IMPROVEMENTS IN AND RELATING TO TRANSMISSION LINE LOUDSPEAKERS

Abstract: The sound wave radiated from the front surface (UF) of a loudspeaker driver diaphragm is of opposite polarity with respect to that radiated from the back surface (UB). If the two signals are directly combined, they will tend to cancel one another. An acoustic phase inversion network (5) is used to ensure that the backwave is in phase with the front wave, and the combined signals are used to drive the inlet of a loudspeaker transmission line (6).

Description: improvements In and Relating to Transmission Line Loudspeakers BACKGROUND OF THE INVENTION 1. Field of the Invention. This invention relates to a method of and apparatus for improving the performance of transmission line loudspeakers and has particular applications in active noise control, high fidelity audio, and sound reinforcement systems. 2. Discussion of the Relevant Art. The simplest form of a loudspeaker system is the direct radiator. Such a loudspeaker radiates sound directly form 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. 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. SUMMARY OF THE INVENTION This invention relates to 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 drive 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 trough 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 used a vented-box system with the transmission line coupled to either the driver aperture or the vent but not both. 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. In one arrangement of the apparatus according to the present invention 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 should be a horn. However, a compact system might use a short tube. Such a tube is too short to exhibit transmission line characteristics. It will act as lumped parameter acoustic mass instead. The transmission characteristics of the improved 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. Another arrangement of the apparatus according to the present invention 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 exists 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 wavelenghts which are long compared to the size of the outlet. Another arrangement of the apparatus according to the current invention whicfe also has particular application in active noise cancellation systems is similar to that described immediately above. However, in this arrangmeent 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. The invention will now be further described by way of examples, with reference to the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS 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 simplfied drawings of the invention, and FIGS. 5 to 9 show two ways Tn 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 analagous circuit. Description of the Preferred Embodiments 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 fornt chamber of the enclosure 3. The sound form 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. FIG. 1 shows the basic signal flow graph of the system using an electroydnamic driver 1. Electric potential E„ is applied accross the driver voice coil which has a resistance R E and a resulting current I vc flows. The electrodynamic coupling Bl of the motor causes a driving force. The sum of this force and the various reaction forces in the system gives in the total force driving the diaphragm F D . This force accelerates the diaphrgam at a rate inversely proportional to the moving mass 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 Bl times the diaphragm velocity u D , sums with the input potential Eg to give the voice coil potential Eγ C . As the diaphragm moves, the front side pushes against the surrounding air and a flow into the front chamber 3 results. This volume D 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 from chamber Up. The conservation of matter requires that the flow into the back chamber U β across the boundary between it and the front chamber be equal to Up but opposite in polarity. The volume velocity u β presurizes 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^g. 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 another component of F D . 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 from chamber 3 pressurizes it. This component of the front chamber acoustic pressure F is equal to the integral of Up with respect to time divided by the acoustic compliance of the front chamber C^p. 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. ∑ AT and AT are frequency dependent line characteristics. The front chamber pressure also causes a reaction force on the diaphragm equal to p times S D . This is another component of F D . Now, assume that the inner vent 5 is no longer blocked. The pressure in the back chamber 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_ now add to form the total flow into the front chamber U p which, in turn, drives the system output U Q . In the arrangement of FIGS. 3 and 4, the analysis of the system is similar, except that the line impedance is simplifed because the short tube presents a lumped parameter element. In this case, the output flow U Q 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^ . The opposing pressure component of F results from the flow losses in the outlet R j ^p. 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. 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 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. 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 7 and the connection to the noisy duct 8. A long, narrow tube 9 allows outsider 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" cause a low pressure region which "draws" the outside air. This air may be useful for cooling or removal of corrosive gases. The analysis and derivation of the analog circuit of the Vented Box Bandpass Loudspeaker is as follows: The symbol used in Figures 10 and 11 and in the calculations are: in Figures 10 and 11 and in the calculations are: LIST OF SYMBOLS C AB Acoustic compliance of Rear Box C AP Acoustic compliance of Front Box C AS Acoustic compliance of Driver (Loudspeaker V AS =P O C2C S^ M AP Acoustic mass of Front Port M A S Acoustic mass of Driver M A B Acoustic mass of Internal Port R AS Acoustic Resistance of Driver R E Electrical Resistance of Driver voice coil S D Driver Diaphragm, M 2 V B Volume of Rear Closed Box (M 3 ) (V B =P 0 C 2 C AB ) V p Volume of Front Box (M 3 ) (V p =P 0 C 2 C Ap ) V d Peak displacement volume driver diaphragm (S D X M ) P Q Mas densily of air (7.18 kg/m 3 ) C Speed of sound in air (345 m/sec) X- j . 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 1 lenght of voice coil in the airgap of driver U Q Volume velocity at the front port U AB v °l ume velocity at the internal port U F Volume velocity inside the front box (Up=U s +U AB ) U β Volume velocity inside the rear box (U B =-U F ) - U s Volume velocity generated at the source P_ Pressure generator (equivalent) E„ Input voltage to the loudspeaker Speaker Parameters η f s (T s = —~ —) Free Air Resonance frequency Q ES Electro-Magnetic Q at f_, Q ms Mechanical Q at f s Q ts Total Q at f s| Q e Q ms Q es +Q ms Vas Volume of air having sam acoustic compliance as driver suspension Vd Peak displacement volume of diaphragm (=S D X M ) S D Effective diaphragm area Xm Peak linear displacement of diaphragm 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 analagous circuit of the Vented Box Bandpass Loudspeaker (VBBP) configuration. The terms R Q and P are determined by the following formulal: R 0 = (BL) P g = V BL) pS D RS D In the following circuit analysis, assumptions are made that there is a lossless enclosure (internal box resistance = O and leakage resistance =α) and that the voice coil inductance is small (Lp«0) The circuit analysis is as follows: 1 1 -_ . U τ (1) P r ( R o +R A S ) +SM AS + - U s + - __ +sM L A_Pτ-U u o sC • AS J sC AS U F "U o (2) = Ξ M Ap -U 0 sC Ap (4) From Equation (2 ) U F = (l+S 2 M Ap C Ap ) U 0 U S +U AB =U F (5) U S =U F -U AB 1 1+sM AP C AP , From equation (3 ) U J A ΛT B 3 .= ~ - — I + τ s β M ja A Λ P| U υ o sM AB ■ sC AB + - .( l+β (M A pC Ap +C AB M Ap ) b 0 5 Λ M A A B IΪ C'-AB Substitute Equations (4) and (6) into the Equation (1) + S 2 ( M AB C AB + M AP C AP + M AP C AB> + 1 (1 + S ' M AP C AP> (8) P g (S 3 C AS M AB C AB )={S 6 M AS C AS M Ap C Ap M AB C AB+ S4M AS C AS M AB C AB + S4M AS C AS M AP C AP +S4M AS C AS M AP C AB +S M AS C AS +S C AS R At M Ap C Ap M AB C AB +S C Ag R AT M AB C AB +S ^S^T^AP^ "1"8 C AS R AT M AP C AB +SC AS R AT + S 4 M Ap C Ap M AB C AB+ S 2 +l + S 2 M AB C AB +S C Ap M AB C AB M Ap +S 4 C AS M Ap M AB C AB }U 0 (9 ) Pg (S 3 C AS M AB C AB ) ={S 6 M AS C AS M A pC Ap M AB C AB -. s5c AS R AT C AP M AB C AB + S < M AS C AS M AB C AB +M AS C AS M AP C AP + M AS C AS C AB +M AP C AP M AB C AB +M AB C AB M AP C AP +M AR C AR M AP C A . . )+S J C AR R AT (M Ap C A p+M A pC AB + M AB C AB> +s2 < M AS C AS +2M AB C AB +M AP C AP +M AP C AB) +SC AS R AT +1 > U o ASSUMPTIONS (10) M AS C AS (11) M AB C AB M AP C AP (13) M Ap C AS (14) M Ap C AB P_C < p 0 c (15) R AT =R 0 +R AS = + W s V AS°__S W S V AS Q MS C AS s°-ts (16) C AS AT (17) P (S) = sM Ap U 0 SYSTEM TRANSFER FUNCTION sM AP ϋ O P θ( s ) (18) G(s) = = P g P g (s) bs 4 (19) G(s) = S b + a 5 S + a 4 S 4 + a-S + a 2 S ;,:: + a^ 1 + a Q when, τ B 2 τ p 2 i τ B 2 τ p 2 τ s 1 s (20) a 5 = x W S °-ts T s 2 T B 2 T p 2 s 2 B 2 T p2 Q ts T s Q ts Q ts (21) a, = (τ s 2 τ B 2 +τ s 2 τ 2 +T_. 2 T. 6 i om __ m ^_ι_rπ ώm _ s -pB ^ 1 p 1 B +1 B 1 ps ■ B <23) " 2 τ B < T S 22T B 2+T P 2 + T - B ) - =(W B 2 W 2 +2 s 2 W 2 +W s 2 B 2 + (25) a Q = , = W S B 2 W m <__rn ώrn __. ^ (26) b= MARCASMABCAB s 2τ B 2 = T ps 2 = W s 2w p 2 W W T^T^ w p£

Claims: CLAIMS 1. Apparatus for achieving improved performance of a transmission line loudspeaker producing a front and back wave comprising one or more electromagnetic driver means mounted in an enclosure means having front and back chamber means connected by a conduit means so that the back wave of the loudspeaker is constructively summed with the front wave in the front chamber means, and a transmission line means attached to the front chamber means comprising the only sound outlet of the system. 2. Apparatus as claimed in claim 1, in which the transmission line means is a short tube means in the enclosure configured to produce a desired band-pass frequency response characteristic. 3. Apparatus as claimed in claim 2, in which a separate pipe means passes through the loudspeaker enclosure means and exits through the short take means for the purpose of noise cancellation. 4. Apparatus as claimed in claim 1, in which a drain means is added between the from and back chamber means to allow trapped liquids to drain from the back chamber. 5. Apparatus as claimed in claim 2, in which a drain tube means is added between the front and back chamber means to allow trapped liquids to drain from the back chamber. 6. Apparatus as claimed in claim 3, in which a drain hole is provided between the front and back chamber means to allow trapped liquids to drain from the back chamber. 7. Apparatus as claimed in claim 2, including a noisy duct means to which the short tube means is connected for the purpose of noise control. 8. Apparatus as claimed in claim 7, in which the short tube means is formed by the area between a plate means in the front chamber means and the connection to the noisy duct or pipe. 9. Apparatus as claimed in claim 7, in which a ventilation tube means is connected between either the front or the back chamber means and the outside air and a venturi-like structure means is in the noisy duct means in order to create a low pressure region to draw outside air through the system. 10. Apparatus as claimed in claim 1 in which said conduit means comprises a hole connecting said front and back chambers. 11. Apparatus for achieving improved performance of a transmission line loudspeaker means producing a front and back wave comprising a driver means, said driver means mounted in an enclosure means, said enclosure means having front and back chamber means connected by a conduit means so that the back wave of the loudspeaker means is constructively summed with the means attached to the from chamber means comprising the sole sound outlet of the system. 12. Apparatus as claimed in claim 11 in which the transmission line means is a short tube means in the enclosure configured to produce a desired band-pass frequency response characteristic. 13. Apparatus as claimed in claim 2, in which a separate pipe means passes through the loudspeaker enclosure means and exits through the short tube means for the purpose of noise cancellation. 14. Apparatus as claimed in claim 1, in which a drain means is added between the front and back chamber means to allow trapped liquids to drain from the back chamber. 15. Apparatus as claimed in claim 2, in which a drain tube means is added between the from and back chamber means to allow trapped liquids to drain from the back chamber. 16. Apparatus as claimed in claim 3, in which a drain hole is provided between the front and back chamber means to allow trapped liquids to drain from the back chamber. 17. Apparatus as claimed in claim 2, including a noisy duct means to which the short tube means is connected for the purpose of noise control. 18. Apparatus as claimed in claim 7, in which the short tube means is formed by the area between a plate means in the front chamber means and the connection to the noisy duct or pipe. 19. Apparatus as claimed in claim 7, in which a ventilation tube means is connected between either the front or the back chamber means and the outside are and a venturi-like structure means is in the noisy duct means in order to create a low pressure region to draw outside air through the system.

Inventors: EGHTESADI KHOSROW, HOGE WILLIAM JOHN JOSEPH, EGHTESADI, KHOSROW, HOGE, WILLIAM, JOHN, JOSEPH
Applicants: NOISE CANCELLATION TECH, NOISE CANCELLATION TECHNOLOGIES, INC.
Application year: 1991
Main classification: H04R25
IPC codes: G10K11, H04R1, H04R25