A source of sound and/or non-sound air motion (such as a loudspeaker) has been coupled to various hollow devices usually called horns or waveguides. These have been used to direct sound and also in attempts to generate low-frequency sound.
One type of waveguide associated with generation of low-frequency sound is the “exponential” horn or waveguide. The name refers to the cross-sectional area (the area of a plane taken across the length of the horn), which increases exponentially with distance from the small end. In such horns the speaker is mounted, usually coaxially, at the smaller end. An exponential horn has no zero or convergence point at which the diameter reaches zero (corresponding to the apex of a cone), and therefore the placement of the speaker cannot constitute any choice of distance from a zero point in an exponential horn.
Another known type of acoustic waveguide has a conical interior space, and this is an example of a waveguide or horn in which the cross-sectional area does not increase exponentially with distance from the small end, but instead increases as the square of the distance from the small end. Unlike an exponential horn, it has a point at which the area is zero, which is referred to as a zero point (for example, the sides of a cone meet at one point, the apex).
A conical horn represents a sector of a sphere. Other sectors of a sphere have the same area development: any closed curve on the surface of a sphere (for example, a rectangle) can define a tapering shape of a waveguide or horn by connecting all the points of the closed curve to the center point of the sphere by respective straight lines. A waveguide or horn in which the cross-sectional area increases as the square of the distance from a zero point will be referred to herein as a “radial” waveguide or horn. However, as discussed in the provisional application, such a radial waveguide or horn is not limited to shapes with straight lines between the zero point and the open end, as long as the cross-sectional area develops substantially as the square of the distance from the zero point (center or convergence point when the waveguide is a projection of straight lines from one point).
Conical waveguides have been used with loudspeakers and other diaphragms. Some early mechanical record players used a mechanically-driven diaphragm (driver) coupled to a conical horn, with the axes of the horn and the driver aligned. Often a flared bell was placed at the end of the horn. The same basic design is still used in some modern public-address equipment, although exponential horns are more common.
U.S. Pat. No. 2,979,149 to Carlsson discloses a conical waveguide with “loudspeaker mechanisms” 5-9 (which may or may not be loudspeakers; they are drawn to resemble wire mesh) mounted at the smaller end and also on a plate 24 that “closes” the larger end of the cone. The loudspeaker mechanism 3′ that is mounted in the plate 24 “is mainly used for low frequencies” while the higher frequencies are produced by a smaller loudspeaker mechanism 5 located at the smaller end (col. 3, lines 54-61). Carlsson teaches generating bass sound with a loudspeaker located at the larger end of a conical waveguide, which is contrary to the invention set out below.
U.S. Pat. No. 4,628,528 shows in FIG. 2 a loudspeaker mounted at one end of a “hard tube 33” (col. 3, line 45) with the axes of the loudspeaker and the tube aligned. This arrangement is said to function as “an acoustic transmission line of length l.” The patent discusses this arrangement according to the conventional theory of organ pipes and the like, which is based on the idea that sound waves reflect from the open end and create a resonance. This theory, which the applicants think is incorrect (because the speed of sound is essentially the same inside and outside of a tube, and therefore there is not actually any impedance mismatch at the open end), fails to take note of the non-resonant effects of a tube on a speaker. FIG. 1 of U.S. Pat. No. 6,278,789 shows a loudspeaker mounted coaxially in a tube, with damping material 14 “near” the driver 11; this patent teaches that damping material at the open end reduces bass output (col. 1, line 60). FIG. 1 of U.S. Pat. No. 3,978,941, likewise, shows a loudspeaker coaxially mounted in a tube, but here the damping material lines the sides of the tube.
A conical waveguide coupled with a coaxial driver is used in the Peavey Quadratic-Throat Waveguide. A white paper on the Peavey Quadratic-Throat Waveguide (http://aa.peavey.com) states, “The weakness of conical horns lies in their acoustical loading characteristics for the transducer, which is insufficient at the low-frequency end.” (Contrary to this teaching, the inventors have demonstrated that a conical horn can exhibit high acoustical loading at bass frequencies.) The Quadratic-Throat Waveguide places the apex of the cone at the surface of the driver (loudspeaker element), which inhibits bass response for reasons discussed below.
The Yorkville company's Unity speaker cabinets use a wide-angle conical horn with speakers mounted on the side of the cone adjacent to the cone's apex. Yorkville describes it as “Summation Aperture Horn Technology, which was invented by loudspeaker designer Tom Danley” (quoting http://www.yorkville.com/products.asp?type=29&cat=38 which cites U.S. Pat. No. 6,411,718 B1). The design uses an axial high frequency compression driver and three midrange drivers all mounted on the side of a single 60°×60° conical horn. The company claims frequencies from 300 Hz to 20 kHz for the horn, and provides a 15-inch subwoofer in the same cabinet as the horn, which demonstrates that the horn does not produce adequate bass.
The Yorkville websites quotes “patent holder Tom Danley” as stating, “with a conical horn, . . . the expansion rate acts as a high pass filter, the low frequency energy does not couple to the mouth. Move a few inches toward the mouth and one finds the expansion rate is much slower and suitable for low midrange, if only that was where the driver was. All one has to do is obtain a mid driver, suitable for efficient horn loading in that frequency band and find the point in the flare where the expansion rate is suitable for that frequency range and couple the sound in at that point. Because the compression driver and each mid driver are less than ¼ wavelength apart, their output combines fully and coherently, something which cannot happen if the driver were further than about ⅓ wavelength apart.”
Thus, the prior art recognizes that mounting a speaker farther from the apex of a cone increases the bass response. However, the prior artisans have not utilized this observation to produce bass, because of what is believed to be a misguided theory. According to the website, “The Unity™ technology takes advantage of the variable flare rate nature of a conical horn. By sectioning the horn according to the expansion rate, the horn can be divided into frequency bands and be loaded with suitable drivers mounted outside the acoustic path.” The applicants base their own design on a completely different theory, and have achieved much better results in producing sound waves that are long in comparison to the size of a horn.
Yorkville's placement of the larger drivers close to the apex, and the failure to truncate the cone immediately below the larger drivers, are inefficient in generating bass, for the reasons set out below. Like the Peavey design, the Yorkville design apparently includes space in the apex of the cone beyond the midrange speakers, which increases the length of the waveguide and also decreases the bass radiating efficiency.
The problem with existing combinations of radial horns and drivers is that their designs do not mimic the action of a large spherical or plane radiator, which are the known models for efficient radiation of bass sound, and therefore cannot generate bass sound efficiently. There has been a need for more efficient production of bass sound.