Patent Grant
7194094
N/A
This invention relates to sound masking systems and, in particular, to sound masking systems for open plan offices.
Freedom from distraction is an important consideration for workers' satisfaction with their office environment. In a conventional enclosed office with full height partitions and doors, any speech sound intruding from outside the office is attenuated or inhibited by the noise reduction (NR) qualities of the wall and ceiling construction. Background noise, such as from the building heating or ventilating (HVAC) system, typically masks or covers up residual speech sound actually entering the office. Under normal circumstances, even very low levels of background nose reduce audibility of the residual speech to a sufficiently low level that the office worker is unable to understand more than an occasional word or sentence from outside and is, therefore, not distracted by the presence of colleagues' speech. In fact, it was shown more than 35 years ago that a standardized objective measure of speech intelligibility called the Articulation Index, or AI, reliably predicts most peoples' satisfaction with their freedom from distraction in the office. “Perfect” intelligibility corresponds to an AI of 1.0, while “perfect” privacy corresponds to an AI of 0.0. Generally, office workers are satisfied with their privacy conditions if the AI of intruding speech is 0.20 or less, a range referred to as “normal privacy” or better.
In recent years, the “open plan” type of office design has become increasingly popular. The open plan design includes partial height partitions and open doorways between adjacent workstations. Due to its obvious flexibility in layout and its advantages in enhancing communication between co-workers, the open plan office design is increasingly popular. However, despite the advantages of the open plan type office, unwanted speech from a talker in a nearby workstation is readily transmitted to unintended listeners in nearby workstation areas.
To reduce the level of unwanted speech in open plan offices, some limited acoustical measures can be employed. For example, highly sound absorptive ceilings reflect less speech, higher partitions attenuate direct path sound signals, particularly for seated workers, and higher partitions also diffract less sound energy over their tops. Additionally, the open doorways can be placed so that no direct path exists for sound transmission directly from workstation to workstation, and the interiors of workstations can be treated with sound absorptive panels. Nevertheless, even in an acoustically well designed open office, the sound level of intruding speech is substantially greater than in an enclosed office space. One other important method that can be used to obtain the normal privacy goal of 0.20 AI in an open plan office is to raise the level of background sound, usually by an electronic sound masking system.
Conventional sound masking systems typically comprise four main components: an electronic random noise generator, an equalizer or spectrum shaper, a power amplifier, and a network of loudspeakers distributed above the office, usually in the ceiling plenum. The equalizer adjusts the white noise spectrum provided by the electronic random noise generator to compensate for the frequency dependent acoustical filtering characteristics of the ceiling and plenum and to obtain the sound masking spectrum shape desired by the designer. The power amplifier raises the signal voltage to permit distribution to the loudspeakers without unacceptable loss in the network lines and ceiling tiles. The generator, equalizer, and power amplifier may be integrated with a speaker or may be located at a central location connected to the loudspeaker distribution network.
The goal of any sound masking system is to mask the intruding speech with a bland, characterless but continuous type of sound that does not call attention to itself. The ideal masking sound fades into the background, transmitting no obvious information. The quality of the masking sound of all currently sold devices is subjectively similar to that of natural random air turbulence noise generated by air movement in a well-designed heating and ventilating system. By contrast, if it has any readily identifiable or unnatural characteristics such as “rumble,” “hiss,” or tones, or if it exhibits obvious temporal variations of any type, it readily becomes a source of annoyance itself.
Obtaining the correct level or volume of the masking sound also is critical. The volume of sound needed may be relatively low intensity if the intervening office construction, such as airtight full height walls, provides a high NR. However, the volume of the masking sound must be a relatively high intensity if the construction NR is reduced by partial-height intervening partitions, an acoustically poor design or layout, or materials that have a high acoustic reflectivity. Even in an acoustically well designed open office, the level of masking noise necessary to meet privacy goals may be judged uncomfortable by some individuals, especially those with certain hearing impairments. However, if the masking sound has a sufficiently neutral, unobtrusive spectrum of the right shape, the intensity of the masking sound can be raised to a sound level or volume nearly equal to that of the intruding speech itself, effectively masking it, without becoming objectionable.
Subjective spatial quality is another important attribute of sound masking systems. The masking sound, like most other natural sources of random noise, must be subjectively diffuse in quality in order to be judged unobtrusive. Naturally generated air noise from an HVAC system typically is radiated by many spatially separated turbulent eddies generated at the system terminal devices or diffusers. This spatial distribution of sources imparts a desirable diffuse and natural quality to the sound. In contrast, even if a masking system provides an ideal spectrum shape and sound level, its quality will be unpleasantly “canned” or colored subjectively if it is radiated from a single loudspeaker or location. A multiplicity of spatially separated loudspeakers radiating the sound in a reverberant (sound reflective) plenum normally is typically used in order to provide this diffuse quality of sound. Almost all plenums use non-reflective ceiling materials and fireproofing materials and require two or more channels radiating different (incoherent) sound from adjacent loudspeakers in order to obtain the required degree of diffusivity. Each loudspeaker normally serves a masking zone of about 100–200 square feet each (i.e. placed on 10′ to 14′ centers). In most cases, the plenum space above the ceiling is an air-return plenum so that the loudspeaker network cable must be enclosed in metal conduit or use special plenum-rated cable in order to meet fire code requirements.
A typical system diffuses the acoustic sound masking signal by placing the loudspeakers in the plenum space facing upward to reflect the acoustic masking signal off the hard deck. As a result, direct path energy from the location of a loudspeaker to the ear of the listener is intentionally minimized by the acoustic sound masking signal that propagates substantially throughout the above ceiling volume and filters down through the ceiling and ceiling elements such as light fixtures, mechanical system grilles, return air openings, etc., at locations somewhat removed from the loudspeaker location. The effectiveness of this approach to diffusion depends on several characteristics. These include the directivity characteristics of the loudspeakers, elements in the plenum such as mechanical system ducts, and on the physical characteristics of the ceiling material itself, such as its density and upper surface acoustical absorption. Costly measures are sometimes needed to improve the uniformity and diffuseness of the masking sound. Some of these measures include employing special vertically directional baffles for the loudspeakers to spread the sound horizontally and coating the upper surface of the ceiling tile with special foils to further spread out the masking sound horizontally. In high density ceilings with large openings for HVAC return air, specially designed acoustical grill “boots” are often necessary to avoid excessive concentration of masking sound, or “hot spots.”
In addition, the sound attenuation characteristics of the ceiling assembly are normally not knowable until after installation and testing. Since masking system loudspeakers are normally installed before the ceiling for reasons of access and economy costly adjustable frequency equalization for the masking sound must be provided to compensate for these site-specific characteristics. Thus, additional time and cost are incurred due to the testing and frequency adjustment that must be performed post installation.
Also, because the acoustic sound masking signal must pass through the acoustical ceiling and be attenuated thereby, a large part of the acoustical power radiated by the loudspeakers is wasted in the form of heat as the acoustic masking signal is attenuated. Accordingly, despite the requirement for only very small amounts of acoustical sound masking power within the listening space itself, relatively high power electrical signals driving large and costly loudspeakers are needed to provide the necessary masking signal strength. Due to the power required, the loudspeaker assemblies are normally large and heavy. Thus, in addition to the costs incurred by the larger amount of power required, the loudspeaker and its enclosure must be supported from additional structure rather than directly by the ceiling tile in order to avoid sagging of the lightweight ceiling material. This additional support structure increases the installation cost, and the placement of the large loudspeakers in the plenum area inhibits access to the above ceiling space, which also complicates the design and installation of the loudspeakers.
Masking loudspeakers sometimes have been installed below higher ceilings, or within the ceiling, in order to overcome some of these limitations. However, their use has been restricted to installation in facilities with atypically high ceiling heights due to appearance, masking sound uniformity, an overly small or crowded plenum area, and cost considerations. When a conventional loudspeaker is attempted below a ceiling in a more typical office environment with ceiling heights of 9′–12′, or within the ceiling, the uniformity of masking sound is found to be unacceptable. In particular, conventional loudspeakers exhibit a narrow beamwidth at higher frequencies, causing “hot-spotting” on their axes. Unlike music or other time varying signals, masking sound has essentially constant bandwidth temporally, and any significant narrowing of beamwidth within the acoustic band is immediately obvious and unpleasant to most individuals. Moreover, unless loudspeakers are mounted within several feet of one another, overall level uniformity is unacceptable due to square law or distance spreading, that is, the sound level attenuates unacceptably with distance from the loudspeaker, drawing attention to its location. This close loudspeaker proximity is unsightly and uneconomic. Thus, in these systems an unacceptable number of these conventional loudspeakers are required to avoid hot-spotting and signal non-uniformity within a masking zone.
Sound masking spectra normally used in open plan offices are well documented. For example, see L. L. Beranek, “Sound and Vibration Control”, McGraw-Hill, 1971, page 593. These spectra were empirically derived over a period of a number of years and are characterized by relatively high levels of sound at lower speech frequencies and by relatively low levels of sound at the higher speech frequencies. Such spectra have been found to provide both effective masking of speech sound intruding into an office and unobtrusive quality of masking sound when used in a typical office with sufficiently high partial height office partitions that act as acoustical barriers between work stations, particularly at high frequencies. These spectra have also been found to work adequately in some other office settings with sufficient high frequency inter-office speech attenuation.
The masking sound level considered unobtrusive by most open office occupants is approximately 48 dBA sound pressure level. As masking levels are increased above 48 dBA, complaints of excessive masking sound increase. Unfortunately, it can be shown that this level of sound with the typically used spectrum is largely ineffective for sound masking in an office setting without significant acoustical barriers to reduce high frequencies of intruding speech sound. If barriers are low or absent, the required distance between workstations to obtain normal speech privacy conditions may exceed 20 feet or more, even with a high quality sound masking system using a typical sound masking spectrum.
Therefore, it would be advantageous to provide a sound masking system that is easier to install, requires fewer adjustments, requires fewer components than the conventional sound masking systems, and provides more privacy in an open plan office.
A sound masking system according to the invention is disclosed in which one or more sound masking loudspeaker assemblies are coupled to one or more electronic sound masking signal generators. The loudspeaker assemblies in the system of the invention have a low directivity index and preferably emit an acoustic sound masking signal that has a sound masking spectrum specifically designed to provide superior sound masking in an open plan office. Each of the plurality of loudspeaker assemblies is oriented to provide the acoustic sound masking signal in a direct path into the predetermined area in which masking sound is needed. In addition, the sound masking system of the invention can include a remote control function by which a user can select from a plurality of stored sets of information for providing from a recipient loudspeaker assembly an acoustic sound masking signal having a selected sound masking spectrum.
In one embodiment, a direct field sound making system provides a direct path sound masking signal into a predetermined area of a building. The direct field sound masking system includes a sound masking signal generator that provides two or more sound masking signals that are mutually incoherent, and a plurality of loudspeaker assemblies coupled to the sound masking signal generator. Each loudspeaker assembly receives the sound masking signal from the sound masking signal generator and produces an acoustic sound masking signal corresponding to the received sound masking signal. Each of the loudspeaker assemblies has a low directivity index, and is oriented to provide the acoustic sound masking signal in a direct path into the predetermined area.
The acoustic sound masking signal can have a predefined spectrum that is defined in terms of intensity at certain frequencies and in certain frequency bands. In one embodiment, the acoustic spectrum has a roll off in intensity of in the range of 2–4 dB between 800–1600 Hz, between 3–6 dB between 1600–3200 Hz, and between 4–7 Hz between 3200–6000 Hz.
In another embodiment, a sound making system for providing a sound masking signal to a predetermined area of a building is disclosed that includes a sound masking signal generator. The sound masking signal generator provides two or more sound masking signal channels of mutually incoherent electrical sound masking signals corresponding to a selected one of a plurality of stored sound masking spectra. A plurality of loudspeaker assemblies are coupled to the sound masking signal generator and receive the electrical sound masking signal therefrom. Each of the plurality of loudspeaker assemblies emits an acoustic sound masking signal corresponding to the electrical sound masking signal. The acoustic sound masking signal has a sound masking spectrum that corresponds to the selected spectrum. A remote control unit is provided and is remotely linked to the masking signal generator via an infrared, radio frequency, ultrasonic, or other signal and provides commands and data to the masking signal generator. In one embodiment, the remote control can be used to select one of a plurality of predetermined sound masking spectra that was stored as sets of information within the masking signal generator for providing from a recipient loudspeaker assembly an acoustic sound masking signal having the selected sound masking spectrum that are stored in the the sound masking signal generator. One of the stored plurality of sets of information is selected and used to provide the one or more electrical sound masking signals
The data and commands can be used to adjust a frequency component of the selected sound masking spectrum, select another of the plurality of stored spectra, or provide other functions such as power on/off.
Other features, aspects, and advantages of the above-described method and system will be apparent from the detailed description of the invention that follows.
The invention will be more fully understood by reference to the following Detailed Description of the Invention in conjunction with the accompanying Drawings of which:
a is a plan view of an office space incorporating effective acoustic barriers between adjacent workstation spaces;
b is a plan view of an office space incorporating short acoustic barriers between adjacent workstation areas;
c is a plan view of an open office space, i.e., an office incorporating no acoustic barriers between adjacent workstation areas;
a is a schematic view of one embodiment of a sound masking system in accordance with the present invention;
b is a schematic view of another embodiment of a sound masking system in accordance with the present invention;
In a sound masking system according to the invention, one or more sound masking loudspeaker assemblies are coupled to one or more electronic sound masking signal generators. The loudspeaker assemblies in the system of the invention have a low directivity index and, preferably, emit an acoustic sound masking signal that has a sound masking spectrum specifically designed to provide superior sound masking in an open plan office. Each of the plurality of loudspeaker assemblies is oriented to provide the acoustic sound masking signal in a direct path into the predetermined area in which masking sound is needed. In addition, the sound masking system of the invention can include a remote control function by which a user can select one of a plurality of stored sets of information for providing from a recipient loudspeaker assembly an acoustic sound masking signal having a selected sound masking spectrum stored in the sound masking signal generator. One of the stored plurality of sets of information is selected and used to provide the one or more electrical sound masking signals. The remote control unit can further be used to control the intensity of at least one frequency component of the selected sound masking spectrum by selecting another one of the stored sets of information. The system of the invention will be more fully explained in the following description of the typical office environment in which the system of the invention can be employed.
a depicts an open plan office 102 that includes first and second office spaces 108 and 110 having a ceiling 106 and a plenum 104. A divider 112, which is placed between the first and second office spaces 108 and 110, extends from the floor to a height that is sufficient to block direct path speech from the adjacent office space, regardless of whether a talker is sitting or standing. As used herein, a talker is a person speaking and a listener is a person, whether intended or not, who is capable of hearing the speech of the talker. Some speech from a talker in office space 108 will leak into the adjacent office space 110. For example, if the divider partition does not extend to the ceiling 106, a speech path 114a and 114b from a standing or sitting talker, respectively, is diffracted over the top of the divider 112, resulting in a diffracted speech path 116 entering the office space 110 from office space 108. Additionally, the noise reduction (“NR”) rating of the divider may be less than 100% so that some of the speech 118a and 118b will be attenuated but still passed as sound 120a and 120b into the office space 110 from the adjacent office space 108. Furthermore, speech reflected from the ceiling and modified by the reflective characteristics of the ceiling is received by a listener in the adjacent office space. The combined effect of the divider characteristic and the resulting allowable acoustic paths is to significantly reduce the high frequency content of the speech spectrum received by the listener relative to the low frequency content.
b depicts an office space 125 that is designed using an open plan office system. In particular, the office space 125 includes a first office space 124 and a second office space 126, which are divided by a divider 128, which is much shorter than the divider 112 in
c depicts a completely open office area 141 with no acoustic barriers between workers. Office area 141 could also be considered as a reception area in a pharmacy or doctor's office in which privacy of people at a reception desk is at issue. In office area 141 there are no individual office spaces, and direct speech paths 142, 144, and 145 exist between individuals. In addition, reflected speech paths 146–148 and 150–152 exist between the individuals as well. In this configuration, the reflected speech paths have little impact and the high frequency content of the received speech spectrum is not reduced at all relative to the low frequency content.
As used herein, the following terms have associated therewith the following definitions. A “direct field sound masking system” is one in which the acoustic sound masking signal or signals, propagating in a direct audio path from one or more emitters, dominate over reflected and/or diffracted acoustic sound masking signals in a particular area referred to as a masking zone. A “direct audio path” is a path in which the acoustic masking signals are not reflected or diffracted by objects or surfaces and are not transmitted through acoustically absorbent surfaces within a masking area or zone. A “reverberant field sound masking system” is one in which the acoustic sound masking signal or signals, propagating in a reflected path from one or more emitters, dominate over direct audio path acoustic sound masking signals in a particular area referred to as a masking zone. A “transition region” is a region in which one or more reflected acoustic sound masking signals from one or more emitters begin to dominate over one or more direct path acoustic sound masking signals from one or more emitters within a region. The location of the transition region relative to one or more emitters is a function of the intensity and directivity of the emitted sound and the emitter, respectively, and of the characteristics of the surface and materials that comprise the reflecting surfaces.
As discussed above, an open plan office often has a sound masking system to compensate for the increased level of sounds that leak between adjacent workstation areas. The sound masking system typically includes a masking signal generator that typically provides two or more mutually incoherent signal channels of sound masking signals to one or more emitters, which typically are loudspeaker assemblies, that emit an acoustic sound masking signal that has a predetermined sound masking spectrum. These emitters are configured and oriented so as to provide a sound masking field that passes through the ceiling tiles, or a reverberant sound masking field such that the acoustic sound masking signals that comprise the sound masking field have as uniform an intensity as possible and as diffuse a field as possible.
Therefore, sound masking systems according to the invention most preferably use a spectrum of the shape of spectrum 204 as depicted in
The spectrum 204 is defined by the roll off in sound intensity within the approximately two and two-thirds octaves within the 800–5000 Hz band. In particular, for the 800–1600 Hz octave, the roll off in attenuation can be between 2–4 dB. For the 1600–3200 Hz octave, the roll off in attenuation can be between 3–6 dB. For the 3200–5000 Hz partial octave, the roll off in attenuation can be between 3–5 dB. Below the 800 Hz frequency, between 200–500 Hz, the spectrum can have a roll off of between 0–2 dB, and between 500–800 Hz, there is approximately a 1–4 dB decline in intensity. Above 5000 Hz, there can be approximately a 3–7 dB roll off between 5000–8000 Hz. Thus, the sound masking spectrum 204 depicted in
It should be appreciated that the intensity of the lowest frequency of the sound masking spectrum described as curve 204 can be arbitrarily set without affecting the shape of the curve. The chosen intensity of the lowest frequency of the sound masking spectrum is a matter of design choice and is selected based on the acoustic characteristics of the area to be masked and the level of ambient background noise.
In some circumstances in the embodiments described herein, it may be advantageous to provide a method of adjusting the sound masking spectrum in order to properly tailor the sound masking spectrum to the particular area to be masked. Often, the masking signal generator is not easily accessible physically after installation, making any post-installation adjustments directly to the masking signal generator difficult and/or time consuming and costly. The sound masking system, according to the invention preferably, is provided with a remote control unit that uses, e.g., infrared, radio frequency, ultrasonic, or other signals to transmit data and commands to a complementary receiver coupled to the masking signal generator. The remote control unit can be used to select one of a plurality of predetermined sound masking spectra that are stored as sets of information in the masking signal generator for providing from a recipient loudspeaker assembly an acoustic sound masking signal having the selected spectrum. This allows a user to select the sound masking spectrum that provides the best AI performance for a specified office design for the space of interest. Alternatively, the remote control unit can act as a remote frequency equalizer and can be used to instruct the masking signal generator to individually adjust the resultant intensity of one or more frequency bands of the currently implemented sound masking spectrum to provide for example, an improved subjective sound masking quality without significantly affecting the achieved AI. Other uses of the remote control unit could include a power on/off function, a volume control function, a signal channel select function, or a sound masking zone select function.
In the embodiments described herein, the loudspeaker assemblies include at least one loudspeaker that has a low directivity index. Referring to
One method of achieving a loudspeaker with a low directivity index is to have the diameter of the effective aperture of emitter 306 less than or equal to the wavelength of the highest frequency of interest in the sound masking spectrum. Such a low directivity index is most easily achieved when the speaker output of each of the loudspeaker assemblies has an effective aperture area that is equal to the area of a circle of an diameter of between 1.25″ and 3″. In a preferred embodiment, the diameter of the effective aperture of the emitter 306 is 1.25″. This diameter of the effective aperture of emitter 306 provides an emitter with an axial directional index at 3000 Hz that is less than 1 dB greater than an infinitesimally small sound source and an axial directional index at 6000 Hz that is less than 3 dB greater than an infinitesimally small sound source. Another method to achieve a loudspeaker with a low directivity index is to place a small reflector in front of the loudspeaker aperture to scatter the high frequency sounds to the sides of the loudspeaker and preventing the high frequency sounds from being axially projected by the loudspeaker. The small effective aperture of the emitter 306 also allows extending the low frequency response in the small airtight enclosure 301 due to the minimization of the mechanical stiffness of the cavity air spring.
a depicts one embodiment of a direct field sound masking system according to the present invention.
Typically when used with the loudspeaker assembly depicted in
In some circumstances, phase effects due to constructive and destructive interference between the acoustic sound masking signals emitted by two or more loudspeaker assemblies may occur. To substantially eliminate this problem, the masking signal generator 401 can produce two or more channels of mutually incoherent sound masking signals. The masking signal generator can be placed in a convenient location such as an equipment room, or the masking signal generator can be secured to a wall, the lower surface of the ceiling and within the office area 402, or the upper surface of the ceiling 404 and within the plenum area 406. The masking signal generator will typically include two or more power amplifiers that are sized according to the number of loudspeaker assemblies that are to be driven with the electrical sound masking signal.
Alternatively,
The masking signal generator can be placed in a convenient location such as an equipment room, or the masking signal generator can be placed adjacent to an emitter assembly and secured to the post or support 436. The sizing of power amplifiers that may be included with the masking signal generator is the same as discussed above with respect to
The advantages of the direct path sound masking systems described herein are primarily in the installation and setup of the sound masking system. In particular, the use of a direct path sound masking system eliminates the need for site specific frequency equalization and spectrum testing. In addition, no combustible, smoke generating, or flame spreading material is introduced into the plenum area. The advantages of the small size and weight of the loudspeaker assemblies 410 or 434 are many. The reduced high frequency beaming and reduced overall cost of the loudspeakers allows more loudspeaker assemblies to be used for a given cost. This permits a higher density of loudspeakers within the overall loudspeaker constellation. In addition, the use of more and smaller loudspeakers reduces the overall power required by each individual loudspeaker, reducing the overall power consumption and improving the overall energy efficiency.
It should be appreciated that a direct field sound masking system of the type described herein can utilize a combination of the ceiling mounted and pole mounted loudspeaker assemblies. The selection of the numbers, the locations and overall constellation of loudspeaker assemblies is a design choice and is a function of the configuration of the particular area to be masked.
In the discussion of
In the embodiment depicted in
In the embodiment depicted in
In the embodiment depicted in
It should be appreciated that other variations to and modifications of the above-described sound masking systems for masking sound within an open plan office may be made without departing from the inventive concepts described herein. For example, the connection between the masking signal generator and the loudspeaker assemblies does not have to be a physical connection via a conductor. Other forms of analog or digital transmission such as infrared, radio frequency, or ultrasonic signals can be used in multiplex system to provide multiple signal channels to one or more sets of loudspeaker assemblies. The receiving loudspeaker assemblies would require additional components to receive and process the transmitted signals. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.
The present application claims priority under 35 USC §119(e) of U.S. Provisional Application No. 60/345,362 titled Sound Masking System, filed Oct. 24, 2001, which is hereby incorporated by reference herein.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3298455 | Kodaras et al. | Jan 1967 | A |
| 3497623 | Todd | Feb 1970 | A |
| 3849679 | Massa | Nov 1974 | A |
| 3879578 | Wildi | Apr 1975 | A |
| 3985200 | Sepmeyer | Oct 1976 | A |
| 3985957 | Torn | Oct 1976 | A |
| 4010324 | Jarvis et al. | Mar 1977 | A |
| 4024535 | Goldstein | May 1977 | A |
| 4052564 | Propst et al. | Oct 1977 | A |
| 4052720 | McGregor et al. | Oct 1977 | A |
| 4054751 | Calder et al. | Oct 1977 | A |
| 4059726 | Watters et al. | Nov 1977 | A |
| 4185167 | Cunningham et al. | Jan 1980 | A |
| 4280019 | Propst et al. | Jul 1981 | A |
| 4319088 | Orfield | Mar 1982 | A |
| 4390748 | Zwicker | Jun 1983 | A |
| 4438526 | Thomalla | Mar 1984 | A |
| 4450321 | Quigley et al. | May 1984 | A |
| 4476572 | Horrall et al. | Oct 1984 | A |
| 4622691 | Tokumo et al. | Nov 1986 | A |
| 4661982 | Kitazato et al. | Apr 1987 | A |
| 4674124 | Horrall et al. | Jun 1987 | A |
| 4686693 | Ritter | Aug 1987 | A |
| 4914706 | Krause | Apr 1990 | A |
| 5167236 | Junker | Dec 1992 | A |
| 5192342 | Baron et al. | Mar 1993 | A |
| 5360469 | Baron et al. | Nov 1994 | A |
| 6310440 | Lansing et al. | Oct 2001 | B1 |
| 20010021259 | Horrall | Sep 2001 | A1 |
| 20020150261 | Moeller et al. | Oct 2002 | A1 |
| 20030048910 | Roy et al. | Mar 2003 | A1 |
| Number | Date | Country |
|---|---|---|
| 1420201 | Jan 1976 | GB |
| Number | Date | Country | |
|---|---|---|---|
| 20030091199 A1 | May 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60345362 | Oct 2001 | US |
