Patent GB-773181-A (GB)
Title: Improvements in sound level meters
IPC: G01H3/00
Filed: 1955-10-21
Abstract: 773,181. Measuring sound intensity. PATENT MANAGEMENT, Inc. Oct. 21. 1955 [Oct. 28, 1954], No. 30180/55. Class 118 (2). [Also in Group XIX] An attenuator system for determining the intensity of a noise and that of selected component frequency bands comprises a transducer for changing the noise into corresponding electrical oscillations which are applied through an attenuator and amplifier to a meter for determining the intensity of the noise itself, and also comprises an analyser for obtaining from the amplifier the required frequency component, a second attenuator and amplifier for applying this component to the meter, the first attenuator being adjustable by a manual control which simultaneously moves a graduated dial and the second attenuator being adjustable by a manual control carrying an index which co-operates with the said graduated dial to give a composite reading which combined with the meter reading gives the intensity of the said required component of the noise. A microphone 24 (Fig. 7) feeds a pre-set amplifier 25 through an attenuator 10, and a meter 29 may be connected to the output via a three-way selector switch 28. The amplifier 25 is also connected to an analyser 30 which transmits only frequencies lying in a selected band to a further attenuator 11 and preset amplifier 31 to which the meter 29 may also be switched when required. In order to preset the gain of amplifiers 25 and 31, or to adjust any variation in gain, a calibration oscillator 80 and loud-speaker 81 are provided, as shown in Fig. 7, to enable sound of predetermined intensity to be applied to the microphone 24. With the meter 29 switched to amplifier 25 the gain is adjusted to make the algebraic sum of the readings of attenuator 10 and meter 29 correspond to the said predetermined intensity. With the meter switched to amplifier 31 and the analyser 30 short-circuited the gain of amplifier 31 is then adjusted until the attenuator 11 and meter 29 together indicate the predetermined value. In the case where background noise is too great to permit the loud-speaker 81 to be used effectively the output of calibration oscillator 80 may be connected by a conductor (not shown) to the input of attenuator 10. A third position of switch 28 enables the meter 29 to be connected directly to the oscillator 80 and itself calibrated. In use the intensity of a noise component is given by the algebraic sum of the readings of attenuator 10, attenuator 11 and meter 29, and to simplify this measurement the controls of attenuators 10 and 11 are arranged as in Figs. 1 and 2 such that a single reading may be taken for the composite settings of the two attenuators. The control knob 20 carrying dial 19 is loosely mounted on the spindle 13 and by means of chain 17 adjusts attenuator 10. Attenuator 11 is adjusted by knob 22 on spindle 13. A fixed index 21 is provided to indicate on dial 19 the setting of attenuator 10. It is arranged that the attenuator 11 and amplifier 31 together have zero gain when an index 23 on knob 22 points to the fixed index 21. The index 23 then indicates on dial 19 the combined attenuation of attenuators 10 and 11 and has merely to be added to the reading of meter 29 (switched to amplifier 31) to give the intensity of the noise component. In the embodiment of Figs. 5 and 6 (not shown), the first attenuator is operated directly by a control knob and by gearing moves a circular dial loosely mounted on the second attenuator spindle as above. The further modification of Figs. 3 and 4 (not shown) in effect merely replaces the fixed index by a moving pointer on the first control knob and a fixed circular dial.