Abstract:
A flexible membrane is attached to an internal wall of the fairing of a flight vehicle. The membrane is inflated with gas to achieve a shape that couples with and thereby attenuates the first acoustic resonance occurring within the volume enclosed by the fairing. A sensor senses the varying ambient atmospheric pressure during flight. A pressure regulating system includes an inflation valve and pressure relief valve. Based on the sensed ambient atmospheric pressure, this system adjusts the membrane&#39;s gauge pressure throughout the flight trajectory to maintain the desired coupling gauge pressure. Acoustic blankets can also be attached to the internal walls of the fairing to abate resonances having frequencies greater than 500 Hz. Inflating the membrane with helium enhances the attenuation otherwise obtained. Multiple membranes can be individually tuned to respectively attenuate multiple acoustic resonances.

Description:
RIGHT OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of acoustic damping. More particularly, the present invention is an inflatable elastic membrane that, when inflated, acts as an acoustic resonator to damp acoustic resonances in an enclosed volume, such as that contained by the fairing of a launch vehicle. 
     Traditional acoustic resonators are fabricated from a rigid material, and enclose a vented volume. A neck usually controls the length of the vent. A well known example of such an acoustic resonator is a wine bottle. An acoustic resonator can be used to reduce acoustic transmission into an enclosed space, such as that of a launch vehicle or an airplane. If the unwanted acoustics are tonal, i.e., occurring at a single frequency or within a narrow bandwidth, the resonator is configured as a reactive device with low damping. Introducing the reactive resonator significantly increases the reactive impedance over a narrow frequency band. 
     By tuning the resonator, the narrow frequency band may be made to coincide with the tonal disturbance. The disturbance is then partially or completely reflected away from the acoustic space, and its transmission into the space is decreased. If the unwanted acoustics are broadband and the acoustic space has a low number of lightly damped resonances, the resonator may be configured as a resistive device with high damping. Introducing a resistive or damped resonator into the acoustic space adds damping to the acoustic resonances. Although all lightly damped acoustic resonances will tend to increase their damping level by introducing a damped resonator, the largest increase in damping will result at the acoustic resonance to which the resonator is tuned. 
     Multiple resonators may be used simultaneously to attenuate multiple tones or multiple band widths. In either case, tuning is achieved by varying the volume and/or neck length of a traditional resonator. This tuning process is well understood and has been described in a number of textbooks, for example, Philip M. Morse &amp; K. Uno Ingard,  Theoretical Acoustics  489-490 (Princeton University Press, 1986), and Leo L. Beranek,  Noise and Vibration Control  370-376 (2d ed., Institute of Noise Control Engineering, 1988). 
     The beneficial effect of the resonator is compromised by the volume it occupies. Furthermore, the use of an acoustic resonator is limited by its weight, especially when multiple resonators are required. Prior art acoustic resonators are described in the following patents: 
     U.S. Pat. No. 6,493,180, “Hard Disk Drive Cover That Contains a Helmholtz Resonator Which Attenuates Acoustic Energy,” issued Dec. 10, 2002; U.S. Pat. No. 6,485,205, “Media Weight Sensor Using an Acoustic Resonator,” issued Nov. 26, 2002; U.S. Pat. No. 6,151,396, “Active Acoustic Resonator for Abating Noise,” issued Nov. 21, 2000; and U.S. Pat. No. 6,116,375, “Acoustic Resonator,” issued Sep. 12, 2000. 
     There is a need in the art for a lightweight, compact resonator that can be tuned to damp low frequency acoustic resonances occurring in an enclosed volume, as well as provide broadband reduction for higher frequency resonances. The present invention fulfills this need in the art. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention is an inflatable elastic membrane that, when inflated, acts as a resistive acoustic resonator to passively damp acoustic resonances in an enclosed volume, and particularly in the fairing of a flight or launch vehicle. Since the elastic material properties and the gas pressure inside the membrane structure determine the acoustic coupling, damping, and resonance for the membrane, the inflatable elastic membrane can be tuned to target specific low-frequency modes that are not attenuated by traditional passive acoustic blankets. 
     The membrane of the present invention can have various geometries, and be inflated with a variety gases, thus providing a lightweight and rugged apparatus. The inflating gases may be other than air, such as helium, for introducing an acoustic impedance mismatch to further reduce the amplitude of acoustic waves propagating in the enclosed volume being damped. 
     The membrane of the present invention can also include a pressure relief valve to vent the membrane during the ascent of a flight vehicle to higher altitude, as well as an inflation valve to inject pressurized gas into the membrane during descent. The present invention can also include a pressure regulator to maintain the desired gauge pressure within a plurality of inflatable membranes by simultaneously controlling their respective inflation and pressure relief valves. Acoustic blankets can also be attached to the internal walls of the vehicle and around the inflated membranes, to abate frequencies greater than 500 Hz. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the radial breathing motion of an inflated spherical membrane of the present invention. 
         FIG. 2  is an illustration of the setup used to test the present invention. 
         FIG. 3  is a frequency response plot showing the acoustic attenuation achieved by the present invention when tested using the setup illustrated in  FIG. 2 . 
         FIG. 4  is an illustration of multiple membranes of the present invention deployed inside a launch vehicle fairing containing a payload. 
         FIGS. 5A and 5B  illustrate side and front views, respectively, of an inflated membrane of the present invention having an elliptical cross-section. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is an acoustic resonator comprised of an elastic membrane inflated with helium or another gas. Since the elastic membrane significantly affects acoustic coupling and damping, the resonator can be configured as either reactive or resistive. A resistive resonator might use a membrane composed of an elastomer, whereas a reactive device might use a stiffer material such as Mylar® polyester film. (Mylar® is a registered trademark owned by Dupont Tejjin Films, for a family of plastic film products made from the resin Polyethylene Terephthalate (“PET”).) Since an inflated membrane would be lightweight, the weight penalty associated with using such a device in a launch vehicle would be negligible. However, in order for the device to function optimally as a resonator, it must be properly tuned. An understanding of the tuning process requires a preliminary comprehension of how the inflated membrane would function as a resistive resonator. 
     An elastic membrane functions most effectively as a passive resistive resonator when its “breathing” mode is tuned to the frequency of interest. The breathing mode for inflatable, elastic spherical membrane  101  inflated with gas  105  is illustrated in  FIG. 1 . The breathing mode for spherical membrane  101  can be predicted by augmenting the equations of motion for a spherical membrane with the spring stiffness of gas  105 . The stiffness of spherical membrane  101  is related to the composition of the membrane material, the internal pressure created by gas  105 , and the volume enclosed by spherical membrane  101 . The equations of motion of spherical membrane  101  are well known by those skilled in the relevant art. Since these equations are nonlinear, the membrane material is highly nonlinear, and the radial breathing motion is relatively substantial for high acoustic levels, the prediction of frequency is approximate and must be augmented by empirical testing. For geometries other than a sphere, empirical testing would be the most efficient method of determining damping ratios and resonance frequencies. 
     To achieve the desired attenuation, it is necessary to obtain a high degree of acoustic coupling of a resistive acoustic resonator to an enclosed acoustic volume containing the resonator. The breathing mode of spherical membrane  101  has extremely good coupling to the acoustic volume that surrounds it for two reasons: the radial breathing motion of the membrane&#39;s surface is uniform in phase, and the mechanical impedance of the membrane material does not restrict the absorption of energy. Membrane  101  “breathes” uniformly, increasing and decreasing in volume, but maintaining its shape. In acoustic literature, a spherical membrane that exhibits a breathing mode would be described as a highly efficient spherical radiator. By reciprocity, a spherical radiator is not only a good generator of acoustics, but is also likely to be easily excited by acoustics. 
     In addition to the radial breathing motion of inflated spherical membrane  101 , the mechanical impedance of the membrane also affects coupling. For example, a relatively compliant, damped device is easily driven into motion by an acoustic excitation, whereas a relatively rigid structure is not. This occurs because, although the breathing mode of both the compliant and rigid device may be tuned so that their respective resonances correspond to the disturbance frequency, the amount of response of each resonator is governed by the moving mass and the amount of damping. Given the same disturbance level and the same damping, the compliant structure would exhibit more motion and absorb more energy, since it has less mass per unit area. For the resonator to couple well and absorb energy, it should have relatively high damping and low mass. There is an optimum value of impedance that maximizes absorption of acoustic energy. The choice of gas, membrane material and inflated volume all determine the degree of coupling and, concomitantly, the effectiveness in attenuating acoustic resonant response. 
     The effectiveness of the inflatable membrane acoustic resonator is not restricted to tuning only the breathing mode to the target acoustic resonance. Significant damping of acoustic modes in a fairing can also be achieved by tuning higher frequency resonances (above the fundamental breathing mode) to coincide with the acoustic resonances. As long as the resonance frequencies of the membrane structure are proximal to the acoustic resonances, there will be efficient coupling, and thus enhanced acoustic damping of the acoustic response. 
     At frequencies for which the membrane resonances do not coincide with acoustic resonances, membrane  101  will still passively absorb incident acoustic waves. Acoustic energy is dissipated as heat when acoustic waves interact with the membrane structure and cause membrane  101  to vibrate. The mechanical vibrations are damped out by friction between the molecules of the membrane material as the mechanical waves propagate along the membrane. As the resulting acoustic waves pass through membrane  101  and into gas  105 , additional energy loss is realized through molecular friction resulting from wave interactions with the gas molecules. 
     If helium gas is used to inflate membrane  101 , the aforementioned loss due to molecular friction is enhanced because helium gas is very dissipative to acoustic energy. Also, the existence of a secondary boundary, i.e., the inflatable membrane structure, within the enclosed volume will cause an impedance mismatch to acoustic waves propagating therein. The acoustic mismatch will be a function of frequency, membrane material, and the nature of the gas used to inflate membrane  101 . The acoustic mismatch will cause scattering of acoustic waves, and hence provide additional dissipation of acoustic energy. This effect will increase with the introduction of more membranes  101  into the enclosed volume. 
     A test was performed to verify that a helium-filled spherical membrane tuned such that its breathing mode corresponds to the first acoustic resonance of a hollow cylinder containing the membrane, will reduce the acoustic response in the cylinder. The experimental setup is shown in  FIG. 2 . Hollow cylinder  200  was 84 inches long, 20 inches in diameter and constructed from ¼-inch thick aluminum. One end of cylinder  200  was terminated with a rigid, 1.5-inch thick particleboard end cap  203 , while the other end was terminated with a relatively compliant ⅛-inch thick aluminum plate  204 . End plate  204  introduces structural dynamics into the acoustic transmission path, which is representative of the acoustic transmission path in real launch vehicle fairings. Acoustic speaker  205 , including diaphragm  206 , was placed directly outside end plate  204  for the purpose of generating random acoustic noise. Accelerometer  207  was attached to diaphragm  206  as a reference sensor to indicate the source strength and spectrum of the acoustic disturbance generated by speaker  205 . Microphone  208  was placed inside cylinder  200  to measure the acoustic response resulting from the external acoustic disturbance created by speaker  205 . 
     Spectrum analyzer  209  was used to generate random noise that was input to speaker  205 , and to measure the signals from microphone  208  and accelerometer  207 . Spectrum analyzer  209  also computed the transfer function (or frequency response function) between accelerometer  207  and microphone  208 . The transfer functions show the structural and acoustic resonances in the transmission path, and their relative magnitudes. The test was performed twice: with cylinder  200  being empty and, again, with spherical (4-inch radius), flexible membrane  210  inflated with helium gas and fixedly located inside cylinder  200 . 
       FIG. 3  shows the transfer function of empty cylinder  200  as a solid line. The spectrum exhibits several lightly damped resonances. The first acoustic resonance of cylinder  200  was at approximately 80 Hz. There were also acoustic modes in cylinder  200  at approximately 160 Hz and 240 Hz. End plate  204  introduced a structural resonance at approximately 175 Hz. 
       FIG. 3  also includes an overlaid plot of the transfer function measured with membrane  210  inflated with helium gas to form a sphere, and attached to the interior of cylinder  200  near rigid end cap  203 . Spherical membrane  210  was tuned to 80 Hz to match the first acoustic resonance of cylinder  200 . As shown by comparing the two plots, the presence of membrane  210  reduced the amplitude of the first resonance (80 Hz) by approximately 8 dB. There was little effect at the second acoustic resonance or the first structural resonance. However, there was significant attenuation of higher order acoustic resonances. The third acoustic resonance at 240 Hz was attenuated by approximately 20 dB. This demonstrates that the present invention can effectively target low frequency resonances, as well as provide broadband reduction for higher frequency resonances. It is noteworthy that the aforementioned reduction in resonance amplitude was achieved with a device, i.e., membrane  210 , that was lighter than air, (uninflated weight of 7.3 grams) and occupied approximately 1% of the volume of cylinder  200 . 
       FIG. 4  illustrates multiple membranes  400  of the present invention, all of which are attached to the inside fairing wall  401  of fairing  402  of flight vehicle  403 . Membranes  400  are each constructed from an elastic polymer material, and each is inflated with helium gas to form a sphere. Membranes  400  are each attached to inside fairing wall  401  with single point attachment hardware  404 , with the attachment hardware including an inflation valve and a pressure relief valve (not shown). Each of membranes  400  is tuned to a different frequency to provide a broad band of noise attenuation; that is, each of membranes  400  is tuned so that at least one of membranes  400  couples with each of the first acoustic resonances occurring in fairing  402 . 
     The pressure relief valve exhausts gas during the ascent of flight vehicle  403  to maintain the desired coupling pressure and prevent the membrane from rupturing due to the decrease of ambient atmospheric pressure. The inflation valve fluidly communicates with a reservoir of pressurized gas (not shown) and provides for inflation of the membrane during descent to maintain the coupling pressure. During flight, a sensor (not shown) senses the ambient atmospheric pressure and continuously sends signals to a regulator (not shown), which controls the respective pressure relief and inflation valves for membranes  400  to maintain the respective coupling pressures. There are a variety of such pressure regulation systems and devices commercially available, and well known to those skilled in the mechanical arts. Any of such systems may be incorporated and used with the present invention in order to regulate the membrane pressures as described herein. 
     Acoustic blankets  405  are attached to inside fairing wall  401 , and abate high frequency noise, e.g., greater than 500 Hz. Fairing  402  also contains payload  406 . Even when fully inflated, membranes  400  do not interfere with the operation or deployment of payload  406 . 
       FIGS. 5A and 5B  illustrate inflatable membrane  500 , another embodiment of the present invention. Membrane  500  is inflated with a gas and attached to the inside fairing wall  501  of a flight vehicle.  FIG. 5A  provides a side view and  FIG. 5B  shows a front view of membrane  500 . Attachment hardware  503  includes a pressure regulating system (not shown) to alternatively exhaust gas from membrane  500  and inject gas into membrane  500  during the trajectory of the flight vehicle, to maintain the gauge pressure of membrane  500  at the level necessary to couple it with the first acoustic resonance of the volume contained by the fairing. Membrane  500  is constructed from elastic polymer material and, as particularly shown by  FIG. 5A , forms a shape that has an elliptical cross-section when inflated. Although having a non-spherical shape when inflated, membrane  500  can still be tuned to act as an acoustic resonator to increase the damping of acoustic resonances, and thereby dissipate acoustic energy in the fairing of a flight vehicle in the manner previously discussed in conjunction with spherical membrane  101 . 
     Other variations of geometry are also intended to be covered by this invention, such as flattened, rectangular-shaped membranes. Furthermore, a membrane of the present invention can be inflated with a gas other than helium, or gas mixtures, such as air. It should also be noted that a membrane of the present invention can be fabricated from one of a variety of elastic non-polymer materials that offer tuning properties different from those of the elastic polymers previously disclosed herein. 
     It is to be understood that the preceding is merely a detailed description of several embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.