Acoustics – Sound-modifying means – Intensifying horn
Reexamination Certificate
2001-07-27
2003-06-24
Lockett, Kim (Department: 2837)
Acoustics
Sound-modifying means
Intensifying horn
C181S152000
Reexamination Certificate
active
06581719
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is generally directed to audio loudspeaker systems and more particularly to such systems, which incorporate sound chambers which transform fan shaped wavefronts issuing from primary waveguides into rectangular planar or curved wavefronts which are directed to sound disseminating secondary waveguides.
2. Description of the Related Art
Large sound systems contain multiple transducers operating in the same frequency band in order to achieve the required sound pressure level (SPL) and the required acoustical coverage of its intended target. The highest efficiency sound systems use the principle of horn loading to achieve maximum SPL. The horn and its associated driver have two particular characteristics of interest: the driver is by definition larger than the throat of the horn; and wavefronts are radiated in a generally spherical shape. The dimensional limit dictates that an array of such devices cannot be constructed so that the horn throats are in close proximity to one another; the throats are displaced by the size of the attached driver. The result of any multi-element combination is that of an array that generates multiple spherical sound waves with significant interference effects. The problems of the resulting multiple source array are well documented in the literature.
Significant effort has been invested by numerous practitioners of the audio arts to find solutions to this problem. The result has been great improvement in the understanding of the principle of horns and better control of the directional characteristics of such devices. D. B. Keele, Jr. has shown many ways to modify the directivity of horns through the use of flat sided conical growth sections combined with arbitrary adjustments to the walls of horns, “What's So Sacred about Exponential Horns?”, D. B. Keele, Jr., Presented at the 51st Convention of the Audio Engineering Society, Los Angeles, May 1975, Preprint 1038. These horns became known as “constant directivity horns”. None of the improvements of individual horns has resulted in an array of horns that eliminates the interference effects found in multi-source arrays.
This work was continued by Clifford A. Hendricksen and Mark S. Ureda with the goal of improved vertical directivity, “The Manta-Ray Horns”, Clifford A. Hendricksen and Mark S. Ureda, Presented at the 58th Convention of the Audio Engineering Society, New York, November 1977, Revised June 1978. This was achieved through a vertically oriented, conical throat section and a constant angle between the top and bottom wall of the horn over most of its length. These horns were referred to as “constant directivity” and “Manta-Ray”horns.
Early Line Arrays
The line array results from a different approach to the problem of interference, H. F. Olson,
Acoustical Engineering,
(Van Nostrand, Princeton, N.J. 1957). In its simplest form, it is a row of closely spaced direct radiators operating in the same frequency band and is dependant on mutual coupling of one driver to the next. Drivers are said to be mutually coupled when they are placed within one wavelength of each other. The benefit of mutual coupling is that drivers operating under such conditions radiate their combined acoustical energy nearly as though they were a single transducer.
Until recently, line arrays have consisted of multiple small direct radiating transducers arranged in a vertical row. Typically the drivers are chosen to be sufficiently small to allow mutual coupling to the highest frequency of concern. For example four inch diameter drivers permit coupling to above 3 Khz, which is sufficient to allow good speech intelligibility. This approach yields a system with a controlled vertical coverage and correspondingly wide horizontal coverage but with at least two significant limitations. Such arrays of direct radiators are low in efficiency and the method does not work for typical horn loaded high frequency drivers because the wavelengths at the highest audio frequencies are generally a factor of ten shorter than the dimensions of the drivers used thus preventing mutual coupling. Furthermore, the use of small direct radiators severely restricts the operating bandwidth.
Waveguides with Linear Inlet Apertures
In October 1987 in New York City, Dr. Earl Geddes introduced the concept of an “Acoustic Waveguide”, through the publication and presentation of a historically important technical paper at the 83rd Audio Engineering Society Convention. Many waveguides are described in the text of the paper with varying degrees of usefulness in the audio world. The publication is titled “Acoustic Waveguide Theory”. The importance of this paper is that it prescribed and consolidated a new approach to the analysis of the boundary of the sound wave as it moves along the length of a horn. Specifically, it is noted that for any horn type device to be considered a true waveguide, it must meet the criteria that the wavefront will always intersect the boundary of the waveguide at a 90 degree angle. If this condition is not met, the wavefront cannot maintain contact with the wall of the horn and it thus should not be considered a waveguide. The reason for this is both obvious and simple. The particle motion in any wave is always in the direction of the travel of the wave and normal to the wavefront. Any boundary not normal to the wavefront will cause a reflection of energy, thus reducing contact with the waveguide wall.
The importance Geddes' work is emphasized by its selection from hundreds of papers for publication in the permanent record of the Journal of the Audio Engineering Society, two years later. (Preprint Number: 2547 Convention: 83 1987-10 AES Journal Vol:Issue: 37:7/8 Page: 555 Year: 1989). Three of the waveguides presented are of pivotal interest in the construction of a new type of line array because their throats approximate the shape of a narrow ribbon, either straight or curved. A characteristic of these waveguides is that they can be joined in an obvious manner to produce a waveguide of extended length. These three are derived from the coordinate systems after which they are named. The cylindrical waveguide is shown in
FIGS. 6
a
-
6
c,
the elliptic cylindrical waveguide is shown in
FIGS. 5
a
-
5
c
and the prolate spheroidal waveguide is shown in
FIGS. 7
a
-
7
c.
A cylindrical waveguide can be configured of infinite length. A waveguide comprised of a number of segments of cylindrical waveguides joined at 90 degrees from the angle theta can also be of infinite length and results in an entrance or throat that is shaped as an extended rectangular slot. From Geddes we know that the height at the mouth must be the same as the height at the throat (i.e. that the two boundary surfaces are parallel) and that the wave is spread out only in the theta direction shown at
FIGS. 5
a
,
6
a
and
7
a.
Geddes comments that all that remains to make this type of waveguide useful is the development of the necessary “phasing plug” to shape a sound wave to match the throat requirements of the waveguide.
The wavefront required to correctly propagate a cylindrical wave to the throat of the waveguide is cylindrical, (curved in the theta direction) and of the correct radius in order to propagate one parameter waves within the waveguide. Any wavefront other than a cylindrical wave of the correct radius will propagate down the waveguide through reflection and higher order modes that are quite undesirable according to Geddes. The compromise of the cylindrical waveguide is used in practice because it is extremely simple to fabricate since all waveguide surfaces are planar.
Geddes also discloses a waveguide suitable for use with a planar rectangular sound source. The elliptical cylindrical waveguide requires a planar wave of rectangular cross section at the throat. This waveguide can also be extended infinitely since two surfaces are parallel in the same manner as the cylindrical waveguide. At this time there is no evidence of any such waveguide in commercial production.
The prolat
Dowell & Dowell , P.C.
Lockett Kim
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