1. Field of the Invention
The present invention relates to horn loudspeakers, and, more particularly, to compact horn loudspeakers suitable for paging or emergency signaling uses in harsh environments and confined spaces such as are found on emergency vehicles.
2. Discussion of the Prior Art
Emergency vehicles (e.g., ambulances) usually carry horn loaded siren loudspeakers for emergency signaling, and in the past it was customary to mount one or more siren loudspeakers on an overhead light bar or on a front bumper.
Traditional horn loaded siren loudspeakers are not well suited for use on some modern vehicles, however. In the past, when it was desired to aim the siren""s sound forwardly, a siren loudspeaker was mounted on the front bumper, in front of the grille covering the radiator. On cars built in the 1980""s, front bumpers projected forwardly four to six inches, providing an exterior surface with ample room for mounting a traditional horn loaded siren loudspeaker.
Many modern vehicles have integrally contoured and painted bumper surfaces and so do not have a front bumper surface well suited to mounting a siren loudspeaker. The siren loudspeaker can be mounted under the hood, behind the grill and in front of the radiator, but this mounting location presents a number of problems. There is very little room between the grille and the radiator, the radiator is often hot, and the siren loudspeaker is exposed to a damp, corrosive environment.
Other loudspeaker designers have sought to solve these problems by fashioning loudspeakers from stacks of metal, planar, plate-like members defining folded or reentrant horns. For example, U.S. Pat. No. 5,970,158, to Beltran, discloses a compact horn loudspeaker intended for use behind the grille of an emergency vehicle or the like. The Beltran horn loudspeaker does achieve a small front to back thickness dimension, but is expensively fabricated from a plurality of planar plate-like aluminum pieces joined in a stack with vulnerable, exposed seams running around the periphery of the horn.
Another problem is that the small space available behind the grille may not be suitable for mounting a siren loudspeaker of sufficient power; there are applications requiring more acoustic output than one compact siren driver can produce. For example, when using the Beltran siren, installers may not be able to find sufficient space to mount two or more of Beltrans"" siren housings. Airflow does have to reach the radiator, after all; one cannot simply cover the interior of the grille with sirens. Another problem with requiring the siren installers to mount two separate sirens is that separate wiring connections have to be made, thus increasing the cost of installation and the opportunity for incorrect wiring. If the siren loudspeakers are wired in parallel, impedance is halved, and if wired in series, impedance is doubled, as compared to a single siren driver. Incorrect wiring can lead to poor acoustic performance or equipment failure due to an overcurrent condition.
Turning to a more general view, the prior art includes a number of compact folded or reentrant horn loudspeakers for use in a variety of settings. For example, the Handbook for Sound Engineers 2d. Edition, pp 545-558, Glen M. Ballou, Editor, 1991, describes a number of xe2x80x9cfolded hornsxe2x80x9d folded to make xe2x80x9ccompactxe2x80x9d products. In the examples cited, the sound path makes one or two 180 degree turns, effectively reversing or folding the sound path back on itself.
U.S. Pat. No. 1,767,812, to W. J. Polydoroff, discloses an xe2x80x9cacoustical hornxe2x80x9d including a compression driver and a xe2x80x9cbodyxe2x80x9d including a continuous winding sound channel having a flared terminal portion constituting the mouth of the horn. The body is, preferably, made in two or three xe2x80x9cmatched discs or sectionsxe2x80x9d independently molded, machined or die pressed and then glued, cemented or fastened together in a stack.
U.S. Pat. No. 1,832,763, to W. M. Campbell, discloses a horn-loaded loudspeaker including what is called an xe2x80x9cacoustic chamberxe2x80x9d intended to reflect or deflect sound waves to redirect sound received in a throat tube 9, directing the sound towards exit slots 7.
In 1928 H. C. Harrison invented an acoustic horn which became the subject of U.S. Pat. No. 1,747,830. Harrison""s acoustic horn was given a xe2x80x9ccompact formxe2x80x9d obtained by xe2x80x9cproviding a reentrant horn in which the sound waves traverse the space within the enclosing walls a plurality of times and expand progressively at a suitable rate in their passage therethrough.xe2x80x9d
U.S. Pat. No. 2,160,166 issued to H. R. Pausin in 1939 and includes a horn for a loudspeaker cast in metal, preferably aluminum, in two parts. The horn is formed of a lower section separably attached to an upper section. The lower section includes the mount for a compression driver and a horn mouth; the upper section is a single casting providing a folded horn throat arcing through 180 degrees and connected to the horn mouth for coupling an acoustic wave to external air.
U.S. Pat. No. 2,957,054, to Levy et al, discloses a transducer or horn-loaded loudspeaker having a driver assembled into an inner compartment of a mechanical acoustic body formed as a horn throat member received within a flared body which forms the horn mouth. Levy""s loudspeaker is characterized as a xe2x80x9creflex horn speakerxe2x80x9d having xe2x80x9cacoustic ductsxe2x80x9d fabricated within two complimentary dieformed members.
U.S. Pat. No. 2,301,459, to A. J. Sanial, disclosing a loudspeaker called an xe2x80x9celectric megaphonexe2x80x9d wherein a loudspeaker driver projects an acoustic wave into a tubular extension projecting forwardly which is in communication with a rearward or reentrant annular space characterized as a reversely extending annular passage for communication with a horn mouth. An outer cylindrical housing is closed at the rear end and the soundwave from the annular passage reflects at a solid rear wall and is reflected forwardly to project out and be coupled to the external air by a flared end or horn mouth.
U.S. Pat. No. 5,804,774, to Ford et al, discloses a ported reflex horn. The background section of this patent discusses xe2x80x9cre-entrantxe2x80x9d horns having horn sections which xe2x80x9cfold backxe2x80x9d on one another, offering a compact alternative to straight horns.
These background references illustrate the many approaches taken in fashioning compact horn-loaded loudspeakers. None of the designs illustrated therein address all of the problems identified by the applicant, as discussed above. In particular, the prior art does not teach or suggest an optimum structure for a compact siren loudspeaker which impervious to the environment, is easy and inexpensive to manufacture, and is easy to install. The prior art is also silent on how one may fashion a siren loudspeaker in a modular configuration, permitting the installer to select a siren having an optimum output power rating for installation under the hood of an emergency vehicle, or the like.
There is a need, therefore, for a modular compact siren assembly adapted for use in small spaces and harsh environments, such as under the hood of an emergency vehicle.
Accordingly, it is a primary object of the present invention to overcome the above mentioned difficulties by providing a modular, compact siren assembly adapted for use in small spaces and harsh environments, such as under the hood of an emergency vehicle.
Another object of the present invention is to provide a modular system of loudspeaker elements adapted for economical manufacture and ease of assembly.
Yet another object of the present invention is to provide a modular loudspeaker which is easily installed under the hood of a vehicle in the confined space between the grille and the radiator.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
A modular horn loudspeaker principally intended for use on emergency vehicles consists of three coaxially aligned cup-shaped parts, a sound chamber, a cover fitted to the sound chamber and a housing. The first embodiment of the modular, compact horn loudspeaker of the present invention is implemented with a single siren driver; the single driver embodiment includes a dome-shaped driver diaphragm integrally formed onto an interior surface of the sound chamber. The sound chamber is an acoustical boundary defining member having curved propagation paths or horn throat channels formed in relief on a cup-shaped exterior surface. The sound chamber is coaxially aligned with and received in a tightly fitted cup-shaped cover member to define a plurality of curved horn throats of expanding rectangular cross sectional dimension directing the sound waves rearwardly around the diaphragm periphery and toward a housing rear wall. A magnet assembly is pressed into a heat sink formed in a larger and concentrically aligned cup-shaped housing having a closed rearwall for redirecting the sound wave forwardly, around the periphery of the cover. The horn mouth is defined by the circular peripheral wall of the external cup-shaped housing and the concentric and coaxially aligned sound chamber/cover assembly.
The largest of the three coaxially aligned, concentrically assembled cup-shaped members is the housing which incorporates a heat sink/magnet assembly, is closed at the rear and open at the front, forming a horn mouth to couple acoustic energy to external air. The cover member opens rearwardly, is closed at the front and is slightly larger in diameter than the sound chamber which fits and is sealed within the cover. The sound chamber is contoured on its interior surface to follow the dome shaped surface of the diaphragm and incorporates an integral phase plug spaced from the diaphragm, as is well known in the art. The outer surface of the sound chamber member incorporates the radially arrayed first, second, third and fourth horn throat sections, respectively. The geometry of each horn throat is defined by combined surfaces of the sound chamber and cover, when coaxially nested together. The one driver modular siren loudspeaker can be characterized as having a folded sound path beginning proximate the diaphragm and ending in front of the horn mouth. The sound chamber horn throats form parallel parts of the sound path. Four diaphragm exits or slots penetrate the sound chamber at the closed end, providing fluid communication between the interior surface and the exterior surface. The sound chamber horn throats begin at the forward, closed end with respective transversely opening diaphragm exits or slots disposed around an integral phase plug that is spaced from the diaphragm. The horn throats diverge transversely or radially from the front, center portion of the of the sound chamber, turning rearwardly and following the contour of the sound chamber""s cup-shaped exterior surface. The four horn throats terminate in an open space proximate the rear wall of the cup-shaped housing. The sound path is effectively recombined at the terminus of the four horn throats and the recombined pressure wave is reflected forwardly, redirected by 180 degrees. The sound path continues in the annular space between the exterior of the cover and the interior of the housing horn mouth toward the front of the housing, exiting into free space through the horn mouth.
A second, two driver embodiment of the modular loudspeaker (hereinafter, the two driver embodiment) involves an alternate assembly method placing two drivers with magnet assemblies back-to-back, so that a first sound chamber""s horn mouth exit apertures face a second sound chamber""s horn mouth exit apertures. The resulting assembly yields eight paths from two sound chambers which blend into one coherent sound source, when placed in a housing adapted to accommodate two drivers. The back-to-back drivers are inserted in a larger bowl shaped back cover or housing having a first open end, a substantially frustoconical sidewall and a closed back end of only slightly smaller diameter, the housing being nearly cylindrical. A front cover is tightly fitted over the front facing sound chamber. The magnet assemblies, being back-to-back, are not received within a fitted heat sink incorporated into the housing, as for the one driver embodiment.
Both the one driver and two driver embodiments are preferably fabricated from diecast aluminum. The one driver embodiment preferably utilizes a Neodymium magnetic circuit for reduced size. Alternatively, the magnetic circuit could be fabricated from conventional Alnico or Ferrite material for applications where size is not an important issue. Aluminum was selected to provide thermal conductive properties for the temperature sensitive Neodymium magnet material. The use of Alnico or Ferrite materials allow the modular loudspeaker to be fabricated from plastic or other cast or molded materials having reduced thermal conductivity.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings, wherein like reference numerals in the various figures are utilized to designate like components.