an 
expansion of the temperature range for the vibration absorber. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.