A three-dimension active silencer includes a measuring unit for detecting a signal relating to a sound radiated from a target sound source which is located in a closed box. A plurality of additional sound sources are arranged around the target sound source. A microphone is arranged at a predetermined position for detecting a sound power at the position. A controller is adapted to control respective amplitudes of the additional sound sources which can be different from an amplitude of the target sound source and respective phases of the additional sound sources, based on the signal detected by the measuring unit, in such a manner that the microphone detects the lowest sound power. The additional sound sources are arranged in such a manner that a position xp of the target sound source, respective relative positions di (i=1, 2, . . . , N) of the N additional sound sources from the target sound source and a wave number k of the sound radiated from the target sound source can substantially satisfy the following expression. According to the feature, it can be achieved to reduce the total sound power in the closed box including the target sound source.

FIELD OF THE INVENTION

This invention relates to a silencer for reducing a sound power radiated from a machine, in particular, to a three-dimension active silencer that is suitable for reducing a sound power radiated from a machine such as a generator in a three-dimension space.

BACKGROUND OF THE INVENTION

There is known an active silencer for a sound transmitted in a one-dimensional manner such as a sound radiated from an exhaust duct of a generator engine. The active silencer is called a one-dimension active silencer, and used in many fields.

FIG. 21is a schematic view of a conventional one-dimension active silencer. As shown inFIG. 21, in opposition to a sound transmitted from a sound source52arranged at an end of the exhaust duct51, the conventional one-dimension active silencer50is intended to reduce a sound power at an outlet53, which is the other end of the exhaust duct51, to zero.

The one-dimension active silencer50has a microphone54arranged at a side of the sound source52in the exhaust duct51in order to detect a sound signal in the exhaust duct51. The one-dimension active silencer50has also a similar microphone55arranged at a side of the outlet53in the exhaust duct51in order to detect a sound signal in the exhaust duct51.

A speaker56which can generate an additional sound is provided between the two microphones54and55in the exhaust duct51. An adaptation controller57is provided for controlling the speaker56. The adaptation controller57is adapted to control the speaker56based on a Filtered-X LMS algorithm, which is often used in general adaptation controlling systems.

The adaptation controller57has a compensation filter58for a transmitting function, a fixed FIR (Finite Impulse Response) filter59and an adaptive FIR filter60. An output signal from the microphone54is adapted to be inputted to the compensation filter58and the fixed FIR filter59. An output signal from the microphone55is adapted to be inputted to the adaptive FIR filter60. An output from the fixed FIR filter59is adapted to be inputted to the speaker56. Then, the adaptation controller57is adapted to automatically determine controlling coefficients in such a manner that the sound level detected by the microphone55is substantially zero.

In such a one-dimension active silencer50, based on the sound detected by the microphone54, the speaker56generates an additional sound having a reverse phase with respect to the detected sound. Thus, the additional sound interferes with the sound transmitted from the sound source52in the exhaust duct51. The result of the interference is monitored by the microphone55. Then, the speaker56continues to be controlled to generate an additional sound in such a manner that the sound power is reduced to substantially zero at the position where the microphone55is arranged. Once the sound power is reduced to zero at a position, the position reflects the sound because of difference in sound impedance. Therefore, the sound is not transmitted to the outlet53any more.

As described above, the one-dimension active silencer50cause the sound power from the sound source to interfere with the additional sound and to reduce to substantially zero when the sound power is transmitted in a one-dimensional manner, for example when the sound power is transmitted through the exhaust duct51. However, the one-dimension active silencer50is not effective in reducing a sound power transmitted in a three-dimensional manner.

There is known a method for reducing a total acoustic power when a sound power from a sound source such as a machine is radiated to a three-dimension space. In the method, many additional sound sources are arranged around the sound source. The respective additional sound sources generate respective additional sounds in order to reduce a sound power leaked from a space surrounded by the additional sound sources as much as possible. Such a conventional three-dimension active silencer is schematically shown in FIG.22.

As shown inFIG. 22, a plurality of additional sound sources113are arranged on a surrounding spherical surface112around a machine111as a target sound source. A plurality of microphone114is arranged close to the plurality of additional sound sources113, respectively.

Each of the plurality of microphones114is connected to a common controller115which is adapted to drive and control the plurality of additional sound sources113. A reference signal relating to a sound radiated from the machine111, for example a vibration signal of an engine if the machine111is the engine, is adapted to be inputted into the controller115.

The controller115conducts a control based on the reference signal in such a manner that each of the sound levels detected by the microphones114is reduced to a substantially minimum, respectively. That is, the controller115determines an amplitude and a phase of each of the additional sound sources113in such a manner that a sum of squares of the sound levels detected by the microphones114is a minimum.

However, in the silencer system shown inFIG. 22, each of the microphones114detects a plurality of additional sounds radiated from the plurality of additional sound sources113, at the same time (multi-overlapping). Thus, a controlling system in the controller115has to be built considering an affect of the multi-overlapping, which makes the controlling system more complex. In addition, a gap between each neighboring two of the additional sound sources113has to be less than half a wavelength of the sound from the target sound source. Thus, if the spherical surface112is defined far from the machine (target sound source)111, the number of the additional sound sources113has to be increased.

In addition, although the conventional active silencing control can achieve lowest sound powers at the positions where the microphones114are placed, it does not necessarily mean that a total acoustic power is a substantially minimum. That is, it is still not achieved to control to reduce a total acoustic power in a three-dimension space including a machine as a sound source.

On the other hand, there are known some passive silencing controls such as sound absorption or sound shading. However, the passive silencing controls is not so effective, especially when a main component of the sound is a low tone for example when the sound is radiated from a generator.

SUMMARY OF THE INVENTION

The object of this invention is to solve the above problems, that is, to provide a three-dimension active silencer that has a simpler controlling system and that is more effective.

In order to achieve the object, a three-dimension active silencer, comprises: a measuring unit configured to detect a signal relating to a sound radiated from a target sound source which is located in a closed box; a plurality of additional sound sources arranged around the target sound source; a microphone arranged at a predetermined position for detecting a sound level at the position; and a controller configured to control respective amplitudes of the additional sound sources which can be different from an amplitude of the target sound source and respective phases of the additional sound sources, based on the signal detected by the measuring unit, in such a manner that the microphone detects the lowest sound level; wherein the additional sound sources are arranged in such a manner that a position xp of the target sound source, respective relative positions di(i

=1, 2, . . . , N) of the N additional sound sources from the target sound source and a wave number k of the sound radiated from the target sound source can substantially satisfy a relationship:1N⁢⁢∑i=1N⁢⁢cos⁢⁢k⁢⁢(xp-di)-cos⁢⁢k·xp≅0

According to the feature, since the N additional sound sources are arranged in such a manner that the above expression is satisfied, a total acoustic power can be reduced to a minimum. As a result, it can be achieved to reduce the total acoustic power in the closed box (a three-dimension space) including the target sound source.

The above silencer is effective, especially when a main component of the sound is a low tone for example when the sound is radiated from an engine in a generator box or an exhaust duct. However, the above silencer is also effective against a target sound not in a closed box such as an engine for a small generator.

The signal detected by the measuring unit may be an accelerating signal obtained from a surface of an engine or the like, a rotating pulse signal, or an AC output signal produced by a generator.

Form many simulation experiments, it is found that the microphone is preferably arranged at a position within one fourth of a wavelength of the sound from the target sound source.

Preferably, the controller has a digital controlling part including a controlling-coefficient calculator which calculate controlling coefficients and a controlling-coefficient processor which calculate sum of products of the controlling coefficients and the signal, and the controlling coefficients are updated substantially every moment.

Preferably, the controller has: an analogue controlling part including a first band-pass filter, a phase adjuster and an amplitude adjuster; and an acoustic-power monitoring part including a second band-pass filter, an AD/DA converter and a digital circuit.

Preferably, the microphone is arranged in an area which is not in a node area of the sound radiated from the target sound source and which is not in node areas of sounds radiated from the additional sound sources.

In addition, a three-dimension active silencer for both a first target sound source and a second target sound source, comprises: a first additional sound source arranged around the first target sound source; second additional sound source arranged around the second target sound source; a first microphone arranged at a first predetermined position for detecting a sound level at the first position; a second microphone arranged at a second predetermined position close to the second target sound source for detecting a sound level at the second position; a second controller configured to control respective amplitudes of the second additional sound sources and respective phases of the second additional sound sources, in such a manner that the second microphone detects the lowest sound level; and a first controller configured to control respective amplitudes of the first additional sound sources and respective phases of the first additional sound sources, in such a manner that the first microphone detects the lowest sound level.

In the case, preferably, a distance between the second position where the second microphone is arranged and the second target sound source is substantially equal to a distance between the second position and the second additional sound source. More preferably, the second position where the second microphone arranged is substantially intermediate between the second target sound source and the second additional sound source.

Preferably, a plurality of second additional sound sources are arranged around the second target sound source, and a volume velocity of the second target sound source is substantially equal to a sum of respective volume velocities of the plurality of second additional sound sources.

The silencer having the above feature is effective especially when a sound radiated from the second target sound source has a substantially same frequency, a delayed amplitude and a delayed phase, with respect to a sound radiated from the first target sound source.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will now be described in more detail with reference to drawings.

First Embodiment

FIG. 1is a schematic view of a first embodiment of a three-dimension active silencer according to the invention. As shown inFIG. 1, the three-dimension active silencer of the first embodiment10is set for canceling sounds from an engine1and an exhaust duct2located in a closed box4. In the case, the engine1and the exhaust duct2form a target sound source.

The engine1is connected to a generator1m, and they form a private electric generator. The private electric generator is surrounded by a closed box4. A capacity of the private electric generator is valuable from a several kVA to a thousand kVA by changing a type of engine. In the embodiment, the private electric generator has a capacity of 60 kVA. The engine1is a type of 4-cylinder and 4-cycle (3000 rpm). In the case, a basic frequency of an engine sound is 100 Hz. Peaks of 103 Hz and 206 Hz are actually measured.

The three-dimension active silencer10includes a measuring unit5for detecting an accelerating signal of a surface of the engine1as a signal relating to the sound radiated from the target sound source. In addition, N (two inFIG. 1) additional sound sources are arranged around the engine1as the target sound source. In the case, the additional sound sources are speakers3. A microphone6is arranged at a predetermined position in the closed box4, for detecting a sound level at the position.

The additional sound sources3, the measuring unit5and the microphone6are connected to a common controller (controlling circuit)7. The controller7is adapted to control respective amplitudes of the additional sound sources3which can be different from an amplitude of the target sound source1and respective phases of the additional sound sources3, based on the signal detected by the measuring unit5, in such a manner that the microphone6detects the lowest sound level.

The additional sound sources3are arranged in such a manner that a sound-center position xpof the engine1and the duct2as the target sound source, respective relative positions di(i

=1, 2, . . . , N) of the N additional sound sources from the sound-center position (the target sound source) and a wave number k of the sound radiated from the target sound source can substantially satisfy a following relationship.1N⁢⁢∑i=1N⁢⁢cos⁢⁢k⁢⁢(xp-di)-cos⁢⁢k·xp≅0

Then, an operation of the first embodiment is explained.

When the engine1is driven, the signal relating to the engine sound is detected by the measuring unit5and is inputted into the controller7. The controller7adjusts the respective amplitudes and the respective phases of the additional sound sources3, based on the signal detected by the measuring unit5, in such a manner that the sound level detected by the microphone6is reduced to a minimum.

According to the above control by the controller7, the sound power at the position where the microphone6is arranged can be reduced. However, since the microphone is fixed to the position, the above control may not always achieve that a total acoustic power in the closed box4is reduced to a minimum. In order to reduce the total acoustic power to the minimum, it is important where the additional sound sources3and the microphone6are arranged. In the embodiment, the additional sound sources3are arranged with a special condition in order to reduce the total acoustic power in the closed box4to the minimum.

The special condition about an arrangement of the additional sound sources3is explained below.

At first, in the embodiment, the sound power is locally high at a cylinder section in the engine1. Thus, the target sound is radiated from the cylinder section in the same phase. That is, it is allowed to assume that the target sound source is a point sound source. Therefore, the engine sound can be treated by using a sound model that has a closed generator box (space)4and a point sound source therein.

When one target sound source1and N additional sound sources3are arranged in a closed space (all sound sources are point sound sources), a total acoustic power in the closed space is expressed by the following expression.W=12⁢Re⁢{p⁡(xp)⁢qp*+∑i=1N⁢p⁡(xsi)⁢qsi*}=ω2⁢ρ⁢⁢c22⁢⁢V⁢∑r⁢[2⁢⁢ξr⁢ωrMr⁢{ωr2-ω2)+(2⁢⁢ξr⁢ωr⁢ω)2}×{qp2⁢{ϕr⁡(xp)}2+qsi2⁢∑i=1N⁢∑j=1N⁢ϕr⁡(xsi)⁢ϕr⁡(xsj)+2⁢qsi⁢qp⁢∑i=1N⁢ϕr⁡(xp)⁢ϕr⁡(xsi)⁢cos⁡(θsi-θp)}](1)

In the above expression (1), xp represents a position of the target sound source, θpa phase of the target sound source, qpan amplitude of the target sound source, xsipositions of the additional sound sources, θsiphases of the additional sound sources, and qsiamplitudes of the additional sound sources (i =1, 2, . . . , N). In addition, ω represents an angular frequency, ρ a density, C a sound velocity, V a volume of the closed space, Mra modal mass, ξra modal damping, ωran characteristic angular frequency, and φr( ) a mode function.

As seen in the expression (1), the total acoustic power changes dependently on the positions xsithe phases θsiand the amplitudes qsiof the additional sound sources3.

A microphone6is arranged in the closed space. Then, the controller7conducts an adaptation control (Filtered-X) to reduce a sound level detected by the microphone6to a minimum. The controlled situation is expressed by the following expression.
P (xm)=[p, Zs1, . . . , ZSN][qp, qs1, qSN]T=0  (2)

In the above expression (2), P represents a sound level and xma position of the microphone. In addition, Z represent terms (functions) representing transmissions from the target sound source and the additional sound sources to the microphone, respectively. In addition, q represents a sound intensity (amplitude). Respective additional characters p and si represent the target sound source1and an i-th (i-listed) additional sound source3.

Then, it is assumed that the N additional sound sources3generate additional sounds having the same sound intensity by means of a common controlling system. The assumptions is expressed by the following expression.
qsi=qs(i=1, 2, . . . , N)

At that time, a ratio of the sound intensities between the target sound source1and the additional sound sources3(qs/qp) is expressed by the following expression.qsqs=-j⁢⁢ω⁢⁢ρ⁢⁢c2V⁢∑r⁢1Mr·ϕr⁡(xm)⁢ϕr⁡(xp)ωr2-ω2+2⁢j⁢⁢ξr⁢ωr⁢ωj⁢⁢ω⁢⁢ρ⁢⁢c2V⁢∑1Mr·ϕr⁡(xm)⁢∑i=1N⁢ϕr⁡(xsi)ωr2-ω2+2⁢j⁢⁢ξr⁢ωr⁢ω(4)

When the relationship is substituted into the above expression (1), the total acoustic power under the adaptation control can be obtained.

Herein, if the additional sound sources3have a common amplitude and a common phase reverse to the target sound source, that is, if the N additional sound sources3satisfies the following expression (The amplitude is 1/N and the phase is anti-phase. The ejπmeans -1.), it is known that the total acoustic power can be considerably reduced if a frequency of the sound is not more than 200 Hz.qsqp=1N⁢ej⁢⁢π(5)
When the relationship of the expression (5) is substituted into the above expression (4), the following relationship can be obtained.1N⁢∑i=1N⁢ϕr⁡(xsi)-ϕr′⁡(xp)=0(6)

Herein, in the first embodiment, a mode function (characteristic mode) in the closed box4may be expressed by the following expression.

In addition, since the N additional sound sources3have a common amplitude and a common phase, if their relative positions with respect to the target sound source1(point sound source) are represented by di (i=1, 2, . . . , N), the following relationship can be obtained from the expressions (6) and (7). It is confirmed that the relationship is satisfied for example if |di|≦1/k=α/2π.1N⁢∑i=1N⁢cos⁢⁢k⁡(xp-di)-cos⁢⁢k·xp≅0(8)

As described above, if the N additional sound sources3are arranged in such a manner that the expression (8) is satisfied, the total acoustic power in the closed box4can be reduced to a minimum. That is, according to the embodiment, the total acoustic power can be reduced to the minimum in the total three-dimensional space.

In addition, if it may be assumed that2madditional sound sources3are arranged in a substantially uniform distribution on a spherical surface around the target sound source1which may be assumed a point sound source, the relationship of the expression (8) may be transformed to the following expression.1m⁢∑i=1N⁢{cos⁢⁢k⁡(xp-di)+cos⁢⁢k⁡(xp+di)}-cos⁢⁢k·xp≅0(9)

Therefore, the following relationship may be obtained.
cos k di≅1  (10)

The relationship means that the additional sound source3are preferably arranged as close as possible to the target sound source.

In addition, the measuring unit5may detect a rotating pulse signal of the engine1or an AC output signal produced by the generator1m, as a signal relating to the sound from the target sound source, instead of the accelerating signal of the surface of the engine1. If the measuring unit5detects the AC output signal produced by the generator1m, since the signal may be produced by only a completely electric process, durability of the silencer may be improved than the case detecting the accelerating signal.

Second Embodiment

The second embodiment of the three-dimension active silencer is explained with reference toFIGS. 2to4.FIG. 2is a schematic view of a second embodiment of the three-dimension active silencer according to the invention.

As shown inFIG. 2, the three-dimension active silencer of the second embodiment is provided for a small generator not contained in a closed box4. N additional sound sources3are arranged in such a manner that a position xpof an engine1as a target sound source, respective relative positions di(i=1, 2, . . . , N) of the N additional sound sources from the target sound source and a wave number k of the sound radiated from the target sound source can substantially satisfy a following relationship.
k|di|≦π/2  (11)

A measuring unit5′ is adapted to detect an AC output signal produced by the generator1mand to make a sound-reference signal.

Other structures and components are substantially the same as the first embodiment shown in FIG.1. The same reference numerals are used in the second embodiment for the same elements as in the first embodiment. The explanations of the same elements as the first embodiment are omitted.

As shown inFIG. 3, an engine sound radiated from the small generator shown inFIG. 2has eleven peaks that start from 25 Hz at intervals of 25 Hz.

Then, an operation of the second embodiment is explained below.

When the engine1is driven, the signal relating to the engine sound is detected by the measuring unit5′ and is inputted into the controller7. The controller7adjusts the respective amplitudes and the respective phases of the additional sound sources3, based on the signal detected by the measuring unit5′, in such a manner that the sound level detected by the microphone6is reduced to a minimum.

The measuring unit5′ detects an AC output signal produced by the generator1m. An example of the detected signal is shown in FIG.4.FIG. 4shows a spectrum of a signal transformed to 2V by a transformer (not shown), from the AC output signal 100 V produced by the generator1m.

As shown inFIG. 4, with regard to the AC output signal detected by the measuring unit5′, a peak of a basic frequency 25 Hz may be distinguished easily, but other peaks (seeFIG. 3) may not be distinguished easily. Thus, it is not preferable to directly use the signal shown inFIG. 4for the control by the controller7.

Therefore, the measuring unit5′ of the second embodiment artificially makes an electric signal shown inFIG. 5, which has peaks that are multiples of a basic frequency 25 Hz. Then, the measuring unit5′ synchronizes (with respect to phases) and overlaps the electric signal with the signal shown inFIG. 4to produce a sound-reference signal. The sound-reference signal resembles the actual sound signal shown inFIG. 3very much and is suitable for using for the control by the controllers7. As a result, as shown inFIG. 6, all of the eleven peaks starting 25 Hz at the intervals of 25 Hz are reduced very well. The above process about the signal is a completely electric process, so that it needs less cost and less consideration for the durability of the silencer.

According to the above control by the controller7, the sound power at the position where the microphone6is arranged can be reduced. However, the above control may not always achieve that a total acoustic power around the small generator is reduced to a minimum. In order to reduce the total acoustic power to the minimum, it is important where the additional sound sources3and the microphone6are arranged.

In the embodiment, the additional sound sources3are arranged with a special condition in order to reduce the total acoustic power around the small generator to the minimum. The special condition about an arrangement of the additional sound sources3is explained below.

At first, in the embodiment, the sound power is locally high at a cylinder section (a substantially center portion) in the engine1. Thus, the target sound is radiated from the cylinder section in the same phase. That is, it is allowed to assume that the target sound source is a point sound source. Therefore, the engine sound can be treated by using a sound model that has a point sound source arranged in an open space.

It is assumed that one target sound source1and N additional sound sources3are arranged in an open space (all sound sources are point sound sources), the N additional sound sources3are arranged on a spherical surface around the target sound source1, and the N additional sound sources have a common amplitude and a common phase. In the case, a ratio r of sound levels of a case wherein the N additional sound sources operate to another case wherein no additional sound sources operate is represented by the following expression.r=WW1=1+(qsqp)2⁢∑i=1N⁢∑j=1N⁢sin⁢⁢c⁡(kdsi-sj)+2⁢N·(qsqp)⁢sin⁢⁢c⁡(kdp-si)⁢cos⁡(θS-θp)⁢(sic⁡(x)=sin⁢⁢xx)(12)

In the above expression (12), θprepresents a phase of the target sound source, qpan amplitude of the target sound source, θsphases of the additional sound sources, and qsamplitudes of the additional sound sources, dsi-sja distance between two (i-th (i-listed) and j-th (j-listed)) additional sound sources; dp-sja distance between the target sound source and a j-th (j-listed) additional sound sources (i, j=1, 2, . . . , N).

As seen in the expression (12), the total acoustic power changes dependently on the position, the phase θsand the amplitude qsof the additional sound sources3.

A microphone6is arranged in the open space. Then, the controller7conducts an adaptation control (Filtered-X) to reduce a sound level detected by the microphone6to a minimum. The controlled situation is expressed by the following expression.
P (xm)=[Zp, Zs1, . . . , ZSN][qp, qs1, . . . , qSN]T=0  (2)

In the above expression (2), P represents a sound level and xma position of the microphone. In addition, Z represent terms (functions) representing transmissions from the target sound source and the additional sound sources to the microphone, respectively. In addition, q represents a sound intensity (amplitude). Respective additional characters p and is represent the target sound source1and an i-th (i-listed) additional sound source3.

Then, it is assumed that the N additional sound sources3generate additional sounds by means of a common controlling system. At that time, a ratio of the sound intensities between the target sound source1and the additional sound sources3(qs/qp) is expressed by the following expression.qsqp=-ZpZs=-sin⁡(khp)khp+j⁢cos⁡(khp)khp∑i=1N⁢{sin⁡(khsi)khsi+j⁢cos⁡(khsi)khsi}(13)

In the above expression (13), hprepresents a distance between the target sound source1and the microphone6, and hsirepresents a distance between the i-th (i-listed) additional sound3and the microphone6. When the above relationship is substituted into the above expression (1), the total acoustic power under the adaptation control can be obtained.

Herein, if the additional sound sources3have a common amplitude and a common phase reverse to the target sound source, that is, if the N additional sound sources3satisfies the following expression (The amplitude is 1/N and the phase is reverse. The ejπmeans -1.), it is known that the total acoustic power can be considerably reduced if a frequency of the sound is not more than 200 Hz.qsqp=1N⁢ej⁢⁢π(5)
Thus, a relationship of hp hp≈hsican be obtained from a condition of the amplitude ratio 1/N in the expression (5).

FIGS. 7A and 7Bshow two examples of arrangements of the additional sound sources3and the microphone6that satisfy the relationship of hp≈hsi.FIG. 8shows relationship between a parameter kd and a reduced total acoustic power, in the case of the arrangement shown inFIG. 7A(dp-si=d), wherein the number of the additional sound sources is1to4. From a result shown inFIG. 8, it is confirmed that the total acoustic power may be considerably reduced if kdp-si(=k |di|)π/2(i=1, 2, . . . , N).

In addition, it is found that arrangements shown inFIGS. 9A and 9Dare much suitable if the additional sound sources3, the microphone6and the target sound source1are arranged in a plane.

As described above, if the N additional sound sources3are arranged in such a manner that the expression (11) is satisfied, the total acoustic power around the small generator can be reduced to a minimum. That is, according to the embodiment, the total acoustic power can be reduced to the minimum in the total three-dimensional space.

Third Embodiment

The third embodiment of the three-dimension active silencer is explained with reference to FIG.10.FIG. 10is a schematic view of a third embodiment of the three-dimension active silencer according to the invention.

As shown inFIG. 10, in the three-dimension active silencer of the third embodiment, a controller7has a digital controlling part7dincluding a controlling-coefficient calculator7awhich calculates controlling coefficients and a controlling-coefficient processor7bwhich calculates a sum of products of the controlling coefficients and the signal. In addition, the controlling coefficients are adapted to be updated substantially every moment.

Other structures and components are substantially the same as the first embodiment shown in FIG.1. The same reference numerals are used in the third embodiment for the same elements as in the first embodiment. The explanations of the same elements as the first embodiment are omitted.

According to the third embodiment, since the controlling coefficients are updated substantially every moment, the control can effectively follow a change caused by the passage of time, such as a temperature change of an inside and/or outside of the generator box4, a pulsation of the engine1or a sound-level (sound-pressure) change caused by a load change. As a result, the sound level detected by the microphone6can be stably reduced to a minimum and the total acoustic power can be also stably reduced to a minimum.

The feature of the third embodiment can be also adopted in the three-dimension active silencer of the second embodiment.

Fourth Embodiment

The fourth embodiment of the three-dimension active silencer is explained with reference to FIG.11.FIG. 11is a schematic view of a fourth embodiment of the three-dimension active silencer according to the invention.

As shown inFIG. 11, in the three-dimension active silencer of the fourth embodiment, a controller7has: an analogue controlling part7gincluding first band-pass filters21, phase adjusters22and amplitude adjusters23; and an acoustic-power (sound-pressure) monitoring part7hincluding second band-pass filters24, a signal switch25, an AD/DA converter26and a digital circuit27.

Other structures and components are substantially the same as the first embodiment shown in FIG.1. The same reference numerals are used in the fourth embodiment for the same elements as in the first embodiment. The explanations of the same elements as the first embodiment are omitted.

The controller7of the fourth embodiment conducts a filtering process to an output signal from the measuring unit5by means of the first band-pass filters21. Then, the controller7advances a phase of the signal by a degree from 0 to 180 or delays the phase by a degree from 0 to 180.

In addition, the controller7always monitors an acoustic-power (sound-pressure) signal detected by the microphone6. The controller7conducts a filtering process to the sound-level signal by means of the second band-pass filters24. The filtered signal is transmitted to the digital circuit27via the signal switch25and the AD/DA converter26.

The digital circuit27transmits an instructing signal to each of the phase adjusters22so that the phase is adjusted to lead the monitored sound level (detected by the microphone) to a minimum. Once it is confirmed that the monitored sounds level is led to the minimum, the phase is fixed. Then, each of the amplitude adjusters23conducts an amplitude-level adjusting process in order to reduce the microphone sound level further. The digital circuit27transmits an instructing signal to each of the amplitude adjusters23so that the phase is adjusted to lead the monitored sound level (detected by the microphone) to a further minimum.

The controller7repeats the above adjustment for every target frequency. Then, finally, the controller7generates a sound having an optimum phase and an optimum amplitudes and outputs it from the additional sound sources3.

According to the fourth embodiment, since the controller7has the analogue controlling part7gand the acoustic-power (sound-pressure) monitoring part7hto automatically adjust the phase and the amplitude for the additional sound sources3to their optima, the control can effectively follow a change caused by the passage of time, such as a temperature change of an inside and/or outside of the generator box4, a pulsation of the engine1or a sound-level (sound-pressure) change caused by a load change. As a result, the sound level detected by the microphone6can be stably reduced to a minimum and the total acoustic power can be also stably reduced to a minimum.

The feature of the fourth embodiment can be also adopted in the three-dimension active silencer of the second embodiment.

Fifth Embodiment

The fifth embodiment of the three-dimension active silencer is explained with reference to FIG.12.FIG. 12is a schematic view of a fifth embodiment of the three-dimension active silencer according to the invention.

As shown inFIG. 12, in the three-dimension active silencer of the fifth embodiment, a microphone6is arranged in an area which is not in a node area of the sound radiated from an engine1as a target sound source in a closed box and which is not in node areas of additional sounds radiated from the additional sound sources3in the closed box.

Other structures and components are substantially the same as the first embodiment shown in FIG.1. The same reference numerals are used in the fifth embodiment for the same elements as in the first embodiment. The explanations of the same elements as the first embodiment are omitted.

In the fifth embodiment, the microphone6is arranged with a special condition. The special condition is led from a fact that: when the microphone6is arranged in the nodal area (minimum-sound-pressure area) made by only the additional sound sources3, a denominator of the above expression (4) becomes zero and the control undesirably diverges; and a fact that: when the microphone6is arranged in the nodal area (minimum-sound-pressure area) made by only the target sound source1, a numerator of the above expression (4) becomes zero and the control is not conducted.

In the fifth embodiment, the phase θsiand the amplitude qsiof the additional sound sources3that can achieve to reduce the total acoustic power W defined by the expression (1) to a minimum, must satisfy the following expression in theory.ⅆWⅆθsi=0,ⅆWⅆqsi=0(14)
Thus, qsi/qpcan be obtained from the expression (14). If the qsi/sp is substituted into the expression (1), a minimum total acoustic power can be obtained. A minimum solution of the total acoustic power in theory is shown in FIG.13.

Values shown inFIG. 13are minima in theory. Thus, the adaptation control by the controller7can achieve the minimum acoustic powers for not every frequency. However, the adaptation control can make a minimum acoustic power for a special target frequency get closer to a theoretical value shown in FIG.13.

For example, a sound in the embodiment has a peak frequency of 103 Hz. Thus, if the frequency of 103 Hz is set as a special target frequency, an overall acoustic power can be reduced effectively.FIG. 14shows a graph of a relationship between the frequencies and the total acoustic powers when the control is conducted for a special target frequency of 103 Hz.

As described above, according to the fifth embodiment, an acoustic power W for a predetermined target frequency can be gotten closer to a theoretical minimum value.

In addition, a calculation of the acoustic power W is conducted for each of many positions where the microphone6is arranged under the above condition. It is found from the calculation that the acoustic power W can be effectively reduced to a minimum when the microphone6is arranged at a corner portion of the closed box4, especially when the microphone6is arranged in such a manner that a distance d from the corner and a wavelength α of the sound satisfy the following expression.d≦λ2⁢⁢π(15)

Herein, in the above embodiments, the target sound source is treated as the point sound source located in the center of the engine1because it is assumed that the sound radiated from the engine1is much larger than the sound radiated from the exhaust duct2. If the sound from the exhaust duct2is so large that it should not be ignored, an optimum arrangement question should be solved with another model having such an additional target sound source.

Sixth Embodiment

The sixth embodiment of the three-dimension active silencer is explained with reference to FIG.15.

In the three-dimension active silencer of the sixth embodiment, the additional sound sources3are arranged in such manner that the following expression is satisfied as described ith regard to the second embodiment.
k|di|≦π/2  (11)
Especially, in the case, the additional sound sources3are arranged in such a manner that the following expression is satisfied.
|di |=d=λ/4(k|di|=π/2)  (16)
In addition, as shown inFIG. 15, the target sound source1and the additional sound sources3are arranged in substantially the same plane, and the microphone6is arranged on a perpendicular line extending from the target sound source1perpendicular to the above plane.

As shown inFIG. 15, when hp and hs represent a distance between the microphone6and the target sound source1and a distance between the microphone6and each of the additional sound sources3, respectively, a phase difference θ of the additional sound sources3relative to the target sound source1is expressed by the following expression.
θ=2πf (h s−h p)/C+π
(h s=√h p2+d2)  (17)

In the sixth embodiment, as seen in the expression, (17), if hp=d=λ/4, the phase is shifted by 37 degrees from 180 degrees which is a condition of the minimum acoustic power. In addition, if hp=d/2=λ/8, the phase is shifted by 55 degrees. However, in a frequency area less than 200 Hz, the acoustic-power can be reduced although the degree of the acoustic-power reduction is lowered. Therefore, if the microphone6is arranged within λ /4 from the target sound source1similarly to the additional sound sources3, the acoustic power can be reduced effectively.

FIGS. 16Ato16C show results of actual experiments. As seen from the results shown inFIGS. 16Ato16C, enough effects of the acoustic-power reduction can be obtained in cases of d=0.10 m, d=0.20 m and d=0.25 m, respectively. Herein, a frequency of 200 Hz corresponds to λ=1.7 m, that is, λ/4=0.425 m.

Seventh Embodiment

The seventh embodiment of the three-dimension active silencer is explained with reference to FIG.17. The three-dimension active silencer of the seventh embodiment30is intended to reduce acoustic powers (sound pressures) radiated from a first target sound source31and a second target sound source32.

As shown inFIG. 17, a first additional sound source41is arranged around the first target sound source31. In addition, a first microphone33is arranged at a predetermined position, for example close to the first target sound source31. In addition, a sensor37such as an acceleration sensor is provided around the first target sound source31, for detecting a signal relating to a sound radiated from the first target sound source31and for transmitting the signal to a first controller (first controlling circuit)35as an inputting signal.

The first controller35has a controlling-coefficient calculator35awhich calculates adaptive controlling coefficients substantially every moment and a controlling-coefficient processor35bwhich calculates a sum of products of the controlling coefficients with the inputting signal land outputs the sum. In addition, the first controller35is adapted to control a phase and an amplitude of the first additional sound source41by using the sum outputted from the controlling-coefficient processor35bin such a manner the sound level detected by the first microphone33is reduced to a minimum.

The first additional sound source41has a speaker, and is adapted to receive the sum outputted from the controlling-coefficient processor35bas an inputted signal to supply an energy for silencing the target sound.

The first microphone33is adapted to detect a sum of the sounds from the first target sound source31and the first additional sound source41, and to function as an error-signal detector that regards the sum as an error signal.

Similarly, a second additional sound source42is arranged around the second target sound source32. In addition, a second microphone34is arranged at a predetermined position, for example close to the second target sound source32. In addition, a sensor38such as an acceleration sensor is provided around the second target sound source32, for detecting a signal relating to a sound radiated from the second target sound source32and for transmitting the signal to a second controller (first controlling circuit)36as an inputting signal.

The second controller36has a controlling-coefficient calculator36awhich calculates adaptive controlling coefficients substantially every moment and a controlling-coefficient processor36bwhich calculates a sum of products of the controlling coefficients with the inputting signal and outputs the sum. In addition, the second controller36is adapted to control a phase and an amplitude of the second additional sound source42by using the sum outputted from the controlling-coefficient processor36bin such a manner the sound level detected by the second microphone34is reduced to a minimum.

The second additional sound source42has a speaker, and is adapted to receive the sum outputted from the controlling-coefficient processor36bas an inputted signal to supply an energy for silencing the target sound.

The second microphone34is adapted to detect a sum of the sounds from the second target sound source32and the second additional sound source42, and to function as an error-signal detector that regards the sum as an error signal.

Then, a model ignoring an effect of the second additional sound source42is assumed. As shown inFIG. 18, when h1represents a distance between the first target sound source31and the first microphone33, h2a distance between the first additional sound source41and the first microphone33, h3a distance between the second target sound source32and the first microphone33, A1an amplitude of the first target sound source31, A2an amplitude of the first additional sound source41, θ a phase of the first additional sound source41with respect to the first target sound source31, and A3an amplitude of the second target sound source32, the acoustic power (sound pressure) detected by the first microphone33is expressed by the following expression (18).P=A1hi⁢ⅇ-j⁢⁢kh1+A2h2⁢ⅇ-j⁡(kh2+θ)+A3h3⁢ⅇ-j⁢⁢kh3(18)

Thus, an optimum amplitude and an optimum phase of the first additional sound source41, which can be obtained under the condition wherein the sound level detected by the first microphone33is reduced to a minimum by the adaptation control, satisfy the following expression (20) under the following expression (19).P2=(A1h1)2+(A2h2)2+(A3h3)2+2⁢A1⁢A2h1⁢h2⁢cos⁡[k⁡(h1-h2)-θ]+2⁢A1⁢A3h1⁢h3⁢cos⁡[k⁡(h1-h3)]+2⁢A2⁢A3h2⁢h3⁢cos⁡[k⁡(h2-h3)+θ](19)∂P2∂A2=0,∂P2∂θ=0(20)
That is, the optimum amplitude and the optimum phase of the first additional sound source41satisfy the following expressions (21) and (22).θ=tan-1⁢αh3⁢sin⁡[k⁡(h2-h3)]1h1-αh3⁢cos⁡[k⁢(h2-h3)](21)A2A1=-1h1⁢h2⁢cos⁡[k⁡(h1-h2)-θ]+αh2⁢h3⁢cos⁡[k⁡(h2-h3)-θ](1h2)2(22)

Herein, as expressed in the following expression (23), α is a ratio of the amplitude A2of the second target sound source32relative to the amplitude A1of the first target sound source31.α=A3A1(23)

If the ratio α=0 (the second target sound source32is not driven), h1=h2satisfy the following expression (24). That is, h1=h2is an optimum condition that can reduce the acoustic power of the first target sound source31to a minimum.θ→π,A2A1→1(24)

However, actually, the ratio α is not 0. Thus, because of the effect of the second target sound source32, the amplitude and the phase of the first additional sound source41may be shifted from their optima expressed by the above expression (24). If the amplitude ratio α is constant, the amplitude and the phase of the first additional sound source41can be compensated from the expressions (21) and (22) by taking the ratio α into consideration. However, if the amplitude ratio α changes during the passage of time, it is difficult to conduct such a compensation.

Therefore, at first, it is intended to silence the sound power from the second target sound source32.

As shown inFIG. 19, h4represents a distance between the second additional sound source42and the first microphone33. If the second additional sound source42is arranged closer to the target sound source32in such a manner that the relationship h3=h4is satisfied and the first microphone33is arranged in such a manner that the relationship h1<h3is satisfied, the sound level detected by the first microphone33is expressed by the following expression (25).P=A1hi⁢ⅇ-j⁢⁢kh1+A3h3⁢ⅇ-j⁢⁢kh3+A4h4⁢ⅇ-j⁡(kh4+π)⁢P2=A12⁢{(1h1)2+(1h3)2+(1h4)2+2⁢α2h1⁢h3⁢cos⁡[k⁡(h1-h3)]-2⁢α2h1⁢h4⁢cos⁡[k⁡(h1-h4)]-2⁢α2h3⁢h4⁢cos⁡[k⁡(h3-h4)]}⁢⁢if⁢⁢h3=h4.⁢P2=A12⁡[(1h1)2+2⁢(αh3)2-2⁢α2(h3)2]→(A1h1)2(25)
That is, in the case, the first microphone33can detect only the sound radiated from the first target sound source3l.

In addition, an optimum condition for the second additional sound source42can be obtained by arranging the second microphone34at a position where a volume velocity of the second target sound source32is substantially equal to a volume velocity of the additional sound source42. That is, the suitable position where the second microphone34should be arranged is in a minimum-acoustic-power area that is formed by the second additional sound source42having a same amplitude and a reverse phase with respect to the second target sound source32. For example, it is assumed that one second additional sound source42is used against one second target sound source32. In the case, the suitable position where the second microphone34should be arranged is located close to the second target sound source32and in such a manner that a distance between the suitable position and the second target sound source32is substantially equal to a distance between the suitable position and the second additional sound source42. Preferably, the suitable position is substantially intermediate between the second target sound source32and the second additional sound source42. If the second microphone34is arranged in such a manner, the second microphone34detects the sound radiated from the second target sound source32relatively more than the sound radiated from the first target sound source31, even if both of the target sound sources31and32are driven at the same time. Thus, the optimum condition for the second additional sound source42can be obtained when the sound level at the position where the second microphone34is arranged is reduced to a minimum by the adaptation control.

In addition, with regard to the number of the second additional sound source, a plurality of the second additional sound sources can be arranged around the second target sound source32. In the case, it is preferable that a blowing volume velocity of the second target sound source32is substantially equal to a sum of absorption volume velocities of the plurality of the second additional sound sources.

The seventh embodiment is suitable when the sound radiated from the second target sound source32has a substantially same frequency, a delayed amplitude and a delayed phase, with respect to the sound radiated from the first target sound source31′. For example, the second target sound source32may be a supplement machine, which is arranged close to the first target sound source31and to which the sound can be transmitted via one or more solid from the first target sound source31. In the case, the second target sound source32is called a secondary sound source.

Then, an effect of the seventh embodiment is explained with results of experiments.

In the experiments, as shown inFIG. 20, the first target sound source31consists of a speaker, and the second target sound source32consists of another speaker. In addition, the first additional sound source41is arranged to face the first target sound source31, and the second additional sound source42is arranged to face the second target sound source32.

The first microphone33is arranged at a position at the same distance from the first target sound source31and from the second target sound source32so that the first microphone33can also detect the sound radiated from the second target sound source32. In addition, the position where the first microphone33is arranged is at the same distance from the first target sound source31and from the first additional sound source41. That is, at the position, the acoustic power of the first target sound source31is minimum when the second target sound source is not driven.

On the other hand, the second microphone34is arranged close to the second target sound source so that the second microphone34is hard to detect the sound radiated from the first target sound source31.

Under the above condition, a sound having a frequency of. 200 Hz was generated by both of the first target sound sources31and the second target sound source32. Gains and phases of respective transmitting functions C11, C12, C21and C22from respective target sound sources31and32to respective microphones33and34were examined. The results are shown in Table 1.

Next, Table 2 shows the effect of the case where only the first target sound source31was driven and only the first additional sound source41was used for the silencing control.

As shown in Table 2, the sound level detected by the first microphone33was reduced from 75.0 dB to 48.3 dB. In addition, the sound level detected by the second microphone34was reduced from 75.0 dB to 47.7 dB. Furthermore, the sound level detected by the third microphone150arranged at a position shown inFIG. 20was reduced from 78.7 dB to 69.4 dB. The experimenter also could confirm the silencing effect.

Next, Table 3 shows the effect of the case where both of the first target sound source31and the second target sound source32were driven and only the first additional sound source41was used for the silencing control.

As shown in Table 3, the sound level detected by the first microphone33was reduced from 81.7 dB to 61.7 dB. However, the sound level detected by the second microphone34was increased from 97.2 dB to 97.7 dB. Furthermore, the sound level detected by the third microphone150arranged at the position shown inFIG. 20was increased from 77.7 dB to 82.0 dB. The experimenter also could not confirm the silencing effect.

It can be assumed in theory that the reason of the above result was that the sound radiated from the second target sound source32affected the microphone33to shift the phase and the amplitude from their theoretical values.

Next, Table 4 shows the effect of the case where both of the first target sound source31and the second target sound source32were driven, at first only the second additional sound source42and then both of the first additional sound source41and the second additional sound source42were used for the silencing control.

As shown in Table 4, because of the control by the second additional sound source42, the sound level detected by the first microphone33was reduced from 81.7 dB to 72.0 dB. In addition, the sound level detected by the second microphone34was reduced from 97.2 dB to 82.5 dB. However, the sound level detected by the third microphone150arranged at the position shown inFIG. 20was reduced only a little from 78.0 dB to 77.6 dB.

Then, after adding the control by the first additional sound source41, the sound level detected by the first microphone33was further reduced to 48.0 dB. In addition, the sound level detected by the third microphone150was reduced to 67.8 dB. The sound level detected by the second microphone34was restrained to 84.2 dB. The experimenter also could confirm the silencing effect.

As described above, according to the seventh embodiment, since a silencing control is conducted in a two-stepped manner for a space including two target sound sources, the silencing control can achieve an excellent silencing effect.