Method of and apparatus for masking-noise generation for architectural spaces and the like
United States Patent [1 1 Allen et al.  
 [111 3,877,412 Apr. 15, 1975 1 1 METHOD OF AND APPARATUS FOR MASKING-NOISE GENERATION FOR ARCHITECTURAL SPACES AND THE LIKE [75] Inventors: Clayton H. Allen, Wellesley; Istvan L. Ver, Arlington, both of Mass.  
 [73] Assignee: Bolt, Beranek &amp; Newman, Inc.,  
 Cambridge, Mass.  
  22 Filed: May 5, 1972 21 Appl. No.: 250,823  
 [52] U.S. Cl. 116/137 R; 181/.5 AG [51] Int. Cl B06b 1/18; B06b 1/20 [58] Field of Search 116/65, 112, 137 R, 137 A, 116/142 R, 147; 253/39 H; 239/519; 181/33 L, .5 AG, .5 R, 45, 65 R; 179/15 R, 1.5 M;  
  5 6] References Cited I UNITED STATES PATENTS 1,812,234 6/1931 Braatelien 239/519 2,678,625 5/1954 Hall et al. 116/142 R 2.8701156 1/1959 Olson et a1 181/.5 R  
 2,893,508 7/1959 Baruch et a1. 181/45 2,975,751 3/1961 Bodine 116/137 A 3,103,941 9/1963 Watters... 137/1 3,144,913 8/1964 Bailey.. 181/33 L 3,229,429 1/1966 Conrad... 181/33 L 3,278,165 10/1966 Gaffney 116/137 A 3,344,766 lO/1967 Cottell 116/137 A 3,721,521 3/1973 Schmidlm 116/137 A X Primary ExaminerRichard= C. Queisser Assistant ExaminerDenis E. Corr Attorney, Agent, or Firm Rines and Rines; Shapiro and Shapiro 57 ABSTRACT This disclosure deals with the generation of broadband masking noise in rooms and the like by a high velocity airjet impinging in a controlled manner in the turbulent mixing region of the jet upon appropriate surfaces.  
 7 12 Claims, 6 Drawing Figures souwo POWER (db RE 16&#39; WATT) PATENTEU 3.871412 SHEET 1 OF 2 Fig. 5.  
 l I l I I 1 2 3 ORIFICE SPACING TO VANES !N INCHES METHOD OF AND APPARATUS FOR MASKING-NOISE GENERATION FOR ARCHITECTURAL SPACES AND THE LIKE The present inventionrelates to methods of and apparatus for making-noise generation for architectural spaces and the like, being more specifically directed to apparatus involving the controlled generation of noise by high velocity air or other fluid jets and the like.  
  The use of masking noise to increase privacy in an office or other space has been widely accepted as a desirable tool. A principal problem residing in such .use, however, is that of obtaining a reliable, controllable, and inexpensive source of broadband, steady noise with an acceptable frequency spectrum.  
  One source of such noise is a loudspeaker supplied with a signal from a broadband electrical noise generator. The signal may conveniently be passed through a filter to control the spectral shape andan amplifier to control the total sound power output. Such a system works very well and produces an acceptable masking noise that is independent of the operation of other devices or services in the architectural space in which it is disposed. The serious drawbacks for such systems include the fact that it is inherently expensive and additionally uses electronic and other components and equipment not familiar to the trades installing and servicing the facilities normally employed in architectural spaces. In addition, when more than one noise generating loud speaker is required in an architectural space, the noise radiated from such speakers, when driven with the same electronic noise generator, is correlated, thereby leading to an interference pattern that produces undesirable, fixed, spatial variation of the noise field due to constructive and destructive addition of acoustic pressure. The use of multiple electronic noise generators circumvents this problem, but substantially increases the cost of the system.  
  An alternative source of masking noise which has been widely used is the air conditioning system itself, particularly the turbulence noise generated at ventilating louvers, diffusers, and other terminal devices. The principal problem involved with such structures, however, is that the noise output is intimately coupled to and varies widely with the primary processes controlling the amount of air flow and its distribution. The efficiency of this noise generating process, moreover, is very low since only a small pressure drop can be tolerated in the air delivery system; the total power required to deliver air being increased significantly when masking noise is generated in this way. While such a noise generating process has been used in spite of its inefficiency and limitations on control, this is only because it is inherently available and serviceable by the trades ordinarily used for installing and servicing ventilation and air conditioning systems, and represents at best a compromise with the problem and the desired performance.  
  In accordance with the present invention, with its vastly improved source of masking noise, advantage is taken of certain properties of jet-produced turbulence as a means of generating noise:  
 1. the efficiency of noise generation increases rapidly with increase in air speed;  
 2. although high speed jets radiate noise due to turbulence generated in the shear layer in the conical mixing region the efficiency is still lowbecause the turbulence here acts as a quadrupole source;  
 3. turbulence acting on a solid surface generates noise as a dipole source with greatly increased efficiency, especially at low frequencies; and  
 4. turbulence acting on an opening can provide monopole radiation with even greater efficiency.  
  An object of the invention, accordingly, it is to provide a new and improved method of and apparatus for masking-noise generation, void of the difficulties and problems above described and operating rather upon the principles of generating noise with jet turbulence in order to form a more efficient, inexpensive, broadband masking-noise source than has been employed heretofore in architectural structures, and to provide therein a controllable spectral shape and a controllable total sound power output independent of other facilities serving the space.  
  A further object is to provide a novel masking-noise generator of more general applicability, as well.  
 Still an additional object is to provide a novel sound source.  
  Other and further objects will be explained hereinafter and are more particularly pointed out in the appended claims. In summary, however, from one of its broader aspects, the invention contemplates the generation of broadband masking noise and the like by interposing an appropriate surface, including an apertured surface and/or a matrix of solid surfaces in the turbulent mixing region of an air or other high velocity fluid jet. While pressure-controlling elements have previously been disposed in fluid streams as described, for example, in US. Pat. No. 2,893,508 of the co-inventor Clayton H. Allen of the present invention, and while cellular air-flow dividers and valving shutters have been used in high velocity streams to control fluid flow and generate specific tones, as described in US. Pat. No. 3,103,941 of B. G. Watters, the present invention, to the contrary, is concerned with transforming otherwise inefficient jet-produced quadrupole radiation substantially to dipole and monopole radiation for efficient noise generation. The jet turbulence region comprises closely spaced, convected pressure sources of audible frequencieswhich by themselves do not radiate sound efficiently because of their close spacing and variations in phase, but when intercepted by a plate or edge, preferably running substantially parallel with the jet, pressure equalization across the plate is prevented and the quadrupole radiation of the jet alone is changed to dipole radiation from one or both sides of the plate; furthermore, when the jet is intercepted by the edges of an open tube, or of a resonator cavity, or small end (throat) of a horn having an open area that is small compared with the physical size of these pressure fluctuations, these pressure fluctuations will have a nearly constant phase over the open area and will produce monopole sound sources that radiate sound into the tube or horn.  
  Where the opening leads to a closed cavity instead of a tube or a horn, the convected pressure fluctuations impinging on the mouth of the cavity cause pressure variations in the cavity proper. Where this cavity has rigid immovable walls, the air in the cavity will be alternately compressed and rarified as a direct result of the pressure fluctuations; however, at and neara particular frequency for which the stiffness of the air in the cavity and the effective mass of the air in the opening form a resonant system, the pressure fluctuations may be amplified and a large oscillating flow of air into and out of the cavity will occur. This augmented flow acts as an impedance transformer and causes the energy inherent in the slowly traveling and closely-spaced pressure fluctuations in the turbulence of the air stream to be efficiently converted into acoustic energy and radiated as sound waves away from the cavity to the surrounding space.  
  Where the cavity has one or more yielding wall surfaces which are large compared with the area of the opening and which can move as a piston or as a diaphragm in response to the pressure variations produced in the cavity of the impingement of the turbulent jet on the mouth of the cavity, sound is produced by direct radiation from the exterior surface of the movable wall member causing a further increase in masking noise output, particularly at low frequencies because of the better radiation efficiency of the large surface area as compared with the small area of the orifice.  
  The invention will now be described with reference to the accompanying drawing,  
  FIG. 1 of which is an isometric view, partly cut away to illustrate constructional details, of a preferred embodiment;  
  FIG. 2, 4, S and 6 are similar views of modifications; and  
  FIG. 3 is a graph illustrating the performance of the system of FIG. 1.  
  In the preferred system of FIG. 1, a high velocity jet of air 1 from a jet orifice 1 impinges upon a matrix of 2 of solid edge-on elements, shown as disposed between a front transverse plate 20 centrally apertured at 20 and a parallel plate 20&#34;, and introduced into the turbulent mixing region 1&#39;&#39; of the jet in an arrangement that utilizes the higher efficiency of the high velocity jet for generating turbulence and simultaneously utilizes the higher efficiency of noise generation, characteristic of the dipole and monopole radiation, resulting from interaction between the turbulent mixing region 1&#34; of the jet 1&#39;, the edges of the aperture 20&#39; and the exposed solid edge surfaces 2 (as opposed to the less efficient quadrupole radiation of the jet itself). This enables the generation of broadband masking noise to be used for increasing privacy in the surrounding architectural spaces 3, the source permitting easy and substantially independent control of both the spectral shape and the total acoustic power output.  
  The noise generator l2 is completely independent of the air conditioning, ventilation, electrical or lighting systems associated with the space 3, but is small enough to be combined with any one of these where such combination may be advantageous. While the source of air supplied along tube 4 and its slidable extension 4, may be l520 psi air-pressure sources commonly used for the control of air conditioning devices, such as thermostats, dampers, mixers and the like, operation has been successfully effected, as later discussed, over a pressure range of about 5 to 60 psi, with total noise power being found to be controllable by such pressure variation or adjustment.  
  The thin vertical vanes 2 extend longitudinally parallel with and cut through the center of the projected jet 1, causing a broadband spectrum with a haystack type radiation distribution of the type desired. The edge-on interaction of the vanes simultaneously advantageously increases the dipole radiation for the noise purposes of the invention, particularly at the low frequencies. The closer the front edges of the vertically oriented radial vanes 2 approach the jet orifice 1, this being adjustable by the clamp 5 moving upon the sliding tube 4&#39; and carrying the matrix 20-2-20&#34; by diverging struts 5, the higher the noise level that is created. The frequency of the maximum acoustic power output (top of the haystack) increases as the vane structure approaches the jet orifice l.  
  The addition of intersecting horizontal vanes 2&#34;, at right angles to the vertical vanes 2&#39;, also centered edgeon the jet, increases the noise output and especially reinforces the low frequencies. Preferably, the longitudinal line of intersection of the vanes 2 2&#34; is centered on the axis of the jet 1, as shown, with the juxtaposed opening 20&#39; in the front plate contributing further monopole type radiation, as well. Intermediate radial vanes 2&#34;, such as at 45, produce &#39;even more output. The structure 2 is shown encased in a sound-absorbing material 6, such as polyurethane foam or the like, bounded by an impervious outer membrane or layer 7 as of Mylar or the like.  
  With such an arrangement using eight vanes 2&#39; 2&#34; 2, one to two inches long axially and extending two to four inches outward from the center, sound power levels of from about 42 to 54 decibels re 10 watt were obtained in the octave band center frequency range of from about 250 to 750 Hz, and declining levels from about 54 to 37 dB over the higher frequency range from about 750 to 4,000 I-Iz, which can provide a sound field closely approaching the preferred background masking noise criteria for typical open office spaces. This performance was obtained with a jet orifice of 0.016 inch, 20 psi air&#39; pressure, a jet orifice to vane edge spacing of about 2.5 inches, a front plate aperture 20 of about 0.75 inch. Successful operation, producing various useful spectral shapes of broadband noise, with distances of A2 inch to 3 inches between the jet orifice l and the vanes 2 and for various pressures, 5 to 60 psi, and for a plurality ofjet orifice sizes (0.008, 0.010, 0.016, 0.024, and 0.032 inches in diameter) has also been attained.  
  It has been found, moreover, that for different frequencies there is an optimum small range ofjet-orificeto-vane spacing for optimum noise sound power generation. This is illustrated in the graph of FIG. 3 as in the 1-to-2 inch distance range along the jet mixing core 1&#39;, for 1,000 Hz. It is evident from test results that satisfactory masking noise can be obtained for a typical office space with the devices of the invention, using a jet orifice of approximately 0.008 to 0.016 inches in diameter operating at 15 to 20 psi and using between 0.0l and 0.1 standard CFM of air depending upon office size and degree of masking desired.  
  The vane and plate structures of the matrix 2 need not assume the particular geometrical configurations of FIG. 1. In the embodiment of FIG. 2, as another example, the rectangular array 2 has been replaced by a circular cylindrical array 12 embodying cylindrical parallel front and rear plates 13 and 13 and intermediate radial vanes 12. The vanes 12&#39; are shown inclined at an acute angle a to the plane of the plates to receive the jet from the orifice 14 after centrifugal turbulent injection along a cylindrical housing 15 through the front plate orifice 16.  
  As another example, the matrix 2 may be formed as the mouth and side walls of a horn structure as shown in FIG. 4. For example, it has been found that with the matrix replaced by the throat of a single or a multicellular horn 22, a similar spectrum is produceable that is also controllable in spectral shape and total sound output by varying the spacing from the jet to the horn throat. It has also been discovered that by thickening and rounding the edges of the horn throat opening 22, as shown, the high frequency components of the noise can be advantageously reduced, giving a spectrum with a greater percentage of low frequency components, as is desirable for some masking purposes. Such horns may be conical, exponential, or of other conventional shapes, providing suitable impedance transformation from the throat to the mouth areas.  
  The aperture 20&#39; may, moreover, be an entrance to a Helmholtz resonator box or cavity 21, FIG. 5, exposing edge-on surfaces 2, if desired, with absorbing material 6 and an outer impervious membrane 7 usable as in the embodiment of FIG. 1.  
  A further refinement is shown in FIG. 6, embodying one or more pointed fins or vanes 8 extending from the opening of the chamber toward the jet orifice to intercept the jet at a point close to the orifice where the turbulent eddies are of small size and high in frequency. These eddies have been rather surprisingly discovered to interact with the fin vane 8 to produce high frequency sounds with an increase in level of 20 to 30 dB or more without substantially altering the output of sound at low frequencies.  
  While the structure of the invention has been illustrated and described in terms of high velocity air jets, it has been demonstrated that jets produced by using larger orifices and higher and lower pressures also can produce satisfactory results for some applications, and it is to be understood that other fluid media may also be similarly employed where desired for more general application. Further modifications will also occur to those skilled in this art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.  
 What is claimed is:  
  l. A masking-noise generator for architectural spaces and the like having, in combination, jet-producing means; means for supplying high velocity fluid to the jet-producing means to project a jet therefrom with a turbulent mixing region; solid surface means disposed substantially edge-on; and means for disposing the solid surface means in said region to produce by interaction with the said mixing region at least one of dipole and monopole noise radiation, the solid surface means comprising a matrix of rigid thin vanes radially extending from a common center line along the axis of the jet to generate a broadband, haystack spectrum with its peak at a predescribed frequency determined by the distance between the jet opening and the leading edge of the vanes of the matrix.  
  2. A masking-noise generator as claimed in claim 1 and in which means is provided for controlling the total noise power output by adjusting the pressure of the fluid supplied to the jet.  
  3. A masking-noise generator as claimed in claim 1 and in which a transverse plate is disposed in the jet forward of the vane matrix, said plate being provided with an aperture located to expose at least the said common center line region of the vane matrix, positioned such that the aperture intercepts at least a part of the turbulent mixing region of the jet.  
  4. A masking-noise generator as claimed in claim 3 and in which means is provided for altering at least one of the spectrum and total power of the noise generated, such means comprising means for moving said plate relative to the jet-producing means.  
  5. A masking-noise generator for architectural spaces and the like having, in combination, jet-producing means; means for supplying high velocity fluid to the jet-producing means to project a jet therefrom with a turbulent mixing region; solid surface means disposed substantially edge-on; and means for disposing the solid surface means in said region to produce by interaction with the said mixing region at least one dipole and monopole noise radiation, said edge-on surface means comprising a matrix having forward and rearward transverse plates the former of which is provided with an opening for the impingement of the turbulent jet.  
  6. A masking-noise generator as claimed in claim 5 and in which said surface means comprise a plurality of intersecting vanes positioned edge-on radially about an axis and impinged thereupon by the jet through said forward plate opening.  
  7. A masking-noise generator as claimed in claim 6 and in which said vanes are tilted at an acute angle to said plates.  
  8. A masking-noise generator as claimed in claim 6 and in which a circular housing is disposed around the jet-producing means and said forward plate opening.  
  9. A masking noise generator as claimed in claim 5 and in which means is provided for modifying the noise spectrum by preferentially absorbing high frequency noise components and reinforcing low frequency components; said means comprising a layer of sound absorbing material wrapped around the open area of said matrix located between said forward and rearward transverse plates, said sound absorbing material being covered over at least part of its outer exposed surface by an impervious membrane.  
  10. A masking-noise generator for architectural spaces and the like having, in combination, jetproducing means; means for supplying high velocity fluid to the jet-producing means to project a jet therefrom with a turbulent mixing region; solid surface means disposed substantially edge-on; and means for disposing the solid surface means in said region to produce by interaction with the said mixing region at least one of dipole and monopole noise radiation, said surface means comprising the edges of an opening in a resonator and edge-on vane means intercepting the turbulent jet.  
  11. A masking-noise generator as claimed in claim 10, and in which at least one large wall of the cavity consists of a movable member such as a light piston, cone, membrane or the like which is forced to move by the pressure variations generated in the interior of the cavity by the action of the turbulent jet impinging upon the mouth aperture of the cavity, the said movable wall thereby producing sound by direct radiation from its exposed or exterior surface and acting as an impedance transformer between the convected pressure fluctuations in the jet and the pressure fluctuations radiated as sound waves to the surrounding air.  
  12. A masking noise generator as claimed in claim 10 and in which said resonator is interiorly provided with acoustically absorbing material to broaden the resonance peak of the broadband noise produced by said masking noise generator.