EP0342353B1 - Arrangement for diminishing the noise level within a motor car - Google Patents

Description

The invention relates to an arrangement for reducing the noise level in the interior of a motor vehicle with the features of the preamble of claim 1.

In a known arrangement of this type (GB-A-2 149 614), noises in an aircraft cabin or another enclosed space are reduced with the aid of microphones distributed in the cabin as noise sensors and of loudspeakers by generating counter-noise (anti-noise system) . An artificially generated sound wave field is superimposed on a sound wave front, which is 180 ° out of phase with the disturbing sound wave front. Such systems have the disadvantage that a relatively large number of microphones must be installed in order to achieve noticeable effects. Furthermore, a high sound power must be generated by the speakers and the required control effort is also great.

The object of the invention is to reduce the noise level in the interior of a vehicle in one or more frequency range (s), the frequencies of which are related to the natural cavity frequencies of the interior, with comparatively little construction effort by actively damping the natural cavity vibration frequencies at all locations in the vehicle interior to lower.

The features of claim 1 serve to achieve this object.

The design of the noise sensor as a Helmholtz resonator with a microphone housed therein ensures stable control and suitable filtering of the recorded noise signal without a sensitive reaction to changes in the acoustic transmission behavior of the vehicle interior, which may result, for example, from opening the doors, windows or sunroof . The Helmholtz resonator thus acts as an acoustic filter with the advantage over conventional electronic filters that it has a very time-stable transmission behavior due to the very low filter attenuation required here.

By tuning the Helmholtz resonator to the cavity natural frequency to be damped, a narrow-band but targeted effect is generated. The damping can be done by a damping material, such as cotton, wool or the like, which is accommodated in the resonator neck or resonator volume of the Helmholtz resonator.

Another advantage of the noise sensor designed according to the invention is that it can be installed very close to the loudspeaker or loudspeakers, which act as actuators in the control loop. In this way, instabilities due to feedback effects can be avoided even at high control loop gains.

For noise reduction in a broad frequency band, it is preferred if a plurality of noise sensors with microphones accommodated in Helmholtz resonators are provided, the individual Helmholtz resonators each being matched to different natural cavity frequencies of the motor vehicle.

The control device can be designed very simply in the arrangement according to the invention. Preferably in the arrangement with several noise sensors described last, the output signals of the noise sensors are connected in parallel, the output signal of each noise sensor being individually amplified or weakened and undergoing a phase shift. The output signals of the noise sensors are then added in at least one summing amplifier and switched to the loudspeakers acting as actuators.

The arrangement with several noise sensors opens up the possibility of accommodating them at several locations in the vehicle interior where high sound pressures of the relevant cavity natural frequencies prevail.

Fig. 1

schematisch eine herkömmliche Anordnung zur Verminderung des Geräuschpegels im Innenraum eines Kraftfahrzeugs;

Fig. 2

die gemessene Schalldruckverteilung für die frequenzniedrigste Hohlraumresonanzschwingung im Innenraum eines Fahrzeuges;

Fig. 3

die Nyquist-Stabilitätsortskurve einer gemäß der Erfindung ausgebildeten Regeleinrichtung, welche eine aktive Dämpfung einer oder mehrerer Hohlraumresonanzschwingungen eines Kraftfahrzeuges bewirkt;

Fig. 4

ein Teilschema einer einfachen Anordnung gemäß der Erfindung;

Fig. 5

eine Gegenüberstellung von Diagrammen, wobei das Diagramm nach Figur 5a den mittels eines Mikrofones erfaßten Schalldruck über der Frequenz und Figur 5b den mittels eines Geräuschsensors nach der Erfindung erfaßten Schalldruck über der Frequenz darstellen;

Fig. 6

eine schematische Draufsicht auf ein Kraftfahrzeug, aus der die Orte der Unterbringung von Geräuschsensor und Lautsprechern bei einer Anordnung nach der Erfindung ersichtlich sind;

Fig. 7

ein Vergleichsdiagramm, welches den Verlauf des Schalldruckes über der Frequenz in einem Kraftfahrzeug ohne die Anordnung nach der Erfindung und mit einer Anordnung gemäß Figur 4 bzw. Figur 6 darstellt; und

Fig. 8, 9

Blockschaltbilder zweier Anordnungen gemäß der Erfindung mit jeweils vier Geräuschsensoren und mehreren Lautsprechern.

The invention is explained in more detail below with reference to schematic drawings of exemplary embodiments. Show it:

Fig. 1

schematically shows a conventional arrangement for reducing the noise level in the interior of a motor vehicle;

Fig. 2

the measured sound pressure distribution for the lowest-frequency cavity resonance vibration in the interior of a vehicle;

Fig. 3

the Nyquist stability locus of a control device designed according to the invention, which actively dampens one or more cavity resonance vibrations of a motor vehicle;

Fig. 4

a partial schematic of a simple arrangement according to the invention;

Fig. 5

a comparison of diagrams, the diagram according to FIG. 5a representing the sound pressure recorded by means of a microphone over the frequency and FIG. 5b representing the sound pressure recorded by means of a noise sensor according to the invention;

Fig. 6

is a schematic plan view of a motor vehicle, from which the locations of the accommodation of the noise sensor and loudspeakers can be seen in an arrangement according to the invention;

Fig. 7

a comparison diagram showing the course of the sound pressure over the frequency in a motor vehicle without the arrangement according to the invention and with an arrangement according to Figure 4 or Figure 6; and

8, 9

Block diagrams of two arrangements according to the invention, each with four noise sensors and several speakers.

In the arrangement according to FIG. 1, a microphone 3 is arranged in the interior 2 of a car 1 in the driver's head area, the output signal of which is fed to a controller 4. The controller 4 generates an actuating signal which is fed to a loudspeaker 5 which is also arranged in the interior, for example the loudspeaker of a radio device.

The sinusoidal signal in the conventional arrangement oscillates with a phase which is shifted relative to the phase of the actual signal detected by the microphone 3 such that the sound signal emitted by the loudspeaker 5 just cancels a certain frequency of the noise signal detected in the driver's head area.

Figure 2 shows the measured sound pressure distribution of the cavity resonance vibration with the lowest frequency of a particular motor vehicle. The natural frequency of this vibration is in the order of 75 Hz for vehicles of the middle and upper class. The next higher natural cavity frequency is in the range of approx. 150 Hz. The numerical values for the sound pressure in decibels (db) show that the maxima of the sound pressure distribution are in the area of the front footwell (117.5 db) and in the area of the rear passenger area (113 db).

FIG. 3 shows in a diagram with a real abscissa and an imaginary ordinate the Nyquist stability locus of an open control loop with a profile, the implementation of which results in active damping leading to a reduction in the noise level at all locations in the vehicle interior. E denotes the cavity natural frequency to be damped.

Figure 4 shows schematically the structure of a simple arrangement according to the invention. In this case, a noise sensor comprises a Helmholtz resonator, which is designed as a rectangular hollow body 10 with an opening 11 in a side face, and a microphone 12, which is received within the hollow body 10 and whose output signal is fed to a control device 13. The control device 13 generates an actuating signal which is forwarded to a loudspeaker 14 arranged at or near the location of a maximum sound pressure (according to FIG. 2). The Helmholtz resonator 10 is tuned to a specific cavity natural frequency to be damped in the respective motor vehicle, the noise level of which is to be reduced in the interior. In order to generate a suitable characteristic, the Helmholtz resonator 10 can be embedded by embedding a porous damping material, such as cotton, wool or the like, can be individually prepared.

The loudspeaker 14 is arranged in places in the interior where maximum sound pressures prevail, in the example according to FIG. 2 in the area of the rear shelf 6 or in the footwell 7, as is customary for radio loudspeakers, so that the loudspeakers which are provided anyway can be used.

FIG. 5 shows the filtering effect of the noise sensor with microphone 12 described in FIG. 4 in the Helmholtz resonator 10: While FIG. 5a) shows several sound pressure peaks in the range of different natural frequencies, which are detected by a simple microphone without a Helmholtz resonator, the Figure 5b) the filter effect when measured at the same location with a microphone, which is arranged according to Figure 4 in a Helmholtz resonator. This enables a targeted approach to reducing the sound pressure generated by a certain natural frequency.

The arrangement according to FIG. 4 is accommodated in a vehicle, for example according to FIG. 6. The same reference numerals as in FIG. 4 are used in FIG. 6. The noise sensor comprising the microphone 12 and the Helmholtz resonator 10 is arranged in the area of the rear shelf 6 near the rear speakers 14, which are also accommodated in this area. In this case, the control device 13 has two outputs, which act on the two loudspeakers 14, 14.

FIG. 7 shows the result of the measure according to FIG. 6, the dashed curve showing the sound pressure versus frequency without the described measures of the invention, and the solid curve showing the sound pressure in the interior versus frequency when used represent the measures according to the invention.

Figures 8 and 9 show arrangements with which a sound pressure reduction for several cavity natural frequencies of a vehicle is possible.

Four noise sensors 10a, 10b, 10c, 10d are provided with corresponding microphones 12a, 12b, 12c, 12d, which are tuned to frequencies f _a , f _b , f _c , f _d , which correspond to particularly pronounced cavity natural frequencies.

The output signals of the microphones 12a to 12d are input into the control device designated 13 as in FIGS. 8 and 9. Here, each sensor signal is input into a series circuit, each with an amplifier 15a to 15d, a phase shifter 16a to 16d and a filter 17a to 17d, the series circuits for the individual sensor signals lying in parallel, and in the embodiment according to FIG. 8 fed to a common summing amplifier 18 , where the sensor signals are summed and input to a power amplifier 19 for three loudspeakers 20, 21, 22. The loudspeakers are arranged at various locations in the vehicle interior which are favorable for noise reduction and, as stated above, can coincide with the usual accommodation locations for the radio loudspeakers.

The embodiment according to FIG. 9, in which the same reference numerals as in FIG. 8 are used for the same components, differs from the embodiment according to FIG. 8 in that the amplified, phase-shifted and filtered sensor signals are divided into two summing amplifiers 18A and 18B connected in parallel, each with downstream power amplifiers 19A and 19B can be entered. Each power amplifier 19A, 19B supplies its own group of three loudspeakers 20, 21, 22 and 23, 24, 25.

In all the embodiments described, the control device 13 can be implemented using analog or digital technology, in which case a digital computer (not shown) can take over the signal processing.

Characteristic of the arrangement according to FIGS. 8 and 9 is the parallel arrangement of the series circuit 15, 16, 17 for each sensor output signal. In this way, individual signal processing can be achieved in adaptation to the individual tuning of the associated noise sensor 10, 12 before the signals are combined in the summing amplifiers 18.

According to the prior art, the filters 17 can comprise low-pass, high-pass, band-pass or blocking filters.

Even if several noise sensors are arranged, the control device 13 has a simple construction, as shown in FIGS. 8 and 9.

Claims (10)

1. An arrangement for reducing the noise level inside a motor vehicle and comprising at least one noise sensor, at least one loudspeaker (14; 20 - 22; 23 - 25) and a regulating device (13) which receives an actual value in the form of the output signal from the noise sensor (10, 12) and delivers a control signal for actuating the loudspeakers (14; 20 - 22; 23 - 25), characterised in that the noise sensor is a microphone (12) disposed in a Helmholtz resonator (10), the Helmholtz resonator (10) is tuned to the natural cavity frequency to be damped, and the loudspeaker (14; 20 - 22; 23 - 25) is disposed at or near the place of maximum sound pressure of the natural cavity frequencies to be damped in the vehicle.
2. An arrangement according to claim 1, characterised in that the Nyquist stability plot of the open loop intersects the positive real axis at a frequency which coincides with the natural frequency E of a cavity resonance.
3. An arrangement according to claim 1 or 2, characterised in that a number of loudspeakers (20 - 22; 23 - 25) are disposed in the region of the hat rack (6) and/or in the front foot region (7).
4. An arrangement according to any of claims 1 to 3, characterised in that a damping material such as wadding or wool is disposed in the neck or resonator space in the Helmholtz resonator (10).
5. An arrangement according to any of claims 1 to 4, characterised in that the Helmholtz resonator (10) and microphone (12) are disposed near the loudspeaker or loudspeakers (14; 20 - 22; 23- 25).
6. An arrangement according to any of claims 1 to 5, characterised in that a number of noise sensors comprising microphones (12a - 12d) in Helmholtz resonators (10a - 10d) are provided, each individual Helmholtz resonator (10a - 10d) being tuned to a different natural cavity frequency of the vehicle.
7. An arrangement according to claim 6, characterised in that the output signals of the noise sensors (10a - 10d, 10a - 12d) are in a parallel circuit, the output signal from each noise sensor being fed into a series circuit comprising an amplifier (15a - 15d), a phase shifter (16a - 16d) and optionally a filter (17a - 17d) or a filter unit (low-pass, high-pass, band-pass).
8. An arrangement according to claim 6 or 7, characterised in that the output signals from the noise sensors (10a - 10d, 12a - 12d) are added in at least one summing amplifier (18; 18A, 18B).
9. An arrangement according to any of claims 1 to 8, characterised in that the regulating device (13) is analog.
10. An arrangement according to any of claims 1 to 8, characterised in that the regulating device (13) contains a digital computer which uses a stored program to process the noise signals, which are acquired in analog form and are converted into digital signals.

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