Patent GB-310493-A (GB)
Title: Improvements in or relating to acoustic resistance devices such as may be used, for example, in gramophones
IPC: G10K11/168
Filed: 1928-09-20
Abstract: 310,493. Electrical Research Products, Inc., (Assignees of Flanders, P. B.). April 28, 1928, [Convention date]. Damping vibrating members; sound-boxes. - An acoustic resistance device, applicable for use as a damping load for a gramophone sound-box as described in Specification 323,043 or as a test-load to absorb the output from a gramophone sound-box during the testing of such instruments to measure the energy of the sound output, comprises means defining a sound wave passage in the form of an open slot having substantially parallel sides, having a length in the direction of wave motion that is small in comparison with the wave lengths of the highest audible frequencies, and having a width between the parallel sides that is small in comparison with the lengths of the " diffusion waves " corresponding to the highest audible frequencies. The width between the parallel sides of the slot is preferably so small with respect to the diffusion wave lengths, i.e. not more than 0.003 inch, that the reactance of the passage is substantially negligible in comparison with its resistance within the range of speech and music frequencies, e.g. the reactance may be 20 per cent or less of the resistance value at wave frequency of 5,000 cycles per second. When using the device as a test load on a gramophone sound-box, the horn or a major portion thereof is removed and the impedance device shown in Fig. 1 is connected by a short tube 6 to the sound-box and adjusted to be equal to the characteristic impedance of the tube or approximate to the resistance of the removed portion of the horn. A small microphone is exposed to the vibrating air between the source and the impedance device. Speech frequency currents of various frequencies but of constant amplitude may be used in the tests. A direct measure is thus obtainable of the varying air pressures available from the source and the characteristics of the sound-box can be plotted. In the devive shown in Fig. 1, the resistive impedance is provided by the annular slot 1 between the tapered wall of a fixed hollow bushing 2 and the correspondingly tapered surface of an adjustable plug 3. The head of the plug, which is perforated at 4, screws on to the outside of the bushing 2, a lock nut 5 holding the parts in their adjusted position. Fig. 2 shows a construction of resistance device in the form of a flat rectangular plate comprising fiat rectangular rods 7 separated by small pieces of foil 8, the whole being held rigid by a clamping frame 9. In a modified form, Fig. 3, a thin metallic ribbon is wound into a closely spaced spiral 10 which is mounted on a metallic supporting frame comprising a ring 11 and a number of radial arms 12. Such a device may be formed by coating a copper ribbon with zinc to a thickness equal to half the desired slot width, the ribbon being then wound into a closely packed spiral of the desired diameter. The spiral is conveniently wound around a small copper rod, the inner end of the ribbon being soldered or welded to the rod. The spiral element is then soldered to its supporting frame and the zinc coating removed by dissolving it in hydrochloric acid. Alternatively, a pair of ribbons of different metals, e.g. copper and aluminium, may be wound side by side into a solid double spiral, the aluminium being dissolved out by a solution of caustic potash. Fig. 4 shows a further modified form of device suitable for providing a resistive leak in a sound conduit and comprising narrow slots between thin flat rings 15 soldered to the ribs 18 of a skeleton cylindrical frame 16. A flange on one end of the ring 16 and a clamping ring 17 on the other end also serve to hold the rings 15 in place. To secure the proper spacing of the rings, temporary spacers of another metal may be inserted during the assembly process, the spacers being ultimately removed by the use of a suitable solvent. Alternatively, the slotted structure may be obtained by winding a coated ribbon edgewise in a close helix on the cylindrical frame and, after soldering to the ribs, dissolving away the coating of the ribbon. Specification 310,505, [Class 13, Bells &c.], also is referred to.