This specification relates generally to the field of loudspeakers. More particularly, this specification relates to a suspension system for the voice coil assembly of a loudspeaker.
In general, a loudspeaker includes a frame, a moving assembly, and a suspension system, which mechanically couples the moving assembly to the frame in a manner that permits the moving assembly to move relative to the frame. The moving assembly includes a diaphragm, which vibrates to radiate pressure waves that are perceived as sound. The suspension system, which may include one or more suspension elements, preferably permits movement along a single axis, so that contact between the moving assembly and the frame, or elements rigidly coupled to the frame, are avoided.
One common type of loudspeaker is a moving coil loudspeaker. In a moving coil loudspeaker, the moving assembly includes a voice coil assembly. The voice coil assembly includes a voice coil former, which is typically a tube with a circular cross section, but which may have some other form of cross section, such as square or rectangular. The moving assembly also includes a voice coil, which is typically formed by tightly winding an electrically conductive wire around the voice coil former. The diaphragm is mechanically coupled to the voice coil assembly. Audio signals, in the form of electrical current in the voice coil, interact with the magnetic field of a magnet assembly which is rigidly coupled to the frame, to cause the diaphragm to vibrate, radiating pressure waves that are perceived as sound.
Another type of loudspeaker is a moving magnet loudspeaker. In a moving magnet loudspeaker, the voice coil assembly is rigidly coupled to the frame, and the moving assembly includes a magnet assembly, mechanically coupled to the diaphragm. Audio signals, in the form of electrical current in the voice coil, interact with the magnetic field of the magnet assembly, to cause the diaphragm to vibrate, radiating pressure waves that are perceived as sound. In this specification, the examples are moving voice coil loudspeakers, but the principles described herein may be applied to moving magnet loudspeakers provided the suspension system has adequate properties such as lateral stiffness.
A first common type of moving voice coil loudspeaker includes a voice coil assembly in which the diameters of the voice coil former and the diaphragm are substantially the same. In these loudspeakers, the outer-most edge of the diaphragm is attached to the upper periphery of the voice coil former. The moving assembly is typically secured to the frame of the loudspeaker by at least a first support element, commonly referred to as a “surround”, which has an inner edge secured to the moving assembly and an outer edge that is secured to the frame. Alternate embodiments may include a second support element, commonly called a “spider”, which includes an inner edge secured to a bottom portion of the voice coil former and an outer edge that is secured to the frame of the loudspeaker. This type of construction is typically found in smaller loudspeakers, such as tweeters, and possibly mid-range speakers.
A typical issue encountered with smaller-sized loudspeakers is that as the loudspeaker becomes smaller, achieving low frequency response becomes more difficult. Low frequency response requires a loudspeaker to displace a larger volume of air to achieve the lower frequencies, as compared to achieving higher frequencies. The volume of air that a loudspeaker can displace is dependent upon the area of the diaphragm and the peak-to-peak excursion of the voice coil assembly that is allowed by the suspension. As the axial stiffness of the suspension is reduced to allow a greater excursion of the voice coil assembly, the radial stiffness of the suspension is usually similarly decreased. To operate at maximum efficiency, the suspension system in smaller loudspeakers should allow a maximum amplitude of axial displacement while constraining the voice coil assembly from moving side to side in order to avoid contact between the voice coil assembly and the other portions of the loudspeaker. As would be expected, as the stiffness of the suspension is reduced, greater side to side motion of the voice coil assembly is usually allowed. This is especially true in those loudspeaker embodiments that only include a single support element securing the voice coil assembly to the frame of the loudspeaker.
In a second type of moving coil loudspeaker, the moving assembly includes a diaphragm that is formed by a cone (or some other planar or non-planar surface, such as a concave surface) having a diameter that is greater than the diameter of the voice coil former. In this type of loudspeaker, an inner periphery of the cone is typically secured to an upper portion of the voice coil former and a first support member has an inner edge secured to an outer periphery of the diaphragm and an outer edge that is secured to the frame of the loudspeaker. A second support member has an inner edge that is secured to a lower portion of the voice coil former and an outer edge that is secured to the frame of the loudspeaker. In this type of loudspeaker, it is often desirable to reduce the outer-most dimensions of the frame of the loudspeaker to facilitate mounting the loudspeaker in various environments where space may be limited.