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Sound absorbers of a novel membrane construction
Applied Acoustics 25 (1988) 197-215
Sound Absorbers o f a Novel Membrane Construction
U. Ackermann, H. V. Fuchs & N. Rambausek Fraunhofer-Institut f'tir Bauphysik, D-7000 Stuttgart 80, FRG (Received 21 March 1988; revised version received 15 April 1988; accepted 21 April 1988)
A BS TRA C T Porous or fibrous materials are used in a great variety of applications to absorb acoustic energy at medium and high frequencies above approximately 200 Hz. There are, however, cases where their open and rough surface entails certain disadvantages with respect to hygiene and cleaning requirements. For medium and low frequencies, which would also require a relatively large absorber thickness and weight, there is a needfor an alternative absorber, the acoustically active components of which are formed exclusively by, ideally, even and smooth membranes. The sound absorber presented herein is of the reactive or resonant type with several different modes of vibration excitable in a complex system of rather thin, though comparatively stiff metal or plastic membranes. The absorption of the vibrational energy as stimulated by the sound waves impinging on the new type of acoustic lining or splitters is brought about solely by frictional forces in bounded shear layers formed in between specially shaped membranes moving against each other and relative to the air volumes adjacent to and between them. When suitably adjusted to the particular noise spectrum, these combined vibrational and damping mechanisms enable the construction of a new generation of sound absorber which no longer requires additional porous material to be incorporated in order to make it effective in a broad band of medium and lowfrequencies. The honeycomb structure, which forms the solidframe for the acoustic absorber, also makes it a relatively light and stable construction element for both room acoustic and industrial applications, where contamination of the absorber by dust in the air or pollution of the environment by abrasive deposits from the damping material is to be avoided under all circumstances. 197 Applied Acoustics 0003-682X/88/$03-50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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U. Ackermann, H. V. Fuchs, N. Rambausek
1 REQUIREMENTS ON SOUND-ABSORBERS IN A I R H A N D L I N G DEVICES In industrial and household appliances air, exhaust gases etc. are transported through flow ducts, containers and outlets. These flow ducts often represent a very undesirable transmission path for airborne sounds between the rooms interconnected by them. In particular they transmit the noise emanating from the flow-producing equipment, e.g. a fan. A reasonably effective way of controlling the noise in such cases is to line the flow duct with sound-absorbing material or to install sound-absorbing splitters in the duct. If sediments are carried with the transported medium, the soundabsorbing elements should not take them up in any circumstances. For understandable reasons of safety, sanitation and fire protection contamination of the absorber is to be avoided. Especially great demands are made on air-handling devices in 'clean rooms' for the production of semi-conductor elements, in hospitals, kitchens for large-scale catering, and in supplying packing appliances in the food industry with sterile air, and therefore, in these cases, it is desirable for the sound-absorbing linings and splitters to be hermetically sealed by gas- and water-tight coverings. In addition, the surface of the absorbers facing the flow should be as smooth and even as possible to keep frictional losses as low as possible. Moreover an additional demand especially made on air-handling devices is to damp sounds at low frequencies efficiently. This is connected, among other things, with the fact that the noise spectrum of a fan shows a falling characteristic with rising frequencies, even at the point of sound emission. A similar tendency can be observed with aerodynamically produced noise in the flow duct, which is also dominated by low frequencies. Even if a noise rating adapted to the human ear is carried out, frequency components around 250 Hz, or even much below that value, often remain dominant at the point of sound immission. Therefore great efforts are made to improve the capability of silencers to reduce noise at low frequencies.
2 A D V A N T A G E S OF T H E M E M B R A N E ABSORBER Conventional absorbers employing porous materials do not fulfill the above-mentioned requirements in all respects. They are susceptible to sediments and can become breeding-places for bacteria. In air-handling devices in hospitals and power-plants the silencers therefore have to be cleaned and decontaminated at regular intervals: The splitters have to be
Sound absorbers o f a novel membrane construction
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removed and cleaned with compressed air or steam, brushes, solvents or decontaminators. The surface of conventional absorber elements is covered with plastic foil and perforated plates or corrugated wire netting. Since, for acoustical reasons, the foil can only be very thin, it is easily damaged during transport, installation and operation, thus moisture can intrude into the porous filling, and the danger of contamination is further increased. For the acoustic efficiency of the absorbing material in the conventional sound absorber splitters the foils covering this material have to be as permeable to sound as possible. That is, the foils have to be very thin, and they must be able to vibrate freely in response to the impinging sound waves. They must not come into close contact either with the absorbing material behind them or with the perforated plates in front of them functioning as mechanical protection. The first condition is hard to realize from the point of view of construction, and the second requirement can easily be affected by deposits during operation. Foils, therefore, cannot guarantee perfect functioning of the silencers. Experience has shown that splitter silencers of conventional construction, therefore, cannot meet all requirements in practice. 3 S O U N D - A B S O R B E R S - - C O M P L E T E L Y M A D E OF MEMBRANES Searching for a sound-absorber for air-handling devices which could be adjusted to low frequencies and striving to protect the conventional fibrous or porous absorber filling against deposits has, after some detours, led the IBP to the development of an absorber which does not incorporate any porous materials.' In cooperation with a commercial firm silencer splitters have been realized which avoid the disadvantages of conventional absorbers and fulfil the above-mentioned requirements of integrity and stability: • The splitters consist only of metal foils. No flammable materials are used. • The splitters are hermetically sealed against the flow, so that no deposits can intrude into the splitters. • All components of the splitters are sufficiently solid, so that neither their mechanical nor their acoustical functioning can be affected during transport, installation, or by the flow. • For comparable installation geometry the acoustic efficiency in the frequency range below 500 Hz is better with the new splitters than with those filled with mineral wool. • Their damping is adjustable to a certain degree, e.g. also to tonal components in the noise spectrum.
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U. Ackermann, H. II. Fuchs, N. Rambausek
• The pressure drop due to the installation of the splitters is kept to an absolute minimum. • Their smooth outer surface gives deposits no chance to settle on them. • The new splitters can be produced with considerably less weight. • They can also be used in an environment which forbids the installation of porous absorbers from a hygienic point of view. The new principle of construction ensures that the load-bearing components of the silencer also function as the framework for the resonance systems and that application of fibrous and porous filling material is avoided.
4 R E S O N A N C E EFFECTS REVISITED If installing an outside membrane of relatively high mass (between 200 and 1000g/m 2) prevents sound-waves transmitted through the flow duct entering the absorber, the frictional effects at the fibres or in the pores of conventional absorbing material can no longer be activated. Instead, however, a twofold membrane system can be constructed in a way that it can be excited into various forms of vibration by the sound-field impinging on the outside membrane. In order to develop sufficiently broadband sound absorbers based on this principle, a wealth of empirical knowledge and practical experience is necessary, because a theoretical treatment so far can only roughly explain the position of the resonance frequencies, and not the damping of these vibrations. It is even rather tedious to estimate the absorption of a simple baffle resonator resulting from energy dissipation in the baffle or from radiation damping in the sealed air cushion. 2 In spite of this two well-known resonance mechanisms may serve as the basis for understanding the principal mode of operation of the new absorber and for approximately selecting construction parameters for adjusting the silencer to a predetermined frequency range. 4.1 Baffle resonators
For an adjustment to low frequencies the best-known absorbers are those consisting of plates or foils installed in front of a wall. Mass M of the plate, spring stiffness F of the air enclosed between the plate and the wall, the density of the plate (PM) and of the air (PL), the sound velocity in air (CL)the
Sound absorbers o f a novel membrane construction
201
thickness o f the plate (s), and the distance from the wall (t) allow us to determine the resonance frequency fp. 1 /--~ fP = ~
% = 2n
p~L1 = 5 4 0 0
L/~__Hz, p
"VPM s t
~q PM s t
(i)
110 e.g. for aluminium: fp = ~ with s and t given in cm. In Fig. 1 the splitter layout (an aluminium plate with s = 0.06 cm, installed 10cm in front of a woodchip board of 1.3 cm) shows that a resonator constructed in this way is only effective in a very narrow frequency band a r o u n d fp = 140 Hz. If higher damping is to be achieved in a broader band, it is recommended in most cases that the air gap is filled with absorbing material.
I
I
I
I
I
30:
g T, 2 0 "E t-.--
10
0
I
I 0
1 O0
I 200
I 300
400
I HZ 5 0 0
Frequency
Fig. 1. Transmission loss of splitter silencersof the simplebaffleresonator type: s = ff06-crn aluminium plate; t = 10-era distance to a woodchip board of 1.3 crn; resonance frequency according to eqn (1): fp = 140Hz; length of the splitter-- 1m.
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U. Ackermann, H. 11".Fuchs, N. Rambausek
In order to reach frequencies around 63 Hz dimensions s = 0.1 cm and t = 30 cm are required. It is known, however, that excitation at this or even lower frequencies is rendered difficult in terms of bandwidth. That is why it has proved to be advantageous 3 to include a second resonance mechanism in the development. 4.2 Helmhoitz resonator
Basically resonances at low frequencies can also be produced by springmass-systems where the vibrating mass M is the air plug in the holes of an inflexible plate in front of a wall (also assumed to be inflexible) and spring F is that of the air cushion between the plate and the wall. The resonance frequency fH is given by: fH = ~
V(I + AI, + Ala) = 5400
V--/*
(2)
where S is the area in cm 2, I* effective length of hole in cm and Vthe enclosed air volume in cm a. The end correction values AIi and Ala stand for the air mass vibrating outside the hole, differing according to the geometry of the hole (see e.g. Ref. 4). Naturally splitter silencers are not usually constructed with Helmholtz resonators only. Fig. 2 shows the typical narrow-band character of damping by such splitters. The maximum damping at fr~ = 170Hz disappears, as expected, if the holes are sealed. To improve the damping characteristic it is recommended, similarly, to fill the air gap, in the baffle resonators, at least partly, with absorbing material. The resonance frequency and the process of damping are also influenced by the form of the holes 4 and the depth of the chambers, s The two main disadvantages of all resonance absorbers constructed with open cavities are, however, that they may take up deposits and that they can generate considerable self-noise by interaction with the flow. Covering the holes e.g. by a foil would prevent--according to prevailing understanding--the excitation of the Helmholtz resonators. A foil directly fixed onto the perforated plate prevents air vibrations, as shown in Fig. 2; according to this philosophy a foil fixed at a certain distance from the plate should be thin (e.g. PE foil < 50 #m, according to Ref. 3) so that sound can penetrate easily into the resonator--so thin that it would be easily damaged under extreme environmental conditions. If a sufficiently broad-band absorber fulfilling the specifications mentioned above is to be constructed without using porous materials and superfine foils, the construction of splitter silencers must be changed in some important respects.
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Sound absorbers o f a novel membrane construction
--t --q --t ____t__ 40 dB 30 o
"F,, on
20
I---
lO
0
I 0
" 100
"1"
" 200
I 300
400
I Hz
500
Frequency
Fig. 2. Transmission loss of splitter silencers of the Helmholtz resonator type: Area of the holes: S = n/4 cm2;Volume of the chambers: V= 1000cm3;End corrections: 1" = All + I A I , 0.8 cm; Resonance frequency according to eqn (2): fM= 170 Hz; Length of splitter: 1m. - holes open; - - - holes sealed.
=
5 ALL-METAL MEMBRANE CONSTRUCTION OF ABSORBERS The newly developed sound absorber combines the baffle resonator with the Helmholtz resonator into broad-band m e m b r a n e absorber without using additional absorbing material. A typical layout of a silencer splitter which is to be used in flow ducts and which fulfils the specification, is shown in Fig. 3. In the layout shown in Fig. 3, which is only one example of m a n y possible and successfully tested ones, a large n u m b e r of chambers is built on each side o f a - - f r o m the acoustical point o f view--stiff bearing plate (5). These chambers a r e - - a l s o from the acoustical point o f view--hermetically sealed against each other by a honeycomb structure (2). By covering each side with a perforated m e m b r a n e (3) each of the newly formed closed chambers is
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U. Ackermann, H. V. Fuchs, N. Rambausek 4
s
3
G;G-2_TS
2
/A
A A /"
I
1 4
3
2
5
Fig. 3. Example of a membrane absorber with cubic honeycomb structure: 1, Frame of the splitter (e.g. 0"5-mm aluminium); 2, Walls of the chambers (e.g. 0"2-mm aluminium); 3, Perforated membrane (e.g. 0"l-mm aluminium); 4, Covering membrane (e.g. 0-2-mm aluminium); 5, Bearing plate (e.g. 0-5-mm aluminium).
provided with a certain hole, which makes them resemble conventional Helmholtz resonators. Finally, both sides of the splitter are covered with a cover membrane (4) which closes all the inside absorber elements with an airand water-tight seal at the frame of the splitter. In order to fulfil its protecting function even under hostile conditions, and in order not to be deformed too much by irregular flow forces and not to buckle under nonsteady operating conditions, the outer membrane is fixed in some places near the partitions of the honeycomb structure (2). With this special example of construction a more or less strong spring/ mass resonance created by the enclosed air cushion as spring and the two membranes as masses, is expected--even according to conventional understanding. For the silencer arrangement shown in the upper part of Fig. 4, however, the narrow-band spectrum of the transmission loss, in fact, indicates an encouraging efficiency of on average 15 dB for a splitter length of L -- i m in a broad frequency range between 150 Hz and 500 Hz. The damping characteristic in Fig. 4 represents an example not yet optimized. It can be adjusted to almost all noise spectra in the medium and lower frequency range by choosing the respective geometric parameters of the chambers and membranes. If a somewhat thicker cover membrane is used,
Sound absorbers o f a novel membrane construction
30
I
I
I
I
205
I
dB
20
o
j I
0
\
I 100
I 200
I aO0
400
I Hz
500
Frequency
Fig. 4. Damping characteristic of a silencer prototype according to Fig. 3: Chamber walls: 0.2-mm aluminium; Perforated membrane: 0.1-mm aluminium; Cover membrane: 0-l-mm aluminium (fixed directly in front of the perforated plate); Depth of the chambers, t: 10 cm; Volume of the chambers, V: 1000cm3; Diameter of the resonator holes, d: 2cm. (Splitter arrangement as in Fig. 1).
the damping can e.g. be shifted to lower frequencies, with, however, a diminution of the damping of higher frequencies. Both the length l of the resonator throat and the distance between perforated and cover membranes play an important role in adjusting the silencer. By throats projecting into the chambers the maximum can be shifted to lower frequencies. If, however, all the other parameters are left unchanged, the level of maximum absorption decreases. But it is still sufficient, as Fig. 5 shows, to exceed the efficiency of porous absorbers of the same geometry at these low frequencies. 6 SILENCER TEST FACILITY The development and testing of the new sound absorbers would have been very expensive and time-consuming because of the great variety of adjustable parameters, if an area of about 12 m 2 had had to be provided for
U. Ackermann, H. I,'. Fuchs, N. Rambausek
206
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0
Q 00
~ - T ' ...
1
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J
0
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a)
dB .J
20
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L~
L-~ I L- I L,,,I
10
0
r "J
S 63
125
1 250
500
" Ik
2k
4k
8kHz
Frequency
Fig. 5. Comparison of the insertion loss of silencer splitters with the same geometrical construction, using: (a) mineral wool; (b) membrane absorbers. each prototype in order to measure the degree of absorption in the reverberation chamber according to DIN 52 212/ISO R354. Furthermore it is not certain whether the resonance mechanisms, which are very sensitive to even small changes in material and construction parameters, could have been recognized correctly and optimized by tests in the reverberation room. Fortunately, a particularly suitable piece of measuring equipment was available for these investigations for the first time: the silencer test facility of the IBP (Fig. 6) allows tests according to the ISODIS 7235 'Measurement procedures for ducted silencers'. 6 For optimizing the prototypes with a maximum efficiency at medium frequencies only 2 m 2 of absorbers had to be produced employing a special glueing technique. The systematic tests carried out simultaneously with this project and the numerous improvements of the whole measuring technique for silencers in ducts will not be dealt with here (see instead Ref. 7). But there is no doubt that it is the available measuring technique which has probably made the
Sound absorbers of a novel membrane construction
Fig. 6.
207
Silencer test facility of the IBP.
development possible at all, or at least easier. Fig. 7, e.g., shows how a microphone, run pneumatically, can be moved along a rail between the silencer splitters to scan the sound-field.
7 A D V A N T A G E S OF T H E N E W SILENCERS The completely smooth cover membranes on both sides of the silencer splitter (according to Fig. 3) guarantee the smallest skin friction resistance possible and thus reduce the pressure drop to an absolute minimum. Under extremely dirty operating conditions, as they may occur e.g. in exhaust stacks, the whole splitter can be cleaned easily by washing, hosing, or dipping.
208
U. Ackermann, H. V. Fuchs, N. Rambausek
Fig. 7. Microphone scanning the sound-field between the silencer splitters.
If the frame (1), all the dividing and reinforcing chamber walls (2), the bearing plate (5), the perforated (3) and the cover (4) membranes are made of metal, the splitters are non-flammable. Hygiene demands can also be fulfilled. If the cover membranes are arranged directly in front of the perforated membranes, the wall does not have to be too thick to achieve a high resistance to mechanical damage of the outer covering. The outer covering of the splitter can be protected against chemical attack and corrosion in a way very similar to that of protecting all the other components of the flow duct. Thus the most important additional demands of practical use are fulfilled. The honeycomb construction makes the splitter highly stable against compression, suction and torsion. The frame (1) taken from conventional silencer splitters with a folded edge of usually 2 cm, all the way round, leaves all the installation spaces unchanged; the new splitters can be transported in the same way as all conventionally built splitters. Since the splitters can be completely constructed from one and the same material, they show no critical behaviour, not even under large temperature changes. Even ice on the smooth outer skin would not drastically reduce their effectiveness. Since there is only hollow space between the honeycomb construction, which does not have to be filled with porous or fibrous
Sound absorbers o f a novel membrane construction
209
material, the weight of the silencer splitters is lower than the weight of those filled e.g. with mineral wool. Silencers with porous absorber fillings cease to function properly if their depth is very small compared with the sound wavelength. Since this lower frequency limit can be somewhat lowered with the new construction, silencers can be adjusted to lower frequencies according to the abovementioned principle for a given space available. These features enable the new all-metal membrane absorber to be used in fields of industrial noise abatement where conventional absorbers cannot be used at all or only in a limited way because of several precautions as mentioned at the beginning of this paper.
8 PRACTICAL APPLICATIONS The application of the membrane absorber is certainly not limited to such special use where less integrity and a less hygienically critical type of absorber is forbidden. In spite of that, current consulting projects at the IBP suggested that for the first practical tests examples should be selected where conventional absorbers had already failed (paper-mill 8) or where they could not be used for reasons of sterility (e.g. bottling-plant9).
8.1 Paper-mill The paper-mill was an example of a very typical emission problem. The noise of the vacuum pumps is radiated at the opening of the exhaust stack and exceeds the rating levels permitted in the neighbourhood of VDI Regulation 2058, Part 1. Tests lasting for years to reduce the emission level by installing conventional silencers with mineral wool filling in the stack, have failed because of both the mechanically (vibration) and chemically (acidity of the air) harsh working conditions. Additionally, after only a very short time, thick deposits of paper refuse appeared on the splitters, which were enclosed by a stainless steel grid. Even relatively thick plastic foils, where the mineral wool filling was pressed in with the help of the above-mentioned grid, resisted only for a few days or weeks--a completely unsatisfactory situation. A test installation of the new absorbers in just this situation was also suggested by the fact that the plant is regularly turned off for a short time for inspection, so that there was an opportunity to observe the behaviour of the membrane silencer over a longer period of time. Figure 8 shows the resulting noise reduction in this case, measured at the opening of the stack. The two noise peaks in the A-weighted spectrum at 160 and 315 Hz were reduced by as much as 12 and 18 dB respectively by installing
210
U. Ackermann, H. V. Fuchs, N. Rambausek 80
dB (A) 70
60
t~
so
if)
40
30
2O
31,5
63
125
250
500
lk
2k
4kHz
Freq uency
Fig. 8. Air-borne sound spectra, measured in third-octave bands and A-weighted, 1 m away from the opening of the stack on the roof of the paper-mill: (a) before, and (b) after installing membrane silencers.
10-cm deep and 1-m long splitters into a chamber in front of the vertical part of the stack. The photograph of Fig. 9 was taken three months after the installation: The walls clearly show the sediment from the flow, inevitable because of their rough surfaces. At the top of the photograph the very dirty covering of the conventional porous lining can be seen. At the bottom of the photograph the stainless steel frame can be seen which enabled the new silencers to be installed in the chamber. In Fig. 10 traces of corrosion can also be seen both on the 0.2-mm thick aluminium membrane and the stainless steel frame, but this corrosion did not reduce their acoustic efficiency at all. Deposits as on the channel wall, however, cannot settle on the ideally smooth surface of the membrane splitters.
Sound absorbers o f a novel membrane construction
Fig. 9.
211
Noise attenuators of the old and the new type in the exhaust stack of a paper-mill.
In a second step a 50-cm deep membrane splitter was adjusted to the noise peak at 40 Hz. In Fig. 11 a photograph shows (in four steps), how this silencer is constructed. The splitter at the bottom has no membranes, the honeycomb structure can be seen very clearly; the splitters in the middle are both covered
Fig. 10.
The membrane absorber after 100days in the exhaust stack.
212
U. Ackermann, H. 11".Fuchs, N. Rambausek
Fig. 11. Membrane silencer for a maximum absorption at 40 Hz.
with perforated membranes, the holes of which have been treated in various ways. The splitter on the top is ready for use, i.e. the covering m e m b r a n e is fixed. Considering the level reduction of 10 dB according to Fig. 8 one has to see that tackling the problem with conventional silencers would not have been equally successful at such low frequencies.
OJ
40
iO
50
70
80
~'ig. 12.
~
L
u~
-
90
100
dB
110
31,5
63
125
500
Frequency
250
lk
2k
4k
8kHz
~ound pressure spectra in the bottling-hall for paper bags not far from a sterile-air fan.
16
....
125
/ ~_._r
-
250
J
Ik
2k
4k
"1. J--L..L.j_.~"
Frequency
500
L
i . . . . . . . . . . . . . .
l
.j
8kHz
II-'~~
_t. u~l"
Fig. 13. Insertion loss of a membrane silencer for the sterile-air mpply device of a bottling-plant adjusted to 400 Hz, measured in the silencer test facility at the IBP. 9
0
0
,
o ,
214
U. Ackermann, H. V. Fuchs, N. Rambausek
8.2 Bottling-plant In the second example a typical problem of noise at the workplace is dealt with. Everywhere in the bottling-hall there is a dominating noise at 400 Hz, which is produced by a sterile-air fan and radiated by the sterile-air ducts and the packing machines for paper bags (Fig. 12). In this project--also still current--a silencer adjusted to 400 Hz was developed, which shows the insertion loss as represented in Fig. 13 (measured according to ISODIS 7235). Sound absorbers of this construction as shown in Fig. 14, are to be inserted between the fan and the connected sterile-air filters in order to prevent transmissions via the relatively light plastic sterile-air ducts. It is quite evident that porous or fibrous absorbing materials cannot be used in such a hygienically extremely sensitive area of the bottling-plant, even if they could be easily acoustically adjusted to the frequency range in question.
Fig. 14. Membrane silencerfor sterile air.
Sound absorbers of a novel membrane construction
215
REFERENCES 1. Ackermann, U., Fuchs, H. V. & Rambausek, N., Neuartiger Schallabsorber aus Metali-Membranen. Gesundheits-Ingenieur, 108 (1987), 67-73. 2. Kiesewetter, N., Schallabsorption durch Platten-Resonanzen. GesundheitsIngenieur, 101 (1980), 57-62. 3. Fasold, W., Schallabsorber und ihr Einsatz in Wohn- und Gesellschaftsbauten. In: Taschenbuch Akustik, ed. W. Fasold, W. Kraak, & W. Schirmer, Verlag Technik, Berlin, 1984, 911-29. 4. Mechel, F. P., Schallabsorption. In: Taschenbuch der Technischen Akustik, ed. M. Heckl & H. A. Miiller, Springer-Verlag, Berlin, 1975. 5. Kurtze, U., Untersuchungen an Kammerd~impfern. Acustica, 15 (1965) 139-150. 6. Mechel, F. P., Fuchs, H. V. & Purshouse, M., A new facility for testing ducted sound attenuators under the influence of flow. In: Noise Control: The International Scene, INTER.NOISE '83, Edinburgh 1983, pp. 387-90. 7. Ackermann, U. & Mechel, F. P., EinfluB der Schallfeldanregung aufdie Wirkung von Schalld~impfern in Kan~ilen. In: Fortschritte der Akustik (DAGA '85), VDEVerlag, Berlin, 1982, pp. 767-70. 8. Ackermann, U., Fuchs, H. V. & Rambausek, N., L/irmminderung im Abluftkanal einer Papierfabrik. IBP-Mitteilung 136 (1987). 9. Ackermann, U., Bestimmung der Einfiigungsd~impfung im Schalld~impferPriifstand. IBP-Mitteilung 106 (1986).
Sound Absorbers o f a Novel Membrane Construction
U. Ackermann, H. V. Fuchs & N. Rambausek Fraunhofer-Institut f'tir Bauphysik, D-7000 Stuttgart 80, FRG (Received 21 March 1988; revised version received 15 April 1988; accepted 21 April 1988)
A BS TRA C T Porous or fibrous materials are used in a great variety of applications to absorb acoustic energy at medium and high frequencies above approximately 200 Hz. There are, however, cases where their open and rough surface entails certain disadvantages with respect to hygiene and cleaning requirements. For medium and low frequencies, which would also require a relatively large absorber thickness and weight, there is a needfor an alternative absorber, the acoustically active components of which are formed exclusively by, ideally, even and smooth membranes. The sound absorber presented herein is of the reactive or resonant type with several different modes of vibration excitable in a complex system of rather thin, though comparatively stiff metal or plastic membranes. The absorption of the vibrational energy as stimulated by the sound waves impinging on the new type of acoustic lining or splitters is brought about solely by frictional forces in bounded shear layers formed in between specially shaped membranes moving against each other and relative to the air volumes adjacent to and between them. When suitably adjusted to the particular noise spectrum, these combined vibrational and damping mechanisms enable the construction of a new generation of sound absorber which no longer requires additional porous material to be incorporated in order to make it effective in a broad band of medium and lowfrequencies. The honeycomb structure, which forms the solidframe for the acoustic absorber, also makes it a relatively light and stable construction element for both room acoustic and industrial applications, where contamination of the absorber by dust in the air or pollution of the environment by abrasive deposits from the damping material is to be avoided under all circumstances. 197 Applied Acoustics 0003-682X/88/$03-50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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U. Ackermann, H. V. Fuchs, N. Rambausek
1 REQUIREMENTS ON SOUND-ABSORBERS IN A I R H A N D L I N G DEVICES In industrial and household appliances air, exhaust gases etc. are transported through flow ducts, containers and outlets. These flow ducts often represent a very undesirable transmission path for airborne sounds between the rooms interconnected by them. In particular they transmit the noise emanating from the flow-producing equipment, e.g. a fan. A reasonably effective way of controlling the noise in such cases is to line the flow duct with sound-absorbing material or to install sound-absorbing splitters in the duct. If sediments are carried with the transported medium, the soundabsorbing elements should not take them up in any circumstances. For understandable reasons of safety, sanitation and fire protection contamination of the absorber is to be avoided. Especially great demands are made on air-handling devices in 'clean rooms' for the production of semi-conductor elements, in hospitals, kitchens for large-scale catering, and in supplying packing appliances in the food industry with sterile air, and therefore, in these cases, it is desirable for the sound-absorbing linings and splitters to be hermetically sealed by gas- and water-tight coverings. In addition, the surface of the absorbers facing the flow should be as smooth and even as possible to keep frictional losses as low as possible. Moreover an additional demand especially made on air-handling devices is to damp sounds at low frequencies efficiently. This is connected, among other things, with the fact that the noise spectrum of a fan shows a falling characteristic with rising frequencies, even at the point of sound emission. A similar tendency can be observed with aerodynamically produced noise in the flow duct, which is also dominated by low frequencies. Even if a noise rating adapted to the human ear is carried out, frequency components around 250 Hz, or even much below that value, often remain dominant at the point of sound immission. Therefore great efforts are made to improve the capability of silencers to reduce noise at low frequencies.
2 A D V A N T A G E S OF T H E M E M B R A N E ABSORBER Conventional absorbers employing porous materials do not fulfill the above-mentioned requirements in all respects. They are susceptible to sediments and can become breeding-places for bacteria. In air-handling devices in hospitals and power-plants the silencers therefore have to be cleaned and decontaminated at regular intervals: The splitters have to be
Sound absorbers o f a novel membrane construction
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removed and cleaned with compressed air or steam, brushes, solvents or decontaminators. The surface of conventional absorber elements is covered with plastic foil and perforated plates or corrugated wire netting. Since, for acoustical reasons, the foil can only be very thin, it is easily damaged during transport, installation and operation, thus moisture can intrude into the porous filling, and the danger of contamination is further increased. For the acoustic efficiency of the absorbing material in the conventional sound absorber splitters the foils covering this material have to be as permeable to sound as possible. That is, the foils have to be very thin, and they must be able to vibrate freely in response to the impinging sound waves. They must not come into close contact either with the absorbing material behind them or with the perforated plates in front of them functioning as mechanical protection. The first condition is hard to realize from the point of view of construction, and the second requirement can easily be affected by deposits during operation. Foils, therefore, cannot guarantee perfect functioning of the silencers. Experience has shown that splitter silencers of conventional construction, therefore, cannot meet all requirements in practice. 3 S O U N D - A B S O R B E R S - - C O M P L E T E L Y M A D E OF MEMBRANES Searching for a sound-absorber for air-handling devices which could be adjusted to low frequencies and striving to protect the conventional fibrous or porous absorber filling against deposits has, after some detours, led the IBP to the development of an absorber which does not incorporate any porous materials.' In cooperation with a commercial firm silencer splitters have been realized which avoid the disadvantages of conventional absorbers and fulfil the above-mentioned requirements of integrity and stability: • The splitters consist only of metal foils. No flammable materials are used. • The splitters are hermetically sealed against the flow, so that no deposits can intrude into the splitters. • All components of the splitters are sufficiently solid, so that neither their mechanical nor their acoustical functioning can be affected during transport, installation, or by the flow. • For comparable installation geometry the acoustic efficiency in the frequency range below 500 Hz is better with the new splitters than with those filled with mineral wool. • Their damping is adjustable to a certain degree, e.g. also to tonal components in the noise spectrum.
200
U. Ackermann, H. II. Fuchs, N. Rambausek
• The pressure drop due to the installation of the splitters is kept to an absolute minimum. • Their smooth outer surface gives deposits no chance to settle on them. • The new splitters can be produced with considerably less weight. • They can also be used in an environment which forbids the installation of porous absorbers from a hygienic point of view. The new principle of construction ensures that the load-bearing components of the silencer also function as the framework for the resonance systems and that application of fibrous and porous filling material is avoided.
4 R E S O N A N C E EFFECTS REVISITED If installing an outside membrane of relatively high mass (between 200 and 1000g/m 2) prevents sound-waves transmitted through the flow duct entering the absorber, the frictional effects at the fibres or in the pores of conventional absorbing material can no longer be activated. Instead, however, a twofold membrane system can be constructed in a way that it can be excited into various forms of vibration by the sound-field impinging on the outside membrane. In order to develop sufficiently broadband sound absorbers based on this principle, a wealth of empirical knowledge and practical experience is necessary, because a theoretical treatment so far can only roughly explain the position of the resonance frequencies, and not the damping of these vibrations. It is even rather tedious to estimate the absorption of a simple baffle resonator resulting from energy dissipation in the baffle or from radiation damping in the sealed air cushion. 2 In spite of this two well-known resonance mechanisms may serve as the basis for understanding the principal mode of operation of the new absorber and for approximately selecting construction parameters for adjusting the silencer to a predetermined frequency range. 4.1 Baffle resonators
For an adjustment to low frequencies the best-known absorbers are those consisting of plates or foils installed in front of a wall. Mass M of the plate, spring stiffness F of the air enclosed between the plate and the wall, the density of the plate (PM) and of the air (PL), the sound velocity in air (CL)the
Sound absorbers o f a novel membrane construction
201
thickness o f the plate (s), and the distance from the wall (t) allow us to determine the resonance frequency fp. 1 /--~ fP = ~
% = 2n
p~L1 = 5 4 0 0
L/~__Hz, p
"VPM s t
~q PM s t
(i)
110 e.g. for aluminium: fp = ~ with s and t given in cm. In Fig. 1 the splitter layout (an aluminium plate with s = 0.06 cm, installed 10cm in front of a woodchip board of 1.3 cm) shows that a resonator constructed in this way is only effective in a very narrow frequency band a r o u n d fp = 140 Hz. If higher damping is to be achieved in a broader band, it is recommended in most cases that the air gap is filled with absorbing material.
I
I
I
I
I
30:
g T, 2 0 "E t-.--
10
0
I
I 0
1 O0
I 200
I 300
400
I HZ 5 0 0
Frequency
Fig. 1. Transmission loss of splitter silencersof the simplebaffleresonator type: s = ff06-crn aluminium plate; t = 10-era distance to a woodchip board of 1.3 crn; resonance frequency according to eqn (1): fp = 140Hz; length of the splitter-- 1m.
202
U. Ackermann, H. 11".Fuchs, N. Rambausek
In order to reach frequencies around 63 Hz dimensions s = 0.1 cm and t = 30 cm are required. It is known, however, that excitation at this or even lower frequencies is rendered difficult in terms of bandwidth. That is why it has proved to be advantageous 3 to include a second resonance mechanism in the development. 4.2 Helmhoitz resonator
Basically resonances at low frequencies can also be produced by springmass-systems where the vibrating mass M is the air plug in the holes of an inflexible plate in front of a wall (also assumed to be inflexible) and spring F is that of the air cushion between the plate and the wall. The resonance frequency fH is given by: fH = ~
V(I + AI, + Ala) = 5400
V--/*
(2)
where S is the area in cm 2, I* effective length of hole in cm and Vthe enclosed air volume in cm a. The end correction values AIi and Ala stand for the air mass vibrating outside the hole, differing according to the geometry of the hole (see e.g. Ref. 4). Naturally splitter silencers are not usually constructed with Helmholtz resonators only. Fig. 2 shows the typical narrow-band character of damping by such splitters. The maximum damping at fr~ = 170Hz disappears, as expected, if the holes are sealed. To improve the damping characteristic it is recommended, similarly, to fill the air gap, in the baffle resonators, at least partly, with absorbing material. The resonance frequency and the process of damping are also influenced by the form of the holes 4 and the depth of the chambers, s The two main disadvantages of all resonance absorbers constructed with open cavities are, however, that they may take up deposits and that they can generate considerable self-noise by interaction with the flow. Covering the holes e.g. by a foil would prevent--according to prevailing understanding--the excitation of the Helmholtz resonators. A foil directly fixed onto the perforated plate prevents air vibrations, as shown in Fig. 2; according to this philosophy a foil fixed at a certain distance from the plate should be thin (e.g. PE foil < 50 #m, according to Ref. 3) so that sound can penetrate easily into the resonator--so thin that it would be easily damaged under extreme environmental conditions. If a sufficiently broad-band absorber fulfilling the specifications mentioned above is to be constructed without using porous materials and superfine foils, the construction of splitter silencers must be changed in some important respects.
203
Sound absorbers o f a novel membrane construction
--t --q --t ____t__ 40 dB 30 o
"F,, on
20
I---
lO
0
I 0
" 100
"1"
" 200
I 300
400
I Hz
500
Frequency
Fig. 2. Transmission loss of splitter silencers of the Helmholtz resonator type: Area of the holes: S = n/4 cm2;Volume of the chambers: V= 1000cm3;End corrections: 1" = All + I A I , 0.8 cm; Resonance frequency according to eqn (2): fM= 170 Hz; Length of splitter: 1m. - holes open; - - - holes sealed.
=
5 ALL-METAL MEMBRANE CONSTRUCTION OF ABSORBERS The newly developed sound absorber combines the baffle resonator with the Helmholtz resonator into broad-band m e m b r a n e absorber without using additional absorbing material. A typical layout of a silencer splitter which is to be used in flow ducts and which fulfils the specification, is shown in Fig. 3. In the layout shown in Fig. 3, which is only one example of m a n y possible and successfully tested ones, a large n u m b e r of chambers is built on each side o f a - - f r o m the acoustical point o f view--stiff bearing plate (5). These chambers a r e - - a l s o from the acoustical point o f view--hermetically sealed against each other by a honeycomb structure (2). By covering each side with a perforated m e m b r a n e (3) each of the newly formed closed chambers is
204
U. Ackermann, H. V. Fuchs, N. Rambausek 4
s
3
G;G-2_TS
2
/A
A A /"
I
1 4
3
2
5
Fig. 3. Example of a membrane absorber with cubic honeycomb structure: 1, Frame of the splitter (e.g. 0"5-mm aluminium); 2, Walls of the chambers (e.g. 0"2-mm aluminium); 3, Perforated membrane (e.g. 0"l-mm aluminium); 4, Covering membrane (e.g. 0-2-mm aluminium); 5, Bearing plate (e.g. 0-5-mm aluminium).
provided with a certain hole, which makes them resemble conventional Helmholtz resonators. Finally, both sides of the splitter are covered with a cover membrane (4) which closes all the inside absorber elements with an airand water-tight seal at the frame of the splitter. In order to fulfil its protecting function even under hostile conditions, and in order not to be deformed too much by irregular flow forces and not to buckle under nonsteady operating conditions, the outer membrane is fixed in some places near the partitions of the honeycomb structure (2). With this special example of construction a more or less strong spring/ mass resonance created by the enclosed air cushion as spring and the two membranes as masses, is expected--even according to conventional understanding. For the silencer arrangement shown in the upper part of Fig. 4, however, the narrow-band spectrum of the transmission loss, in fact, indicates an encouraging efficiency of on average 15 dB for a splitter length of L -- i m in a broad frequency range between 150 Hz and 500 Hz. The damping characteristic in Fig. 4 represents an example not yet optimized. It can be adjusted to almost all noise spectra in the medium and lower frequency range by choosing the respective geometric parameters of the chambers and membranes. If a somewhat thicker cover membrane is used,
Sound absorbers o f a novel membrane construction
30
I
I
I
I
205
I
dB
20
o
j I
0
\
I 100
I 200
I aO0
400
I Hz
500
Frequency
Fig. 4. Damping characteristic of a silencer prototype according to Fig. 3: Chamber walls: 0.2-mm aluminium; Perforated membrane: 0.1-mm aluminium; Cover membrane: 0-l-mm aluminium (fixed directly in front of the perforated plate); Depth of the chambers, t: 10 cm; Volume of the chambers, V: 1000cm3; Diameter of the resonator holes, d: 2cm. (Splitter arrangement as in Fig. 1).
the damping can e.g. be shifted to lower frequencies, with, however, a diminution of the damping of higher frequencies. Both the length l of the resonator throat and the distance between perforated and cover membranes play an important role in adjusting the silencer. By throats projecting into the chambers the maximum can be shifted to lower frequencies. If, however, all the other parameters are left unchanged, the level of maximum absorption decreases. But it is still sufficient, as Fig. 5 shows, to exceed the efficiency of porous absorbers of the same geometry at these low frequencies. 6 SILENCER TEST FACILITY The development and testing of the new sound absorbers would have been very expensive and time-consuming because of the great variety of adjustable parameters, if an area of about 12 m 2 had had to be provided for
U. Ackermann, H. I,'. Fuchs, N. Rambausek
206
I ~ . . . . . . ~ , , . . , ~
0
Q 00
~ - T ' ...
1
T(
J
0
30
a)
dB .J
20
rj
L~
L-~ I L- I L,,,I
10
0
r "J
S 63
125
1 250
500
" Ik
2k
4k
8kHz
Frequency
Fig. 5. Comparison of the insertion loss of silencer splitters with the same geometrical construction, using: (a) mineral wool; (b) membrane absorbers. each prototype in order to measure the degree of absorption in the reverberation chamber according to DIN 52 212/ISO R354. Furthermore it is not certain whether the resonance mechanisms, which are very sensitive to even small changes in material and construction parameters, could have been recognized correctly and optimized by tests in the reverberation room. Fortunately, a particularly suitable piece of measuring equipment was available for these investigations for the first time: the silencer test facility of the IBP (Fig. 6) allows tests according to the ISODIS 7235 'Measurement procedures for ducted silencers'. 6 For optimizing the prototypes with a maximum efficiency at medium frequencies only 2 m 2 of absorbers had to be produced employing a special glueing technique. The systematic tests carried out simultaneously with this project and the numerous improvements of the whole measuring technique for silencers in ducts will not be dealt with here (see instead Ref. 7). But there is no doubt that it is the available measuring technique which has probably made the
Sound absorbers of a novel membrane construction
Fig. 6.
207
Silencer test facility of the IBP.
development possible at all, or at least easier. Fig. 7, e.g., shows how a microphone, run pneumatically, can be moved along a rail between the silencer splitters to scan the sound-field.
7 A D V A N T A G E S OF T H E N E W SILENCERS The completely smooth cover membranes on both sides of the silencer splitter (according to Fig. 3) guarantee the smallest skin friction resistance possible and thus reduce the pressure drop to an absolute minimum. Under extremely dirty operating conditions, as they may occur e.g. in exhaust stacks, the whole splitter can be cleaned easily by washing, hosing, or dipping.
208
U. Ackermann, H. V. Fuchs, N. Rambausek
Fig. 7. Microphone scanning the sound-field between the silencer splitters.
If the frame (1), all the dividing and reinforcing chamber walls (2), the bearing plate (5), the perforated (3) and the cover (4) membranes are made of metal, the splitters are non-flammable. Hygiene demands can also be fulfilled. If the cover membranes are arranged directly in front of the perforated membranes, the wall does not have to be too thick to achieve a high resistance to mechanical damage of the outer covering. The outer covering of the splitter can be protected against chemical attack and corrosion in a way very similar to that of protecting all the other components of the flow duct. Thus the most important additional demands of practical use are fulfilled. The honeycomb construction makes the splitter highly stable against compression, suction and torsion. The frame (1) taken from conventional silencer splitters with a folded edge of usually 2 cm, all the way round, leaves all the installation spaces unchanged; the new splitters can be transported in the same way as all conventionally built splitters. Since the splitters can be completely constructed from one and the same material, they show no critical behaviour, not even under large temperature changes. Even ice on the smooth outer skin would not drastically reduce their effectiveness. Since there is only hollow space between the honeycomb construction, which does not have to be filled with porous or fibrous
Sound absorbers o f a novel membrane construction
209
material, the weight of the silencer splitters is lower than the weight of those filled e.g. with mineral wool. Silencers with porous absorber fillings cease to function properly if their depth is very small compared with the sound wavelength. Since this lower frequency limit can be somewhat lowered with the new construction, silencers can be adjusted to lower frequencies according to the abovementioned principle for a given space available. These features enable the new all-metal membrane absorber to be used in fields of industrial noise abatement where conventional absorbers cannot be used at all or only in a limited way because of several precautions as mentioned at the beginning of this paper.
8 PRACTICAL APPLICATIONS The application of the membrane absorber is certainly not limited to such special use where less integrity and a less hygienically critical type of absorber is forbidden. In spite of that, current consulting projects at the IBP suggested that for the first practical tests examples should be selected where conventional absorbers had already failed (paper-mill 8) or where they could not be used for reasons of sterility (e.g. bottling-plant9).
8.1 Paper-mill The paper-mill was an example of a very typical emission problem. The noise of the vacuum pumps is radiated at the opening of the exhaust stack and exceeds the rating levels permitted in the neighbourhood of VDI Regulation 2058, Part 1. Tests lasting for years to reduce the emission level by installing conventional silencers with mineral wool filling in the stack, have failed because of both the mechanically (vibration) and chemically (acidity of the air) harsh working conditions. Additionally, after only a very short time, thick deposits of paper refuse appeared on the splitters, which were enclosed by a stainless steel grid. Even relatively thick plastic foils, where the mineral wool filling was pressed in with the help of the above-mentioned grid, resisted only for a few days or weeks--a completely unsatisfactory situation. A test installation of the new absorbers in just this situation was also suggested by the fact that the plant is regularly turned off for a short time for inspection, so that there was an opportunity to observe the behaviour of the membrane silencer over a longer period of time. Figure 8 shows the resulting noise reduction in this case, measured at the opening of the stack. The two noise peaks in the A-weighted spectrum at 160 and 315 Hz were reduced by as much as 12 and 18 dB respectively by installing
210
U. Ackermann, H. V. Fuchs, N. Rambausek 80
dB (A) 70
60
t~
so
if)
40
30
2O
31,5
63
125
250
500
lk
2k
4kHz
Freq uency
Fig. 8. Air-borne sound spectra, measured in third-octave bands and A-weighted, 1 m away from the opening of the stack on the roof of the paper-mill: (a) before, and (b) after installing membrane silencers.
10-cm deep and 1-m long splitters into a chamber in front of the vertical part of the stack. The photograph of Fig. 9 was taken three months after the installation: The walls clearly show the sediment from the flow, inevitable because of their rough surfaces. At the top of the photograph the very dirty covering of the conventional porous lining can be seen. At the bottom of the photograph the stainless steel frame can be seen which enabled the new silencers to be installed in the chamber. In Fig. 10 traces of corrosion can also be seen both on the 0.2-mm thick aluminium membrane and the stainless steel frame, but this corrosion did not reduce their acoustic efficiency at all. Deposits as on the channel wall, however, cannot settle on the ideally smooth surface of the membrane splitters.
Sound absorbers o f a novel membrane construction
Fig. 9.
211
Noise attenuators of the old and the new type in the exhaust stack of a paper-mill.
In a second step a 50-cm deep membrane splitter was adjusted to the noise peak at 40 Hz. In Fig. 11 a photograph shows (in four steps), how this silencer is constructed. The splitter at the bottom has no membranes, the honeycomb structure can be seen very clearly; the splitters in the middle are both covered
Fig. 10.
The membrane absorber after 100days in the exhaust stack.
212
U. Ackermann, H. 11".Fuchs, N. Rambausek
Fig. 11. Membrane silencer for a maximum absorption at 40 Hz.
with perforated membranes, the holes of which have been treated in various ways. The splitter on the top is ready for use, i.e. the covering m e m b r a n e is fixed. Considering the level reduction of 10 dB according to Fig. 8 one has to see that tackling the problem with conventional silencers would not have been equally successful at such low frequencies.
OJ
40
iO
50
70
80
~'ig. 12.
~
L
u~
-
90
100
dB
110
31,5
63
125
500
Frequency
250
lk
2k
4k
8kHz
~ound pressure spectra in the bottling-hall for paper bags not far from a sterile-air fan.
16
....
125
/ ~_._r
-
250
J
Ik
2k
4k
"1. J--L..L.j_.~"
Frequency
500
L
i . . . . . . . . . . . . . .
l
.j
8kHz
II-'~~
_t. u~l"
Fig. 13. Insertion loss of a membrane silencer for the sterile-air mpply device of a bottling-plant adjusted to 400 Hz, measured in the silencer test facility at the IBP. 9
0
0
,
o ,
214
U. Ackermann, H. V. Fuchs, N. Rambausek
8.2 Bottling-plant In the second example a typical problem of noise at the workplace is dealt with. Everywhere in the bottling-hall there is a dominating noise at 400 Hz, which is produced by a sterile-air fan and radiated by the sterile-air ducts and the packing machines for paper bags (Fig. 12). In this project--also still current--a silencer adjusted to 400 Hz was developed, which shows the insertion loss as represented in Fig. 13 (measured according to ISODIS 7235). Sound absorbers of this construction as shown in Fig. 14, are to be inserted between the fan and the connected sterile-air filters in order to prevent transmissions via the relatively light plastic sterile-air ducts. It is quite evident that porous or fibrous absorbing materials cannot be used in such a hygienically extremely sensitive area of the bottling-plant, even if they could be easily acoustically adjusted to the frequency range in question.
Fig. 14. Membrane silencerfor sterile air.
Sound absorbers of a novel membrane construction
215
REFERENCES 1. Ackermann, U., Fuchs, H. V. & Rambausek, N., Neuartiger Schallabsorber aus Metali-Membranen. Gesundheits-Ingenieur, 108 (1987), 67-73. 2. Kiesewetter, N., Schallabsorption durch Platten-Resonanzen. GesundheitsIngenieur, 101 (1980), 57-62. 3. Fasold, W., Schallabsorber und ihr Einsatz in Wohn- und Gesellschaftsbauten. In: Taschenbuch Akustik, ed. W. Fasold, W. Kraak, & W. Schirmer, Verlag Technik, Berlin, 1984, 911-29. 4. Mechel, F. P., Schallabsorption. In: Taschenbuch der Technischen Akustik, ed. M. Heckl & H. A. Miiller, Springer-Verlag, Berlin, 1975. 5. Kurtze, U., Untersuchungen an Kammerd~impfern. Acustica, 15 (1965) 139-150. 6. Mechel, F. P., Fuchs, H. V. & Purshouse, M., A new facility for testing ducted sound attenuators under the influence of flow. In: Noise Control: The International Scene, INTER.NOISE '83, Edinburgh 1983, pp. 387-90. 7. Ackermann, U. & Mechel, F. P., EinfluB der Schallfeldanregung aufdie Wirkung von Schalld~impfern in Kan~ilen. In: Fortschritte der Akustik (DAGA '85), VDEVerlag, Berlin, 1982, pp. 767-70. 8. Ackermann, U., Fuchs, H. V. & Rambausek, N., L/irmminderung im Abluftkanal einer Papierfabrik. IBP-Mitteilung 136 (1987). 9. Ackermann, U., Bestimmung der Einfiigungsd~impfung im Schalld~impferPriifstand. IBP-Mitteilung 106 (1986).