Loudspeaker circuitry

Abstract

Electrical circuitry adapted to drive a dual-coil loudspeaker having a primary voice coil and a second voice coil connected in parallel with the primary voice coil, the second voice coil being in series with an LC resonant circuit of impedance Z.sub.mf, further comprising an inductance compensation filter of impedance Z.sub.if in parallel with the LC resonant circuit.

Claims

1. An electrical circuitry adapted to drive a dual-coil loudspeaker having a primary voice coil and a second voice coil connected in parallel with the primary voice coil, the second voice coil being in series with a resonant circuit of impedance Z.sub.mf, further comprising an inductance compensation filter of impedance Z.sub.if in parallel with the resonant circuit.

2. The electrical circuitry according to claim 1, in which the impedance of the inductance compensation filter is given by $Z_{if}=R_{e1}{}^2/j\omega L_{e1}\left(\omega \right)$ where R.sub.e1 is the resistance of the primary voice coil, j is the imaginary operator, ω is the circular frequency and L.sub.e1(ω) is the frequency-dependent inductance of the primary voice coil, and where $L_{e1}\left(\omega \right)=\left(Z_{eb}\left(\omega \right)-Z_{eb}\left(0\right)\right)/\left(j\omega \right),$ Z.sub.eb(ω) being the frequency dependent blocked impedance and Zeb(0) being the DC blocked impedance.

3. The electrical circuitry according to claim 1, in which the impedance of the resonant circuit is given by $Z_{mf}=Z_m{\left(R_{e1}/B{\mathcal{l}}_1\right)}^2$ where Z.sub.mf is the mechanical load seen by the loudspeaker, R.sub.e1 is the resistance of the primary voice coil and Bℓ.sub.1 is the force factor of the primary voice coil.

4. The electrical circuitry according to claim 1, in which the inductance compensation filter comprises a capacitor C1, or a capacitor C1 in series with a resistor R1.

5. The electrical circuitry according to claim 4 in which the inductance compensation filter comprises a capacitor C1 in series with a resistor R1 and, in series, a further capacitor C2 in parallel with a resistor R2.

6. The electrical circuitry according to claim 5 in which the inductance compensation filter further comprises a further capacitor C3 in parallel with a further resistance R3.

7. The electrical circuitry according to claim 1, further comprising a voltage divider R4-R5 located in series between: (i) the parallel-connected resonant circuit and the inductance compensation filter, and (ii) the second voice coil.

8. The electrical circuitry according to claim 1, further comprising a Zobel network in parallel with the primary voice coil and in parallel with the secondary voice coil, the resonant circuit and the inductance compensation filter.

9. The electrical circuitry according to claim 1, in which the primary and second voice coils are coaxial and share the same magnetic gap.

10. The electrical circuitry according to claim 1, in which the primary and second voice coils are coaxial and operate in separate magnetic gaps.

11. The electrical circuitry according to claim 9, in which the primary and second voice coils are separated by an acoustic chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will now be described by way of example and with reference to the accompanying figures, in which;

[0017] FIG. 1(a) is a schematic illustration of the dual-coil drive arrangement in US3838216, and FIG. 1(b) is the equivalent electrical circuit;

[0018] FIG. 2 is a sound pressure/frequency graph showing an example of a closed box loudspeaker using a conventional a single coil and a dual-coil system;

[0019] FIG. 3 is an impedance/frequency graph comparing the impedance of a closed box loudspeaker using a conventional a single coil and a dual-coil system;

[0020] FIG. 4 is a graph giving a comparison of the loudspeaker impedance when the inductance is not ignored;

[0021] FIG. 5 shows the equivalent electrical circuit of the dual coil arrangement including a Zobel network Z.sub.z;

[0022] FIG. 6 is a plot of the loudspeaker impedance with and without a Zobel network;

[0023] FIG. 7 is an electrical circuit in accordance with the invention to cancel the effect of the inductance of the primary and secondary voice coils;

[0024] FIG. 8 is a comparison of the loudspeaker impedance of a conventional single coil system, and the impedance of the loudspeaker arrangement of FIG. 7;

[0025] FIG. 9 is the electrical circuit of FIG. 7 including a Zobel network;

[0026] FIG. 10 is a passive circuit Z.sub.mf required for use of a dual-coil loudspeaker driver in free air, in a baffle or a closed box;

[0027] FIG. 11 is a passive circuit Zmf required for a dual-coil loudspeaker driver in a vented box;

[0028] FIG. 12 is an example of a simple inductance-cancelling passive circuit Z.sub.if;

[0029] FIGS. 13 and 14 are examples of passive circuit Z.sub.if required for semi-inductance LR2 and LR3, respectively;

[0030] FIG. 15 shows the circuit of FIG. 7 incorporating a voltage divider R4 - R5;

[0031] FIG. 16 shows the circuit of FIG. 15 when a Zobel network is used;

[0032] FIG. 17 shows the pressure response showing the control of the Q-factor enabled by the circuit of FIG. 15 or of FIG. 16;

[0033] FIGS. 18(a) and 18(b) show single gap and dual-gap voice coil arrangements, respectively, and

[0034] FIGS. 19(a) and 19(b) show single driver and dual-driver arrangements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] FIGS. 1 to 6 relate to the prior art and are described in the introduction above.

[0036] FIG. 7 shows the basic circuit in accordance with the invention to cancel the effect of the voice coil inductance, It consists of an inductance compensation filter Z.sub.if — typically but not exclusively, a capacitor in series with an optional resistor — in parallel to the original circuit Z.sub.mf driving the second voice coil, voice coil 2, which is driven in parallel with the primary voice coil, voice coil 1. FIG. 8 shows that when the circuit of FIG. 7 is used not only is the effect of the inductance annihilated (the monotonic rise at high frequencies), but more importantly no dip is present in the impedance.

[0037] FIG. 9 shows the circuit of FIG. 7 adapted to cancel the effect of the inductance with a Zobel network Z.sub.z adapted to cancel the effect of any residual inductance.

[0038] The mathematical description of the system of the invention will now be described. The Z.sub.mf circuit compensates the mechanical load Z.sub.m seen by the loudspeaker. Its impedance is substantially

[00004]$Z_{mf}=Z_m{\left(R_{e1}/B{\mathcal{l}}_1\right)}^2$

where R.sub.e1 and Bℓ.sub.1 are respectively the resistance (in ohms) and the force factor (in N/A) of the primary voice coil. Some adjustments are sometimes required to consider the resistance of the secondary voice coil, so in most embodiments better results and greater sensitivity may be achieved with a resistance value in the Z.sub.mf circuit lower than that given by the equation above.

[0039] The Z.sub.if circuit compensates the inductance of the loudspeaker. Its impedance is substantially

[00005]$Z_{if}=R_{e1}{}^2/j\omega L_{e1}\left(\omega \right)$

where j is the imaginary operator, ω is the circular frequency and L.sub.e1(ω is the complex frequency-dependent inductance (in H) of the primary voice coil, where

[00006]$L_{e1}\left(\omega \right)=\left(Z_{eb}\left(\omega \right)\text{-}{Z}_{eb}\left(0\right)\right)/\left(j\omega \right)$

and Z.sub.eb(ω) is the frequency dependent blocked impedance and Z.sub.eb(0) is the DC blocked impedance.

[0040] The impedances Z.sub.mf and Z.sub.if being in parallel, the overall impedance Z.sub.ef of the circuit that is in series with secondary coil is therefore substantially

[00007]$Z_{ef}=Z_{mf}{Z}_{if}/\left(Z_{mf}+Z_{if}\right)$

[0041] The Z.sub.mf circuit compensates the mechanical load seen by the loudspeaker; therefore, its topology depends on the type of environment in which the loudspeaker is placed. If used in free air, in a baffle or a closed box, the RLC (resistor R inductor L capacitor C) circuit shown in FIG. 10 is sufficient to flatten the impedance. If the loudspeaker is used in a ported enclosure, the Z.sub.mf circuit is instead as shown in FIG. 11, and comprises: a first branch R1-L1-C1 that compensates the loudspeaker; a second branch R2-C2 that compensates the box, and a third branch R3-L3 that compensates the vent.

[0042] The Z.sub.if circuit compensates the inductance of the loudspeaker and is shown in FIG. 12. The simplest circuit uses a single capacitor C1 but sometimes a resistor R1 in series is needed for fine tuning. In certain circumstances, the inductance is frequency-dependent and it is required to use a so-called semi-inductance model, involving several branches. FIG. 13 and FIG. 14 show respectively compensation circuits LR2 and LR3 which are the most common semi-inductance models.

[0043] The dual-coil arrangement gives an opportunity unachievable with a conventional single coil driver: the control of the Q-factor without changing the input impedance. The principle is to insert a voltage divider R4 - R5 between the electrical circuit of impedance Z.sub.ef and the secondary voice coil, as shown in FIG. 15. When a Zobel network Z.sub.z — typically a capacitor in series with a resistor — is used to compensate any residual effects of the inductance such as depicted in FIG. 9, the voltage divider may use two inductors L1 and L2 respectively in series with the resistors R4 and R5, as shown in FIG. 16. The effect, depicted in FIG. 17, is to allow control of the pressure response at low frequency, giving more flexibility for the user in the loudspeaker placement in a room for example.

[0044] As in US3838216, the motor system described above uses a single magnetic gap shared by the two voice coils, as shown in FIG. 18a. An alternative is, while still using the same motor system, to use one gap per voice coil, as in FIG. 18b, where the diaphragm of the second voice coil is behind the diaphragm of the primary voice coil and radiates rearwardly. In the equivalent electrical circuits, two motors could drive the same diaphragm as in FIG. 19(a), or a small acoustic chamber could be placed between two drivers as in FIG. 19(b); the latter arrangement is an isobaric arrangement.

[0045] It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the present invention is principally described with reference to circular voice coils (in the form of a substantially planar ring with a central hole); however, the invention applies equally to non-circular arrangements, such as oval, elliptical or race track shaped (figure of eight, or triangular/square/polygonal with rounded corners) voice coils, or any shape being symmetrical in one or two orthogonal directions lying in the general plane perpendicular to the voice coil axis and having a central hole.

[0046] Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.