Abstract:
In an acoustic control system an operation range of an active type noise control device (an ANC device) and an operation range of an active type effect sound control device (an ASC device) are exchanged in accordance with the number of working cylinders of an engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a National Stage entry of International Application No. PCT/JP2009/060242, having an international filing date of Jun. 4, 2009, which claims priority to Japanese Application No.: 2008-276368, filed Oct. 28, 2008, the disclosure of each of which is hereby incorporated in its entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to an active acoustic control system (active type acoustic control system) including an active noise control apparatus and an active sound control apparatus. 
     BACKGROUND ART 
     There are known an active noise control apparatus (hereinafter referred to as an “ANC apparatus”) and an active sound control apparatus (hereinafter referred to as an “ASC apparatus”) as apparatuses for controlling acoustics in relation to noise within the passenger compartment of a vehicle. 
     The ANC apparatus generates a canceling sound for canceling a noise such as a noise (muffled engine sound) that is generated in the passenger compartment of the vehicle by the operation (vibration) of the engine and a noise (road noise) that is generated in the passenger compartment by the contact between the wheels and the road while the vehicle is traveling, and reduces the noise with the canceling sound. Some ANC apparatuses are selectively turned on and off depending on the number of engine cylinders in operation and change frequencies to be controlled (see, for example, U.S. patent application Publication No. 2004/0258251). 
     An ASC apparatus generates a sound effect (quasi-engine sound) in synchronism with the muffled engine sound to enhance an acoustic effect in the passenger compartment, e.g., to emphasize a change in the speed of the vehicle (see, for example, U.S. patent application Publication No. 2006/0215846). 
     There has also been developed an active acoustic control system which employs an ANC apparatus and an ASC apparatus in combination (see, for example, International Publication No. WO 90/13109 and U.S. patent application Publication No. 2006/0269078). According to International Publication No. WO 90/13109, the ANC apparatus and the ASC apparatus are always in operation. According to U.S. patent application Publication No. 2006/0269078, in order to prevent the ANC apparatus and the ASC apparatus from interfering with each other, the ANC apparatus and the ASC apparatus are activated and inactivated in relation to each other depending on a combination of an engine rotational frequency [Hz] and a change per unit time in the engine rotational frequency (engine rotational frequency change) [Hz/s] (see FIG. 5 of U.S. patent application Publication No. 2006/0269078). 
     SUMMARY OF INVENTION 
     The invention disclosed in U.S. patent application Publication No. 2006/0269078 still remains to be improved for using the ANC apparatus and the ASC apparatus in more appropriate situations. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide an active acoustic control system which is capable of controlling an ANC apparatus and an ASC apparatus more appropriately. 
     According to the present invention, an active acoustic control system comprises an active noise control apparatus (ANC apparatus) for outputting a canceling sound (CS) to cancel a passenger compartment noise, an active sound control apparatus (ASC apparatus) for outputting a quasi-engine sound, and an operation switcher for switching between operation of the ANC apparatus and operation of the ASC apparatus, based on an operational range of the ANC apparatus and an operational range of the ASC apparatus which are related to at least one of a vehicle speed, an engine rotation frequency, a vehicle speed change, and an engine rotation frequency change, wherein the operation switcher changes the operational range of the ANC apparatus and the operational range of the ASC apparatus depending on an active cylinder number of an engine. 
     According to the present invention, the operation switcher changes the operational range of the ANC apparatus and the operational of the ASC apparatus, which are related to at least one of a vehicle speed, an engine rotation frequency, a vehicle speed change, and an engine rotation frequency change, depending on the active cylinder number of the engine. It is thus possible to perform an acoustic control process depending on the active cylinder number. As a result, the ANC apparatus and the ASC apparatus can be used in a more appropriate situation. 
     If the operational range of the ANC apparatus and the operational range of the ASC apparatus are defined by at least the engine rotation frequency, then the engine rotation frequency may have a minimum value for operating the ANC apparatus, the minimum value being set to a quotient obtained by dividing a minimum value of frequencies to be controlled by the ANC apparatus, by the order, with respect to the engine rotation frequency, of a chiefly generated frequency component of the passenger compartment noise depending on the active cylinder number, and the engine rotation frequency may have a maximum value for operating the ANC apparatus, the maximum value being set to a quotient obtained by dividing a maximum value of the frequencies to be controlled by the ANC apparatus, by the order. In this manner, an operational range for the ANC apparatus can be set appropriately. 
     If the operational range of the ANC apparatus and the operational range of the ASC apparatus are defined by at least the vehicle speed change or the engine rotation frequency change, then as the active cylinder number is greater, a minimum value of the vehicle speed change or the engine rotation frequency change for operating the ASC apparatus may be set to a lower value. Generally, as a torque which the engine is required to produce is higher, the active cylinder number is greater, and when the torque is high, the driver of the vehicle often wants to drive the vehicle in a sporty way. According to the present invention, as the active cylinder number is greater, the minimum value of the vehicle speed change or the minimum value of the engine rotational speed change for operating the ASC apparatus is set to a lower value to make the ASC apparatus operable more easily. Thus, the ASC apparatus is operated in a manner to meet the demands of the driver. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of a vehicle which incorporates an active acoustic control system according to an embodiment of the present invention; 
         FIG. 2  is a diagram showing a relationship between a rotational angle of the crankshaft of an engine and an explosion stroke of a cylinder of the engine when the engine is in an all-cylinder mode; 
         FIG. 3  is a diagram showing a relationship between a rotational angle of the crankshaft and an explosion stroke of a cylinder of the engine when the engine is in a two-cylinders-deactivation mode; 
         FIG. 4  is a diagram showing a relationship between a rotational angle of the crankshaft and an explosion stroke of a cylinder of the engine when the engine is in a three-cylinders-deactivation mode; 
         FIG. 5  is a block diagram of an acoustic controller of the active acoustic control system according to the embodiment of the present invention; 
         FIG. 6A  is a diagram showing an operational range defining table in the all-cylinder mode; 
         FIG. 6B  is a diagram showing an operational range defining table in the two-cylinders-deactivation mode; 
         FIG. 6C  is a diagram showing an operational range defining table in the three-cylinders-deactivation mode; and 
         FIG. 7  is a flowchart of an operation sequence in which an operation switcher of the acoustic controller selects an operational range defining table. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Embodiment 
     1. Overall and Componential Arrangement 
     (1) Overall Arrangement: 
       FIG. 1  is a schematic view of a vehicle  10  which incorporates an active acoustic control system  12  (hereinafter referred to as an “acoustic control system  12 ”) according to an embodiment of the present invention. The vehicle  10  may be a gasoline-powered vehicle, an electric vehicle, a fuel cell vehicle, or the like. The acoustic control system  12  has the functions of both an ANC apparatus and an ASC apparatus. 
     The acoustic control system  12  includes an acoustic controller  14 , a speaker  16 , a microphone  18 , and an amplifier  20 . In the acoustic control system  12 , a fuel injection controller  22  {hereinafter referred to as “FI ECU  22 ” (Fuel Injection Electronic Control Unit  22 )} for controlling fuel injection of an engine E inputs engine pulses Ep from the engine E and an active cylinder number signal Scy to the acoustic controller  14 . When the acoustic controller  14  is operating as the ANC apparatus, an error signal e is input through the microphone  18  to the acoustic controller  14 . Based on the engine pulses Ep, the active cylinder number signal Scy, and the error signal e, the acoustic controller  14  generates and outputs a combined control signal Scc representative of the waveform of a control sound CS through the amplifier  20  to the speaker  16 . The speaker  16  outputs the control sound CS represented by the combined control signal Scc. When the acoustic controller  14  is operating as the ANC apparatus, the control sound CS is a canceling sound for a muffled engine sound NZe. When the acoustic controller  14  is operating as the ASC apparatus, the control sound CS is a quasi-engine sound. When the acoustic controller  14  is operating as the ANC apparatus, the microphone  18  detects a residual noise left after the canceling sound has canceled the muffled engine sound NZe, and outputs an electric signal (error signal e) representative of the detected residual noise to the acoustic controller  14 . The acoustic controller  14  uses the error signal e in generating the control sound CS serving as the canceling sound. 
     (2) Engine E and FI ECU  22 : 
     In the present embodiment, the engine E is an engine having six cylinders each operating in four strokes (intake→compression→explosion→exhaust). The six cylinders are combined with one crankshaft. The six cylinders are arranged such that when all of them are in operation, explosion strokes take place in the cylinders at equal rotational angles. 
     In order for each cylinder to operate in the four strokes, the crankshaft needs to make two revolutions, with the intake and compression strokes in the first revolution and the explosion and exhaust strokes in the second revolution. Therefore, it is necessary that each explosion stroke takes place at every angle of 120° of the crankshaft rotation which is calculated by dividing the angle through which the crankshaft makes two revolutions (720°=360°×two revolutions) by six (the number of cylinders). The three sets of two cylinders are arranged with respect to the crankshaft at angular intervals of 120°, and while one of the two cylinders which are disposed at the same angular position is in the explosion stroke, the other cylinder is in the intake stroke. 
       FIG. 2  is a diagram showing the relationship between the rotational angle of the crankshaft of the engine and the explosion strokes of the cylinders when the engine is in an all-cylinder mode in which all the cylinders are active. In the all-cylinder mode, when the crankshaft rotates through 120°, the first explosion occurs in the first cylinder. When the crankshaft further rotates through 120° (when the crankshaft rotates through a total of)240°, the second explosion occurs in the second cylinder. When the crankshaft further rotates through 120° (when the crankshaft rotates through a total of 360°), the third explosion occurs in the third cylinder. When the crankshaft further rotates through 120° (when the crankshaft rotates through a total of 480°), the fourth explosion occurs in the fourth cylinder. When the crankshaft further rotates through 120° (when the crankshaft rotates through a total of 600°), the fifth explosion occurs in the fifth cylinder. When the crankshaft further rotates through 120° (when the crankshaft rotates through a total of 720°), the sixth explosion occurs in the sixth cylinder. 
     The engine E according to the present embodiment also operates in cylinders-deactivation modes in which some cylinders are deactivated, for the purpose of achieving better mileage when the engine E produces a low torque and rotates at a high engine rotational speed (such as when the vehicle is cruising). The cylinders-deactivation modes include a two-cylinders-deactivation mode in which four of the six cylinders are activated and the remaining two cylinders are deactivated and a three-cylinders-deactivation mode in which three of the six cylinders are activated and the remaining three cylinders are deactivated. 
     As the crankshaft and the cylinders are physically connected to each other, the correlation between the rotational angle of the crankshaft and the angular positions of the explosion strokes cannot be changed. In the two-cylinders-deactivation mode, the explosion strokes take place as shown in  FIG. 3 , for example. In the three-cylinders-deactivation mode, the explosion strokes take place as shown in  FIG. 4 , for example. 
     In the two-cylinders-deactivation mode, as shown in  FIG. 3 , when the crankshaft rotates through 120°, the first explosion occurs in the first cylinder. When the crankshaft further rotates through 240° (when the crankshaft rotates through a total of 360°), the second explosion occurs in the third cylinder (no explosion occurs in the second cylinder). When the crankshaft further rotates through 120° (when the crankshaft rotates through a total of 480°), the third explosion occurs in the fourth cylinder. When the crankshaft further rotates through 240° (when the crankshaft rotates through a total of 720°), enters the fourth explosion occurs in the sixth cylinder (no explosion occurs in the fifth cylinder). 
     In the three-cylinders-deactivation mode, as shown in  FIG. 4 , when the crankshaft rotates through 240°, the second cylinder enters the first explosion stroke (the first cylinder does not enter the explosion stroke). When the crankshaft further rotates through 240° (when the crankshaft rotates through a total of 480°), the fourth cylinder enters the second explosion stroke (the third cylinder does not enter the explosion stroke). When the crankshaft further rotates through 240° (when the crankshaft rotates through a total of 720°), the sixth cylinder enters the third explosion stroke (the fifth cylinder does not enter the explosion stroke). 
     Whether the engine E is to operate in the all-cylinder mode, the two-cylinders-deactivation mode, or the three-cylinders-deactivation mode is determined by the FI ECU  22  which controls the ignition timings, etc. of the engine E depending on parameters including a torque which the engine E is required to produce. 
     The FI ECU  22  controls the fuel injection and ignition of the engine E, and sends engine pulses Ep and an active cylinder number signal Scy to the acoustic control system  12 . 
     An engine pulse Ep output by the FI ECU  22  is a signal which goes high when the piston (not shown) in each cylinder reaches the top dead center. Since the engine E according to the present embodiment has six cylinders, the engine pulse signal goes high six times every two revolutions of the crankshaft, i.e., the engine pulse signal goes high three times every one revolution of the crankshaft, irrespective of which mode the engine E operates in. 
     The active cylinder number signal Scy is representative of the number of active cylinders (active cylinder number Ncy). According to the present embodiment, the active cylinder number signal Scy represents six in the all-cylinder mode, four in the two-cylinders-deactivation mode, and three in the three-cylinders-deactivation mode. 
     (3) Acoustic Controller  14 : 
     (a) Overall Arrangement: 
       FIG. 5  shows an internal arrangement of the acoustic controller  14 . The acoustic controller  14  includes an engine rotation frequency detector  30  (hereinafter referred to as a “detector  30 ”), an ANC circuit  32 , an engine rotation frequency change detector  34  (hereinafter referred to as a “detector  34 ”), an ASC circuit  36 , an operation switcher  38 , and an adder  40 . 
     (b) Engine Rotation Frequency Detector  30 : 
     The detector  30  detects an engine rotation frequency fe [Hz] based on the engine pulses Ep from the FI ECU  22 , and outputs the detected engine rotation frequency fe to the ANC circuit  32 , the detector  34 , the ASC circuit  36 , and the operation switcher  38 . As described above, the engine pulse Ep goes high three times during one revolution of the crankshaft, irrespective of which mode the engine E operates in. One period of the engine pulse Ep is equal to the time period in which the engine E makes a ⅓ revolution. Based on this relationship, the engine rotation frequency fe can be calculated by detecting the time from the rising edge of an engine pulse Ep to the rising edge of the next engine pulse Ep, for example. 
     (c) ANC Circuit  32 : 
     The ANC circuit  32  generates a control signal Sc 1  based on the engine rotation frequency fe from the detector  30  and the error signal e from the microphone  18 , and outputs the generated control signal Sc 1  to the adder  40 . The control signal Sc 1  represents the waveform of the control sound CS serving as a canceling sound for canceling the muffled engine sound NZe. The ANC circuit  32  generates a reference signal (canceling sound reference signal) for the control sound CS based on the engine rotation frequency fe, and performs an adaptive filtering process on the canceling sound reference signal thereby to generate the control signal Sc 1 . In the adaptive filtering process, the canceling sound reference signal is passed through an adaptive filter. The adaptive filter has filter coefficients which are set to minimize the error signal e based on a reference signal, which is generated by correcting the canceling sound reference signal based on transfer characteristics from the speaker  16  to the microphone  18 , and the error signal e. The ANC circuit  32  may be one of the circuits disclosed in U.S. patent application Publication No. 2004/0258251 and U.S. patent application Publication No. 2006/0269078, for example. 
     As described later, when the ANC circuit  32  receives an output stop signal Sw 1  from the operation switcher  38 , the ANC circuit  32  reduces the amplitude of the control signal Sc 1  to zero, essentially eliminating its output signal. 
     (d) Engine Rotation Frequency Change Detector  34 : 
     The detector  34  calculates an engine rotation frequency change Δaf (a change in the engine rotation frequency fe per unit time) [Hz/s] based on the engine rotation frequency fe from the detector  30 , and outputs the engine rotation frequency change Δaf to the ASC circuit  36  and the operation switcher  38 . 
     (e) ASC Circuit  36 : 
     The ASC circuit  36  generates a control signal Sc 2  based on the engine rotation frequency fe from the detector  30  and the engine rotation frequency change Δaf from the detector  34 , and outputs the control signal Sc 2  to the adder  40 . The control signal Sc 2  represents the waveform of the control signal CS serving as a sound effect (quasi-engine sound) in synchronism with the muffled engine sound NZe. The ASC circuit  36  generates a reference signal (sound effect reference signal) for the control sound CS based on the engine rotation frequency fe, and performs various sound pressure adjusting processes on the sound effect reference signal thereby to generate the control signal Sc 2 . The sound pressure adjusting processes include a process for increasing a gain used for the sound effect reference signal in response to increase in the engine rotation frequency change Δaf (Δaf-specific sound pressure adjusting process). The ASC circuit  36  may generate a plurality of sound effect reference signals depending on the order (1st order, 1.5th order, 3rd order, etc.) of the engine rotation frequency fe. If the ASC circuit  36  generates a plurality of sound effect reference signals, then the ASC circuit  36  may perform different amplitude adjusting processes on the sound effect reference signals depending on the engine rotation frequency and the order thereof, combine the amplitude-adjusted sound effect reference signals into a combined sound effect reference signal, and then perform the Δaf-specific sound pressure adjusting process on the combined sound effect reference signal. The ASC circuit  36  may be one of the circuits disclosed in U.S. patent application Publication No. 2006/0215846 and U.S. patent application Publication No. 2006/0269078, for example. 
     As described later, when the ASC circuit  36  receives an output stop signal Sw 2  from the operation switcher  38 , the ASC circuit  36  reduces the amplitude of the control signal Sc 2  to zero, essentially eliminating its output signal. 
     (f) Adder  40 : 
     The adder  40  combines the control signal Sc 1  from the ANC circuit  32  and the control signal Sc 2  from the ASC circuit  36  into a combined control signal Scc, and outputs the combined control signal Scc through the amplifier  20  to the speaker  16 . 
     (g) Operation Switcher  38 : 
     The operation switcher  38  generates an output stop signal Sw 1 , an output stop signal Sw 2  or both of them based on the active cylinder number signal Scy from the FI ECU  22 , the engine rotation frequency fe from the detector  30 , and the engine rotation frequency change Δaf from the detector  34 . The operation switcher  38  sends the output stop signal Sw 1  to the ANC circuit  32  and sends the output stop signal Sw 2  to the ASC circuit  36  for thereby controlling operation of the ANC circuit  32  and operation of the ASC circuit  36 . 
     Specifically, the operation switcher  38  selects an operational range defining table depending on the active cylinder number signal Scy from a plurality of operational range defining tables. The operational range defining tables serve to define an operational range of the ANC circuit  32  and an operational range of the ASC circuit  36  based on the engine rotation frequency fe and the engine rotation frequency change Δaf. According to the present embodiment, the operational range defining tables include an all-cylinder table corresponding to the all-cylinder mode (see  FIG. 6A ), a two-cylinders-deactivation table corresponding to the two-cylinders-deactivation mode (see  FIG. 6B ), and a three-cylinders-deactivation table corresponding to the three-cylinders-deactivation mode (see  FIG. 6C ). Each of  FIGS. 6A through 6C  has a horizontal axis representing an engine rotational speed Ne [rpm] which is equal to 60 times the engine rotation frequency fe and a vertical axis representing an engine rotational speed change ΔNe [rpm/s] which is equal to 60 times the engine rotation frequency change Δaf. 
     The operation switcher  38  switches between operation of the ANC circuit  32  and operation of the ASC circuit  36  based on the selected operational range defining table, the engine rotation frequency fe, and the engine rotation frequency change Δaf. For example, if the all-cylinder table shown in  FIG. 6A  is selected, the engine rotational speed Ne is 3000 [rpm], and the engine rotational speed change ΔNe is 50 [rpm/s], then the operation switcher  38  sends the output stop signal Sw 1  to the ANC circuit  32  and does not send the output stop signal Sw 2  to the ASC circuit  36 , thereby operating the ASC circuit  36 . If the two-cylinders-deactivation table shown in  FIG. 6B  is selected, the engine rotational speed Ne is 3000 [rpm], and the engine rotational speed change ΔNe is 50 [rpm/s], then the operation switcher  38  sends the output stop signal Sw 2  to the ASC circuit  36  and does not send the output stop signal Sw 1  to the ANC circuit  32 , thereby operating the ANC circuit  32 . 
     In the all-cylinder table shown in  FIG. 6A , the operation switcher  38  operates the ANC circuit  32  if the engine rotational speed Ne is in the range from 700 to 2000 [rpm] and the engine rotational speed change ΔNe is in the range from −150 to 100 [rpm/s]. Also, the operation switcher  38  operates the ASC circuit  36  if the engine rotational speed Ne is equal to or higher than 2200 [rpm] or the engine rotational speed change ΔNe is equal to or higher than 150 [rpm/s]. Further, the operation switcher  38  does not operate either of the ANC circuit  32  and the ASC circuit  36  (i.e., sends the output stop signal Sw 1  to the ANC circuit  32  and the output stop signal Sw 2  to the ASC circuit  36 ) if the engine rotational speed Ne and the engine rotational speed change ΔNe are in ranges other than the above. 
     In the two-cylinders-deactivation table shown in  FIG. 6B , the operation switcher  38  operates the ANC circuit  32  if the engine rotational speed Ne is in the range from 2100 to 6000 [rpm] and the engine rotational speed change ΔNe is in the range from −150 to 150 [rpm/s]. Also, the operation switcher  38  operates the ASC circuit  36  if the engine rotational speed Ne is equal to or higher than 6200 [rpm] or the engine rotational speed change ΔNe is equal to or higher than 200 [rpm/s]. Further, the operation switcher  38  does not operate either of the ANC circuit  32  and the ASC circuit  36  if the engine rotational speed Ne and the engine rotational speed change ΔNe are in ranges other than the above. 
     In the three-cylinders-deactivation table shown in  FIG. 6C , the operation switcher  38  operates the ANC circuit  32  if the engine rotational speed Ne is in the range from 1400 to 4000 [rpm] and the engine rotational speed change ΔNe is in the range from −150 to 300 [rpm/s]. Also, the operation switcher  38  operates the ASC circuit  36  if the engine rotational speed Ne is equal to or higher than 4200 [rpm] or the engine rotational speed change ΔNe is equal to or higher than 400 [rpm/s]. Further, the operation switcher  38  does not operate either of the ANC circuit  32  and the ASC circuit  36  if the engine rotational speed Ne and the engine rotational speed change ΔNe are in ranges other than the above. 
     In each of the operational range defining tables, the maximum and minimum values of the engine rotational speed Ne for operating the ANC circuit  32  are determined depending on the minimum and maximum values of frequencies to be controlled by an ANC apparatus. The ANC apparatus is made up of the detector  30 , the ANC circuit  32 , the amplifier  20 , the speaker  16 , and the microphone  18 . The minimum value of the frequencies to be controlled by the ANC apparatus according to the present embodiment is 35 [Hz], and the maximum value thereof is 100 [Hz] (i.e., the ANC apparatus cancels a noise in the frequency range from 35 to 100 Hz). 
     As shown in  FIG. 2 , when the engine E is operating in the all-cylinder mode, three explosions occur at equal angular intervals (i.e., every 120°) each time the crankshaft of the engine E makes one revolution. Therefore, the muffled engine sound NZe generated at this time chiefly includes a 3rd-order component of the engine rotation frequency fe. A quotient obtained by dividing the minimum value of the frequencies to be controlled by the AMC apparatus by 3 (i.e., 35÷3) represents the minimum value of the engine rotational speed fe for operating the ANC circuit  32 . Then, the quotient is multiplied by 60 thereby to obtain the minimum value of the engine rotational speed Ne of 700 [rpm] (=35÷3×60). Similarly, a quotient obtained by dividing the maximum value of the frequencies to be controlled by the ANC apparatus by 3 (i.e., 100÷3) represents the maximum value of the engine rotational speed fe for operating the ANC circuit  32 . The quotient is multiplied by 60 thereby to obtain the maximum value of the engine rotational speed Ne of 2000 [rpm] (=100÷3×60). 
     As shown in  FIG. 3 , when the engine E is operating in the two-cylinders-deactivation mode, two explosions occur each time the crankshaft of the engine E makes one revolution. At this time, the explosions do not occur at equal angular intervals. More specifically, the first explosion and the second explosion are angularly spaced by an angular interval of 240°, the second explosion and the third explosion are angularly spaced by an angular interval of 120°, and the first explosion and the third explosion are angularly spaced by an angular interval of 360°. As these angular intervals appear once each time the crankshaft of the engine E makes one revolution, the muffled engine sound NZe chiefly includes a 1st-order component)(360°), a 1.5th-order component)(240°), and a 3rd-order component)(120°) of the engine rotation frequency fe. Of these components, the 1st-order component is the lowest. A quotient obtained by dividing the minimum value of the frequencies to be controlled by the ANC apparatus by 1 (i.e., 35÷1) represents the minimum value of the engine rotational speed fe for operating the ANC circuit  32 . Then, the quotient is multiplied by 60 thereby to obtain the minimum value of the engine rotational speed Ne of 2100 [rpm] (=35÷1×60). Similarly, a quotient obtained by dividing the maximum value of the frequencies to be controlled by the ANC apparatus by 1 (i.e., 100÷1) represents the maximum value of the engine rotational speed fe for operating the ANC circuit  32 . Then, the quotient is multiplied by 60 thereby to obtain the maximum value of the engine rotational speed Ne of 6000 [rpm] (=100÷1×60). 
     As shown in  FIG. 4 , when the engine E is operating in the three-cylinders-deactivation mode, three explosions occur at equal angular intervals of 240° each time the crankshaft of the engine E makes two revolutions. Stated otherwise, the engine E undergoes the explosion 1.5 times during one revolution of the crankshaft. Therefore, the muffled engine sound NZe generated at this time chiefly includes a 1.5th-order component of the engine rotation frequency fe. A quotient obtained by dividing the minimum value of the frequencies to be controlled by the ANC apparatus by 1.5 (i.e., 35÷1.5) represents the minimum value of the engine rotational speed fe for operating the ANC circuit  32 . Then, the quotient is multiplied by 60 thereby to obtain the minimum value of the engine rotational speed Ne of 1400 [rpm] (=35÷1.5×60). Similarly, a quotient obtained by dividing the maximum value of the frequencies to be controlled by the ANC apparatus by 1.5 (i.e., 100÷1.5) represents the maximum value of the engine rotational speed fe for operating the ANC circuit  32 . Then, the quotient is multiplied by 60 thereby to obtain the maximum value of the engine rotational speed Ne of 4000 [rpm] (=100÷1.5×60). 
     In each of the operational range defining tables, when attention is focused only on the engine rotational speed Ne, the minimum value of the engine rotational speed Ne for operating the ASC circuit  36  is determined depending on the maximum value of the frequencies to be controlled by an ASC apparatus. In other words, a value calculated by adding 200 [rpm] to the maximum value of the frequencies to be controlled by the ANC apparatus represents the minimum value of the engine rotational speed Ne for operating the ASC circuit  36 . The ASC apparatus is made up of the detector  30 , the detector  34 , the ASC circuit  36 , the amplifier  20 , and the speaker  16 . 
     In each of the operational range defining tables, when attention is focused only on the engine rotational speed change ΔNe, the minimum value of the engine rotational speed change ΔNe for operating the ASC circuit  36  is set to a lower value as the active cylinder number Ncy of the engine E is greater. Specifically, the minimum value of the engine rotational speed change ΔNe in the all-cylinder mode in which the active cylinder number Ncy is six, is lower than that in the two-cylinders-deactivation mode in which the active cylinder number Ncy is four. Also, the minimum value of the engine rotational speed change ΔNe in the two-cylinders-deactivation mode in which the active cylinder number Ncy is four, is lower than that in the three-cylinders-deactivation mode in which the active cylinder number Ncy is three. The reasons for such settings are as follows: Generally, as a torque which the engine E is required to produce is higher, the active cylinder number Ncy is greater, and when the torque is high, the driver of the vehicle often wants to drive the vehicle in a sporty way. Therefore, as the active cylinder number Ncy is greater, the minimum value of the engine rotational speed change ΔNe for operating the ASC apparatus is set to a lower value thereby to make the ASC apparatus operable more easily. Thus, the ASC apparatus is operated in a manner to meet the demands of the driver. 
     (4) Speaker  16 : 
     The speaker  16  outputs the control sound CS based on the combined control signal Scc from the acoustic control system  12 . Therefore, when the acoustic control system  12  operates as the ANC apparatus, the speaker  16  outputs a canceling sound for canceling the muffled engine sound NZe, and when the acoustic control system  12  operates as the ASC apparatus, the speaker  16  outputs a sound effect as a quasi-engine sound. 
     (5) Microphone  18 : 
     The microphone  18  detects the difference, i.e., an error, between the muffled engine sound NZe and the control sound CS serving as the canceling sound, as a residual noise, and outputs an error signal e representative of the residual noise to the ANC circuit  32  of the acoustic control system  12 . 
     2. Selection of an Operational Range Defining Table 
       FIG. 7  is a flowchart of an operation sequence in which the operation switcher  38  selects an operational range defining table. 
     In step S 1  shown in  FIG. 7 , the operation switcher  38  receives the active cylinder number signal Scy from the FI ECU  22 . In step S 2 , the operation switcher  38  determines whether the active cylinder number Ncy represented by the active cylinder number signal Scy is six (all-cylinder mode) or not. If the active cylinder number signal Scy indicates the all-cylinder mode (S 2 : Yes), then the operation switcher  38  selects the all-cylinder table ( FIG. 6A ) in step S 3 . 
     If the active cylinder number signal Scy does not indicate the all-cylinder mode (S 2 : No), then the operation switcher  38  determines whether the active cylinder number Ncy represented by the active cylinder number signal Scy is four (two-cylinders-deactivation mode) or not in step S 4 . If the active cylinder number signal Scy indicates the two-cylinders-deactivation mode (S 4 : Yes), then the operation switcher  38  selects the two-cylinders-deactivation table ( FIG. 6B ) in step S 5 . 
     If the active cylinder number signal Scy does not indicate the two-cylinders-deactivation mode (S 4 : No), then the operation switcher  38  determines whether the active cylinder number Ncy represented by the active cylinder number signal Scy is three (three-cylinders-deactivation mode) or not in step S 6 . If the active cylinder number signal Scy indicates the three-cylinders-deactivation mode (S 6 : Yes), then the operation switcher  38  selects the three-cylinders-deactivation table ( FIG. 6C ) in step S 7 . If the active cylinder number signal Scy does not indicate the three-cylinders-deactivation mode (S 6 : No), then it is considered that the acoustic control system  12  is in operation, but the engine E is not operating (e.g., the engine key is in an “accessory” position). In this case, the operation switcher  38  does not select any of the operational range defining tables, and does not operate either of the ANC circuit  32  and the ASC circuit  36 . 
     3. Advantages of the Present Embodiment 
     According to the present embodiment, as described above, an operational range defining table is selected depending on the active cylinder number Ncy of the engine E, thereby changing the operational ranges of the ANC circuit  32  and the ASC circuit  36 . It is thus possible to perform an acoustic control process depending on the active cylinder number Ncy. As a result, the ANC circuit  32  and the ASC circuit  36  can be used in a more appropriate situation. 
     According to the present embodiment, the minimum value of the engine rotational speed Ne for operating the ANC circuit  32  is set to a quotient obtained by dividing the minimum value of the frequencies to be controlled by the ANC apparatus by the order (3 for the all-cylinder mode, 1 for the two-cylinders-deactivation mode, and 1.5 for the three-cylinders-deactivation mode) of a chiefly generated frequency component of the muffled engine sound NZe with respect to the engine rotation frequency fe, and the maximum value of the engine rotational speed Ne for operating the ANC circuit  32  is set to a quotient obtained by dividing the maximum value of the frequencies to be controlled by the ANC apparatus by the above order. In this manner, an operational range for the ANC circuit  32  can be set appropriately. 
     According to the present embodiment, as the active cylinder number Ncy is greater, the minimum value of the engine rotational speed change ΔNe for operating the ASC circuit  36  is set to a lower value. Generally, as a torque which the engine E is required to produce is higher, the active cylinder number Ncy is greater, and when the torque is high, the driver of the vehicle often wants to drive the vehicle in a sporty way. According to the present embodiment, as the active cylinder number Ncy is greater, the minimum value of the engine rotational speed change ΔNe for operating the ASC circuit  36  is set to a lower value thereby to make the ASC circuit  36  operable more easily. Thus, the ASC circuit  36  is operated in a manner to meet the demands of the driver. 
     B. Applications of the Present Invention 
     The present invention is not limited to the above embodiment, and it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims. For example, the following structures may be adopted. 
     In the above embodiment, the operation switcher  38  switches between operations of the ANC circuit  32  and the ASC circuit  36  based on the engine rotational speed Ne and the engine rotational speed change ΔNe. However, the operation switcher  38  may switch between operations of the ANC circuit  32  and the ASC circuit  36  based on either one of the engine rotational speed Ne and the engine rotational speed change ΔNe. Alternatively, the operation switcher  38  may switch between operations of the ANC circuit  32  and the ASC circuit  36  based on a vehicle speed and a change in a vehicle speed. 
     In the above embodiment, the engine E has six cylinders. However, the engine E is not limited to six cylinders, but may have four cylinders, eight cylinders, ten cylinders, twelve cylinders, or the like. 
     In the above embodiment, the engine rotational speed Ne for operating the ANC circuit  32  is set based on the minimum and maximum values of the frequencies to be controlled by the ANC apparatus. However, such a setting scheme is not limitative. In the above embodiment, as the active cylinder number Ncy is greater, the minimum value of the engine rotational speed change ΔNe for operating the ASC circuit  36  is lower. However, the minimum value of the engine rotational speed change ΔNe for operating the ASC circuit  36  may be set according to another procedure, e.g., may be set to one value irrespectively of the active cylinder number Ncy.