Acoustic noise reduction headset

Abstract: A module adapted for use in an active noise reduction headset includes an enclosure having walls separating the interior of the enclosure from the exterior of the enclosure. A driver having a diaphragm is mounted in one of the walls. The driver radiates sound waves that cancel ambient noise. A port, having a resonance, connects the interior and the exterior. A resonance damper damps the port resonance.

Description: The invention relates to active noise reduction headsets. For background, reference is made to U.S. Patent Nos. 4,644,581, 4,922,542 and 5,305,387. JP-A-04227396 discloses a headphone consisting of an acoustic transducer, a case and ducts. EP-A-0688143 discloses a supra aural headphone, comprising an earphone with a shell and a cushion. It is an important object of the invention to provide an improved active noise reduction headphone. According to the invention there is provided an active noise reduction headsest as defined in claim 1. Other features, objects and advantages will become apparent from the following detailed description, which refers to the following drawings in which: FIG. 1 is a plan view of a noise reduction headset; FIGS. 2A-2C are diagrammatic cross-sectional views of earcups employing the invention; FIGS. 3A and 3B are perspective views of a module according to the invention; FIGS. 4A-4D are cross sectional views of alternate embodiments of a module according to the invention; and FIG. 5 is a plot of driver response vs. frequency for the modules of FIGS. 4A-4D. With reference now to the drawings and more particularly to FIG. 1, there is shown an active noise reduction headset according to the invention. A headband 56 is attached to two earcup assemblies 58. Each of the earcup assemblies 58 includes an earcup portion 50 and a conformable pad 54. The headband can be one of many types, including headbands in which the length of the headband or the position of the earcup assemblies on the headband can be adjusted. The connection between the headband 56 and the earcup assemblies may allow for adjustment along one or more axes relative to the headband. In use, the headband 56 holds the earcup assemblies 58 over the user's ears, either enclosing the ear (circumaural) or over the ear (supraural) . Referring to FIGS. 2A-2C, there are shown diagrammatic cross-sectional views of the earcup assembly 58. In FIG. 2A, earcup shell 50 encloses a front cavity 52. Conformable pads 54 seal against the side of the user's head so that they seal a user's ears from a substantial portion of the ambient noise. Front cavity 52 encloses active noise reduction module 10. Active noise reduction module 10 includes driver 18 which includes sound wave radiating diaphragm 21. Passing through earcup shell 50 are port 16 and acoustically resistive opening 17 in parallel which will be described in more detail below. In FIGS. 2B and 2C, passageway 19 connects port 16 and resistive opening 17 to the ambient environment. The dimensions of the passageway 19 are such that the passageway acts neither as a port nor a waveguide for sound waves radiated by diaphragm 21; the acoustic effect is the same as if the port 16 and the resistive opening 17 both connect directly to the ambient environment as in FIG. 2A. In front cavity 52 may be placed damping material 53 (for example, open cell foam) to reduce resonances in the front cavity and assist in passive attenuation. An embodiment as in FIGS. 2B and 2C is advantageous, because it allows flexibility in the placement of the active noise reduction module, and it makes the active noise reduction module 10 implementable in a variety of different earcups with minimal modification to the earcup. In operation, microphone 22 measures sound pressure. Circuitry (not shown) compares sound pressure measured by microphone 22 with sound pressure due to sound waves radiated by diaphragm 21 to detect ambient noise, and transmits to driver 18 signals which cause diaphragm 21 to radiate sound patterns similar to the ambient noise, but out of phase with ambient noise, thereby significantly attenuating it. Referring to FIGS. 3A-3B, there are shown views of active noise reduction module 10. An enclosure 12 encloses a back cavity 14 (FIGS. 4A-4D). Passing through a wall of enclosure 12 are port 16 and acoustically resistive opening 17. Mounted in a surface of enclosure 12 is driver 18. Microphone 22 is mounted on the exterior of enclosure 12. In this embodiment, microphone 22 is mounted at a position approximately at the edge of diaphragm 21, with the pickup surface facing radially inwardly, that is such that a line perpendicular to the pickup surface is approximately orthogonal to the axis 24 of driver 18. In one embodiment of the invention, enclosure 12 is generally cylindrical, with driver 18 being approximately 42 mm in diameter and mounted in one of the planar surface of the cylinder. Back cavity 14 enclosed by enclosure 12 has a volume of typically approximately 10 to 20 cubic centimeters, and front cavity 52 typically has a volume of about 100-200 cubic centimeters. The acoustic mass of port 16 and compliance of back cavity 14 are typically tuned to a frequency of about 300 Hz. Acoustically resistive opening 17 provides an acoustic resistance of about 1 x 16 7 ohms. Resistive opening 17 may be an opening 17a with an acoustically resistive covering 17b, or may be an opening filled with acoustically resistive material. Referring now to FIGS. 4A-4D, there are shown the active noise reduction module 10 with certain elements omitted or modified to better explain the invention. FIGS. 4B-4D will be used below to explain the effect of some of the features of the embodiment of FIG. 4A. In FIG. 4B, port 16 and resistive opening 17 are omitted. In FIG. 4C, resistive opening 17 is omitted. In FIG. 4D, resistive opening 17 is omitted and acoustically resistive material 16a is inserted in port 16. (Alternatively, the openings of port 16 may be covered with an acoustically resistive material.) In the embodiment of FIG. 4D, the acoustic resistance of the acoustic material 16a connects back cavity 14 with the ambient environment in series with the mass of the air in port 16. In the embodiment of FIG. 4A, the acoustic resistance of the resistive opening 17 connects back cavity 14 with ambient environment in parallel with the mass of the air in port 16. Referring now to FIG. 5, there is shown plots of a computer simulation of the response of driver 18 as a function of frequency, illustrating the effect of various elements of the embodiment of FIG. 3A. Curve 24 represents the response of driver 18 in the embodiment of FIG. 4B. Curve 26 represents the response of driver 18 in the embodiment as shown in FIG. 4C. As compared with curve 24, curve 26 shows better response at lower frequencies, but also has low output at the port resonance frequency, in this case 300 Hz. Curve 28 represents the response of driver 18 in the embodiment of FIG. 4D. As compared with curve 26, curve 28 shows a smoother frequency response and improved response at low frequencies. Curve 30 represents the response of the embodiment of FIG. 4A. As compared with curves 24, 26 and 28, curve 30 shows smooth frequency response and improved response at low frequencies. An headset according to the invention is advantageous over conventional headsets because the port and resistive path configuration of the earcup allows active attenuation to be attained with a significantly smaller back cavity. This feature, in turn, allows for a given earcup size, a larger front cavity, which improves passive attenuation. The smooth frequency response facilitates using simpler electronic circuitry, and combined with the more efficient operation of the driver, allows for significantly reduced power consumption, a particular advantage in battery-powered active noise reduction headsets.

Claims: An active noise reduction headset comprising: an earcup (58) ; and an active noise reduction module (10) comprising, an enclosure (12) having walls separating an interior of said enclosure (12) from the exterior of the headset, a driver (18) with a diaphragm (21), a resonance damper comprising an acoustically resistive opening (17) connecting said interior and said exterior of said headset, and a microphone (22) having a pickup surface, said microphone (22) being mounted at approximately an outer edge of said diaphragm (21), and characterised by the earcup (58) being a circumaural earcup, a port (16) connecting said interior and said exterior, said port (16) having a mass that coacts with the compliance of the enclosure (12) to establish a resonant circuit having a resonance at a given resonant frequency, and said acoustically resistive opening (17) of said resonance damper being coupled to said resonant circuit. An active noise reduction headset in accordance with claim 1, wherein said port (16) and said acoustically resistive opening (17) connect said interior and said exterior of the headset in parallel. An active noise reduction headset in accordance with claim 1, wherein said port (16) and said acoustically resistive opening (17) connect said interior and said exterior of the headset in series. An active noise reduction headset in accordance with claim 1, wherein the pickup surface of said microphone (22) is oriented perpendicularly to the axis of said driver (18). An active noise reduction headset in accordance with claim 1, wherein the pickup surface of said microphone (22) faces radially inward. An active noise reduction headset in accordance with claim 1, wherein said interior has a volume of less than approximately 20cm 3 .

Inventors: SAPIEJEWSKI ROMAN, SAPIEJEWSKI, ROMAN, Sapiejewski, Roman c/o Bose Corporation
Applicants: BOSE CORP, BOSE CORPORATION
Application year: 1998
Main classification: H04R1
IPC codes: H04R5, H04R1