(19) |
|
|
(11) |
EP 0 539 940 B1 |
(12) |
EUROPEAN PATENT SPECIFICATION |
(45) |
Mention of the grant of the patent: |
|
24.04.1996 Bulletin 1996/17 |
(22) |
Date of filing: 28.10.1992 |
|
(51) |
International Patent Classification (IPC)6: G10K 11/16 |
|
(54) |
Active noise cancellation system
Aktives Lärmunterdrückungssystem
Système actif de suppression du bruit
|
(84) |
Designated Contracting States: |
|
BE DE FR GB IT NL SE |
(30) |
Priority: |
31.10.1991 FI 915143
|
(43) |
Date of publication of application: |
|
05.05.1993 Bulletin 1993/18 |
(73) |
Proprietor: NOKIA TECHNOLOGY GmbH |
|
D-75175 Pforzheim (DE) |
|
(72) |
Inventor: |
|
- Kuusama, Juha
SF-33720 Tampere (FI)
|
(56) |
References cited: :
EP-A- 0 098 594
|
FR-A- 2 531 023
|
|
|
|
|
- MACHINE DESIGN, vol. 59, no. 29, 10th December 1987, page 70, Cleveland, Ohio, US;
ANONYMOUSLY: "Low-frequency noise gets waved back"
|
|
|
|
Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
|
[0001] The present invention as it is defined in the appended claims relates to an active
noise cancellation system for a motor vehicle, comprising means for generating one
or more electrical signals proportional to the noise in the target area, an electronic
means receiving these signals, such as an adaptive filter, for generating a cancellation
noise signal, one or more sound sources connected to said electronic means for generating
cancellation noise in the target area, and one or more sensors for detecting residual
noise in the target area and transmitting it in an electrical form to the electronic
means to tune its operation.
[0002] Figure 1 in the accompanying drawings represents schematically the main principle
of an active noise cancellation system, and Figure 2 shows a block diagram corresponding
to this basic schematic drawing. A conventional system for active noise cancellation
of the kind shown in Figure 1 comprises noise detection sensors 1 generating an electrical
signal proportional to the noise present in the target area. These detection sensors
can be either microphones, in which case the signal generated by them corresponds
to the noise in the target area, or the detection sensors may also be of another type,
in which case they generate for instance only a reference signal proportional to the
rotation speed of the vehicle motor. Thus the signals generated by the detection sensors
1 need not correspond to the actual noise, but only be correlated with it. These detection
sensor signals are conveyed to an electronic means 2 generating a noise cancellation
signal, which most commonly is an adaptive filter. This adaptive filter calculates
by means of a suitable algorithm, such as an LMS algorithm or a Fan-Vemuri algorithm,
a signal which in amplitude corresponds to the produced noise signal but is of an
opposite phase thereto. Such a cancellation noise signal is reproduced by cancellation
noise sources, such as loudspeakers 3, to generate cancellation noise in the target
area. Attenuation of the noise is achieved as a combined result of the actual noise
and the cancellation noise of an opposite phase. In practice, noise can be cancelled
by such an arrangement about 10 -30 dB for instance at the motor noise frequency.
In order that the means for generating the cancellation noise signal may be effectively
controlled and adapted to the conditions in each case, the system further comprises
residual noise sensors 4 which have been adapted to detect residual noise in the target
area and control the means 2 for generating the cancellation noise in response to
this.
[0003] Figure 2 shows a block diagram corresponding to the system of Figure 1. Therein the
signal generated by the noise detection sensor 1 has been denoted by the reference
character x, which signal is inputted in a means 2 for generating a cancellation noise
signal, which generates a signal y that is a function of the noise signal x. This
signal y is then reproduced by a loudspeaker 3, in which situation the signal proceeds
as a sound wave via a transmission route H to a sensor 4 for residual noise. This
sensor 4 for residual noise, which may in practice be for instance a microphone, generates
an electrical signal e which may be represented as a function H(y)+n, wherein n is
noise in the microphone 4. The document FR-A-2 531 023 discloses an active noise cancellation
system to be used in motor vehicles.
[0004] In practice, it has been found that the degree of cancellation achieved by the systems
of Figures 1 and 2 is greatly dependent on the amplitude gain of H(y). This gain dependence
leads in practice to such a situation that a system of the kind described in Figure
2 is only stable with specific gains and even so that the gains that are available
are greatly dependent on the frequency band in which one wishes the active noise cancellation
to be the most effective.
[0005] Thus a problem is encountered with tuning the control system so as to operate effectively
in different frequency bands without presenting problems in the operation of the system.
This is achieved with the system of the invention, which is characterized in that
an amplifier has been disposed between the sound source or sources of cancellation
noise and the means for generating a cancellation noise signal to control the gain
of the cancellation noise signal in response to a signal proportional to the rotation
speed of the vehicle motor. Thus the stability problem has now been solved by varying
the gain of the cancellation noise signal in dependence on the frequency and specifically
in dependence on the rotation speed of the vehicle motor. Preferably the gain of the
cancellation noise signal is increased when the rotation speed of the motor decreases.
Even though the amplitude of the noise prevailing in the vehicle actually decreases
when the rotation speed of the motor decreases simultaneously as the frequency of
the noise decreases, yet in that case the gain must be increased on account of the
practical limitations presented by the smallness of the interior of the vehicle and
the properties of the sound sources available. Conventional loudspeakers reproduce
low frequencies rather poorly, and the dimensions of the reproduction space also have
a significant effect of restricting the possibility of reproduction of low frequencies.
[0006] In the following, the system of the invention will be described in more detail with
reference to the enclosed drawing, wherein
Figure 1 shows schematically the principle of construction of an active sound cancellation
system,
Figure 2 shows a block diagram corresponding to the system of Figure 1, and
Figure 3 shows a block diagram of the sound cancellation system of the invention.
[0007] Figure 3 represents a block diagram of the sound cancellation system of the invention,
wherein an additional amplifier 5 has been disposed between the sound source of cancellation
noise 3 and the means 2 for generating the cancellation noise signal, said additional
amplifier being controlled as a function of the signal z. In accordance with the invention,
this signal z is proportional to the rotation speed of the vehicle motor. Preferably
the gain of the signal y generated by the amplifier 5 is an inverse function of the
frequency of this signal z. Thus, as the frequency of the signal z increases the gain
of the amplifier 5 is reduced, and respectively as the frequency of the signal z decreases,
i.e. the rotation speed of the vehicle motor decreases, the gain is boosted. Increasing
the gain at low frequencies is necessary in order for the loudspeaker 3 to be able
to reproduce also these frequencies with a sufficient amplitude.
[0008] When an amplifier 5 providing a gain inversely correlated with the frequency of the
principal source of noise for the cancellation of which the system is intended is
incorporated into the system, problems related to the instability of the control system
are avoided and the level of noise cancellation can be substantially enhanced. In
order for optimum operation to be achieved in each use application, the gain of the
amplifier 5 and specifically its variation as a function of the frequency of signal
z must be adapted to the practical conditions prevailing in each case. However, the
essential feature is that the gain of the amplifier 5 is controlled in dependence
on the rotation speed of the vehicle motor.
1. An active noise cancellation system for a motor vehicle, comprising means (1) for
generating one or more electrical signals proportional to the noise in the target
area, an electronic means (2) receiving these signals, such as an adaptive filter,
for generating a cancellation noise signal, one or more sound sources (3) connected
to said electronic means for generating cancellation noise in the target area, and
one or more sensors (4) for detecting residual noise in the target area and transmitting
it in an electrical form to the electronic means (2) to tune its operation, characterized in that an amplifier (5) has been disposed between the sound source or sources (3)
of cancellation noise and the means (2) for generating a cancellation noise signal
to control the gain of the cancellation noise signal in response to a signal (z) proportional
to the rotation speed of the vehicle motor.
2. A system as claimed in claim 1, characterized in that the gain of the amplifier (5) is inversely proportional to the frequency
of the signal (z) controlling it.
1. Ein aktives Geräuschunterdrückungssystem für ein Kraftfahrzeug, umfassend Mittel (1)
zum Erzeugen von einem oder mehreren zu dem Geräusch in dem Zielbereich proportionalen
elektrischen Signalen, ein elektronisches Mittel (2), wie zum Beispiel ein Adaptivfilter,
zum Empfang dieser Signale zwecks Erzeugung eines Unterdrückungsgeräuschsignals, eine
oder mehrere zur Erzeugung von Unterdrückungsgeräusch in dem Zielbereich dienende
Schallquellen (3), die mit dem besagten elektronischen Mittel verbunden sind, sowie
einen oder mehrere Sensoren (4) zum Nachweis von Restgeräuch in dem Zielbereich und
zu dessen Übertragung in elektrischer Form an das elektronische Mittel (2), um dessen
Betrieb abzustimmen, dadurch gekennzeichnet, daß ein Verstärker (5) zwischen der Schallquelle bzw. den Schallquellen (3) zur Erzeugung
von Unterdrückungsgeräusch und dem Mittel (2) zur Erzeugung eines Unterdrückungsgeräuschsignals
angeordnet ist, um die Verstärkung des Unterdrückungsgeräuschsignals im Einklang mit
einem zu der Drehzahl des Fahrzeugmotors proportionalen Signal (z) zu regeln.
2. Ein System nach Anspruch 1, dadurch gekennzeichnet, daß die durch den Verstärker (5) bewirkte Verstärkung zu der Frequenz des sie regelnden
Signals (z) umgekehrt proportional ist.
1. Système actif d'élimination de bruit pour un véhicule à moteur, comprenant des moyens
(1) pour créer un ou plusieurs signaux électriques proportionnels au bruit dans la
surface cible, des moyens électroniques (2) recevant ces signaux, tels qu'un filtre
adaptatif, pour créer un signal d'élimination de bruit, une ou plusieurs sources acoustiques
(3) relié(e)s auxdits moyens électroniques pour créer une élimination de bruit dans
la surface cible, et un ou plusieurs capteurs (4) pour détecter le bruit résiduel
dans la surface cible et le transmettre sous une forme électrique aux moyens électroniques
(2) pour accorder son fonctionnement, caractérisé en ce qu'un amplificateur (5) a
été disposé entre la ou les sources acoustiques (3) d'élimination de bruit et les
moyens (2) pour créer un signal d'élimination de bruit afin de commander le gain du
signal d'élimination de bruit en réponse à un signal (z) proportionnel à la vitesse
de rotation du véhicule à moteur.
2. Système selon la revendication 1, caractérisé en ce que le gain de l'amplificateur
(5) est inversement proportionnel à la fréquence du signal (z) le commandant.