Patent GB-1342799-A (GB)
Title: Method of and device for enhancing marine navigational sound signals in the presence of acoustic noise
IPC: H03G9/10
Filed: 1971-05-27
Abstract: 1342799 Selective amplification of sound signals SPERRY RAND CORP 27 May 1971 [19 June 1970] 17370/71 Heading H4J  Detection of information signals (e.g. fog signals) in the presence of noise signals (e.g. marine acoustic noise) is assisted by converting the information and noise signals into electrical signals and amplifying the electrical signals by an amplifier having a gain versus frequency characteristic adjusted to have a slope opposite to the slope of the amplitude versus frequency characteristic of the noise signal. The electrical noise signals of frequency lower than the lowest frequency of the information signals are filtered out and the remaining signals are utilized, e.g. by a recorder and headphones or a loud-speaker. A typical ship's background noise due to bow wake, ship's engine, machinery, &c. varies in a fairly regular manner from 31À5 to 4,000 Hz with a negative slope of 6 db. per octave (Fig. 1, not shown, the region between 31À5 and 125 Hz being of particularly high intensity. Fog warning sounds produced by ship's bells, horns and whistles are in the frequency range 125 to 4000 Hz (Fig. 3, not shown). A circuit (Fig. 4) makes use of these facts. A cardioid microphone 11 supplies a preamplifier 12 whose gain is adjustable by a control 13 but has a slope lying between 4 and 8 db. per octave, preferably 6 db. per octave. The output of the preamplifier is applied to a filter network 14 having a low frequency cut-off control 15 and a high frequency cut-off control 16. The response of the preamplifier 12 and filter network 14 is bounded by the curves 100 and 102 (Fig. 3) and is typically the curve 101 having a rising slope of 6 db. per octave in portion BC. The output of the filter network 14 is supplied to a compression amplifier 17, controlled in conventional manner by an amplifier 19 and rectifier 20. The negative and positive slopes of 6 db. per octave of the noise curve and of the preamplifier, combined with volume compression, allow the ship's lookout to hear all sounds within the information signal frequency range at about the same sound level. The 4 to 8 db. rising portion of the characteristic curve (Fig. 3) is beneficial in detecting navigational sounds emitted by distant sound sources. The lookout wears earphones or listens to a loud-speaker in the shelter of the pilot house, a power amplifier 22 being provided. The preamplifier 12 (Fig. 5) includes a matching transformer 31 and an operational amplifier 12a, having two double ended stages 34, 35 and 41, 42 and a single ended class AB output stage 46, 47. Transistors 48 and 49 respectively co-operate in the first and second stages in determining current flow levels. A damping network 60 having components 61, 62, 63, which may be adjustable, prevents high frequency oscillation. Feedback from the output 50 to the transistor 35 controls the slope gain of the amplifier 12a. Capacitors 51 and 55 are adjusted to give the desired (e.g. 6 db. per octave) increase in gain with frequency. Variable resistors 54 and 56 are overall gain-determining resistors, and may be gang controlled. Circuit 12b is an emitter-follower circuit.