Patent GB-672090-A (GB)
Title: Improvements relating to instruments for measuring sound radiation pressure
IPC: G01H3/10
Filed: 1949-03-24
Abstract: 672,090. Measuring sounds and like vibrations. SIEMENS-REINIGER-WERKE AKT.- GES. March' 24, 1949. [Dec. 30, 1948] No. 8131/49. 'Class 118 (ii). An instrument for measuring sound radiation pressure comprises a casing 13 of polyvinylchloride attached to a brass base 1 and having a circular opening at the top closed by an extremely thin polyamide diaphragm 23 which is permeable to sound, the whole being filled with water and containing a sound pressure responsive reflector 10 supported by a mechanical linkage system so as to be capable of displacement, against a reacting force, in the direction of sound radiation received through the diaphragm 23. The supporting system comprises two U-shaped brass brackets 2 and 3 mounted on the base 1 and respectively carrying bronze torsional ribbons 4 and 5 which are clasped at their centres by rigid members 6 and 7 to the centre of two similar torsional ribbons 8 and 9 fixed on the saddle-roof shaped reflector 10 made of plastic material. Sound waves reflected from member 10 are absorbed by the members 21 and 22 comprising fibres of plastic material. Radiation pressure on reflector 10 causes distortion of the parallelepipedic frame-like supporting system and the pointer 11 attached to the linking member 7 indicates the displacement on a scale 12 which may be calibrated in watts. Deflection due to the weight of the reflector and linking members is compensated by the adjustable weights 14, 15 and 16 and zero . point adjustment is made by rotation of knob 20 which moves the lever 19 attached near the end of torsion ribbon 4 causing slight distortion thereof. The linking member 7 carries a damping flap 17 moving in cylinder 18. The casing 13 extends into a ring 25 around the diaphragm 23 to form a container to receive a sound transmitter and a coupling liquid. Other material and liquid may be used but if their coefficients of expansion are unequal an expansion chamber must be provided -but the thin diaphragm 23 may serve this purpose. Instead of being thin the diaphragm may consist of a material having the characteristic sound impedance of the conducting liquid, or it may be an even multiple of half-wavelengths thick. Fig. 2 (not shown), illustrates an embodiment wherein the counterweights 14, 15 and 16 are replaced by a secondary parallelepipedic framework connected to the first one in a specific way, and damping is effected by simple flaps attached to each rigid linking member. This arrangement provides stability of the system for angles of the first framework approaching 180 degrees and permits a working angle of greater than 90 degrees. The reflector consists of a foamy plastic material or of doubled metal foil and is made liquid proof, so as to keep a thin air laver between its surfaces. Instead of compensating the weight of the meter system by counterweights (which exert bending strain on the torsion ribbons) the rigid parts may be made of a material whose specific weight is equal to that of the sound conducting liquid, or an additional part of smaller specific weight may be added.