This invention relates to a unique means of powering a loudspeaker, namely, by the use of a hydraulic drive system.

BACKGROUND

Traditional loudspeaker drivers have been powered with an electromagnetic moving-coil actuator. A loudspeaker powered by this type of actuator is not extremely efficient, but has properties which make it well-suited to the reproduction of audio signals. Recent advances in the manufacture of hydraulic servovalves and pumps and in control systems make it possible to use a hydraulic actuator and an open-loop control system rather than an electromagnetic actuator to drive a loudspeaker cone such that high sound pressure levels are produced even at low frequencies.

One of the problems with electromagnetic voice-coil devices is that in designing a device which can exert high forces, one ends up with a device which also dissipates significant amounts of energy. This electromagnetic damping has been used to good effect by the audio industry in making loudspeakers which have fairly smooth bandpass characteristics. However, it is now possible to use modem control systems to "equalize" dynamic systems which have highly irregular bandpass characteristics. This makes it possible to use a hydraulic actuator in concert with a servovalve, which effectively produces a velocity source, with virtually no damping, to drive the loudspeaker cone. Advances in the manufacture of servovalves have made it possible to manufacture the devices inexpensively, and small, high pressure hydraulic pumps are now commonly available. Asami et al, in Patent No. 5,060,274, propose a hydraulically driven speaker which involves the use of a high frequency pressure control device and pressure and position feedback. The pressure control device, feedback transducers and control electronics of such a device are not easily or inexpensively obtained. The proposed invention circumvents these shortcomings by using commonly available components.

Additionally, the device may be operated using pneumatic power rather than hydraulic power.

Accordingly it is an object of this invention to provide a loudspeaker with a hydraulic actuator and servovalve. Another object of this invention is to provide a hydraulic power source to drive the cone in a loudspeaker.

A further object of this invention is to provide a loudspeaker actuator with essentially no damping characteristics.

These and other objects will become apparent when reference is had to the accompanying drawings in which:

Fig. 1 shows the plan and side view of a typical electromagnetic loudspeaker driver,

Fig.2 shows a diagrammatic view of a conventional bandpass loudspeaker,

Fig. 3 shows a graph of the plot of SPL versus frequency, Fig. 4 shows a diagrammatic view of a hydraulic powered driver,

Fig. 5 shows a diagrammatic view of a hydraulic bandpass loudspeaker,

Fig. 6 shows a graph of the plot of SPL versus frequency for the loudspeaker of

Fig. 5, and

Fig. 7 shows a diagrammatic view of a complex hydraulic bandpass enclosure.

DESCRIPTION OF THE DEVICE

A typical loudspeaker driver 10 is shown in Figure 1. The cone 11 which displaces air to make sound, is driven by a voice coil 12 which sits inside a magnetic structure 13. The electromagnetic coupling between the voice coil and the magnet allows a current passed through the coil to result in a force on the voice coil. This coupling also induces a back-EMF as a result of the voice coil\'s velocity which is essentially a dissipator of energy. This limits the efficiency of the device at certain frequencies. When the typical loudspeaker is placed in a simple enclosure like that in

Figure 2, a bandpass characteristic results as in Figure 3. Enclosure 20 has speaker 21 and tuned aperture 22.

If a hydraulic actuator and servovalve is used in place of the electromagnetic device, the resulting arrangement is more like Figure 4, in which the hydraulic actuator 21 is coupled directly to the speaker cone 22. A servovalve driven actuator is basically a velocity source since flow through the servovalve is proportional to the electrical input signal. The driver of a conventional speaker is a source of force in general. The above-mentioned cone may be any shape or material required to displace air and support the operating loads. Indeed, another advantage of the present invention is that low cone mass is no longer a necessary design condition. If this hydraulic loudspeaker is placed in the bandpass box of Figure 5, a bandpass characteristic similar to that shown in Figure 6 results. If this is too irregular, it is possible to build a more complex enclosure with more degrees of freedom. This type of enclosure 30 like that in figure 7, results in a flatter bandpass characteristic, although some type of damping may need to be added to the system. These enclosures are only used as examples. The device can be used with virtually any enclosure geometry. These measures should not be necessary with the advent of digital devices or even analog circuits which can "equalize" the response of such a device. The electrical signal driving the servovalve can be conditioned through an analog or digital circuit which would artificially "flatten" the response of the device. This circuit can easily be designed such that no feedback of parameters is required to maintain acceptable operation of the speaker. Enclosure 30 has speaker 31, tuned aperture 32, and wall 33 with tuned aperture 34.

PREFERRED EMBODIMENT In one embodiment of the hydraulic-powered loudspeaker, the loudspeaker cone can be a 12" diameter cone. The maximum stroke of the device will be +/- 12mm. The maximum force required from the actuator is approximately 200 Newtons. A hydraulic supply of 10 MPa (1500 PSI) is required, so the piston area of the hydraulic piston will be at least 2E-5 square meters. If the cylinder is to displace +/-

12mm at 100 Hz, the minimum required flow rate is 5E-5 cubic meters per second (3 cubic inches per second). There will be some leakage which requires this flow rate to be higher since a hydraulic cylinder is generally designed with leakage and "dither" movement to improve its linearity. There also should be a centering spring to center the cylinder so a position transducer is not required and a small bias flow from the servovalve can be tolerated. This centering stiffness may be the stiffness of the speaker cone support or may be an additional stiffness added to the cone or the hydraulic actuator.

While the preferred embodiments have been shown and described it will become apparent to those of ordinary skill in the art that many changes and modifications can be made to the invention without departing from the scope of the appended claims.