FI94563B - Active noise canceling system - Google Patents

Description

! 94563

The present invention relates to an active noise attenuation system for a motor vehicle, the system 5 comprising means for generating an electrical signal proportional to the noise of one or more target areas, generating an anti-noise signal for receiving electronic signals such as an adaptive filter, one or more and one or more detectors for detecting residual noise in the target area and transmitting it in electronic form to an electronic device for tuning its operation.

Fig. 1 of the accompanying drawings schematically shows the basic principle of an active noise attenuation system and Fig. 2 shows a block diagram corresponding to this principle diagram. A conventional system for active noise attenuation 20 as shown in Fig. 1 comprises noise detectors 1 which generate an electrical signal proportional to the noise prevailing in the target area.

These detectors can be either microphones, in which case the signal they generate corresponds to the noise prevailing in the target area, or the detectors can be of other types, in which case they generate, for example, only a reference signal proportional to the engine speed of the vehicle. The signals generated by the detectors 1 thus do not have to correspond to the actual noise but only to correlate with it. These detector signals are applied to an electronic device 2 which generates a counter-noise signal liC, which is most conventionally an adaptive filter. This adaptive filter calculates, by means of a suitable algorithm, such as the LMS algorithm or the Fan-Vemuri algorithm, a signal which corresponds to the noise signal expressed in amplitude but is in the opposite phase. Such an anti-noise signal is reproduced with anti-noise sound sources, such as speakers 3, to generate anti-noise in the target area. The combined effect of the actual noise and the counter-phase noise opposite to it results in noise attenuation. In practice, noise can be attenuated by such a system by about 10-30 dB, for example at the noise frequency of the vehicle engine. In order to be able to effectively control and adapt the anti-noise signal generating device to the respective conditions, the system further comprises 10 residual noise detectors 4 adapted to detect residual noise in the target area and to control the anti-noise generating device 2 accordingly.

Figure 2 shows a block diagram corresponding to the system of Figure 1. In it, the signal generated by the noise detector 1 is denoted by the reference x, which is input to the anti-noise signal generating device 2, which generates the signal y, which is a function of the noise signal x. This signal y is then reproduced by the speaker 3, whereby the signal propagates as a sound wave via the transmission path H to the residual noise detector 4. This residual noise detector 4, which may in practice be a microphone, generates an electrical signal e which can be represented as H (y) + n, where n there is noise in the microphone 4.

In practice, it has been found that the amount of attenuation provided by the systems of Figures 1 and 2 is highly dependent on the amplitude gain of H (y). This gain dependence in practice leads to a situation where a system such as that illustrated in Figure 2 is not stable except at certain gains 30 and still so that the available gains are highly dependent on the frequency range in which active noise attenuation is desired to be most effective.

The problem thus becomes that the control system is tuned in such a way that it operates efficiently in different frequency ranges without leading to problems in the operation of the system. This is achieved by a system according to the invention, characterized in that an amplifier is arranged between the anti-noise source or sources and the anti-noise signal generating means for adjusting the gain of the anti-noise signal on the basis of a signal proportional to the engine speed of the vehicle. Thus, the stability problem has now been solved by changing the gain of the counter-noise signal in a frequency-dependent manner and in particular in a vehicle engine speed. Preferably, the gain of the response signal is increased as the engine speed decreases. In fact, although the amplitude of the noise in the vehicle decreases as the engine speed decreases while the noise frequency decreases, the gain then has to be increased due to the practical limitations of the small interior of the vehicle and the characteristics of the available sound sources. After all, conventional speakers reproduce low frequencies quite poorly, and the dimensions of the playback space also have a significant effect on limiting the possibility of low-frequency reproduction.

The system according to the invention will now be described in more detail with reference to the accompanying drawing, in which Figure 1 schematically shows the basic structure of an active silencing system, Figure 2 shows a block diagram corresponding to the system of Figure 1 and Figure 3 shows a block diagram of a silencing system according to the invention.

.10 Figure 3 shows a block diagram of a sound attenuation system according to the invention, in which an additional amplifier 5 is arranged between the anti-noise source 3 and the anti-noise signal generating device 2, which is controlled as a function of the signal z. According to the invention, this signal z is 35 proportional to the engine speed of the vehicle. Preferably, the gain y 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 correspondingly, as the frequency of the signal z decreases, i.e., as the engine speed of the vehicle decreases, the gain is increased. It is necessary to increase the gain at low frequencies in order to make the speaker 3 reproduce these frequencies with sufficient amplitude.

By adding an amplifier 5 to the system, the gain of which is inversely dependent on the frequency of the main noise source for which the system is intended to be attenuated, problems related to the instability of the control system are avoided and the level of noise attenuation can be substantially improved. In order to achieve optimal operation in each operating application, the gain of the amplifier 5, and in particular its change as a function of the frequency of the signal z, must be adapted to the prevailing practical conditions. However, it is essential that the gain of the amplifier 5 is controlled 20 depending on the rotational speed of the vehicle engine.

Claims (2)

An active noise cancellation system for a motor vehicle, comprising means (1) for generating one or more electrical signals proportional to the noise within the target signal, an electronic means (2) such as an adaptive filter receiving these signals for generation of a counter-noise signal, one or more sound sources (3) connected to said electronic means for generating counter-noise within the target area and one or more detectors (4) for detecting residual noise within the target area and for transmitting the same in electrical form to the electronic means ( 2) for tuning its function, characterized in that an amplifier (5) is arranged between the counter-noise source (s) (3) and the means (2) which generates the counter-noise signal for controlling the counter-noise signal on the basis of a signal (z) proportional to the speed of the vehicle engine. 2. A system according to claim 1, characterized in that the gain of the amplifier (5) is inversely proportional to the frequency of the signal (z) controlling it. • ·