Classifications

• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/162—Selection of materials
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N1/00—Silencing apparatus characterised by method of silencing
  • F01N1/24—Silencing apparatus characterised by method of silencing by using sound-absorbing materials
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
  • F24F13/00—Details common to, or for air-conditioning, air-humidification, ventilation or use of air currents for screening
  • F24F13/02—Ducting arrangements
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
  • F24F13/00—Details common to, or for air-conditioning, air-humidification, ventilation or use of air currents for screening
  • F24F13/24—Means for preventing or suppressing noise
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N2310/00—Selection of sound absorbing or insulating material

Definitions

• the present invention relates to an absorbent for a dissi- pative absorption of sound.
• the invention relates to a sound attenuator incorporating the absorbent and to a method for sound reduction in a system for transport of a gaseous medium.
• a transport system comprises a ventilation system.
• the gas transport system comprises an exhaust gas system and in particular an exhaust gas system for an internal-combustion engine, for example in a ship.
• the device and the method relate to a channel, from the wall or outlet of which noise is generated, which may be subjected to acoustic requirements.
• the invention is also advantageously applicable to other elongated gas transport systems, such as in exhaust gas plants in, for example, vehicles with internal-combustion engines or in flue-gas cleaning devices for plants for, for example, production of electric power.
• sound By sound is meant a physical phenomenon which gives rise to hearing sensations.
• sound is regarded as a wave motion in a gaseous medium. Sound may, however, also be transported in other media, such as fluids and solid materials.
• the sound propagates as a longitudinal wave motion at a velocity of about 340 m/s. However, the velocity is dependent on the temperature of the medium.
• the audible sound comprises frequencies from about 20 Hz to about 20,000 Hz.
• the wavelength of the audible sound in air with a normal temperature thus varies from the order of magnitude of 3 m at low frequencies (-100 Hz) , 30 cm for sound at intermediate frequencies (-1000 Hz) , and 3 cm for sound at high frequencies (-10,000 Hz).
• the sound may vary greatly with both amplitude (sound intensity) and time.
• a traditional absorbent sound energy is transformed into heat by flow resistance of the absorbent.
• Such an absorbent substantially exhibits resistive attenuation.
• Other words for this are dissipative or viscous attenuation.
• the ratio of the thickness of the absorbent to the length of the sound waves which are included in the sound has proved to be decisive for the attenuation at lower frequencies in such a traditional absorbent.
• a satisfactory attenuation is attained at these absorbents for sound frequencies at which the thickness of the absorbent is larger than a quarter of a wavelength of the sound.
• the sound attenuating properties then decrease drastically for sound with lower frequencies, which has a larger wavelength.
• Well-known materials for the manufacture of a resistive absorbent are mineral wool and glass wool.
• the wool is retained by an adhesive which causes a homogeneous structure in the absorbent.
• these absorbents are less suited since bacteria may develop in the absorbent fibres may loosen.
• a common requirement in such hygienic environments is that an absorbent shall be capable of being flushed.
• the known absorbent has proved to be less resistant and may retain moisture for a long period of time. After repeated flushing, the absorbent is gradually dissolved.
• the known wool is built up of brittle fibres, in which case a less good mechanical strength is obtained in the absorbent. In case of heavy vibrations, the structure is decomposed in course of time.
• porous absorbents are available on the market and their sound-absorbing properties are known by measurements.
• the porous absorbents are characterized by thickness and density.
• One problem in the manufacture of circular attenuators for, for example, ventilation systems is that the absorbent, which is usually made flat, must be bent to fit into the attenuator. Depending on the original thickness of the absorbent, it will have a varying density in the circular design. On the inside of the sound attenuator, the density will be high and tendencies to folding will arise. On the outside, cracks will sometimes arise as a result of the hard bending.
• the known absorbent In environments with high gas velocities, the known absorbent is also less good. A surface-wiping gas tears with it fibres and particles from the absorbent. Successively, these end up in channels and spaces where they have a negative influence on the environment. The torn-off particles also result in the absorbent being gradually worn down and, in the end, disappearing entirely. In these contexts, it is known to coat the absorbent with a more stable layer, for example of thin plastic or a- perforated sheet. These coatings involve extra operations during manufacture and thereby tend to increase the cost.
• the designation sound attenuator here means a device which is capable of consuming sound energy. This may occur by transforming the sound energy into some other form of energy, such as, for example, heat.
• the designation resistive attenuator refers to a device which is capable of absorbing sound in a gas channel, that is, to transform the sound energy into another form of energy.
• the designation attenuator in the following text, means a device which is capable of reducing sound, and attenuation means the property of reducing sound.
• a resistive attenuator is a circular or square tube, the sides of which, exposed to the gas flow, are coated with an absorbent or a porous medium of small coupled cavities .
• a common such sound attenuator intended for a ventilation system is described in the patent document GB 2,122,256. From the patent document US 2,826,261, another resistive attenuator intended for an exhaust system is previously known.
• absorbent there is used a resistive absorbent of the type described above.
• the absorbent may also be protected by an air-permeable surface layer, for example a perforated sheet, to attain a longer service life and better mechanical stability at high gas speeds.
• Such a resistive attenuator will have a sound-attenuating property which covers a wide frequency range.
• the attenuation is also dependent on the thickness and flow resistance of the absorbent, the exposed absorbent surface, any surface protection such as, for example, a perforated sheet and the dimen- sions of the attenuator, such as the length and the diameter thereof .
• the sound- attenuating properties are also dependent on where in the system the sound attenuator is placed. It often turns out that the properties which are achieved in a laboratory, especially at low frequencies, and which are described in pamphlets, are seldom achieved in practice. This often leads to oversizing in order to attain a desired sound attenuation with sufficient certainty.
• Another known way of reducing the sound emission from a gas transport system is to prevent the sound from propagating in the channel. This may be achieved by arranging a reflecting obstacle in the gas channel. Such an obstacle is obtained by creating a sound which is in opposition to the sound in the channel, thus achieving extinction.
• One such technique is active sound attenuation. In connection with active sound attenuation, a sound is added which is directed in a direction opposite to the sound progressing in a channel. This oppositely directed. sound is then created by a loudspeaker placed in the channel.
• controllable conditions are required for an active system to function well.
• a reactive attenuator substantially operates according to two principles.
• the first type is a reflection attenuator. This comprises an increase of the cross-section area, whereby the area increase gives rise to a reflection wave which propagates in a direction opposite to the propagation of the sound.
• the function is a broadband function.
• the second type is a resonance attenuator.
• the function is a narrow-band function and may almost be regarded as a filter which eliminates pure tones from the sound.
• the orifice of a resonance attenuator must be placed in a pressure maximum of the sound field in the channel. The resonance attenuator is thus very sensitive to the position in the channel .
• Sound attenuator devices in transport systems for gas implies further complications since the wavelength of the sound is changed with the temperature. If, for example, the temperature of the gas is increased from 20°C to 900°C, the sound velocity and hence the wavelength increase twofold.
• An attenuator which operates well at normal temperature therefore suffers deteriorated properties, especially at low frequencies when the gas is heated. This usually results in sound attenuating devices in transport systems with hot gases becoming very bulky.
• the object of the present invention is to suggest ways and means of achieving an absorbent which has good absorbing properties within a wide frequency range and which is inex- pensive to manufacture. It shall be less space-demanding than prior art absorbents and be applicable to environments L J t t
• the inventive concept also comprises threads formed of other solid materials, such as, for example, metal.
• the thickness of the absorbent should be smaller than about 5 % of the cross-section area of the channel. Such a small limitation of the channel area only entails a minor pressure increase.
• the mat is reinforced with a net of, for example, metal.
• the absorbent is arranged elongated and penetrates through a greater part of the channel system.
• the absorbent is shaped as a guide vane, for example at bends and descents in the channel system.
• the absorbent according to the invention is also adapted to be placed in a resistive attenuator.
• such an attenuator is arranged in- combination with one or more reactive attenuators.
• the sound field in the channel may be locally controlled and optimized attenuating properties be obtained.
• Figure 1 shows the specific flow resistance versus the frequency of an absorbent according to the invention
• Figure 2 shows a cross section of an absorbent according to the invention
• Figure 3 shows the absorption versus the frequency of a few embodiments of an absorbent according to the present invention
• Figure 4 shows a cross section in the longitudinal direction of part of a system for transport of a gaseous medium according to the invention
• Figure 5 shows alternative cross section shapes of part of a system for transport of a gaseous medium according to the invention.
• the dynamic flow resistance has one resistive part and one reactive part.
• the resistive part of the resistance is a viscous attenuation which is independent of the frequency of the sound.
• the reactive part is mass-dependent and exhibits a resistance which increases with the frequency.
• the reactive resistance dominates in the frequency range of interest, that is, the frequency range where a good absorption is desired.
• it is thus the frequency-dependent reactive flow resistance which determines the absorption properties. Since the absorption decreases with increased flow resistance, the absorption in the frequency range of interest decreases.
• a majority of known porous absorbents have a great reactive resistance in the frequency range where absorption is desired. The known absorbents thus do not fulfil the condition that the normalized flow resistance is limited between one and two within a wide frequency range .
• Figure 1 shows the specific flow resistance versus the frequency of a porous absorbent.
• the resistive flow resistance is designated z and the reactive flow u> t t 1 H 1
• the properties of the absorbent thus manufactured are adjustable and optimizable for a desired purpose .
• an absorbent which, at normal temperature, has less good absorption properties will thus receive much better properties at higher temperatures .
• One absorbent which has this property is perforated sheet.
• Such an absorbent is suitably manufactured from a sheet with a thickness of 1 mm or less, with a degree of perforation which is less than 10 % and with holes which are about 1 mm or less. For a normal temperature, the holes would need to be smaller than one-tenth of a millimetre. Such a perforated sheet is difficult and costly to manufacture.
• Figure 2 shows a typical absorbent according to the invention. It consists of a thin mat 1 of long elastic fibres, which cross each other in all directions in an irregular pattern.
• the threads are manufactured of a plastic such as, for example, polyester.
• An advantage of this material is that, in case of fire, it is decomposed into water and carbon dioxide.
• other materials of elongated bendable threads or fibres are also possible.
• the figure also shows an advantageous embodiment of the absorbent in which a thin foil 2 is attached as protection in front of the thin mat. In the shown example, the foil is fused to the mat in a line pattern 3.
• the foil primarily consists of a polyethylene film but may also be another plastic material or a metal foil.
• Figure 3 shows the influence of a covering foil on the absorbent. Depending on the thickness or weight of the foil, an absorption - decreasing with the frequency - is obtained at high frequencies .
• the figure shows a typical basic absorption of a porous absorbent and the effect of three different thicknesses, 5, 10 and 20 ⁇ m, of such a foil.
• the foil across the greater part of the absorbent surface, should lie loosely adjacent to the mat. In the shown case, this problem is solved in that the foil is fixed to the mat in lines only. In the case of direct contact, such as by gluing or if the foil is pressed against the absorbent of, for example, perforated sheet, the absorption is deteriorated at high frequencies.
• a foil prevents particles from penetrating into the absorbent. It is thus suitable for use in environments involving environmental requirements.
• the foil-clad absorbent will also have better long-term properties since particles do not penetrate into and stop up the porous channels .
• Figures 4 and 5 show a transport system designed for a gaseous medium with a first 4, a second 5 and a third 6 channel section containing an absorbent 1 according to the invention. Since the absorbent is thin, it has very little influence on the cross-section area and thus gives rise to an extremely small pressure drop across the channel section. Because of its plasticity, the absorbent is suited to be arranged as a guide vane in the system, as shown in the example. The length of the absorbent is not, as in known sound attenuators, limited to the length of the attenuator itself but may be arranged optionally along the channel system.
• Figure 5 shows a few examples of how the absorbent is intended to be arranged in the transverse direction of the channel.
• the absorbent 1 is arranged in a laminated pattern 8, in a cross pattern 9, and in a circular pattern 10.
• Other shapes are also possible within the scope of the invention.
• the absorbent according to the invention is exceedingly suited to be arranged as a resistive attenuator together with a reflection or reaction attenuator in a channel system. By suitably dimensioning the properties of such attenuators, a very efficient attenuation may be obtained over a frequency interval such as, for example, a third octave band.
• the channel system is not limited to comprise a channel system with a circular-cylindrical cross section.
• the invention may, with an equivalent result, be applied to systems with a multi-edge cross section as well as to systems with longitudinally bent sections.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Multimedia (AREA)
• Soundproofing, Sound Blocking, And Sound Damping (AREA)
• Absorbent Articles And Supports Therefor (AREA)
• Saccharide Compounds (AREA)
• Medicines Containing Plant Substances (AREA)
• Exhaust Silencers (AREA)
• Gloves (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)
• External Artificial Organs (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 2000
  • 2000-09-18 SE SE0003349A patent/SE0003349D0/xx unknown
• 2001
  • 2001-09-17 EP EP01967894A patent/EP1319156B1/fr not_active Expired - Lifetime
  • 2001-09-17 AT AT01967894T patent/ATE321248T1/de not_active IP Right Cessation
  • 2001-09-17 DE DE60118221T patent/DE60118221T2/de not_active Expired - Lifetime
  • 2001-09-17 AU AU2001288177A patent/AU2001288177A1/en not_active Abandoned
  • 2001-09-17 US US10/380,850 patent/US20040099477A1/en not_active Abandoned
  • 2001-09-17 WO PCT/SE2001/001982 patent/WO2002023099A1/fr active IP Right Grant
• 2003
  • 2003-03-17 NO NO20031212A patent/NO321542B1/no not_active IP Right Cessation

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