Acoustics – Sound-modifying means – Muffler – fluid conducting type
Utility Patent
1999-08-13
2001-01-02
Dang, Khanh (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S252000, C181S255000
Utility Patent
active
06167984
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device and a method for sound reduction in a transport system for gaseous medium The gas transport system is primarily intended for an exhaust system arranged in an internal-combustion engine of a ship, whereby the noise generated from the outlet of the exhaust system is to fulfil certain predetermined requirements with respect to sound. However, the invention may be advantageously applied also to ventilation plants, exhaust gas plants in, for example, vehicles with internal-combustion engines, or flue gas cleaning devices for plants for production of electric power.
BACKGROUND OF THE INVENTION
For the purpose of reducing the sound which is emitted especially from the orifice of a ventilation system or an exhaust system, it is known to arrange one or more sound attenuators in the gas channel of the system. The term “sound attenuator” usually means a device with the ability to consume sound energy. This can take place by the sound energy being transformed into some other energy form, such as, for example, heat, the energy of which may be diverted and cooled. As discussed below, the term “resistive attenuator” constitutes a device in a gas channel which is capable of absorbing sound, that is, of transforming the sound energy into another energy form. The term “attenuator”, as raised herein, means an apparatus which is capable of reducing sound, and attenuation means the property of reducing sound.
One typical embodiment of a resistive attenuator is a round 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 patent document GB 2,122,256. From the patent 2,826,261, another resistive attenuator intended for an exhaust system is previously known. As absorbent there is usually used mineral wool or glass wool including some adhesive which causes the absorbent to have a bonded structure. The absorbent may also be protected by an air-permeable surface layer, for example a perforated plate, to attain greater 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 and is dependent, besides on the thickness and the rate of flow of the absorbent, also on the length and the inner area of the attenuator.
The ratio of the absorbent thickness to the length of the acoustic waves which are part of the sound is determining for the attenuation at lower frequencies. A satisfactory attenuation is achieved for sound frequencies at which the thickness of the absorbent is larger than a quarter of a wavelength of the sound. The sound attenuation properties then decrease drastically for sound of lower frequencies which has a greater wavelength. Even when the ratio of the wavelength to absorbent thickness is about 1/8, the absorption is only half as great, and at the ratio 1/16 it is only 20% of the absorption which is obtained at the ratio 1/4. Since a certain absorption capacity still remains, in many cases a sufficient absorption may be obtained by increasing the length of the total absorbent in the gas transport system. Also, the cross-section area of the gas transport system is of importance for the sound reduction obtained since the reduction in the upper frequency range of the sound decreases with increased cross-sectional area.
A problem with the resistive attenuator is thus that the absorbing layer must be made thick to be able to absorb low frequencies. This entails a large volume. A smaller absorbent thickness may, however, be compensated by a larger total length of the attenuator. This leads to an increased cost of the sound reduction obtained. Another problem is that the pressure reduction in the system must be limited. This leads to a relatively large cross-section area of the system. The sound reduction at the upper frequency range of the sound is thus reduced. The sound-attenuating properties are also dependent on where in the system the sound attenuator is placed. It often appears that the properties which are obtained in a laboratory, especially at low frequencies, and which are described in pamphlets, are seldom obtained in practice. This leads to a great oversizing in order to ensure a sufficient sound attenuation.
Another known way of reducing the sound emission from a gas transport system is to prevent the sound from propagating in the channel. This can be achieved by arranging reactive obstacles in the gas channel. One such obstacle is obtained by creating a sound which is out of phase with the sound in the channel, whereby extinction occurs. This technique is used preferably in connection with so-called active sound attenuation. The oppositely directed sound is then created by a loudspeaker placed in the channel. However, extremely controllable conditions are required in order for an active system to function.
One further way of reducing the sound which reaches the orifice is to arrange an obstacle to the progressing acoustic wave in the channel. This type of sound attenuator actually consumes no energy and is usually named reactive attenuator. A reactive attenuator substantially operates according to two principles. The first type is a reflection attenuator. This comprises an increase of the cross-sectional area, whereby the area increase gives rise to a reflection wave which propagates in a direction opposite to the propagation of the sound. From a functional point of view, the obstacle may be regarded as a wall, from which the sound rebounds. The second type of abstacle is a resonance attenuator, which influences the propagation of the sound in a channel. In this case, the obstacle may be regarded as a pitfall, into which the progressing sound falls on its way towards the orifice.
Resonance sound attenuators comprise two main types, namely, quarter-wave attenuators and so-called Helmholtz resonators. The latter is tuned to one frequency only, whereas a quarter-wave attenuator is tuned to a certain tone but also influences its odd harmonics. The quarter-wave attenuator usually comprises a closed pipe which is connected to the channel and which corresponds to a quarter wavelength of the sound to be attenuated. Its attenuating properties usually cover a very narrow frequency range. One problem with a reactive attenuator is that the volume must be tuned to the frequency of the sound to be prevented. Another, and much more difficult, problem to overcome with regard to a reactive attenuator is that it is very sensitive to where it is located in the system. By regarding the sound as something that propagates in steps and the obstacle as a pitfall, into which the progressing sound is to fall, it is easily realized that it is important to place the orifice of the pitfall correctly in relation to the length of step. An incorrectly placed pitfall implies that the sound may step over without resistance. To obtain a maximum attenuating effect, the orifice of the quarter-wave attenuator must thus be placed in a pressure maximum of the sound field in the channel.
There are also a great number of devices which in various ways combine the methods mentioned above. However, the problem is usually that the various comnponents end up in different locations where they are not effective. To compensate for the unforeseeable properties, conventional sound attenuator systems are often greatly oversized, which leads to expensive, heavy and space-demanding plants with high pressure drops.
Sound attenuator devices in transport systems for gas, where the gas changes temperature, implies further complications since the wavelength of the sound is changed with the temperature. If 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
Götmalm {umlaut over (O)}rian
Johansson Claes-Goran
ABB Flakt AB
Dang Khanh
Pollock Vande Sande & Amernick
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