Field of the Invention This invention generally relates to elevator systems. More particularly, this invention relates to elevator cab ceiling ventilation that has sound reduction characteristics.

Description of the Relevant Art An elevator cab ceiling typically includes a ventilation duct or channel that allows airflow between an elevator cab and a hoistway. A ventilation fan facilitates airflow within the ventilation channel. Traditionally, the ventilation channel is formed as a vertical duct that extends straight through the ceiling. Typically, the ventilation channel extends straight down from an upper opening at a top portion of the elevator cab to a lower opening in the ceiling within the elevator cab. An elevator machine includes a drive that operates a rope or belt system to move the elevator cab within the hoistway. Various noise sources such as the elevator machine, rope interaction with sheaves, rope vibration and radiation, and the ventilation fan generate noise that can be easily transmitted through the ventilation channel into the elevator cab. Such noise can disturb a passenger, and thus can be a detriment to perceived ride quality and comfort. The ventilation channel in the elevator ceiling is one of the main noise transmission paths. The typical ventilation channel provides a direct noise path into the elevator cab. One prior solution to this problem involved using long air ducts lined with acoustic absorptive materials, however acoustic absorptive materials can be expensive and difficult to install. Another solution has been to use an active noise control system, which utilizes a speaker, microphones, and a controller to actively monitor and cancel noise generated during elevator operation. Disadvantages with these prior solutions include a lack of system robustness, need for regular maintenance, increased manufacturing and installation complexity, and failure to fully address all frequency bands of interest. There is a need for an improved ventilation arrangement that provides reduced airborne noise transmission into an elevator cab. Disclosed embodiments of this invention utilize offset inlet and outlet ducts in combination with an intermediate ceiling ventilation cavity, which avoid the difficulties mentioned above.

SUMMARY OF THE INVENTION In general terms, this invention is an elevator cab ceiling that includes offset inlet and outlet ventilation ducts to reduce noise levels and improve ride quality. An example ceiling includes an upper ceiling panel and a lower ceiling panel spaced apart from each other with an intermediate cavity between them. An inlet duct portion is associated with the upper ceiling panel and a separate outlet duct portion is associated with the lower ceiling panel. The intermediate cavity fluidly connects the inlet duct portion and the outlet duct portion to form a ventilation path. The combination of separate inlet and outlet duct portions and the intermediate cavity reduces airborne noise transmissions that might otherwise enter an elevator cab through the ventilation path, which improves ride quality. In one example, the upper and lower ceiling panels are vertically spaced apart from each other to form the intermediate cavity. The inlet and outlet duct portions are horizontally spaced apart from each other and extend at least partially into the intermediate cavity. The inlet duct portion defines an inlet opening for air from an elevator hoistway and the outlet duct portion defines an outlet opening to direct air into an elevator cab. By horizontally spacing the inlet and outlet duct portions, the inlet and outlet openings are arranged in a non-overlapping relationship. In one example, at least one baffle is installed within the intermediate cavity between the inlet and outlet duct portions to further reduce noise. The baffle reduces noise by interrupting an acoustic transmission path within the intermediate cavity. A plurality of baffles can also be used with at least one baffle being supported by the upper ceiling panel and at least one baffle being supported by the lower ceiling panel. By alternating baffles between the upper and lower ceiling panels, a serpentine flow path is formed and noise reduction characteristics are enhanced. The elevator cab ceiling includes a unique ventilation channel that improves ride quality by reducing undesirable noise transmission into an elevator cab. The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates a side view of an elevator cab that has a two-panel ceiling designed according to an embodiment of this invention. Figure 2 is an isometric view of the elevator cab of Figure 1. Figure 3 is a graph of predicted noise reduction spectra comparing noise reduction for a traditional ventilation duct configuration and noise reduction for an elevator ceiling incorporating an embodiment of the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As seen in Figures 1 and 2, an elevator cab 10 includes a passenger compartment 12 defined by a floor 14, a pair of side walls 16, a back wall 18, a front wall 20, and a ceiling 22. An elevator machine (not shown) is used to move the elevator cab 10 within an elevator hoistway 24. The ceiling 22 includes a first ceiling panel 26 and a second ceiling panel 28. The first and second ceiling panels 26 and 28 are vertically spaced apart from each other and are positioned in an overlapping relationship. An intermediate ceiling cavity 30 exists between the ceiling panels 26 and 28. In this example, the ceiling panels 26 and 28 establish the walls of the cavity 30. In another example, a separate structure such as a large duct or channel is inserted between the ceiling panels 26 and 28. A first duct portion 32 is associated with the first ceiling panel 26 and a second duct portion 34 is associated with the second ceiling panel 28. The first and second duct portions 32, 34 are separated and offset from each other by being horizontally spaced apart from each other. Each example duct portion extends at least partially within the intermediate ceiling cavity 30. In the example shown in Figures 1 and 2, the first duct portion 32 includes an inlet opening that receives air from the elevator hoistway 24. The second duct portion 34 defines an outlet opening to direct air into the passenger compartment 12. By horizontally spacing the first and second duct portions 32, 34, the inlet and outlet openings are arranged in a non-overlapping relationship. The intermediate ceiling cavity 30 fluidly connects the first and second duct portions 32, 34 to form a ventilation path or channel. The first and second duct portions 32, 34 are fractional or partial length ducts. This means that the first and second duct portions 32, 34 each have a length that is only a fractional dimension of the overall height between the first and second ceiling panels 26, 28. In the illustrated example, the first and second ceiling panels 26, 28 are separated by a first height and the length of the example first and second duct portions 32, 34 is less than the first height. Thus, there is no continuous duct extending directly downward from the first ceiling panel 26 to the second ceiling panel 28 to form the ventilation channel. Instead a discontinuous or fractional channel is formed by separating the first and second duct portions 32, 34. This discontinuous or fractional configuration provides significant noise attenuation capability because noises originated in the hoistway 26 cannot follow a straight, uninterrupted path directly into the cab 12. The term "duct" as used in this description does not necessarily require a closed channel or a specific shape. The illustrated example includes generally rectangular ducts. Another example includes at least one duct wall positioned to deflect flow within the cavity 30 at least in the vicinity of the corresponding opening. To further reduce noise, baffles 40 are installed within the example intermediate ceiling cavity 30. In the example shown, the baffles 40 are positioned between the first and second duct portions 32, 34 to interrupt a flow path from the inlet to the outlet. The baffles 40 can be supported by either the first or second ceiling panels 26, 28. In the example shown, the baffles 40 are alternately mounted to the first and second ceiling panels 26, 28 to form a generally serpentine flow path, allowing airflow to change direction multiple times. As shown in Figure 2, the intermediate ceiling cavity 30 is defined by a height dimension H, a depth dimension D, and a width dimension W. The baffles 40 are shown as being longer in the direction of the depth dimension D than the corresponding dimension of the first and second duct portions 32, 34. This configuration ensures that airflow is directed as needed within the intermediate ceiling cavity 30. It should be understood that while only a few baffles 40 are shown in Figures 1 and 2, only one baffle 40 may be required, or additional baffles 40 may be required depending on the desired level of noise reduction. Those skilled in the art who have the benefit of the description will be able to configure baffles to meet their particular needs. Figure 3 shows a graph of predicted noise reduction spectra for a frequency range of approximately 0 to 4000 Hz extending along the x-axis. The magnitude of noise reduction is shown on the y-axis in decibels (dB). The noise reduction for a traditional ventilation duct configuration is indicated at 50 and the noise reduction for an elevator ceiling 22 incorporating an embodiment of the subject invention is shown at 60. The maximum noise reduction 50 for the traditional ventilation duct configuration never exceeds a magnitude of 30 dB while the minimum noise reduction for the elevator ceiling 22 incorporating an embodiment of the subject invention is at least 30 dB. Thus, the concept of using offset partial length ducts located at the inlet and outlet openings provides significant noise reduction capability when compared to the traditional ventilation configuration. The acoustic performance of this ventilation configuration can be increased by displacing the inlet and outlet openings within the intermediate ceiling cavity 30, and by adding baffles 40 located at selected positions within the intermediate ceiling cavity 30 to provide airborne noise reduction within an even wider frequency range. This configuration can be used in elevators of any duty, size, or speed. High speed and tighter hoistway elevator designs could especially benefit from this low-cost and simple method for reducing airborne noise transmission. Further, enhancements to noise reduction performance can be provided by adding acoustic absorption material and by increasing the thickness of the first and second ceiling panels 26, 28. The displacement of the inlet and outlet openings relative to each other provides high-frequency noise reduction by directing high frequency acoustic waves along the interrupted path within the cavity 30. The baffles 40 provide increased high frequency noise reduction due to acoustic wave directivity, and can be tailored to modify the modal characteristics of the intermediate ceiling cavity 30. The location of the inlet and outlet openings within the first and second ceiling panels 26, 28 can be determined by using the Boundary Element Method (BEM) model simulation. The operation of this model simulation is well-known in the art. Further, the partial first and second duct portions 32, 34 act as waveguides, attenuating oblique incident sound waves at lower frequencies, resulting in increased noise reduction. In addition, the location of the inlet and outlet openings, and the lengths of the first and second duct portions 32, 34, can be tuned to avoid exciting particular modal frequencies of the intermediate ceiling cavity 30. One advantage of the disclosed configuration is that all of these noise reduction enhancements can be incorporated into a standard two-panel ceiling without adding different materials to the construction and with only minor changes to existing manufacturing processes. The construction can also accommodate light fixtures, however, an extra wall may be required between the intermediate ceiling cavity 30 and a fixture enclosure (not shown). Current mechanical and electrical interfaces with the elevator cab 10 will not have to be modified. Thus, a simple, low-cost, and robust ventilation channel configuration is provided that significantly reduces airborne noise when compared with traditional configurations. The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this invention. The scope of legal protection given to this invention can only be determined by studying the following claims.