Abstract:
A personal sound masking system for use in an individual workspace provides an optimized acoustic background environment by delivering a sound masking signal that is specifically matched to the individual user&#39;s location and physical relationship to other nearby offices. The sound masking system employs multiple loudspeakers and multiple mutually incoherent channels in order to obtain a desired degree of diffuseness. A control module includes an erasable programmable read-only memory (EPROM) that stores data representing a number of samples of a masking signal segment, addressing logic that accesses the samples in the memory sequentially and repetitively to generate different series of data values each representing a different masking signal, digital to analog converters that convert the series of samples into analog masking signals, and power amplification circuitry that amplifies the analog masking signals to levels suitable for driving the loudspeakers. The sound masking system also includes a user-accessible volume control to enable the user to adjust the sound level to achieve optimum sound masking in his or her individual workspace.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) of U.S. provisional patent application No. 60/077,535, filed Mar. 11, 1998, entitled “Personal Sound Masking System”, the entire disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     It is well known that 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. Residual speech sound actually entering the office is normally masked or covered up by even very low levels of background noise, such as from the building heating or ventilating system. Under normal circumstances, the resulting speech audibility is sufficiently low 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&#39; 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, could be used to reliably predict most people&#39;s 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”. 
     In recent years, the open plan type of office design has become increasingly popular due to its obvious flexibility and communication advantages. In contrast to conventional closed offices, the open plan design has only partial height partitions and open doorways, and unwanted speech readily transmits from a talker to unintended listeners in adjacent offices. Limited acoustical measures can be employed to reduce the level of the resulting speech that is transmitted. Highly sound absorptive ceilings reflect less speech, and higher partitions diffract less sound energy over their tops. Additionally, doorways are placed so that no direct line of sight or sound transmission exists from office to office, and the interiors of offices are 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 most enclosed offices. In order to obtain the normal privacy goal of 0.20 AI, acousticians know that the level of background sound in the open office must be raised, usually by electronic sound masking systems. Indeed, a considerable proportion of larger contemporary open offices use electronic sound masking systems, sometimes called “white sound” systems. However, few smaller offices use such systems due to prohibitive costs. 
     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 throughout the office. The equalizer adjusts the spectrum to compensate for the frequency dependent acoustical filtering characteristics of the ceiling and plenum or air space above and to obtain the 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. The generator, equalizer, and power amplifier are typically located at a central location connected to the loudspeaker distribution network. A typical system uses loudspeakers serving about 100-200 square feet each (i.e. placed on 10′ to 14′ centers); the loudspeakers are usually concealed above an acoustical tile ceiling in the plenum space. In most cases, the plenum 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. 
     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 is subjectively similar to the natural random air turbulence noise generated by air movement in a well-designed heating and ventilating system. The overall shape of the masking spectrum is of paramount importance if the goal of unobtrusiveness is to be met. 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. However, if the sound has a sufficiently neutral, unobtrusive spectrum of the right shape, it can be raised, without becoming objectionable, to a sound level or volume nearly equal to that of the intruding speech itself, effectively masking it. 
     Although a distributed, ceiling mounted sound masking system has numerous advantages, such a system has significant disadvantages that interfere with the effectiveness of the system at the level of the individual office worker. For example, mechanical system ducts and other physical obstructions, as well as acoustical variations in the above-ceiling plenum and ceiling components such as vented light fixtures and air return grilles, pose significant challenges to the designer in achieving adequately uniform spectral quality. In many installations, cavity resonances in the plenum occur and cannot be completely ameliorated by equalization or other techniques. As a consequence, the acoustical spectrum obtainable at any one office worker location may be substantially compromised compared to the ideal spectrum desirable at his or her particular location. This non-ideal spectrum and spatial variation throughout the office places an effective upper limit on the effectiveness of the masking system. 
     Obtaining the correct level or volume of the masking sound also is critical. The volume of sound needed may be relatively low if the intervening office construction, such as airtight full height walls, provides high NR, but it must be relatively high in level if the construction NR is compromised by partial-height intervening partitions or acoustically poor design or materials. Even in an acoustically reasonably 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. Some systems use volume controls on each masking loudspeaker to permit their adjustment for good spatial uniformity. Even with this costly measure, variations in level of 3-6 dB throughout an office are typical. This amount of variation typically corresponds to differences in AI of 0.1 to 0.2 and sentence intelligibility differences of more than 80% at different locations throughout the office. Such variations are clearly undesirable. Additionally, masking noise may not be desired in larger conference rooms or other communication spaces sharing ceiling plenums with masked areas, and it is impossible for the designer to fully satisfy both requirements. 
     Subjective spatial quality is a third 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 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 essential in order to provide this diffuse quality of sound. With some non-reflective ceiling materials and fireproofing materials used in plenums, it is necessary to resort to two or more channels radiating different (incoherent) sound from adjacent loudspeakers in order to obtain a limited degree of diffuseness. Some contemporary masking systems use such techniques, adding significantly to their installation complexity and cost. Despite careful consideration and design, the degree of diffuseness typically obtained is further limited by the economically dictated need to place many of the ceiling loudspeakers on the same signal distribution channel. 
     Finally, intentional lack of any user accessible controls is a requirement of conventional masking system design. Because the background sound affects the privacy of all occupants in the office, it is not appropriate to permit individual users to control the characteristics of the masking sound, which are relatively critical. Any temporal changes in the masking level throughout the office are seriously objectionable. Controls are typically locked by various security devices, including physical cabinet locks and electronic password controls to generators and other centrally located electronic components. 
     In addition to the conventional sound masking systems described above, several self-contained general-purpose devices have been used to provide masking sound in offices. These include mechanical devices using fans and various types of electronic sleep aids and “ambient nature environment” units. Although some of these devices have incorporated “white noise” generators, no one system is able to provide the three essential characteristics, for sound masking application, of tailored spectral shaping, adjustable level, and diffuse spatial quality. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a personal sound masking system is disclosed that provides each individual workspace with an optimized acoustic background environment by delivering a sound masking signal that is specifically matched to the individual user&#39;s location and physical relationship to other nearby offices. The sound masking system employs multiple loudspeakers and multiple mutually incoherent channels in order to obtain a desired degree of diffuseness. In a preferred embodiment the sound masking signals are generated from a number of masking signal samples stored in a memory, and the samples are specifically synthesized to minimize memory requirements while avoiding audible transients or sample singularities. 
     The sound masking system also includes a conveniently accessible volume control to enable the user to adjust the sound level, in order to achieve optimum sound masking in his or her individual workspace. 
     The personal sound masking system of the invention is useful in any workspace or personal space where acoustic privacy from intruding background conversation is desirable. People occupying open office plan cubicles, occupants of closed offices or group work spaces, and residents of dormitory or hospital rooms can benefit from the optimized acoustic background environment possible with the system of the invention. 
     Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is an elevation view of a personal sound masking system installed in an open plan office in accordance with the present invention; 
     FIG. 2 is a plan view of the installation of FIG. 1; 
     FIG. 3 is a system level assembly diagram of a personal sound masking system in accordance with the present invention; 
     FIG. 4 is an exploded assembly diagram of a control module in the personal sound masking system of FIG. 3; 
     FIG. 5 is an exploded assembly diagram of a loudspeaker module in the personal sound masking system of FIG. 3; 
     FIG. 6 is a schematic diagram of control circuitry on a printed circuit board in the control module of FIG. 4; 
     FIG. 7 is a plot of acoustic spectra of interest in the personal sound masking system of FIGS. 1-3; and 
     FIG. 8 illustrates an alternative mounting scheme for the loudspeaker module of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show a typical open-plan office, often referred to as a “cubicle.” The offices are separated by partitions  10  whose height is typically in the range of 4.5 to 7 feet. The office occupant may sit at a desk  12  or other station. A sound masking system includes a control module  14  mounted on an inside inner panel of the desk  12 , using for example mating hook-and-pile tabs secured to the desk  12  and control module  14  respectively. The control module  14  is connected to left and right channel loudspeakers  16  via telephone-type multi-conductor cables  18 . The loudspeakers  16  are secured to a partition  10  using suitable means, examples of which are described below. 
     FIG. 3 shows the elements of the personal sound masking system. The control module  14  has a user-accessible volume control  20 . The loudspeaker cables  18  connect to the control module  14  using telephone-type modular plugs and jacks. The control module  14  also contains a jack for receiving a mating plug  22  of an external AC adapter that provides DC power at approximately 7 volts. It will be appreciated that in alternative embodiments DC power may be supplied at other convenient voltages. 
     FIG. 4 shows the elements of the control module  14 . 
     The control module  14  includes a top  30 , base  32 , and a printed circuit board (PCB) assembly  34  containing electronic circuitry that generates sound masking signals that are provided to the loudspeakers  16 . The PCB assembly  34  includes the volume control  20 , which extends through an opening  36  in the top  30  when the control module  14  is fully assembled. The PCB assembly  34  also includes a DC power jack  38  and dual modular jacks  40  for connection to the loudspeakers  16 . A light pipe  42  is used to transmit an indication of the presence of DC power from the PCB assembly  34  to an external user via an opening  44  in the top  30 . The top  30 , base  32 , and PCB assembly  34  are secured together using machine screws  46 . Adhesive-backed hook-and-pile tab pairs  48  are secured to the outside of the base  32  for removably securing the control module  14  to a hard external surface. 
     FIG. 5 shows the elements of a loudspeaker module  16 . The outer components include a base  50 , a top  52 , and a grill  54 . A loudspeaker  56  is secured to an insert  58  using machine screws  60 . The loudspeaker module  16  includes a dual modular jack component  62  connected to the loudspeaker  56  by wires (not shown). The various components of the loudspeaker module  16  are secured together using machine screws  64 . Adhesive-backed hook-and-pile tab pairs  66  are secured to the outside of the base  50  for securing the loudspeaker module  16  to an external hard surface. An identifying label  68  is also secured to the outside of the base  50 . 
     Notably, the loudspeaker  56  in the loudspeaker module  16  of FIG. 5 faces toward the base  50  rather than toward the grill  54 . This arrangement is preferred in order to reduce an undesirable acoustical interference effect caused by loudspeaker placement relative to reflective surfaces. Sound radiated directly to a listener from a loudspeaker travels a shorter distance than is sound reflected from nearby surfaces. If the reflected sound path at a given frequency is ½ wavelength longer that the direct sound path, the reflected sound suffers a 180 degree relative phase shift and cancels the direct sound. Similarly if the reflected sound travels a full wavelength further than the direct sound, the reflected sound reinforces the direct sound, causing a peak in the response. Similar effects obtain at other even and odd multiples of ½ wavelength. These alternating dips and peaks, or comb filtering action, severely compromise the frequency response and cannot be effectively corrected by frequency equalization. However, if the radiating surface of the loudspeaker is close to the reflecting surface, this effect occurs at only short wavelengths or higher frequencies. Inverting the loudspeaker so that the distance from the loudspeaker cone to the reflecting surface is minimized moves the effect above the frequency range of interest. 
     FIG. 6 shows the electrical circuitry employed on the PCB assembly  34  to generate the sound masking signals. 
     Data representing samples of left-channel and right-channel sound masking signals are stored in an erasable programmable read-only memory (EPROM)  80 . The samples represent approximately 3 to 4 seconds of each signal, and are accessed in a repetitive fashion to continually reproduce the 3-to-4-second interval for each channel. The samples are created in a manner that minimizes audible transients or singularities that may be objectionable in the masking signal over numerous repetitions of the segment. In particular, the beginning and ending of each signal segment is located at a zero crossing in order to provide for a smooth transition between repetitions of the signal segment. 
     A set of counters  82  driven by a crystal oscillator  84  sequentially address the samples in a repetitive fashion to produce the masking signal for each channel. Alternating values generated by the counters  82  select samples from the left and right channels, and these values are loaded into a corresponding digital-to-analog converter (DAC)  86 -L or  86 -R. Low-pass filters  88 -L and  88 -R remove high frequency alias noise, and power amplifiers  90 -L and  90 -R amplify the signals to levels suitable for driving the respective loudspeakers  56  (FIG.  5 ). The gain of the amplifiers  90 -L and  90 -R is established by a control signal from a potentiometer Rl, which is part of the volume control  20  of FIGS. 3 and 4. 
     The outputs from the amplifiers  90 -L and  90 -R are provided to two modular jacks J 2  and J 3  in the manner shown. Because both the right and left channel signals are available at each jack J 2  and J 3 , the control module  14  may be connected to the loudspeaker modules  16  in a variety of ways. For example, each loudspeaker module  16  may be connected to a different one of the jacks J 2  and J 3  with a separate cable  18 , as shown in FIGS. 1 and 3. Alternatively, it may be desirable to use a “daisy chain” configuration, in which the control module  14  is connected to a first one of the loudspeaker modules  16  using one jack J 2  or J 3 , and the first loudspeaker module  16  is then connected to the other loudspeaker module  16  in order to forward the corresponding masking signal. Such daisy chaining can also be used in an alternative embodiment having four independent channels rather than two. In such an embodiment, different pairs of loudspeakers are daisy-chained to a corresponding jack J 2  or J 3 , and different pairs of four independent channels are connected to corresponding ones of the jacks. 
     FIG. 6 also shows power supply circuitry on the PCB assembly  34 , including a jack J 1  for receiving a plug from an AC adapter, a fuse F 1 , and a protection diode D 1 . The input power is filtered by capacitor C 1  to provide a DC supply voltage Vp of approximately 6 volts. The supply Vp is used by the power amplifiers  90 -L and  90 -R as well as a 5-volt regulator  92 . The output from the regulator  92  is a supply voltage Vcc filtered by a second capacitor C 2 . 
     While the illustrated embodiment does not include a power switch, it may be desirable to include a user-controlled ON/OFF switch in alternative embodiments. 
     Also shown in FIG. 6 is a dual inline package (DIP) switch S 1  used to generate two additional address inputs for the EPROM  80 . The switch S 1  can be used to select from among four different sets of sound masking signals programmed into the EPROM  80 . As discussed below, it may be desirable to provide sound masking signals having different spectra for use in different surroundings having different acoustic characteristics. By programming the different spectra into the EPROM  80  and providing a configuration switch S 1 , the sound masking system can be readily adapted for use in such different surroundings, while avoiding the need to maintain different versions of the system or version-specific components. 
     FIG. 7 shows a plot of different spectra of interest in the personal sound masking system. The plotted values are sound pressure or loudspeaker terminal voltage levels, as appropriate, in ⅓-octave bands around corresponding center frequencies. Curve  1 A represents a typical desired acoustical background spectrum for sound masking in an open plan type office, office “A,” based on an articulation index of 0.20 and typical values of acoustical isolation between the office and an intruding source location, such as an adjacent office. Curve  2  represents the frequency response of the loudspeaker modules  16 . Curve  3 A is calculated as the difference between curves  1 A and  2 , and represents the required voltage spectrum generated by the control module  14  in order to achieve the background masking sound spectrum shown in curve  1 A. It will be appreciated that the spectrum of curve  2  will generally be different in alternative embodiments employing different types or configurations of loudspeakers. It is generally desirable that the spectrum of curve  3 A be matched to that of curve  2  so that the resulting background masking sound follows the spectrum of curve  1 A. 
     Curve  1 B represents a typical desired acoustical background spectrum for sound masking in another type of open office, office “B,” having different ceiling materials and partition heights. Curve  3 B illustrates the corresponding voltage spectrum required at the loudspeaker terminals assuming the same loudspeaker response as in case described above. 
     FIG. 8 shows a technique for mounting each loudspeaker  16  to a cloth-covered surface, such as the wall of a typical open-plan office. A plastic pin plate  100  is secured to the adhesive-backed surface of the tab pairs  66 . The pin plate  100  has embedded hooks  102  and  104  that taper to a point. The hooks  102  and  104  can be inserted into the cloth surface and then pressed downward to retain the loudspeaker on the wall. 
     While in the foregoing description the personal sound masking system includes two separate loudspeaker modules  16  and a separate control module  14 , it may be desirable in alternative embodiments to integrate the PCB assembly  34  with one of the loudspeakers  56  in a combined control/loudspeaker module. Alternatively, to enhance portability the PCB assembly  34  and both loudspeakers  56  may be integrated into a single housing. As another variant, the loudspeaker modules  16  may be configured to be removably attachable to the control module  14  for enhanced portability, in a manner similar to portable stereo music systems or “boom boxes.” 
     Regarding the signal-generating circuitry, it may be desirable that the memory used to store the signal samples be field programmable, for example to enable fast and cost-effective updating. Thus in alternative embodiments the EPROM  80  may be replaced by an electrically erasable device such as an EEPROM or a flash-programmable RAM. 
     In the illustrated embodiment the spectrum of the sound-masking signal is determined primarily by the collection of samples stored in a memory and sequentially played out via the DACs  86 . It may be desirable in alternative embodiments to generate each masking signal using a cascaded circuit including a pseudo-random noise generator and a spectrum-shaping filter, where the noise generators for the different channels are mutually incoherent. The filters may be either digital or analog, and may include programmability features in order to provide flexibility in matching the spectra of the generated masking signals with the response of the loudspeaker modules. 
     In the foregoing, the sound masking system has been described as a distinct entity apart from other elements of a typical office. In alternative embodiments it may be desirable to integrate the sound masking function into another component, such as for example a multimedia personal computer (PC) used in the office. In such an embodiment the masking signal data may be recorded on a computer memory device such as a magnetic disk or optical disk, or it may be loaded into system memory from a network. Audio player software running in the background can play the masking signal through the PC&#39;s loudspeakers. 
     It will be apparent to those skilled in the art that modification to and variation of the above-described methods and apparatus are possible without departing from the inventive concepts disclosed herein. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.