JPH02192541A - Active muffling device - Google Patents

Abstract

PURPOSE:To prevent a disturbance flow and noise in a duct and further prevent a howling with a low cost means by a method wherein a surface of a sound absorber material and surfaces of a sensor and a sound wave oscillator are covered by a thin film member made of non-porous resilent material. CONSTITUTION:An air blowing duct 2 is made such that its cross sectional area is expanded from an upstream side slightly upward of a microphone to a downstream end of a speaker 5. An increased cross sectional area forms a step at each of the surfaces and this step part is coated with a sound absorbing material 6 formed into a plate by foam material such as polyurethane foam or the like. A surface of a sound absorbing member 6 and the surfaces of a microphone 3 and a speaker 5 are covered by a thin film member 7 formed by forming chloroprene rubber of non-porous resilient material into a thin film.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an active muffling device that actively muffles noise propagating within a duct.

[Prior Art] Active noise reduction devices (see Japanese Patent Application Laid-open No. 1983-9397) have conventionally been used to actively eliminate noise by emitting into the duct a sound with a phase opposite to that of the noise propagating in the duct. Proposed.

This system consists of a microphone that detects noise propagating in the duct, a control circuit that generates low-frequency sound with the opposite phase to the noise detected by the microphone, and a control circuit that transmits the low-frequency sound created by the control circuit to the duct. The system consists of a speaker that emits air inside the duct, and performs sound pressure synthesis of the noise propagating inside the duct and the low-frequency sound emitted from the speaker, thereby eliminating noise.

In such active noise reduction devices, as a means to prevent howling that occurs when the sound emitted from the speaker is fed back to the microphone, a low-pass filter is used that attenuates and blocks medium and high frequency components that are likely to induce howling. It is assembled in such a way that the power is directed towards the open end of the duct so that the low frequency sound emitted from the speaker is not transmitted to the microphone.

[Problems to be Solved by the Invention] However, a sufficient howling prevention effect cannot be obtained using only a low-pass filter, and in order to improve the howling prevention effect, the control circuit becomes complicated and the cost of the device increases.

On the other hand, when the speakers are placed toward the open end of the duct, it is necessary to increase the number of speakers in order to obtain a silencing effect, resulting in an increase in the size of the device.

In addition, the presence of microphones and speakers exposed inside the duct may disrupt the air flow passing through the duct.
Due to increased ventilation resistance, turbulence noise could occur.

The present invention has been made based on the above-mentioned circumstances, and an object thereof is to provide an active muffling device that prevents turbulent noise in a duct and also prevents howling using low-cost means.

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present invention includes a detector for detecting noise propagating in a duct, and a sound having a phase opposite to the noise detected by the detector in the duct. In the active sound damping device, a sound absorbing material made of a porous elastic material is attached to an inner wall surface of the duct at least/partly between the detector and the sound wave oscillator. In addition, at least the surface of the sound absorbing material and the surfaces of the detector and the sound wave oscillator exposed in the duct are made of a non-porous bone material so that the inner wall surfaces of the duct are smooth and flush. Covering with a thin film member is adopted as a technical means.

[Operations and Effects of the Invention] The present invention having the above configuration has a sound absorbing material attached to the inner wall surface of at least a portion of the duct between the detector and the sonic wave oscillator, thereby reducing medium and high frequency components that tend to induce howling. In addition to attenuating the sound energy, it is possible to reduce the amount of un-cancelled sound emitted from the sound wave oscillator that feeds back and reaches the detector. Furthermore, by suppressing reflection on the inner wall surface of the duct, it is possible to reduce the generation of standing waves within the duct.

As a result, howling can be prevented from occurring. Furthermore, depending on the frequency of the noise propagating inside the duct,
Howling can be effectively prevented by changing the length or thickness of the sound absorbing material. For example, if the noise is a high frequency, the length of the sound absorbing material may be small, but if the noise is a low frequency, the length of the sound absorbing material may be longer.
Or it is necessary to make it thicker.

On the other hand, by covering at least the surface of the sound absorbing material and the surfaces of the detector and sound wave oscillator exposed in the duct with a thin film member, the inner wall surface of the duct is smooth and becomes the same surface (flush structure). It is possible to reduce turbulent noise generated on the surface of a detector, a sound wave oscillator, or a sound absorbing material.

Furthermore, since the surfaces of the detector and the sonic oscillator are covered with a thin film member, it is possible to prevent wind pressure and static pressure in the duct from being applied to the detector 2 and the sonic oscillator. Therefore, it is possible to prevent the noise detection ability of the detector from deteriorating, and also to prevent the accuracy of the sound reproduced by the sonic oscillator from deteriorating and to improve the durability of the sonic oscillator.

[Example] Next, an active silencing device of the present invention will be explained based on an example shown in the drawings.

FIG. 1 is a cross-sectional view of a duct to which the active silencing device of the present invention is applied.

The active silencer 1 of this embodiment is applied to, for example, a blower duct 2 of an air conditioner, and is a microphone (detector of the present invention) for detecting noise propagating inside the blower duct 2.
3, a control circuit 4 that generates a sound with a phase opposite to the noise detected by the microphone 3, and a speaker (sonic wave oscillator of the present invention) 5 that emits the sound generated by the control circuit 4 into the ventilation duct 2. has been done.

The ventilation duct 2 has a rectangular cross section, and a microphone 3 and a speaker 5 are assembled on the negative side thereof.

The microphone 3 is installed near a noise source (for example, a blower), and the speaker 5 is installed downstream from the microphone 3.

This ventilation duct 2 is provided with an enlarged cross-sectional area from a position slightly upstream of the microphone 3 (left side in Figure 1) to the downstream end of the speaker 5 (right side in Figure 1). A sound absorbing material 6 made of a porous elastic material such as polyurethane foam or the like formed into a plate shape is attached to each stepped portion of each surface formed by this process.

In addition, since the sound absorbing material 6 is attached to the inner wall surface of the ventilation duct 2, it is preferable that the sound absorbing material 6 is easily processed into a plate shape and has a particularly large sound absorbing effect in a low frequency range (below H2 in 1).

Further, the surface of the sound absorbing material 6 and the surfaces of the microphone 3 and speaker 5 are covered with a thin film member 7 made of chloroprene rubber, which is a non-porous elastic material, formed into a thin film.

At this time, the thickness of the sound absorbing material 6 and The mounting positions of the microphone 3 and speaker 5 are taken into consideration.

Note that it is desirable that the thin film member 7 has a smooth surface, is easy to process into a thin film, and does not cause acoustic resistance.

When attaching the sound absorbing material 6 to the inner wall surface of the ventilation duct 2,
When covering the surface of the sound absorbing material 6 with the thin film member 7, the work can be easily done by using double-sided tape or by attaching adhesive tape to one side of the sweat absorbing material 6 or the thin film member 7 in advance. .

The control circuit 4 is well-known! It is made of R construction and has the function of electrically amplifying the noise detected by microphone 3, blocking medium and high wave sound, and then adjusting the low frequency noise signal to the opposite phase of the noise detected by microphone 3. be.

Next, the operation of this embodiment will be explained.

Since the noise reduction control by the active noise reduction device 1 is conventionally well known, here, the effect caused by using the sound absorbing material 6 and the thin film member 7 will be explained.

By pasting the sound absorbing material 6 between the microphone 3 and the speaker 5, the sound energy of mainly medium and high frequency components is attenuated and silenced, and the canceled sound emitted from the speaker 5 is fed back to the microphone. 3
The amount reached can be reduced. Moreover, by suppressing reflection on the inner wall surface of the ventilation duct 2, it is possible to reduce the generation of standing waves within the ventilation duct 2, and improve acoustic characteristics (reduce reverberation).

As a result, the noise propagation characteristics within the air duct 2 are simplified, and the occurrence of howling can be prevented.

By simplifying the noise propagation characteristics in this way, the characteristics that must be realized by the control circuit 4 become simple, and the burden on the control circuit 4 is reduced. Therefore, the precision and reliability of the control circuit 4 can be improved, and the cost of the control circuit 4 can also be reduced.

In addition, since the amount of noise to be canceled transmitted to the downstream side of the air duct 2 can be reduced, the energy of the cancellation sound required for noise suppression is small, and therefore the amplifier ( (not shown) may be small.

On the other hand, the surface of the sound absorbing material 6 and the surfaces of the microphone 3 and the speaker 5 are covered with a thin film member 7, and the inner wall surface of the air duct 2 is made into a smooth flush structure without steps X-. It is possible to suppress an increase in pressure loss due to a change in the cross-sectional area of the air duct 2, and to reduce ventilation resistance within the air duct 2.

As a result, it is possible to reduce turbulence noise, frictional noise, etc. on the surfaces of the sound absorbing material 6, microphone 3, and speaker 5 that would otherwise occur when the thin film member 7 is not used.

Furthermore, since the surfaces of the microphone 3 and the speaker 5 are covered with the thin film member 7, it is possible to prevent wind pressure and static pressure in the air duct 2 from being applied to the microphone 3 and the speaker 5. This has effects such as preventing a decrease in noise detection ability, preventing a decrease in accuracy of sound reproduced by the speaker 5, and improving the durability of the speaker 5.

As described above, the active muffling device 1 according to the present invention can be achieved by simply attaching the sound absorbing material 6 and the thin film member 1 between the microphone 3 and the speaker 5, without increasing the size of the device as in the past. In addition, howling can be prevented at low cost, and turbulence noise, friction noise, etc. in the ventilation duct 2 can be reduced.

(Modification) In the above embodiment, the sound absorbing material 6 is pasted between a position slightly upstream of the microphone 3 and the speaker 5. A straddle material 6 is provided at least in part between the
Just paste it.

For example, if the noise is a high frequency, the sound absorbing material 6 may be provided in a part, and if the noise is a low frequency, the sound absorbing material 6 needs to be made longer or thicker.

Although polyurethane foam is used as the sound absorbing material 6, it is not necessary to be limited to polyurethane foam, and a porous material having elasticity such as glass wool, soft synthetic resin foam, or nonwoven fabric may be used.

Furthermore, although chloroprene rubber is used as the thin film member 7, a thin film made of a rubber material other than chloroprene rubber, a thin resin film, paper, or the like may also be used.

Although the cross section of the ventilation duct 2 is rectangular, it may be square, circular, or polygonal other than rectangular.

Although the microphone 3 is used as a detector, a vibration detector may be attached to a vibration source that causes noise generation, a vibrating wall surface, or the like.

Although the method of adhering the sound absorbing material 6 and the thin film member 7 with double-sided tape or the like has been shown, they may also be attached to the wall of the ventilation duct by tightening and fixing them with screws.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a blower duct to which the active silencing device of the present invention is applied. In the figure 1...Active silencer 2...Blower duct (duct) 3...Microphone (detector) 4...Control circuit 5...Speaker (sound wave oscillator) 6...Sound absorbing material 7.・・Thin J model parts

Claims (1)

[Claims] 1) An active silencer comprising: a detector that detects noise propagating within a duct; and a sonic oscillator that emits sound in phase opposite to the noise detected by the detector into the duct. A sound absorbing material made of a porous elastic material is attached to the inner wall surface of the duct at least in a portion between the detector and the sound wave oscillator, and the inner wall surface of the duct is smooth and flush. An active sound damping device characterized in that at least the surface of the sound absorbing material and the surfaces of the detector and the sound wave oscillator exposed in the duct are covered with a thin film member made of a non-porous elastic material.