Patent GB-1436093-A (GB)
Title: Battery-driven sound level indicator with luminous display
IPC: G01H3/12
Filed: 1974-03-13
Abstract: 1436093 Sound intensity measurement FELTEN & GUILLEAUME CARLSWERK AG 13 March 1974 [14 March 1973] 11247/74 Heading H4D A battery operated indicator for measuring sound levels incoporates an optical display using light emitting diodes. The device is portable and is housed in a cylindrical casing 1, Fig. 2, having at one end a conical shaped portion which may be provided with a plug-on cylindrical screen against mechanical and acoustical disturbances, and at the other end a detachable cap 10 in which are mounted batteries 16. A metallic sleeve is resiliently mounted at the end of printed circuit board 6 and houses (preferably a capacitor) microphone K. The circuit Fig. 4, is mounted on circuit board 6 and the output from the microphone K is, fed, via a FET high input impedance stage to a first variable negative feedback amplifier G1, adjustable in steps, by means of switch S3, each step corresponding to a sound pressure level difference of 10 dBA. Switch S3 (Fig. 4) is embodied in sliding switch 15 (Fig. 2) having an operating knob 14 projecting through an opening 18 which opening 18 may be sealed by, e.g. two lips of foam rubber. The device is pointed at the sound source, and slide switch 14 is adjusted to the lowest position at which diode 12 is illuminated (Fig. 1) and the corresponding sound level read from the scale. In the circuit, Fig. 4, the output of amplifier G1 is fed to a filter N, the weighting characteristics of which are standardized to the human ear. A second feedback amplifier G2 is fed from filter N and has a limited gain adjustment to calibrate the device. The amplified signal is rectified by a peak value rectifying circuit G, and integrated by a circuit I. Switch S1 operates as the on-off switch for the device, by connecting battery B1 into the circuit. The display part A is equipped with a separate battery B2. When a sufficiently large signal is applied to the input of the display part A (depending the gain setting on amplifier G1 and the sound pressure intensity applied to microphone K), the light-emitting diode L is activated. A battery test facility is provided by actuating switch S2, so that it disconnects the output from integrator I and connects a voltage. from the battery B1 to the input of the display part L, whereby the light emitting diode L is activated, provided the batteries B1 and B2 are in a state of sufficient charge to allow accurate operation of the device. Switches S1 and S2 may be embodied as microswitches (13, Fig. 3, not shown), projecting through holes (7) in each side of the tubular casing 1. A resilient plastics sleeve 11, having apertures for the operating knob 14 and diode or diodes 12, is fitted over the tubular casing 1, and actuation of microswitches 13 is performed through this resilient sheath. A terminal, for testing and recording purposes may be connected to amplifier G1 or G2. In further embodiments (Figs. 6, 7, not shown) duplication of stages G2, G, I, A and L (Fig. 6) or more simply stage A (Fig. 7), so that each separate parallel amplifying and display part has a different gain, enables, e.g. two other L.E.D's to be provided to operate respectively when the measured sound level is equal to or greater than the switch setting above of below the switch setting corresponding to the measured sound level.