The apparatus disclosed and claimed herein pertains generally to the field of calibrating a hydrophone array comprising a plurality of hydrophones which are electrically interconnected in a generally elongated configuration.  More particularly, the invention pertains to such calibration wherein apparatus is employed which is portable or highly transportable, and which provides a sealed cavity in which the hydrophone array and a selected fluid are contained during calibration.  Even more particularly, the invention pertains to the field of calibrating an elongated or linear array of hydrophones down to frequencies on the order of 1-2 Hz, in the absence of a substantial or non-transportable body of water. 
It is presently well known in the art to electrically interconnect individual hydrophones into arrays which are to be used for gathering acoustic data.  In such arrays, which have important commercial and military applications in detecting acoustic signals present in ocean areas, all of the individual hydrophones cooperate or interact to provide a beam forming or direction-sensing capability.  Such arrays are usually linear, and comprise a number of hydrophones distributed over a length which may be in excess of 300 feet.  However, it is likewise well known that the detection capabilities of individual hydrophones in an array may unpredictably vary over a period of time.  Consequently, to accurately interpret acoustic information sensed by a hydrophone array, it is necessary to calibrate the array, that is, to determine the response provided by the cooperatively functioning hydrophones thereof to various frequencies over an acoustic frequency range. 
Calibration of a linear hydrophone array in the VLF range, i.e. the frequency range of approximately 1 to 100 Hz, has been found to be particularly difficult.  In the past, it has been found necessary to deploy a linear array in open ocean, and project VLF signals thereto from an acoustic generator spaced many miles therefrom.  More recently, U.S. Pat.  No.  3,859,620, issued Jan. 7, 1975 to Joseph L. Percy, discloses apparatus for calibrating a linear hydrophone array, or line array, down to a frequency of 10 Hz, the apparatus including a cylindrically shaped wire mesh framework.  The array is helically coiled within the frameworks, and then both framework and coiled array are suspended into a water-filled test tank, into which acoustic signals are projected over a calibration frequency range.  The Percy patent teaches that it is very important that the framework enclosing the array be transparent to acoustic waves.  According to Percy, the array should not be contained in an enclosure having solid walls, to prevent the generation of interfering acoustic energy, or standing waves, during calibration.  Also, Percy stresses the need to make the test tank acoustically non-reflective, so that a test facility may have to be specially designed for hydrophone array calibration. 
The United States Navy makes extensive use of linear hydrophone arrays.  Prior to the present invention, it was necessary for the Navy to transport virtually all of its arrays to an immobile specially constructed facility for VLF calibration.  A hydrophone array could therefore experience much more movement and handling than was necessary for its actual deployment and operation as an acoustic sensor, whereby the cost of gathering acoustic data was measurably increased. 
In the past, individual hydrophones have been calibrated in sealed enclosures or cavities.  However, linear arrays of hydrophones may comprise twenty or more discrete hydrophone elements, distributed over lengths in excess of 300 feet.  The prior art generally taught, as indicated by the Percy patent, the undesirability of enclosing such arrays in a solid shell during calibration.  Yet it has been anticipated that if a linear array could be calibrated within a sealed enclosure, the requirement that a special test tank for VLF calibration could be eliminated.  In addition, if the structure enclosing an array during calibration was sufficiently transportable to be moved to sites at which the array was deployed and operated, the need to move and handle arrays would be substantially reduced, and significant cost savings could be achieved.