Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Electromagnetic
Reexamination Certificate
1999-01-11
2001-04-17
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Electromagnetic
C381S423000, C181S171000
Reexamination Certificate
active
06219432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to loudspeaker drive units.
2. Description of Related Art
Known loudspeaker drive units comprise a diaphragm of which the outer portion is connected to a chassis member by way of a flexible surround.
The surround stops sound radiated by the rear surface of the diaphragm from passing round the outer edge of the diaphragm and thus cancelling out radiation from the front surface of the diaphragm. The surround allows the cone to move freely in an axial direction but restrains movement of a rocking kind or in a non-axial direction.
The surround thus has an important role in the operation of a loudspeaker drive unit, particularly, if the drive unit is to be used in a hi fi audio system.
In fact, both the surround and the diaphragm influence the quality of sound reproduction from a loudspeaker drive unit and it is exceedingly difficult to come close to an “ideal” loudspeaker drive unit using currently available materials.
OBJECTS AND SUMMARY
It is an object of the invention to provide a loudspeaker drive unit with an improved surround.
In one aspect thereof, the present invention provides a loudspeaker drive unit comprising:
a diaphragm made of a first material;
a chassis member: and
a surround made of a second, different, material connecting the outer portion of the diaphragm to the chassis member; wherein the bending wave impedance of the surround is substantially equal to the bending wave impedance of the diaphragm.
The surround provides mechanical damping to waves of bending that travel up the diaphragm and enter the surround and it reduces the possibility that these waves will be reflected back down into the diaphragm again. This first aspect of the invention is based on the realisation that when the bending wave impedance of the diaphragm is substantially equal to the bending wave impedance of the surround, the matching of the two impedances can avoid the reflection of waves back into the diaphragm again because the diaphragm is correctly “terminated”.
The characteristic impedance of a medium is the velocity of the type of wave in question multiplied by the density of the medium. For a bending wave the velocity is given by the following formula:
(1.8f*h){circumflex over ( )}0.5(e/ro){circumflex over ( )}0.25
where ro=density, e=Young's modulus, h=thickness and f=frequency.
If, for the sake of example, the thickness of the surround was equal to the thickness of the diaphragm but the density of the surround was ⅛ the density of the diaphragm, and the Young's modulus of the surround was ⅛ the Young's modulus of the diaphragm, then the bending wave velocity would be the same in both the surround and the diaphragm but the bending wave impedance of the surround would be ⅛ that of the diaphragm. If the thickness of the surround were now made instead 8*8 (=64) times the thickness of the diaphragm, matching of the bending wave impedances would be achieved.
When bending waves are reflected back into a diaphragm, standing waves tend to appear and the diaphragm seems to “break up” into sections instead of acting uniformly. This “break up” can be avoided by matching the bending wave impedance of the surround to that of the diaphragm.
According to another aspect thereof, the present invention provides a loudspeaker drive unit comprising:
a diaphragm;
a chassis member: and
a surround connecting the outer portion of the diaphragm to the chassis member; wherein the surround is made of a body of foam material arranged to be compressed against the chassis member when the diaphragm moves towards the chassis member.
In such a construction, axial movement of the diaphragm alternately compresses and decompresses the material of the surround rather than bending it as in a conventional surround.
The use of a surround made of a body of foam material arranged to be compressed against the chassis member when the diaphragm moves towards the chassis member provides a particularly effective and practical solution to avoiding sound radiation from the surround and one which lends itself particularly well to matching of the bending wave impedances as in the first-mentioned aspect of the invention.
Preferably, substantially all parts of the surround located between the diaphragm and the chassis member and capable of radiating sound are arranged parallel or at an acute angle with respect to the longitudinal axis of the loudspeaker drive unit.
This aspect of the invention is based on the realization that the surround has its own resonant frequencies and that by arranging that substantially all parts of the surround located between the diaphragm and the chassis member and capable of radiating sound are arranged parallel or at an acute angle with respect to the longitudinal axis of the loudspeaker drive unit the adverse effect of these resonances can be reduced. In such an arrangement sound is not radiated forwards from the surround with the sound from the diaphragm but is directed away to the side or at an angle. The effect of resonances of the surround is therefore less objectionable to a listener positioned in front of the loudspeaker drive unit.
The invention may be applied to either an active loudspeaker drive unit, that is, for example, one with a magnet system and voice coil for driving the diaphragm, or to a passive radiator, that is, a unit in which there is no direct electromagnetic drive to the diaphragm. Passive radiators, sometimes called “drone cones” are used in ports of loudspeaker enclosures.
Preferably, the outer portion of the diaphragm lies axially beyond the chassis member. The diaphragm then lies closest to the listener and the surround is located behind it.
The surround may be joined to the diaphragm at a location spaced from the periphery of the diaphragm. Such a construction enables the diaphragm to be made larger than the surround.
Advantageously, the surround is of integral construction with the diaphragm. By that means problems of making a connection between the surround and the diaphragm can be avoided.
The surround may be made of a resilient polymeric material, for example, rubber, for example, silicone rubber. A surround made of such material has particularly good flexibility.
The surround may be made of a foam material, for example, foam plastics material or foam rubber material. A surround made of such a material has particularly good damping properties.
The surround may be made of plastics material.
The surround may be made of a woven material.
Advantageously, the surround is corrugated. That is a simple way of giving the diaphragm freedom to move.
The surround may be substantially “C”-shaped in cross-section and preferably the open mouth of the “C” faces the said axis.
The surround may be substantially “>”-shaped in cross-section and preferably the point of the “>” faces the said axis. Such a construction comprises in effect two straight sections joined by an integral hinge portion.
The surround may be of square cross-section, two opposite sides of the square running substantially parallel to the said axis and the two remaining sides being joined to the diaphragm and chassis member respectively. Such a construction is particularly simple to realize using foam material. The surround may be rectangular instead of square in cross-section.
The surround may be in the form of bellows running substantially parallel to the said axis.
The surround may be in the form of a ring, preferably a ring of hollow cross-section.
The ring may be of circular cross-section or of elliptical cross-section.
The interior of the ring when of hollow form may be sealed and optionally the interior of such a ring is filled with a gas, for example, air. Optionally, the gas is under pressure. Such constructions provide a cushioning effect somewhat analogously to the inner tube of a bicycle tire.
The interior of a hollow ring may instead be open to ambient air by way of slits or holes.
Preferably, the diaphragm is cone-shaped and the periphery of the cone is joi
Fryer Peter Alexander
Nevill Stuart Michael
Roe Stephen Philip Barnham
B&W Loudspeakers Limited
Burns Doane Swecker & Mathis L.L.P.
Kuntz Curtis
Ni Suhan
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