EP0916817B1 - Active silencer - Google Patents

Description

The invention relates to a silencer, in particular for exhaust systems of motor vehicles.

For sound damping, for example in the automotive sector, active and passive systems are used. In the latter arrangements are often used in which there is a good sound attenuation if and only if a strong deviating from the characteristic impedance of this channel sound field impedance is generated at one point of an interfering sound channel. By this mismatch arises at this point a reflection of the noise. Therefore, such arrangements are referred to as reflection muffler. A strongly deviating from the characteristic impedance of Störschallkanales sound field impedance results, for example, at an open outlet of a hollow body when it is energized with its resonant frequency. In this case, the sound field impedance at the output of the hollow body goes to zero. A structurally particularly simple embodiment of this hollow body is a so-called λ / 4 resonator. Such resonators are known for example from " exhaust silencer for motor vehicles", The Library of Technology Bd. 83, p. 21, Fig. 13, verlag modeme industrie AG, 1993, ISBN 3-478-93076-6 . It is known that a closed pipe section is branched off in the manner of a resonator of a channel leading the background noise or a pipe. The branching point forms an acoustic coupling point of the resonator. In this resonator, a sound field is in resonance when ¼ of the wavelength or an odd multiple thereof fits into the cavity of the resonator. A disadvantage of this resonator is that a good sound attenuation is always achieved only if exactly ¼ of the wavelength or an odd multiple thereof is equal to the acoustically effective length of the resonator. The reason for this lies in the according to the structural design of the resonator fixed predetermined physical conditions At the closed resonator end, sound waves without phase jump are reflected at the resonator end forming an open acoustic resonator end and at 180 ° phase jump at the open resonator end at the closed resonator end. Therefore, resonances and thus good sound attenuation are possible only at certain wavelengths. Another disadvantage of this resonator is that for damping low frequencies particularly large and thus space-consuming resonator lengths are required.

Not least the mentioned disadvantages of the passive systems led to the development of the active systems, in which loudspeakers are used for influencing the Störschalls. From DE-A-4 226 885 an arrangement for active sound absorption in motor vehicles is known. This arrangement includes a Helmholtz resonator in which a loudspeaker is integrated to simulate different void volumes of the Helmholtz resonator. This active Helmholtz resonator absorbs that in its environment, e.g. in the passenger compartment of the vehicle existing noise. For this purpose, the sound field in the vicinity of the Helmholtz resonator energy is removed by sound energy is converted by the bottle neck-like entrance of the Helmholtz resonator into heat energy.

From DE-A-4 317 403 and from WO-A-93 05282 arrangements for active sound attenuation are known, in which a chamber containing a loudspeaker is branched off from an interference sound channel. The loudspeaker should emit compensation sound such that the two sound fields of background noise and compensation sound cancel each other as much as possible by superimposing or attenuating each other. In WO-A-93 05282, a control microphone is positioned at an unspecified location. This control microphone, in conjunction with a control unit, should control the compensation sound of the loudspeaker so that attenuation or extinction is established at the location of the control microphone.

The known active systems have in common that they are designed for the greatest possible attenuation of noise. However, such extensive sound attenuation is not desirable in every case. For example, a motor vehicle must develop audible sounds from other road users for safety reasons. In addition, it may be necessary to attenuate certain interfering frequencies as well as possible, but to allow others to pass at an adjustable rate, e.g. to create a specific sound image.

The invention has for its object to provide a muffler that allows both simple means a damping and a targeted change in the sound of a background noise.

This object is achieved by the combination of features of claim 1.

The active silencer according to the invention comprises at least one hollow body having a loudspeaker or a loudspeaker diaphragm, which is acoustically coupled via a hollow body opening to a channel carrying a noise. When this is referred to as speaker diaphragm, it also includes other bodies or fabrics that can be excited to oscillate, for example, piezoelectric elements. Furthermore, a device is provided which compares an adjustable setpoint value with an actual value of a sound field impedance present in the region of the acoustic coupling point and determines the movement of the loudspeaker diaphragm for regulating the actual value to the setpoint value. The desired value is arbitrarily adjustable within a range of values which extends from values which effect a maximum attenuation of the disturbing sound up to a value which produces minimum attenuation. The possibilities of Störschallbeeinflussung are thus much greater than in known mufflers. In particular, the noise can be qualitatively changed not only in terms of its sound amplitude but also in terms of a sound design. In a preferred embodiment, in contrast to the known active systems, not only the sound pressure, but also the sound velocity is measured, whereby the sound field impedance between very small and very large, in extreme cases between zero and infinity lying words einregelbar. The proposed muffler generally works according to the adaptation principle, ie, in the area of the acoustic coupling point, an acoustically variable sound field impedance is generated which is adapted to the respective task, sound attenuation and / or sound design. The influence thus achieved is, for example, a total reflection of the noise at a so-called adaptation point in the background noise channel, so that - as seen in the flow direction - no background noise propagates beyond the adaptation point or the coupling point. In another case, a target value of the sound field impedance is adjusted so that the noise passes through the adjustment point completely undamped. If necessary, the nominal value of the sound field impedance is adjusted so that the noise at the adjustment point is partially reflected and partially transmitted, so that the background noise propagates beyond the adjustment point with a smaller sound amplitude. The setting of the setpoint may be dependent on certain frequencies of background noise. With the help of the variable adjustment of the sound field impedance, an artificial sound design (sound design) thus arises, in that the sound attenuation can be deliberately influenced by selecting a desired sound field impedance. Thus, with the help of the active silencer of one and the same noise source desired different sound images can be generated. Due to the described adaptation of the sound field impedance - eg extreme mismatch - so different or deliberately no sound attenuation can be achieved for different frequencies of noise, if desired or required. As a result, the muffler according to the invention over prior art mufflers is versatile in at the same time structurally simple and inexpensive design. This is especially true when the noise is guided in a pipe, as is the case for example in exhaust systems or ventilation ducts.

The muffler thus allows using the speaker diaphragm at a variety of frequencies an adjustable reflectance of the Störschallwellen in the acoustic coupling point. The temporal movement function of Loudspeaker diaphragm is determined or controlled by amount and phase so that for a frequency or a plurality of frequencies of noise, the sound field impedance in the acoustic coupling point is more or less adapted as needed to the characteristic impedance of the Störschallkanales.

The determination of the actual value of the sound field impedance is preferably carried out by measuring the two parameters determining the sound field impedance, namely sound pressure and sound velocity. This can be done with the help of a sound pressure transducer and a Schallschnellewandlers. However, the above parameters can also be measured alone with a sound pressure transducer or a sound velocity transducer. The sound velocity can be determined, for example, with a pressure gradient microphone. Advantageously, the sound pressure transducer is provided by a commercially available component, e.g. realized cost-effectively by an electromechanical transducer, in particular by a microphone. The Schallschnellewandler can also be realized by two spaced apart sound pressure transducer. The measuring device or the transducers used need not necessarily be positioned in the measuring point itself. On the contrary, it is also conceivable to position the transducers away from the measuring point and to detect them by a probe, e.g. a tube probe to connect to the respective measuring point. By remote from the measuring point positioning the measuring device is protected from hot exhaust gases or other harmful influences.

In an advantageous embodiment, the muffler comprises two hollow bodies with a common region of an acoustic coupling point, wherein the loudspeaker membranes of the two hollow bodies are arranged opposite one another. As a result, the required displacement of a single membrane can be divided into two membranes and achieve a reduction of the mechanical and / or electrical power to control the individual speaker membranes. The two loudspeaker membranes of the hollow bodies preferably oscillate in opposite phase to one another.

Fig.1

die Prinzipdarstellung eines erfindungsgemäßen Schalldämpfers,

Fig.2

eine erste Ausführungsform mit einem einzigen Hohlkörper,

Fig.3

eine zweite Ausführungsform mit zwei Hohlkörpern,

Fig.4

einen Querschnitt gemäß Schnittlinie IV-IV in Fig.2 mit einer Darstellung von höheren Moden des Hohlkörper-Schallfeldes.

The subject invention will be explained in more detail with reference to the embodiments illustrated in FIGS. Show it:

Fig.1

the schematic diagram of a silencer according to the invention,

Fig.2

a first embodiment with a single hollow body,

Figure 3

a second embodiment with two hollow bodies,

Figure 4

a cross section along section line IV-IV in Figure 2 with a representation of higher modes of the hollow body sound field.

The basic structure and operation of a silencer according to the invention will first be explained with reference to FIG. 1: it comprises at least one hollow body 1, which is connected via an acoustic coupling point 2 in the manner of a spur line with a Störschallkanal 3, approximately formed by the exhaust pipe 4 of a motor vehicle is. The hollow body 1 is cylindrical, for example, in cross-section and extends with its central longitudinal axis 5 substantially transversely to the interference sound channel 3. The coupling point 2 is formed in the simplest case by an opening in the exhaust pipe. In order to prevent or at least reduce the penetration of exhaust gas into the hollow body, a perforated pipe section or a pipe-permeable material may be present instead of an opening. In the coupling point 2 facing away from the end face of the hollow body 1, a speaker 6 is inserted with a loudspeaker diaphragm 7. In the region of the coupling point 2, a measuring device 8 for determining the actual value of the sound field impedance present in this area is arranged within the hollow body. The measuring device is designed so that both the sound pressure and the sound velocity can be determined with it. It contains, for example, a sound pressure transducer and a sound velocity transducer and is connected via a signal line 9 on the input side to a control unit 10. If necessary, several measuring devices may be present or the measuring device may include a plurality of optionally arranged at spatially separated positions sound pressure and Schallschnellewandler. The control unit 10 in turn is connected via a signal line 11 on the output side to the speaker 6. About a selection unit 12 is the desired at the interface 2 Setpoint of the sound field impedance adjustable.

The goal of conventional active silencers is to maximize the sound attenuation in the noise channel and, accordingly, to set the sound pressure and thus the sound field impedance to a minimum. With a silencer according to the invention, however, it is possible to set virtually any impedance values at the acoustic coupling point 2 and thereby specifically determine the extent of sound attenuation (total reflection, partial or maximum attenuation of the Störschallamplitude) in Störschallkanal 3 at a matching point 3a adjacent to the coupling point. If the acoustic coupling point is regulated to a minimum sound pressure level, the sound field impedance also becomes minimal in accordance with the equation Z = p / v. On the other hand, however, a very large sound field impedance at the coupling point can be achieved if the sound velocity (v) is controlled to a minimum in this area. In conjunction with the positioning of the measuring device can thus generally make a spatial alignment of the sound field impedance. While a large sound field impedance in the interfering channel itself practically causes a large sound attenuation, such an impedance at the acoustic coupling point 2 means that the interfering sound passes through the matching point 3a unchanged. Due to the adjustable between the extreme values zero and infinity sound field impedance thus a very variable influence of the noise is possible. Thus, it can not only be damped, but its sound can also be changed by setting a high sound field impedance that leaves this unchanged at the coupling point 2 and, for other frequency ranges, setting this minimum sound field impedance virtually out of filter. The generation of the desired sound field impedance takes place in that the signals transmitted via the signal line 9 to the control unit 10 of the measuring device 8 are processed by means of an algorithm, eg an LMS algorithm and the signal line 11 the speaker signals are supplied to the sound field of the hollow body. 1 in the direction of the desired setpoint.

In the case of the silencer according to FIG. 1, the central longitudinal axis 5 of the hollow body 1 extends transversely to the longitudinal extension of the interference sound channel 3. The same applies essentially to the direction of movement of the loudspeaker diaphragm 7. In the arrangements shown in FIGS. 2 and 3, the hollow bodies 1a extend The approximately barrel-shaped hollow body 1 a, 1 b are interspersed by the exhaust pipe 4 eccentrically, this one on the one hollow body end face 13,13a on and on the other end face 14,14a again exit and its central longitudinal axis 16 parallel to the Längserstrekkung of the hollow body 1a, 1b or to the central longitudinal axis 17 extends. The embodiments according to FIGS. 2 and 3 differ with regard to the number of loudspeakers and with regard to the arrangement of the acoustic coupling point. The hollow body of the arrangement according to FIG. 2 has only one loudspeaker 6a, which is arranged on the front end side 13 viewed in the flow direction 15, its loudspeaker diaphragm 7 being located substantially upstream of the plane covered by the end face 13. The to a motor (not shown) connected portion 4a of the exhaust pipe 4 extends in the flow direction 15 into the hollow body and ends with axial distance in front of the end face 14 of the hollow body 1a. The area between the downstream end of the portion 4a and the end face 14 forms the acoustic coupling point 2a. From the end face 14, a section 4b of the exhaust pipe 4, which discharges the exhaust gas into the atmosphere, extends away. The two pipe sections 4a, 4b have the same central longitudinal axis 16. The measuring device 8 is, as in the embodiment described above, arranged near the acoustic coupling point 2.

As long as the transverse dimension of the hollow body 1,1a, 1b is small to the wavelength, propagate substantially uniform sound waves in the hollow body 1, so that for the positioning of the measuring device 8 from an acoustic perspective, each place in a plane parallel to the sectional plane IV - IV extending and Area of the coupling point 2 lying level is suitable. In the case of passing hot exhaust gases through the Störschallkanal 3, the measuring device 8 can therefore be positioned transversely to the flow direction 7 away from the hot exhaust gas without In this case, the sound field impedance control at the coupling point 2 or the interference sound interference at the adjustment point 3a is impaired. If the pipe sections 4a, 4b carry hot exhaust gases, additional protection for the hollow body 1 and the loudspeaker 6 arranged in it against the hot exhaust gases can be achieved by the section 4a of the exhaust pipe 4 extending to the end face 14 and in the region of the coupling point 2 perforated, in particular with high porosity to improve the acoustic transmission.

In another, not shown embodiment, the sound pressure and or rapid velocity measurement takes place at the coupling point 2 such that the measuring point is still positioned in the region of the coupling point 2, the corresponding transducer or sensors themselves but spaced from the measuring point and positioned via a probe connected with it. A sound pressure transducer is e.g. designed as a probe microphone.

The active silencer according to FIG. 3 also contains a substantially barrel-shaped hollow body 1b in whose two end faces 13a, 14a a loudspeaker 6a, 6b is arranged and which, as described above, passes eccentrically through an interference sound channel 3 or an exhaust pipe 4 is. The hollow body 1 b can also be considered as two hollow bodies 1 c, 1 d set abutting each other with their open end sides. The acoustic coupling point 2 is formed by the fact that two pipe sections 4c, 4d projecting approximately equally far into the hollow body 1b end with the same central longitudinal axis 16 with axial spacing to form the acoustic coupling point 2a. But the coupling point can also by a perforation or the like. be formed. The two speakers 6a, 6b are connected on the output side with two signal lines 11a, 11b to the control unit 10 connected to a selection unit 12. By the mentioned embodiment, the required stroke of the loudspeaker diaphragm of a single loudspeaker according to FIG. 1 can be divided between the two diaphragms 7b, 7c of the loudspeakers 6b, 6c according to FIG. This advantageously results in a power reduction of the individual speaker. Control unit 10 is the output side connected to both speakers 6b, 6c, wherein the in the control unit 10 contained transfer functions or algorithms are adapted to the required operation of the speakers 6b, 6c.

Occur in the hollow body 1,1a, 1 b frequencies above the range of planar sound waves, so arise in the sound field of the hollow body - depending on its cross-sectional shape - pressure node lines of higher modes than the fundamental mode. On these print node lines the sound pressure of the higher modes is always zero. In a preferred embodiment of the active muffler, therefore, at least the measuring point corresponding to the position of the measuring device 8 lies on a pressure node line of a higher mode. Also, the cross-sectional center of the Störschallkanales 3 or its central longitudinal axis 16 may lie on a pressure node line of a higher mode. The aforesaid positioning of the measuring point and possibly of the interfering sound channel makes it possible to avoid an undesired measurement of the sound pressure of the higher mode during the sound pressure measurement at the acoustic coupling point 2,2a, 2b without complex compensation measures (for example additional algorithms in the control unit 10).

Thus, e.g. in Fig. 4, the measuring point or the position of the measuring device 8 on the pressure node line 18 of the first radial mode and at the same time on a first pressure node line 19 of the second circumferential mode. The radial center or the central longitudinal axis of the interference sound channel 3 is located approximately at the intersection of the pressure node line 18 and the first pressure node line 19 of the second circumferential mode. A second pressure node line 20 of the second circumferential mode is also shown in FIG. Depending on the cross-sectional shape of the hollow body 1,1a, 1b - e.g. circular as in Fig. 4 or rectangular - the measuring point and the central longitudinal axis 16 of the Störschallkanales can lie on different pressure node lines a higher mode.

In the embodiments according to FIGS. 2 and 3, the direction of movement of the loudspeaker diaphragm 7a, 7b, 7c runs parallel to the direction of propagation 15 of the background noise, whereby the loudspeaker diaphragm within the silencer space-saving can be arranged and transverse to the direction of propagation no additional space must be considered. In addition, a movable in the propagation direction 15 speaker diaphragm can be well protected against the acoustic coupling point 2,2a position, if this is required, for example, due to hot exhaust gases in Störschallkanal. In this way, the optionally sensitive material of the speaker diaphragm is protected against premature wear improved. Moreover, with this arrangement it is possible to arbitrarily position a measuring point for measuring an actual value of the sound field impedance in the plane of the hollow-body sound field in the area of the acoustic coupling point, without falsifying the measurement of the sound pressure. This is possible at least at low frequencies, ie at even sound waves of the hollow body sound field. In this way, the measuring point can be positioned relatively far away from the acoustic coupling point 2,2a, whereby, for example, a measuring device 8 (eg pressure transducer, microphone) arranged in the measuring point is better protected against hot exhaust gases in the background noise channel for recording the sound pressure.

Finally, it should be noted that all types are conceivable for the hollow body 1 and the Störschallkanal 3 and the exhaust pipe 4. These components do not necessarily have to be cylindrical. Rather, for example, rectangular, elliptical or semi-circular cross sections of these components are conceivable. Moreover, it is expedient if the section 4b, 4d of the exhaust pipe 4 located in the flow direction 15 after the acoustic coupling point has a slightly larger inner diameter, at least in the region of the coupling point, than the pipe section 4a, 4c located in front of the coupling point. With this configuration, the entry of expanding exhaust gas into the hollow body 1 can be at least reduced at the transition point between said pipe sections. Furthermore, it may be expedient in terms of a compact and space-saving design, if the speaker diaphragm 7 is arranged concentrically to the Störschallkanal 3, ie if this or him forming the exhaust pipe 4 passes through the loudspeaker or the concentric.

LIST OF REFERENCE NUMBERS

1

hollow body

2

Acoustic interface

3

Störschallkanal

3a

adjustment point

4

exhaust pipe

4a, b, c, d

section

5

central longitudinal axis

6

speaker

7

Speaker cone

8th

measuring device

9

signal line

10

control unit

11

signal line

12

selecting unit

13

front

14

front

15

flow direction

16

central longitudinal axis

17

central longitudinal axis

18

Pressure node line of the first radial mode

19

first print node line of the second circumferential mode

20

second pressure node line of the second circumferential mode

Claims (8)

1. Active silencer, with at least one hollow body (1, 1') which has at least one loudspeaker (6) and is acoustically coupled via a hollow-body orifice to a noise duct (3) suitable for the routing of noise, a regulating unit (10) being provided, which

   - compares an adjustable desired value with an actual value of sound-field impedance, the said actual value occurring in the region of the acoustic coupling point (2), and

   - determines the movement of the loudspeaker membrane (7) in order to regulate the actual value to the desired value, and

   - the adjustable desired-value range extending from values causing a maximum damping of the noise to values causing a minimum damping of the noise.
2. Silencer according to Claim 1, a measuring device (8) for measuring the actual value of the sound-field impedance being provided, by means of which both the sound pressure and the sound velocity can be determined.
3. Silencer according to Claim 2, the measuring device (8) having at least one sound-pressure converter and/or at least one sound-velocity converter.
4. Silencer according to Claim 2 or 3, the measuring device (8) being arranged in the region of the acoustic coupling point (2, 2a).
5. Silencer according to one of the preceding claims, with two hollow bodies (1b, 1c) and with a common acoustic coupling point (2a), the loudspeaker membranes (7b, 7c) of the two hollow bodies (1a, 1b) being arranged opposite one another.
6. Silencer according to Claim 5, the loudspeaker membrane (7b, 7c) of the two hollow bodies (1b, 1c) oscillating in counterphase to one another.
7. Silencer according to Claim 4, 5 or 6, in which the common acoustic coupling point (2a) is at about an equal distance from the loudspeaker membranes (7b, 7c) opposite one another.
8. Silencer according to one of the preceding claims, the noise duct (3) passing through the at least one hollow body (1a, 1b, 1c) in the direction of the longitudinal mid-axis (17) of the latter.

Images