Patent Publication Number: US-4648275-A

Title: Underwater acoustic impedance measuring apparatus

Description:
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to an automatic underwater acoustic impedance measuring apparatus. 
     2. Description of the Prior Art 
     A pulse tube has traditionally been used as the apparatus for impedance measurement commencing with the efforts of Erwin Meyer and Eugen Skudryzk, German professors, to develop underwater acoustic absorbers for the German Navy during World War II. Physically, the pulse tube is a thick walled steel structure filled with water and instrumented with acoustic transducers. A gated sine wave with about five cycles to approximate a plane wave is the acoustic signal that is employed. Some tubes are equipped to provide temperature and hydrostatic pressure control for studying samples subjected to varying environmental stimuli. 
     Typical dimensions of various pulse tubes are as follows: 
     
         ______________________________________                                    
Length (feet)    7     42       21   42                                   
Inner Diameter (inches)                                                   
                 2     5.56     2.50 3.40                                 
Wall Thickness (inches)                                                   
                 1     2.22     2.75 2.30                                 
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     An acoustic signal is generated at the bottom of the tube to be reflected from the sample, mounted at the top, which is attached to a high impedance backing, or mounted against a layer of gas acting as a low impedance backing or mounted in the center of the tube so that there is a water backing. A portion of the signal is reflected and its phase is shifted by the sample. The percentage of energy reflected is expressed as the reflection coefficient. Acoustic impedance can be obtained from a knowledge of the reflection coefficient and of the phase shift. 
     The conventional method of measuring impedance in a water filled impedance tube processes the reflected signal by nulling it with a known signal of equal amplitude and opposite phase. This is conventionally manually performed by an operator at discrete frequencies and represents a long and tedious process subject to error and to operator fatigue. 
     One recent example, U.S. Pat. No. 4,305,295 illustrates a portable apparatus for measuring acoustic impedance in air at the surface of curved sound absorber wherein the apparatus consists of a circular, flexible disc held at a constant distance from a curved absorbing surface by pins or flexible ribs. Sound from a loudspeaker is fed to the center of the discs and allowed to propagate radially in the space between the disc and absorber. Radial arrays of microphones on the disc surface sense sound pressure amplitude and phase, from which impedance is calculated. Such apparatus is used for determining the acoustic properties of absorbing linings as installed in ducts of jet engines. It is also applicable for measuring the acoustic impedance of other absorptive surfaces, including for example, acoustic wall and ceiling panels. Another example is illustrated in U.S. Pat. No. 4,289,143 wherein there is described a method of and apparatus for audiometrically determining the acoustic impedance of a human ear in air. 
     The measuring of underwater acoustic impedance of various materials and acoustic structures is required in the use of such materials and structures such as absorbers, decouplers, sonar domes, and transducers. The acoustic impedance of a material or of an acoustic structure provides significant information regarding the operating parameters or attributes of the structure or of its components. The interpretation and examination of an impedance locus reveals the magnitude of acoustic absorption, reflection, sonic velocity, elastic moduli, dissipation and resonance frequencies. These qualities are measured for purposes related to material development, for evaluation, for product development and for quality control. Some of the hydroacoustic applications derived from acoustic impedance information are manifested as underwater absorbers, reflectors, transducers, sonar domes, and baffles. Dynamic moduli, useful in non-acoustic and in air acoustic applications, are also determined from acoustic impedance information obtained with the use of an underwater acoustic impedance measuring apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention provides an automatic underwater acoustic impedance measuring apparatus comprising transducer means properly located on said apparatus, one transducer means positioned so that it senses the incident signals and reflected signals to compute reflection factor, a second transducer means positioned at the face of sample material at the plane of said transducer means so that is senses the incident and reflected waves to compute the phase angle, said transducer means properly interfaced with a computer means for automatically computing and a printing means for printing out in real time the acoustic impedance. 
     OBJECTS OF THE INVENTION 
     An object of the invention is to provide an automatic underwater acoustic impedance measuring apparatus. 
     Another object of the invention is to provide an automatic underwater acoustic impedance measuring apparatus with enhanced performance. 
     Still another object of the invetion is to provide an automatic underwater acoustic impedance measuring apparatus which automatically computes and prints out a frequency range of interest. 
     Other objects and many of the attendant advantages and usage of the invention will be readily observed and appreciated as the same becomes better understood by reference to the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a cross-sectional view of the impedance tube of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing shows an embodiment of the automatic underwater acoustic impedance measuring apparatus of the invention. Three hydrophones, transducer means, are flush mounted in the wall of a steel tube that is filled with water. A sample material is mounted so that its face is at the plane of hydrophone H 1  when the backing is a high impedance such as a heavy mass or a low impedance such as a gas. The sample material may be mounted so that its face is at the plane of hydrophone H 2  when the sample is water backed. If required, or desired for some technical reason, a polymer film hydrophone or a flexible ceramic hydrophone of very small thickness can be attached to the face of the sample and its electrical output can be led out through the backing located at the top of the tube, in place of hydrophone H 1  or hydrophone H 2 . 
     The electrical signal produced by a gated sine wave at the hydrophone located at the sample face, H 1  or H 2 , is the sum of the incident (Pi) and reflected signals (Pr). Incident pressure is &#34;Pi&#34;. The reflected signal (Pr) is modified by the reflection factor &#34;R&#34; of the sample material and by its associated phase &#34;θ&#34;. Reflection factor R=(Pr/Pi) 2 . Through algebraic manipulation the magnitude of the hydrophone voltage is determined by the following formula: 
     
         [Pi(1+2R Cos θ+R.sup.2)].sup.1/2 
    
     The reflection factor is obtained from the output of hydrophone H 3  by obtaining the ratio of the reflected to the incident signals. Consequently, it is possible to determine the phase angle from the output of hydrophone H 1 , or H 2 , and thus the acoustic impedance of the sample. 
     An integral part of the apparatus is a dedicated computer. First, a frequency range of interest is selected, e.g. 2k Hz to 9k Hz, with fine increments such as 100 Hz and the dedicated computer is programmed to sweep the selected range automatically. Second, the reflection factor (R) is obtained from hydrophone H 3  and it is applied to the output of H 1  or H 2  which computes the phase angle θ. Third, the values of &#34;R&#34; and &#34;θ&#34; are fed into a program for automatically calculating, printing and plotting acoustic impedance. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.