Waveguide electroacoustical transducing

Abstract: A waveguide system for radiating sound waves. The system includes a low loss waveguide for transmitting sound waves, having walls are tapered so that said cross-sectional area of the exit end is less than the cross-sectional area of the inlet end. In a second aspect of the invention, a waveguide for radiating sound waves, has segments of length approximately equal to where l is the effective length of said waveguide and n is a positive integer. The product of a first set of alternating segments is greater than the product of a second set of alternating segments, in one embodiment, by a factor of three. In a third aspect of the invention, the first two aspects are combined.

Description: The invention relates to acoustic waveguide loudspeaker systems, and more particularly to those with waveguides which have non-uniform cross-sectional areas. For background, reference is made to US 4628528, US5373564 and US 6278789. It is an important object of the invention to provide an improved waveguide. WO 96/11558 discloses a waveguide system for radiating sound waves, comprising: a source of sound waves; a low loss acoustic waveguide for transmitting sound waves, said waveguide comprising a first terminus adapted to be coupled to the source of sound waves; a second terminus adapted to radiate said sound waves to the external environment; a centerline running the length of said waveguide; and walls enclosing cross-sectional areas in planes perpendicular to said centerline, said walls being tapered such that the cross-sectional area at said second terminus is less than the cross-sectional area at said first terminus. According to the present invention, such a waveguide system is characterised by the features of claim 1. Other features, objects, and advantages will become apparent from the following detailed description, which refers to the following drawings in which: Figure 1 is a cross-sectional view of a waveguide loudspeaker system according to the invention; and Figures 2a and 2b are computer simulated curves of acoustic power and driver excursions, respectively vs. frequency for a waveguide according to the invention and for a conventional waveguide. With reference now to the drawings and more particularly to Figure 1, there is shown a loudspeaker and waveguide assembly according to the invention. A waveguide 14 has a first end or terminus 12 and a second end or terminus 16. Waveguide 14 is in the form of a hollow tube of narrowing cross-sectional area. Walls of waveguide 14 are tapered, such that the cross-sectional area of the waveguide at the first end 12 is larger than the cross-sectional area at the second end 16. The second end 16 may be slightly flared for acoustic or cosmetic reasons. The cross-section (as taken along line A-A of Figure 1, perpendicular to the centerline 11 of the waveguide 14) may be circular, oval, or a regular or irregular polyhedron, or some other closed contour. The waveguide 14 may be closed-ended or open-ended. Both ends may radiate into free air as shown or one end may radiate into an acoustic enclosure, such as a closed or ported volume or a tapered or untapered waveguide. For clarity of explanation, the walls of the waveguide 14 are shown as straight and the waveguide 14 is shown as uniformly tapered along its entire length. In a practical implementation, the waveguide may be curved to be a desired shape, to fit into an enclosure, or to position one end of the waveguide relative to the other end of the waveguide for acoustical reasons. The cross section of the waveguide 14 may be of different geometry, that is, have a different shape or have straight or curved sides, at different points along its length. Additionally, the taper of the waveguide may vary along the length of the waveguide. An electroacoustical transducer 10 is positioned in the first end 12 of the waveguide 14. In one embodiment of the invention, the electroacoustical transducer 10 is a cone type 65 mm driver with a ceramic magnet motor, but may be another type of cone and magnet transducer or some other sort of electroacoustical transducer. Either side of electroacoustical transducer 10 may be mounted in the first end 12 of the waveguide 14, or the electroacoustical transducer 10 may be mounted in a wall of the waveguide 14 adjacent the first end 12 and radiate sound waves into the waveguide 14. Additionally, the surface of the electroacoustical transducer 10 that faces away from the waveguide 14 may radiate directly to the surrounding environment as shown. Interior walls of waveguide 14 are essentially lossless acoustically. In the waveguide may be a small amount of acoustically absorbing material 13. The small amount of acoustically absorbing material 13 may be placed near the transducer 10, as described in US 6278789 so that the waveguide is low loss at low frequencies with a relatively smooth response at high frequencies. The small amount of acoustically absorbing material damps undesirable resonances and provides a smoother output over the range of frequencies radiated by the waveguide but does not prevent the formation of low frequency standing waves in the waveguide. In one embodiment of the invention, the waveguide is a conically tapered waveguide in which the cross-sectional area at points along the waveguide is described by the formula where A represents the area, where y = the distance measured from the inlet (wide) end, where B = x AR AR -1 , where x = the effective length of the waveguide, and where AR = A inlet / A inlet . The first resonance, or tuning frequency of this embodiment is closely approximated as the first non-zero solution of αf = tan βf, where α = 2πχ c 0 AR AR -1 β = 2πχ c 0 , and c 0 = the speed of sound. After approximating with the above mentioned formulas, the waveguide may be modified empirically to account for end effects and other factors. in one embodiment, the length x of the waveguide 14 is 660 mm (26 inches). The cross-sectional area at the first end 12 is 4130 mm 2 (6.4 square inches) and the cross-sectional area at the second end 16 is 581 mm 2 (0.9 square inches) so that the area ratio (defined as the cross-sectional area of the first end 12 divided by the cross-sectional area of the second end 16) is about 7.1. Referring now to Figures 2a and 2b, there are shown computer simulated curves of radiated acoustic power and driver exhaustion vs. frequency for a waveguide loudspeaker system according to the invention (curve 32), without acoustically absorbing material 13 and with a length of 660 mm (26 inches), and for a straight walled undamped waveguide of similar volume and of a length of 914 mm (36 inches) (curve 34). As can be seen from Figures 2a and 2b, the bass range extends to approximately the same frequency (about 70 Hz) and the frequency response for the waveguide system according to the invention is flatter than the untapered waveguide system. Narrowband peaks (hereinafter "spikes") in the two curves can be significantly reduced by the use of acoustically absorbing material (13 of Figure 1).

Claims: A waveguide system for radiating sound waves, comprising: a source of sound waves (10); a low loss acoustic waveguide (14) for transmitting sound waves, said waveguide comprising a first terminus (12) adapted to be coupled to the source of sound waves (10); a second terminus (16) adapted to radiate said sound waves to the external environment; a centerline (11) running the length of said waveguide; and walls enclosing cross-sectional areas in planes perpendicular to said centerline; said walls being tapered such that the cross-sectional area at said second terminus (16) is less than the cross-sectional area at said first terminus (12); characterised in that ; a first surface of the source of sound waves radiates into the waveguide; a second surface of the source of sound waves radiates into the environment; and said cross-sectional area progressively decreases as a function of distance from said first terminus (12). A waveguide system in accordance with claim 1, wherein said cross-sectional areas vary according to a formula where A is the cross-sectional area, A inlet is the cross-sectionai area at said first terminus (12), y is the distance measured from said first terminus (12), and B = x AR AR -1 , where x is the length of the waveguide (14), and AR is the cross-sectional area at said first terminus (12) divided by the cross-sectional area at said second terminus (16). A waveguide system in accordance with claim 1, wherein the cross-sectional area at said second terminus (16) is less than one half the cross-sectional area at said first terminus (12). A waveguide system in accordance with claim 3, wherein the cross-sectional area at said second terminus (16) is on the order of one seventh of the cross-sectional at said first terminus (12).

Inventors: FROESCHLE THOMAS A, HOEFLER JEFFREY, PARKER ROBERT P, SCHREIBER WILLIAM P, WENDELL JOHN H, FROESCHLE, THOMAS A., HOEFLER, JEFFREY, PARKER, ROBERT P., SCHREIBER, WILLIAM P., WENDELL, JOHN H.
Applicants: BOSE CORP, BOSE CORPORATION
Application year: 1999
Main classification: H04R1
IPC codes: H04R1, G10K11