NOISE CONTROL APPARATUS

Description

[0001] This invention relates to noise control apparatus.

[0002] The simplest form of a loudspeaker system is the direct radiator. Such a loudspeaker radiates sound directly from the enclosure aperture(s)--the driver diaphragm and, in the case of vented-box systems, a vent or port. There are no additional devices through which the sound passes.

[0003] A transmission line loudspeaker adds an additional device, such as a horn for impedance matching, through which some or all of the sound passes. Prior forms of transmission line systems may be divided into three classes. A type A transmission line system consists of a closed-box direct radiator loudspeaker with a transmission line added to the driver aperture. All radiated sound passes through the transmission line. A Type B or C system consists of a direct radiator system with the transmission line coupled to the back chamber of the enclosure. Both the Type B or Type C form exhibit the fault that the transmission line presents an acoustical short circuit to the back of the driver at least at some frequencies. This can cause serious dips in the system response.

[0004] US-A-4665549 discloses acoustic attenuation apparatus for a duct which guides an acoustic wave. A silencer is provided for passively attenuating the acoustic wave in the duct, and a cancelling speaker is provided within the silencer. The combination provides hybrid active/passive combined attenuation. Various rectangular and circular structures are disclosed, together with multi-path and multi-speaker arrangements.

[0005] JP-A-2072395 discloses in its abstract an active noise control method for application to a large noise of low frequency including connecting a branch pipe to a noise propagation conduit and generating a control sound through the branch pipe. The branch pipe is fitted midway along the duct which connects a noise source such as an air blower and an air compressor to an air intake (or air outlet). The control sound is generated by a speaker installed at an end of the branch pipe.

[0006] US-A-4875546 discloses a loudspeaker for a hi-fi audio system having means for acoustically impeding excursion of the transducer diaphragm and means for acoustically attenuating the output of acoustic vibrations of frequencies above a preselected frequency. The loudspeaker includes first and second subchambers separated by a dividing wall in which the transducer is mounted. A first port acoustically couples the first subchamber with the second subchamber and a second port acoustically couples the second subchamber with the outside environment surrounding the loudspeaker.

[0007] There is described herein an improved form of the Type A system in which the signals from both the drive aperture and the port of a vented-box direct radiator system are combined to drive the transmission line. The original form of the Type A system prevents the back wave from the driver diaphragm from interfering with the front wave by trapping the back wave in the closed cavity behind the driver. The improved system passes the back wave through an acoustic phase inverter so that it may be combined with the output from the front side of the driver. This doubles the energy available to drive the transmission line. This improvement should not be confused with hybrid systems which use a vented-box system with the transmission line coupled to either the driver aperture or the vent but not both.

[0008] The improved performance is roughly analogous to that seen in a vented-box direct radiator system as compared to a closed-box system. Either the efficiency or the bandwidth may be increased; or the system size may be decreased; or a tradeoff may be made among these possible benefits.

[0009] The present invention provides noise control apparatus according to Claim 1.

[0010] Noise control apparatus according to the pre-characterising portion of Claim 1 is known from the abstract of JP-A-2072395.

[0011] In one preferred arrangement of the apparatus one or more electrodynamic loudspeaker drivers is installed in a vented-box direct radiator enclosure, and a cover is added to the front of the enclosure. This cover forms a front chamber into which both the driver and the vent radiate sound. The sound passes through the front chamber and into the transmission line. For applications requiring high efficiency over a wide bandwidth the transmission line may be a horn. However, a compact system might use a short tube, configured to produce a desired band-pass frequency response characteristic.

[0012] The transmission characteristics of the system using a horn will be determined in great measure by the horn and the acoustic load it presents to the front chamber, but will, in general, be high-pass in nature. The transmission characteristics of the short tube system will be band-pass in nature. The driver and the vented-box portion of the enclosure will provide a 4-pole high-pass response, and the front chamber and the outlet tube will provide a 2-pole low-pass response.

[0013] A preferred arrangement of the apparatus is particularly useful for active noise cancellation applications such as exhaust mufflers or duct silencers. In this arrangement the pipe or duct through which the noisy signal is flowing passes through the enclosure and exits through the outlet tube. The end of the noisy pipe or duct is aligned with the end of the loudspeaker and the two are coaxial. Thus, the antinoise signal radiated by the loudspeaker during the active cancellation is coaxial with the noise. Very good cancellation may be obtained at frequencies with wavelengths which are long compared to the size of the outlet.

[0014] Another preferred arrangement of the apparatus which also has particular application in active noise cancellation systems is similar to that described immediately above. However, in this arrangement the pipe or duct containing the noisy flow does not pass through the loudspeaker. Instead, the loudspeaker outlet tube connects the loudspeaker front chamber to the pipe as a tee fitting into the pipe. In this case, the pipe need not end at the point where the noise and antinoise are mixed. This arrangement is useful for "in duct" cancellation.

[0015] The invention will now be further described by way of examples, with reference to the accompanying drawings, in which:

FIGS. 1 and 3 are signal flow graphs of the improved loudspeaker with a long transmission line and a short outlet tube,

FIGS. 2 and 4 are simplified drawings of the invention, and

FIGS. 5 to 9 show two ways in which the invention may be put to practical use in noise cancellation applications.

FIG. 10 shows a general form of a vented box bandpass loudspeaker.

FIG. 11 shows a simplified acoustical analogous circuit.

[0016] Consider first FIGS. 1 and 2. FIG. 2 shows an electrodynamic loudspeaker driver 1 mounted in an enclosure 2 so that one side of the driver diaphragm radiates sound into the front chamber of the enclosure 3. The sound from the other side of the driver passes through the acoustic phase inverter comprising the back chamber 4 and the inner vent 5 which connects the front and back chambers. The total system output consists of the sum of the front wave and the phase corrected back wave flowing through the front chamber and out via the transmission line 6.

[0017] FIG. 1 shows the basic signal flow graph of the system using an electrodynamic driver 1. Electric potential E_g is applied accross the driver voice coil which has a resistance R_E and a resulting current I_VC flows. The electrodynamic coupling B1 of the motor causes a driving force. The sum of this force and the various reaction forces in the system gives the total force driving the diaphragm F_D. This force accelerates the diaphragm at a rate inversely proportional to the moving mass M_MS of the driver. The resulting acceleration of the diaphragm a_D is integrated once with respect to time (the 1/s operation in the LaPlace domain) to find the velocity of the diaphragm u_D and a second time to find the displacement of the diaphragm x_D. Now, moving the diaphragm results in some reaction forces. As the diaphragm is displaced against the mechancial springs in its suspension, an opposing force inversely proportional to the mechanical compliance C_MS of the driver is added to the total force F_D. Another opposing force results from the motion through the mechanical losses R_MS of the system and is equal to the product of R_MS and u_D. Also, as the voice coil moves through the magnetic field of the motor a back emf is generated which tends to oppose the driving potential. This back emf, which is equal to the electromagnetic coupling B1 times the diaphragm velocity u_D, sums with the input potential Eg to give the voice coil potential E_VC.

[0018] As the diaphragm moves, the front side pushes against the surrounding air and a flow into the front chamber 3 results. This volume velocity U_D is equal to the product of the diaphragam velocity u_D and its effective area S_D. This volume velocity is one of the components of the total flow into the front chamber U_F. The conservation of matter requires that the flow into the back chamber U_B across the boundary between it and the front chamber be equal to U_F but opposite in polarity. The volume velocity U_B pressurizes the back chamber 4. The acoustic pressure of the back chamber p_B is equal to the integral of U_B with respect to time divided by the acoustic compliance to the back chamber C_AB. This pressure exerts another reaction force against the back of the diaphragm which is equal to the pressure p_B times the diaphragm area S_D. This is another component of F_D.

[0019] For the purpose of an orderly description of the system, assume that the inner vent 5 is blocked. This is equivalent to the unimproved form of the transmission line loudspeaker. The flow into the front chamber 3 pressurizes it. This component of the front chamber acoustic pressure p_F is equal to the integral of U_F with respect to time divided by the acoustic compliance of the front chamber C_AF. The front chamber pressure drives the flow through the transmision line 6 at a rate inversely proportional to the input reactance X_AT of the line. The resistive part of the line impedance R_AT causes a reaction pressure which is -also a component of p_F. X_AT and R_AT are frequency dependent line characteristics. The-front chamber pressure also causes a reaction force on the diaphragm equal to p_F times S_D. This is another component of F_D.

[0020] Now, assume that the inner vent 5 is no longer blocked. The pressure in the back chamber p_B will drive a flow through the inner vent with a volume velocity U_p which is equal to the integral of the pressure p_B with respect to time divided by the acoustic mass of the vent M_AP. The volume velocity components U_D and U_p now add to form the total flow into the front chamber U_F which, in turn, drives the system output U_O.

[0021] In the arrangement of FIGS. 3 and 4, the analysis of the system is similar, except that the line impedance is simplified because the short tube presents a lumped parameter element. In this case, the output flow U_O is equal to the front chamber pressure p_F integrated with respect to time and divided by the acoustic mass of the outlet vent M_AF. The opposing pressure component of p_F results from the flow losses in the outlet R_AF.

[0022] Analysis of the signal flow graphs yields the approriate design equations which allow the correct driver and enclosure parameters to specify for a desired system.

[0023] FIGS. 5 to 8 show views of a practical loudspeaker system using the present invention which has particular application in active noise control systems. In this apparatus an additional component, a flow tube 7 containing for the noisy flow (such as the exhaust of an engine), has been added. Also, a drain tube 8 has been added between the front and back chamber so that water or other liquids trapped in the back chamber may escape. If the loudspeaker were used in an active noise cancellation system on a vehicle and if the vehicle were driven through deep water, the muffler could be flooded. The drain tube would allow the trapped water to flow out of the back chamber. The drain tube must be sized so that it acts as an acoustic mass rather than an acoustic leak between the chambers. Its mass must either be considered when adjusting the enclosure tuning or be trivial compared to the inner vent 5 so that the effect of the drain may be ignored.

[0024] FIG. 9 shows an apparatus using the present invention which also has particular application for active noise control. In this instance, the- short tube 6 is formed by the area between the heat shield plate 17 and the connection to the noisy duct 7. A long, narrow tube 9 allows outside air to enter the enclosure. This tube, like the drain tube discussed above, should be sized so that it has no adverse effect on the system acoustic performance. It may enter the enclosure through either the front or back chamber. Air is forced through the system because of the venturi-like detail 10 in the noisy duct. The flow through the duct over the "venturi" causes a low pressure region which "draws" the outside air. This air may be useful for cooling or removal of corrosive gases.

[0025] The analysis and derivation of the analog circuit of the Vented Box Bandpass Loudspeaker is as follows: The symbols used in Figures 10 and 11 and in the calculations are:

LIST OF SYMBOLS

[0026] 

C_AB

Acoustic compliance of Rear Box

C_AP

Acoustic compliance of Front Box

C_AS

Acoustic compliance of Driver (Loudspeaker V_AS=P_oC²C_AS)

M_AP

Acoustic mass of Front Port

M_AS

Acoustic mass of Driver

M_AB

Acoustic mass of Internal Port

R_AS

Acoustic Resistance of Driver

R_E

Electrical Resistance of Driver voice coil

S_D

Driver Diaphragm, M²

V_B

Volume of Rear Closed Box (M³) (V_B=P_oC²C_AB)

V_P

Volume of Front Box (M³) (V_p=P_oC²C_AP)

V_d

Peak displacement volume driver diaphragm (S_DX_M)

P_o

Mas densily of air (7.18 kg/m³)

C

Speed of sound in air (345 m/sec)

X_m

Peak linear displacement of driver diaphragm

S_p

Area of the front port

S_B

Area of the internal port

B

Magnetic flux density in driver airgap

l

length of voice coil in the airgap of driver

U_o

Volume velocity at the front port

U_AB

Volume velocity at the internal port

U_F

Volume velocity inside the front box (U_F=U_S+U_AB)

U_B

Volume velocity inside the rear box (U_B=-U_F)

U_S

Volume velocity generated at the source

P_g

Pressure generator (equivalent)

E_g

Input voltage to the loudspeaker

Speaker Parameters

[0027] 

Q_ES

Electro-Magnetic Q at f_s

Q_ms

Mechanical Q at f_s

Q_ts

Total Q at

Vas

Volume of air having sam acoustic compliance as driver suspension

Vd

Peak displacement volume of diaphragm (=S_DX_M)

S_D

Effective diaphragm area

Xm

Peak linear displacement of diaphragm

[0028] Referring now to Fig. 10, there is shown the general form of the Vented Box Bandpass Loudspeaker (VBBP) configuration. Fig. 11 shows the simplified acoustical analogous circuit of the Vented Box Bandpass Loudspeaker (VBBP) configuration. The terms R_o and P_g are determined by the following formulae:

[0029] In the following circuit analysis, assumptions are made that there is a lossless enclosure (internal box resistance = O and leakage resistance =a) and that the voice coil inductance is small (L_F≈O)
The circuit analysis is as follows:

[0030] From equation (3)

[0031] Substitute Equations (4) and (6) into the Equation (1)

ASSUMPTIONS

[0032] 

SYSTEM TRANSFER FUNCTION

[0033] 



when,

[0034] It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the claims.

Claims

1. Noise control apparatus comprising:

an enclosure means (2);

at least one loudspeaker driver means (1) mounted in the enclosure means (2); and

a duct means (7), the enclosure means being arranged to generate anti-noise to noise in the duct means (7);

   characterised in that:

the enclosure means has front (3) and back (4) chamber means connected by a conduit means (5);

the loudspeaker driver means is adapted to produce a front and back wave so that at a desired frequency the back wave is constructively summed with the front wave in the front chamber means (3); and

transmission line means (6) is provided attached to said front chamber means (3) and acoustically coupled to the duct means (7), said transmission line means (6) comprising the only outlet of anti-noise from the enclosure means to the duct means (7).

2. Apparatus as claimed in Claim 1, in which the transmission line means is a horn.

3. Apparatus as claimed in Claim 1, in which the transmission line means is a short tube means.

4. Apparatus as claimed in Claim 1, 2 or 3, in which the duct means (7) passes through the enclosure means (2) and exits through the transmission line means (6) for the purpose of noise control.

5. Apparatus as claimed in any of the preceding claims, in which a drain means (8) is provided between the front and back chamber means to allow trapped liquids to drain from the back chamber, the drain means preferably being in the form of a hole or a tube.

6. Apparatus as claimed in Claim 3, or Claim 4 or 5 as dependent on Claim 3, in which the short tube means (6) is formed by the area between a plate means in the front chamber means and the connection to the duct means.

7. Apparatus as claimed in any of the preceding claims, in which a ventilation tube means (9) is connected between either the front or the back chamber means and the outside air and a venturi-like structure means is in the duct means in order to create a low pressure region to draw outside air through the system.

8. Apparatus as claimed in any of the preceding claims, in which said conduit means (5) comprises a hole connecting said front and back chambers.

Ansprüche

1. Geräuschsteuerungs- bzw. -unterdrückungsvorrichtung, die umfaßt:

ein Gehäusemittel (2),

mindestens ein Lautsprechertreibermittel (1), das in dem Gehäusemittel (2) angebracht ist; und

ein Hohlleitungs- bzw. Röhrenmittel (7), wobei das Gehäusemittel ausgelegt ist, um gegen ein Geräusch in dem Hohlleitungs- bzw. Röhrenmittel (7) ein Antigeräusch zu erzeugen,

   dadurch gekennzeichnet, daß:

das Gehäusemittel ein vorderes Kammermittel (3) und ein hinteres Kammermittel (4) aufweist, die über ein Leitungsmittel (5) verbunden sind;

das Lautsprechertreibermittel ausgelegt ist, um eine vordere Welle und eine hintere Welle zu erzeugen, so daß die hintere Welle bei einer gewünschten Frequenz konstruktiv mit der vorderen Welle in dem vorderen Kammermittel (3) summiert wird; und

ein Übertragungsleitungsmittel (6) vorgesehen ist, das an dem vorderen Kammermittel (3) angebracht ist und akustisch mit dem Hohlleitungs- bzw. Röhrenmittel (7) gekoppelt ist, wobei das Übertragungsleitungsmittel (6) den einzigen Auslaß für Gegengeräusch von dem Gehäusemittel zu dem Hohlleitungs- bzw. Röhrenmittel (7) umfaßt.

2. Vorrichtung nach Anspruch 1, bei der das Übertragungsleitungsmittel ein Horn darstellt.

3. Vorrichtung nach Anspruch 1, bei der das Übertragungsleitungsmittel ein kurzes Rohrmittel ist.

4. Vorrichtung nach Anspruch 1, 2 oder 3, bei der das Hohlleitungs- bzw. Röhrenmittel (7) sich durch das Gehäusemittel (2) erstreckt und über das Übertragungsleitungsmittel (6) zum Zwecke einer Geräuschsteuerung- bzw. -unterdrückung austritt.

5. Vorrichtung nach irgendeinem der vorhergehenden Ansprüche, bei der zwischen dem vorderen Kammermittel und dem hinteren Kammermittel ein Ablaufmittel (8) vorgesehen ist, damit eingefangene Flüssigkeiten von der hinteren Kammer ablaufen können, wobei das Ablaufmittel vorzugsweise die Form eines Lochs oder eines Rohrs hat.

6. Vorrichtung nach Anspruch 3 oder Anspruch 4 oder 5, wenn diese von Anspruch 3 abhängen, bei der das kurze Rohrmittel (6) durch den Bereich zwischen einem Plattenmittel in dem vorderen Kammermittel und der Verbindung zu dem Hohlleitungs- bzw. Röhrenmittel ausgebildet wird.

7. Vorrichtung nach irgendeinem der vorhergehenden Ansprüche, bei der ein Ventilationsrohrmittel (9) entweder zwischen dem vorderen Kammermittel oder dem hinteren Kammermittel und der Außenluft verbunden ist und bei der sich ein venturiartiges Strukturmittel in dem Hohlleitungs- bzw. Röhrenmittel befindet, um einen Bereich niedrigen Drucks zu erzeugen, um Außenluft durch das System zu ziehen.

8. Vorrichtung nach irgendeinem der vorhergehenden Ansprüche, bei der das Leitungsmittel (5) ein Loch umfaßt, das die vordere Kammer und die hintere Kammer verbindet.

Revendications

1. Dispositif de suppression de bruit comprenant :

une enceinte (2) ;

au moins un moyen d'excitation de haut-parleur (1) monté dans l'enceinte (2) ; et

un conduit (7), l'enceinte étant agencée de manière à engendrer un anti-bruit pour le bruit dans le conduit (7) ;

caractérisé en ce que :

l'enceinte comprend une chambre avant (3) et une chambre arrière (4) connectées par un conduit (5) ;

le moyen d'excitation de haut-parleur est prévu pour produire une onde avant et une onde arrière de sorte que, à une fréquence désirée, l'onde arrière est. ajoutée par construction à l'onde avant dans la chambre avant (3) ; et

il est prévu une ligne de transmission (6) fixée à ladite chambre avant (3) et acoustiquement couplée au conduit (7), ladite ligne de transmission (6 ) comportant la seule sortie d'anti-bruit de l'enceinte vers le conduit (7).

2. Dispositif selon la revendication 1, dans lequel la ligne de transmission est un pavillon.

3. Dispositif selon la revendication 1, dans lequel la ligne de transmission est un tube court.

4. Dispositif selon la revendication 1, 2 ou.3, dans lequel le conduit (7) traverse l'enceinte (2) et sort à travers la ligne de transmission (6) pour la fonction de suppression du bruit.

5. Dispositif selon une quelconque des revendications précédentes, dans lequel une purge (8) est prévue entre la chambre avant et la chambre arrière pour permettre aux liquides emprisonnés de s'évacuer de la chambre arrière, la purge étant de préférence sous la forme d'un trou ou d'un tube.

6. Dispositif selon la revendication 3, ou la revendication 4 ou 5 lorsqu'elle dépend de la revendication 3, dans lequel le tube court (6) est formé par la région entre une plaque placée dans la chambre avant et la connexion au conduit.

7. Dispositif selon une quelconque des revendications précédentes, dans lequel un tube de ventilation (9) est connecté entre la chambre avant ou arrière et l'air-extérieur, et une structure de type venturi est prévue dans le conduit afin de créer une région.de basse pression pour aspirer l'air extérieur à travers le système.

8. Dispositif selon une quelconque des revendications précédentes, dans lequel ledit conduit (5) comprend un trou faisant communiquer lesdites chambres avant et arrière.

Drawing