Metal on metal sound when driving a jeep

Reference number:

The invention relates to an active exhaust silencer for pipes carrying exhaust gas according to the features in the preamble of claim 1.

Exhaust silencers are provided in exhaust systems of motor vehicles in order to dampen the intensity of the exhaust sound emanating from the engine to an acceptable level. The sound pressure level can be up to 160 dB (A). Legal requirements therefore provide a legal framework for sound absorption.

Passive silencers work on the principle of sound absorption and are made up of fibrous or open-pored materials that have large surfaces that are as strongly structured as possible. In this way, the exhaust sound should be diverted into absorbing and sound-absorbing labyrinths and reflected so that the sound energy is reduced until the exhaust sound has fallen below a desired level. Passive silencers, which work on the principle of sound absorption, accumulate the exhaust gas. In this respect, they reduce the performance of the engine.

Exhaust silencers have also been proposed which make use of the principle of counter-sound, with disturbing sound being superimposed with compensation sound. The compensation sound should have the same frequency and intensity as the interfering sound, but be 180 ° out of phase. The interference dampens the disturbing sound, which in the ideal case leads to complete cancellation.

Counter-noise can be generated passively by specially designed resonators or actively via loudspeakers. Resonators can, for example, be designed as λ / 4 tubes and be coupled laterally to pipes carrying exhaust gas. At the end of the λ / 4 tube, sound is reflected out of phase by 180 °. The reflected sound waves are superimposed on the disturbing sound and thus cause the sound attenuation. However, the time it takes for the sound to propagate twice over the length of the λ / 4 pipe must be taken into account. Sometimes the frequency of the exhaust sound has changed in the meantime due to the dynamic conditions in pipelines carrying exhaust gas, so that the desired compensation is not achieved when the reflected sound is superimposed. In addition, the frequencies at which cancellation is possible are limited from the outset to multiples of λ / 4 for physical reasons.

Due to these limitations, active exhaust silencers have been developed that generate compensation sound through loudspeakers and, for example, from the publications



emerge. Control loops and control loops are used to generate the compensation sound. In control circuits, sensors pick up parameters relevant to the exhaust sound, such as B. the engine speed, the load condition of the engine and the temperature of the exhaust gas. On the basis of these input signals, a control unit generates corresponding output signals with which the loudspeaker arranged on the pipe carrying the exhaust gas is controlled. Such control circuits are relatively easy to manufacture and provide acceptable results at low cost.

The sound attenuation can be further improved by a control loop. In this case, the sensor system is preferably supplemented by a pressure sensor or a microphone. On the basis of the exhaust sound recorded by this sensor system, the control unit, in conjunction with the loudspeaker, can then generate precisely matched compensation sound which is adapted to the dynamic changes in the exhaust sound and therefore has a high damping effect.

The physical framework conditions for the attenuation of exhaust sound are extremely difficult. As already described above, the exhaust sound has a sound pressure level of up to 160 dB (A), with the exhaust gas flow pulsating. The exhaust gas temperature in the exhaust gas pipeline can be up to 600 ° C, which increases the speed of sound from 330 m / s to 600 m / s. In addition, exhaust gas is extremely aggressive chemically.

The conventional cone loudspeakers previously used for exhaust noise damping cannot withstand the harsh physical conditions. The membrane and magnets wear out very quickly. Titanium membranes were therefore used on a trial basis; however, these are too expensive for mass production. In addition, large-area membranes and heavy magnets are required to generate low frequencies. B. no installation space is available in motor vehicles and which are not considered for series production due to their weight.

A new design for loudspeakers is in the

described. It is a loudspeaker, the membrane of which is set into longitudinal oscillating movements in a direction perpendicular to the membrane expansion by an electromagnetic transducer. The design enables a particularly flat design of the loudspeaker and is characterized by particularly advantageous sound characteristics.

Proceeding from this, the invention is based on the object of creating an active exhaust silencer which, regardless of the vehicle type, superimposes the exhaust noise with compensation noise in the required intensity and which is also characterized by a compact design and durability or reliability.

This object is achieved with the features specified in claim 1.

The core idea here is to superimpose the exhaust sound with compensation sound from a loudspeaker that works as an electroacoustic transducer based on flexural waves. Loudspeakers of this type have a membrane on the surface of which bending waves and shear waves propagate when they are excited to vibrate by a transducer.

Wave propagation in membranes takes place in different ways. While density and stretch waves are dominant in thicker membranes, bending and shear waves also arise in thinner media. The bending wave components have proven to be suitable for use in loudspeakers because of their amplitude and their propagation behavior. The propagation behavior of flexural waves in a membrane is significantly influenced by the flexural rigidity of the membrane. The bending stiffness depends on the frequency. At the so-called coincidence frequency, the phase velocity of the wave in the membrane is identical to the phase velocity in the air. At this frequency, the wave separates from the membrane at an angle of approx. 0 °. Above the coincidence frequency, the angle increases up to 90 °, whereby the efficiency increases by leaps and bounds. The coincidence frequency therefore represents the lowest frequency at which bending waves are converted into air sound waves. Below this frequency, pure piston vibration is predominant.

Loudspeakers of this type are particularly advantageously characterized by their flat design. The membrane is thin and preferably flat or slightly curved. The transducer is attached to and coupled to the rear of the membrane. The membrane is held in a frame of the housing of the loudspeaker. The housing is located on the side of the membrane facing away from the exhaust gas flow and also encloses the converter.

The membrane must be designed in such a way that it can withstand the special chemical and thermal requirements of a pipeline carrying exhaust gas. Another prerequisite is that it must be gas-tight.

The loudspeaker is controlled by a control unit that is coupled to a sensor system via signal lines. The exhaust silencer can thus be constructed both as a control circuit and as a control circuit and thus actively and with a high degree of efficiency attenuate the exhaust noise.

Advantageous embodiments and developments emerge from the dependent claims 2 to 19.

In the development of the invention according to claim 2 it is provided to arrange the membrane in an opening in the wall of the exhaust-gas pipeline. This development enables a particularly compact design, with the loudspeaker being located in the immediate vicinity of the pipeline carrying the exhaust gas. The direct coupling of the loudspeaker with the exhaust gas stream simplifies the attenuation considerably, because fewer factors have to be taken into account when generating the counter-sound.

Alternatively, it is possible according to claim 3 to connect a resonator to an opening in the wall of the exhaust-gas pipeline and to assign a membrane to this resonator at a distance from the opening. Due to the distance from the pipeline, the membrane is thermally decoupled from the exhaust gas flow, so that low demands can be made on the load-bearing capacity of the membrane. The resonator can also be used to align the sound and focus it on the exhaust gas flow.

It when the membrane is designed to be flat, as provided in claim 4, is particularly advantageous. In this way, the calculation of the bending waves and the design of the membrane are significantly simplified. In addition, a compact design is achieved with large membranes, which can be placed, for example, in the immediate vicinity of the pipe leading to the exhaust gas on the body floor of a motor vehicle.

However, the membrane can also be designed to be adapted to the contour of the wall of the exhaust-gas-carrying pipeline (patent claim 5). Such a slightly curved membrane can be integrated particularly easily into the exhaust-gas pipeline. The available installation space is therefore used efficiently.

In the advantageous development according to claim 6 it is provided that the membrane is vaporized with metal on the side facing the exhaust gas flow. This feature significantly increases the chemical resistance and temperature resistance of the membrane. Alternatively, the membrane according to claim 7 can be covered with a stainless steel foil on the side facing the exhaust gas flow.

The properties of the exhaust sound are essentially influenced by the engine, i.e. its speed level and its load condition, as well as by the temperature of the exhaust gas flow. Accordingly, it is provided in claim 8 to couple the control unit to engine control electronics. Motor data can be continuously transmitted to the control unit during operation via the direct connection - without the need for cumbersome signal conversion with the associated loss of efficiency and time. For this purpose, appropriate interfaces are to be provided on the output side of the engine control electronics and on the input side of the control unit.

In claim 9, the sensor system comprises a temperature sensor for measuring the exhaust gas temperature in the exhaust-gas pipeline. Since the speed of sound, as already shown, is particularly dependent on the temperature of the exhaust gas, taking the exhaust gas temperature into account increases the efficiency of the exhaust silencer significantly.

According to claim 10, the sensor system comprises a throttle valve sensor for detecting the throttle valve position. The throttle valve position allows conclusions to be drawn about the load condition of the engine. Such sensors are common in today's motor vehicles and can also be used within the scope of the claimed invention.

In addition, the sensor system can include a sensor for detecting the engine speed (patent claim 11).

A particularly high degree of efficiency in damping the exhaust sound is achieved if a control loop is set up to generate the compensation sound. Therefore, it is provided in claim 12 that the sensor system comprises a pressure sensor or a microphone for detecting the exhaust sound in the exhaust-gas pipeline. Like the loudspeaker, this sensor system is then designed to be emission-resistant.

In the advantageous further development according to claim 13, the control unit is controlled by a microprocessor. This feature enables flexible adaptations of the exhaust silencer to different practical conditions. The control behavior of the control unit is now program-controlled. The programs can be changed or exchanged via a corresponding interface on the control unit. In this way, a design of the exhaust sound in the sense of a sound design is made possible. A microprocessor-controlled control unit also simplifies the vehicle-independent installation of the exhaust silencer, since the software only needs to be adapted while the hardware remains the same.

It is also provided in claim 15 that the control behavior of the control unit can be adjustable. In this way, the driver is able to use switches or rotary controls to directly influence the noise of the vehicle, e.g. to make it sound particularly sporty or quiet.

According to claim 16, the transducer is a voice coil. Voice coils are characterized by their compact design and the fact that they contain only a few moving parts. Alternatively, the converter of claim 17 comprises an electric motor, on the drive shaft of which an eccentric is attached, which is coupled to the membrane via a connecting rod. In this way, an oscillation of the membrane at a frequency can be generated in a particularly simple and robust manner. As a result of the arrangement, the electric motor can be arranged spatially separated from the exhaust gas flow, so that the thermal stress on the converter is significantly reduced. A transducer that is attached directly to the membrane should be designed to be particularly heat-resistant (claim 18).

According to claim 19, a housing is provided, the membrane and the transducer being arranged in the housing. The housing essentially serves two purposes, namely to protect against environmental influences and to facilitate assembly, in that the transducer with the membrane and the housing can be delivered to the assembly as a prefabricated assembly.

The invention is explained in more detail below with reference to exemplary embodiments shown schematically in the drawings. Show it:

Figures 1 and 2
Longitudinal sections through two embodiments of an active exhaust silencer.

In Figure 1, an active exhaust silencer 1 is shown for an exhaust pipe 2 using the example of a motor vehicle. Through the active exhaust silencer 1, the sound of the exhaust gas is superimposed with a 180 ° phase-shifted compensation sound from a loudspeaker 3 and thus attenuated or extinguished.

The flat loudspeaker 3 is inserted in a lateral opening 4 in the wall 5 of the exhaust-gas pipeline 2 of a motor not shown in detail. The loudspeaker 3 has a diaphragm 7 arranged in a housing 6 and a voice coil 8. The membrane 7 is thin and made of a multilayer material, so that it has a particularly low coincidence frequency and a broad frequency spectrum within which it oscillates. The exhaust gas resistance and impermeability of the membrane 7 is ensured by the fact that it is vapor-coated with metal. The membrane 7 is oriented so that its surface 9 comes into contact with the exhaust gas flow AS. The voice coil 8 is fastened on the rear side of the diaphragm 7 and excites the diaphragm 7 to vibrate, whereby flexural waves propagate on the surface 9 of the diaphragm 7 facing the exhaust gas. The voice coil 8 is also designed to be heat-resistant. The loudspeaker 3 has sufficient potential to generate compensation sound in the required intensity.

The loudspeaker 3 is controlled by a microprocessor-controlled control unit 10, which is attached separately to the body of the vehicle, which is not shown in detail. This has further interfaces 11-16 for a sensor system, for an operating unit 17, for data transfer and for power supply. On the basis of the signals transmitted by the sensor system, the control unit 10 calculates a compensation oscillation. This is converted into an electrical oscillation by a digital / analog converter 18 and amplified by an amplifier 19 belonging to the control unit before it is passed on to the loudspeaker 3. The compensation oscillation is calculated under program control. The programs can be exchanged via the data transfer interface 12. Specific programs are provided for different vehicle types. The control behavior of the control unit 10 can be modified by the driver via an operating unit 17, e.g. B. to give the sound of the vehicle a sporty or gentle note or to make the vehicle noise quieter or louder.

The sensor system comprises a direct coupling of the control unit 10 to engine control electronics 20, a temperature sensor 21 and a microphone 22, which are attached to the pipe 2 carrying the exhaust gas. The signals are transmitted via suitable signal lines 27. The engine control electronics 20 transmit the speed and the load condition of the engine to the control unit 10 via a special output interface 23. The temperature sensor 21 records the temperature of the exhaust gas flow AS in the pipeline 2 in the immediate vicinity of the loudspeaker 3. It is designed to be exhaust-gas resistant. The emission-resistant microphone 22 is inserted upstream of the loudspeaker 3 in a second opening 24 in the wall 5 of the pipeline 2.

An operating unit 17 is provided to influence the vehicle sound. This is arranged so that it can be operated by the driver while driving. It comprises switches 25 and rotary control 26. The operating unit 17 is coupled to the control unit 10 via a signal line 27.

The exhaust silencer 1 shown in FIG. 2 corresponds in terms of its basic structure to that described above.Corresponding components or component components are therefore provided with the same reference symbols.

The difference between the embodiment variants of FIG. 1 and FIG. 2 is that the loudspeaker 3 is attached to a resonator 28 which is inserted into the lateral opening 4 in the wall 5 of the exhaust-gas pipeline 2. The resonator 28 is funnel-shaped and consists of sheet metal. The membrane 7 is now arranged at a distance A from the opening 4. The sound generated by the membrane 7 propagates in the resonator 28 and is superimposed in the pipe 2 with the exhaust sound after it has passed the opening 4.

1 -
Exhaust silencer
2 -
3 -
4 -
5 -
8 -
Voice coil
Control unit
11 -
Interface to engine electronics
12 -
Data transfer interface
13 -
Microphone input
14 -
Interface for temperature sensor
15 -
Input for power supply
16 -
Interface for control unit
17 -
Control unit
18 -
Digital / analog converter
19 -
20 -
Motor control electronics
21 -
Temperature sensor
22 -
23 -
Output interface
24 -
25 -
26 -
Rotary control
27 -
Signal line
28 -
A -
AS -
Exhaust gas flow