JPH03285533A - Active muffler - Google Patents

Abstract

PURPOSE:To enhance muffler effect by disposing a secondary muffler sound source in a duct through which sound wave propagates from a primary sound source and disposing a sound feedback detecting means at the outside of the duct in the rear of the secondary muffler sound source. CONSTITUTION:A secondary sound source, i.e., a speaker 4, is mounted on the inner wall of a duct 2 through which sound wave propagates from a primary sound source 1. A sound wave detecting microphone 3 is disposed at a position separated by a distance L1 in the vertical direction to the duct 2 from the speaker 4. A microphone 6 for detecting sound feedback is disposed at a position separated by a distance L2 from the speaker 4 in a speaker box 7. The distance L2 is adjusted such that the signals detected through the microphones 3, 6 have reverse phases. A signal processing circuit comprises a preamplifiers 8, 9, an adder 10, an inverted amplifier 11 and a power amplifier 12 and cancel the sound feedback detected through the microphone 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an active noise reduction device, and particularly to an active noise reduction device that suppresses noise from a blower, an engine, or the like.

[Background Art and Problems to be Solved by the Invention] Active noise reduction devices are examples of devices that suppress noise from blowers, engines, and the like. This active silencer suppresses noise by emitting sound of the same amplitude with a 180° phase shift from a secondary sound source.
Figure 3 shows its basic configuration. In Figure 3, sound source 1
The sound waves generated from the pipe pass through the pipe 2 and are emitted from the open end of the pipe. Inside the conduit 2, a sound wave detection microphone 3 is provided on the sound source 1 side, and a secondary sound source 4 for muffling is provided on the open end side of the conduit.
are installed a predetermined distance apart. The signal detected by the sound wave detection microphone 3 is inverted and amplified by the signal processing circuit 5, sent to the secondary sound source 4, and output into the pipe line 2.

Here, the signal processing circuit 5 stores the time required for the sound wave generated from the sound source 1 to propagate the distance from the position of the sound wave detection microphone 3 to the position of the secondary sound source 4,
The sound waves output from the secondary sound source 4 are redirected to the microphone 3.
It must have characteristics that take into account acoustic feedback, etc. However, it is difficult to configure a stable system that takes into account the acoustic feedback that propagates in the cross mode in the conduit 2, that is, the sound waves that propagate in the conduit while reflecting off the walls.
If the distance between the secondary sound source 4 and the secondary sound source 4 is shortened, the feedback gain between the secondary sound source 4 and the microphone 3 increases, causing howling.

On the other hand, close-coupled active noise reduction devices have been studied in which the distances in Fig. 3 are set to 0 and L to make the system more compact and the signal processing circuit simplified, and the basic configuration thereof is shown in Fig. 4. Shown below. According to this method, the time delay between the microphone 3 and the secondary sound source 4 is almost negligible in the frequency band to be muted, so the signal processing circuit 5 simply inverts and amplifies the output of the microphone 3. Can be configured.

However, in such a tightly coupled active silencing device, howling is likely to occur due to the cross mode in the conduit 2, as described above, so it is not possible to increase the gain in the signal processing circuit 5. The output from the next sound source is limited, making it impossible to obtain a sufficient silencing effect. For this reason, at present, little consideration has been given to the basic structure of tightly coupled active noise damping devices, and as shown in Figure 5, a large number of tightly coupled active noise damping devices are arranged in cascade to increase the noise reduction effect. It is only being considered.

An object of the present invention is to suppress howling that occurs in the above-described close-coupled active silencer, and to enhance the noise reduction effect of the closely-coupled silencer.

[Means for Solving the Problems] An active noise reduction device according to the present invention includes a conduit through which a sound wave emitted from a primary sound source propagates, a sound wave detection means for detecting the sound wave propagating through this conduit, and a Signal processing that performs predetermined signal processing on the outputs of the secondary sound source for silencing and the sound wave detection means, which are provided in the silencing secondary sound source, and sends them to the secondary silencing sound source to generate sound waves of opposite phase from the secondary silencing sound source. and the sound wave detection means and the silencing secondary sound source are installed so as to be located on one plane perpendicular to the pipe.

The active noise reduction device according to claim (1) is provided with acoustic feedback detection means for detecting acoustic feedback outside the conduit behind the secondary sound source for silencing, and the output of the acoustic feedback detection means and the sound wave detection means are provided. The acoustic feedback detected by the acoustic wave detection means in the conduit is canceled by combining the outputs of the two.

The active sound damping device according to claim (2) is characterized in that the sound wave propagation time τ=L/C (C: speed of sound) determined by the distance L in the direction perpendicular to the pipe line between the sound wave detection means and the secondary sound source for noise reduction is the signal. By subtracting a signal obtained by delaying the output of the processing means from the output signal of the sound wave detection means, acoustic feedback from the secondary sound source to the sound wave detection means is canceled out.

[Operations] In the invention according to claim (1), in order to remove the acoustic feedback from the secondary sound source from the signal detected by the sound wave detection means in order to suppress howling, the L such that an acoustic feedback detection means is provided. When the acoustic feedback detection means is installed at the same distance as the distance between the secondary sound source and the sound wave detection means, the signal detected by the acoustic feedback detection means is a signal that is in opposite phase to the acoustic feedback detected by the sound wave detection means. becomes. By adding these signals, the acoustic feedback detected by the acoustic wave detection means is canceled out.

In the invention according to claim (2), by predicting the acoustic feedback to the sound wave detection means from the input signal of the secondary sound source and subtracting this predicted amount from the output signal of the sound wave detection means,
Stay at L to cancel out the acoustic feedback. The input of the secondary sound source is input for a certain period of time τ=L/C (C:
The predicted amount described above can be obtained. This suppresses acoustic feedback, thereby suppressing howling.

[Example] First example The configuration of a first embodiment of this invention is shown in FIG.

In FIG. 1, a secondary sound source speaker 4 is installed on the inner wall of the conduit 2.
is installed, and a sound wave detection microphone 3 is installed at a distance L1 from the speaker 4 in a direction perpendicular to the conduit 2.
A tightly coupled active noise reduction device is constructed. A speaker box 7 having a size determined by the size of the conduit 2 is installed behind the speaker 4. An acoustic feedback detection microphone 6 is installed in the speaker box 7 at a distance L2 from the speaker 4. The distance L2 is the sound wave detection microphone 3
The distance is adjusted so that the signal detected by the microphone 6 and the detection signal of the acoustic feedback detection microphone 6 are in opposite phases, and the acoustic feedback detection microphone 6 is installed so that L2=L1. The signal processing circuit of the active silencer includes preamplifiers 8, 9 . Adder 10° phase inversion amplifier 11. It is composed of a power amplifier 12. The output signal of the microphone 3 installed in the conduit 2 is combined with the output signal of the acoustic feedback detecting microphone 6 by an adder 10, and the acoustic feedback detected by the microphone 3 is canceled out. Adder 10
The phase of the output is inverted by an inverting amplifier 11, the gain is adjusted to an appropriate gain, and the output is output to the speaker 4 at L through a power amplifier 12.

Second embodiment FIG. 2 shows the configuration of a second embodiment of this invention.

In FIG. 2, a secondary sound source speaker 4 is installed on the inner wall of the conduit 2.
is installed, and a sound wave detection microphone 3 is installed at a distance from the speaker 4 in a direction perpendicular to the conduit 2, thereby forming a tightly coupled active silencer.

The signal processing circuit of the active silencer includes a preamplifier 9.
Subtractor 14, gain adjusters 15 and 19. Delay circuit 1
6. Consists of a low-pass filter 171, a phase inverter 18, and a power amplifier 12.

The sound wave signal inside the conduit 2 detected by the microphone 3 is
The signal passes through a preamplifier 9 and a low-pass filter 17, is turned into an opposite phase signal by a phase inverter 18, has its gain adjusted by a gain adjuster 19, is passed through a power amplifier 12 at L, and is output to a speaker 4.
A sound wave with an opposite phase is output. The output of the phase inverter 18 is also provided to a delay circuit 16 to determine the predicted amount of acoustic feedback. In the delay circuit 16, a signal is delayed for a time τ=L/C (C: speed of sound) determined by the distance L between the speaker 4 and the microphone 3. However, when the group delay characteristics of speaker 4 cannot be ignored,
The group delay characteristic of the speaker 4 is also considered and the delay time τ is corrected. This delayed signal is further applied to the subtracter 14 with the gain adjusted L by the gain adjuster 15, thereby canceling out the acoustic feedback.

In this second embodiment, the portion indicated by reference numeral 20 in the feedback loop is such that the output of the phase inverter 18 is outputted from the speaker 4, propagates in the conduit 2, is detected by the microphone 3, and is detected by the preamplifier 9. It imitates the transfer characteristics from to output.

[Effects of the Invention] According to the invention claimed in claim (1), in a tightly coupled active noise reduction device, the microphone installed behind the secondary sound source speaker is used to detect the sound wave detected by the microphone installed in the conduit. By detecting a signal with the opposite phase of the detected acoustic feedback and combining this signal with the output signal of the sound wave detection microphone installed in the conduit, the acoustic feedback is canceled out. Compared to the tightly coupled active noise damping device, the occurrence of howling is suppressed and the sound damping effect can be enhanced.

According to the invention according to claim (2), in the tightly coupled active noise reduction device, the delay time of the sound wave is taken into consideration depending on the distance between the secondary sound source and the microphone, and the acoustic feedback from the secondary sound source to the microphone is predicted by L, By subtracting this predicted amount from the microphone output, L is set to cancel out the acoustic feedback, so it is possible to suppress the occurrence of howling and increase the silencing effect compared to conventional tightly coupled active silencers. can.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a close-coupled active silencer according to a first embodiment of the present invention. FIG. 2 is a diagram showing a closely coupled active silencer according to a second embodiment of the present invention. FIG. 3 is a diagram showing the basic configuration of an active noise reduction device, which is the background of the present invention. FIG. 4 is a diagram showing the basic configuration of a tightly coupled active noise reduction device, which is the background of the present invention. FIG. 5 is a diagram showing a structure in which closely coupled active noise reduction devices are connected in cascade, which is the background of the present invention. In the figure, 1 is a primary sound source, 2 is a pipe, 3 is a microphone for detecting sound waves, 4 is a secondary sound source for silencing, 6 is a microphone for detecting acoustic feedback, 10 is an adder, 11 is an inverting amplifier, and 12 is a power Amplifier, 14 is a subtracter, 16
1 represents a delay circuit, and 19 represents a gain adjuster. Figure 1

Claims (2)

[Claims] (1) A conduit through which a sound wave emitted from a primary sound source propagates, a sound wave detection means for detecting the sound wave propagating through the conduit, a secondary sound source for silencing provided within the conduit, and the sound wave a signal processing means that performs predetermined signal processing on the output of the detection means and sends it to the silencing secondary sound source, and generates a sound wave of opposite phase from the silencing secondary sound source, the sound wave detection means and In a close-coupled active noise reduction device installed such that the secondary sound source for silencing is located on one plane perpendicular to the pipe, a sound source is placed outside the pipe behind the secondary sound source for silencing. Acoustic feedback detecting means for detecting feedback is provided, and the signal processing means includes a synthesizing means for synthesizing the output of the acoustic feedback detecting means and the output of the sound wave detecting means, An active silencer characterized in that the acoustic feedback detected by the detection means is canceled out. (2) a conduit through which a sound wave emitted from a primary sound source propagates; a sound wave detection means for detecting the sound wave propagating through the conduit; a secondary sound source for silencing provided within the conduit; and the sound wave a signal processing means that performs predetermined signal processing on the output of the detection means and sends it to the silencing secondary sound source, and generates a sound wave of opposite phase from the silencing secondary sound source, the sound wave detection means and In the close-coupled active noise reduction device, the signal processing means is arranged such that the secondary sound source for silencing is located on one plane perpendicular to the conduit, and the signal processing means includes the sound wave detection means and the silencing secondary sound source. The propagation time of the sound wave determined by the distance L in the direction perpendicular to the pipe from the next sound source = L/C
(C: speed of sound), including means for subtracting a signal obtained by delaying the output of the signal processing means from the output signal of the sound wave detection means, canceling out acoustic feedback from the secondary sound source for silencing to the sound wave detection means. An active silencer characterized by: