Patent Publication Number: US-7711124-B2

Title: Method and device for attenuating the noise generated at the outlet of an exhaust line

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method and a device for attenuating the noise generated at the outlet of an exhaust line. 
   Eighty percent of the noise emitted by a motor vehicle powered by a heat engine is caused by the engine. The exhaust system of the vehicle therefore has to be designed to reduce the sound level detected by residents. 
   The provision of an active noise control device in the exhaust line is known. This device allows the noise that is to be attenuated to be neutralized by causing interference between the noise to be attenuated and a counter-noise, which has the same frequency and the same amplitude but is in opposition of phase. The counter-noise is produced electronically using signal processing algorithms, in order to generate destructive interference with the noise to be attenuated. 
   The sound wave forming the counter-noise is generated by electromagnetic loudspeakers. In the known solutions, the loudspeakers directly produce a sound wave having the characteristics of the counter-noise. In order to be effective, said loudspeakers have to have an electrical power from 100 to 150 W and a mass of between 2 and 4 kg. 
   The current active noise control solutions are therefore relatively heavy and bulky. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to propose a lighter device for attenuating noise generated by a heat engine. 
   The invention accordingly relates to a method for attenuating noise generated at the outlet of an exhaust line, wherein it comprises the steps of:
         defining a signal representing the noise to be attenuated,   emitting a first high-frequency sound wave from a first transducer into an attenuation zone of the exhaust line, which first high-frequency sound wave is inaudible and has a carrier frequency of higher than 50 kHz, and   emitting a second high-frequency sound wave from a second transducer into the attenuation zone of the exhaust line, the first and second transducers being configured for generating interference between the first and second sound waves in the attenuation zone, which second sound wave is inaudible and has as its carrier frequency the carrier frequency of the first high-frequency sound wave and contains a low-frequency counter-noise signal, which is in opposition of phase to the signal representing the noise to be attenuated.       

   According to particular embodiments, the attenuation method utilizes one or more of the following characteristics:
         the frequency of the counter-noise signal is between 10 and 1,000 Hz, and   the carrier frequency is substantially equal to 100 kHz.       

   The invention also relates to a device for attenuating the noise generated at the outlet of an exhaust line, wherein it comprises;
         means for defining a signal representing the noise to be attenuated,   means for producing a low-frequency counter-noise signal, which is in opposition of phase to the signal representing the noise to be attenuated,   a first and a second transducer arranged in an attenuation zone of the exhaust line, the first and second transducers being configured for generating interference between the sound waves that are produced and present in the attenuation zone,   means for controlling the first transducer for emitting a first high-frequency sound wave, which first high-frequency sound wave is inaudible and has a carrier frequency of higher than 50 kHz, and   means for controlling the second transducer for emitting a second high-frequency sound wave, which second high-frequency sound wave is inaudible and has as its carrier frequency the carrier frequency of the first high-frequency sound wave and contains the low-frequency counter-noise signal, which is in opposition of phase to the signal representing the noise to be attenuated.       

   According to particular embodiments, the device comprises one or more of the following characteristics:
         the first and second transducers are piezoelectric transducers,   said piezoelectric transducers are lead zirconate titanate-based, said means for defining a noise signal comprise a microphone for recording the residual noise at the outlet of the exhaust line, and   said means for defining a noise signal comprise a unit for monitoring the ignition frequency of the engine.       

   Finally, the invention relates to an installation for powering a motor vehicle, wherein it comprises a heat engine, an exhaust line and a noise attenuation device as described above, the first and second transducers being arranged on the exhaust line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention will be facilitated by reading the following description, which is given solely by way of example and with reference to the drawings, in which: 
       FIG. 1  is a schematic view of an installation for powering a motor vehicle, which installation is provided with a noise attenuation device according to the invention; and 
       FIG. 2  is a schematic view illustrating the interference phenomenon utilized in the noise attenuation device according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The power installation illustrated in  FIG. 1  basically comprises a heat engine  12 , at the outlet of which an exhaust line  14  is connected. 
   The exhaust line comprises, as is known per se, an exhaust silencer  16 . It opens at an end  18  for releasing exhaust gases. 
   The power installation is provided with a sound attenuation device  20 , which comprises two sound wave sources  22 ,  24 , which are capable of producing sound waves in the silencer  16 , which delimits a noise attenuating space  26 . 
   The two sources  22 ,  24  are formed by piezoelectric transducers. These transducers are advantageously lead zirconate titanate or “PZT”-based. 
   The two transducers are joined to the wall of the silencer  16  and are configured so as to generate interference, on the one hand, between the sound waves produced by these transducers and, on the other hand, between the waves produced by these transducers and the sound wave produced by the circulation of the exhaust gases. 
   The main emission axes of the two transducers are preferably angularly offset by approximately 45°. 
   The device comprises a generator  30 , which is capable of supplying a high-frequency signal F 1  for energizing a first transducer  22  at a constant frequency and base amplitude. This frequency is preferably higher than 50 kHz so as to produce a first sound wave that is inaudible from the first transducer  22 . 
   This frequency is, for example, substantially equal to 100 kHz. 
   The generator  30  is formed by an oscillator of any suitable type. 
   The device  20  also comprises a unit  32  for generating a counter-noise. This unit is connected to a unit  34  for controlling the engine. This unit  34  controls, as is known per se, the ignition of the engine at an ignition frequency. The unit  32  comprises means for picking up a signal ΔF representing the ignition frequency of the engine. 
   The device  20  also comprises a microphone  36  arranged at the outlet  18  of the exhaust line. This microphone is connected to the unit  32  for generating the counter-noise. The unit receives from the microphone a signal of the residual noise Δε corresponding to the attenuated noise measured at the outlet of the exhaust line. 
   From the signals ΔF and Δε, the unit  32  for generating the counter-noise produces a low-frequency counter-noise signal Δf cb . The counter-noise signal Δf cb  has the same frequency and the same amplitude as the signal, denoted by Δf b  and representing the noise to be attenuated, but is in opposition of phase relative to this noise. The frequency of the counter-noise signal is therefore between 10 and 1,000 Hz. 
   At the outlet of the unit  32 , the device  20  comprises a mixer  38 , to which the source  30  is connected. The mixer  38  is therefore capable of mixing the signals Δf cb  and F 1 . The signal denoted by F 1 +Δf cb , which is obtained at the outlet of the mixer  38 , has a carrier frequency that is identical to the frequency of the first sound wave and contains the counter-noise signal Δf cb  corresponding to the signal Δf b  representing the noise to be attenuated, but is in opposition of phase thereto. 
   The outlet of the mixer  38  is connected to the second transducer  24 . 
   The attenuation device operates in the following manner: 
   From the residual noise signal Δε and the operating frequency Δf cb  of the engine, the unit  32  produces a counter-noise signal Δf cb . A high-frequency signal F 1  is applied to the first transducer  22 , whereas a high-frequency signal F 1 +Δf cb  is applied to the second transducer  24 . The sound waves produced by the first and second transducers are inaudible, these waves having a very high carrier frequency. 
   In the attenuation space  26 , as illustrated in  FIG. 2 , the sound wave caused by the circulation of the exhaust gases and the two sound waves derived from the transducers  22  and  24  interfere with one another. The sound wave produced by the exhaust gases has the same frequency and the same amplitude as the counter-noise, but is in opposition of phase relative thereto. The transducer  22  supplies a high-frequency sound wave corresponding to the signal F 1 , whereas the second transducer  24  supplies a high-frequency wave containing the signal of the counter-wave Δf cb . The interference of these three sound waves produces at the outlet a sound wave of which one of the components, resulting from the interference of the signals  2 F 1 +Δf cb +Δf b , is inaudible, since it has a very high frequency, and of which the other component, resulting from the interference of the signals F 1 −(F 1 +Δf cb )+Δf b , is continuous.  2 F 1 +Δf cb +Δf b  and F 1 −(F 1 +Δf cb )+Δf b  correspond to the superimposition of the waves. 
   It will thus be seen that the audible component of the exhaust gas noise produced by the motor is eliminated at the outlet of the exhaust line. 
   Moreover, since the sound sources  22  and  24  used are controlled so as to produce a high-frequency sound wave, these sources may be very small and consist, in particular, of piezoelectric elements. The noise attenuation device therefore has a very light weight.