Accession Number : AD0009108


Title :   RESEARCH PROGRAM ON NOISE MITIGATION IN SONAR II


Descriptive Note : Final progress rept.


Corporate Author : ITT FEDERAL LABS NUTLEY NJ


Personal Author(s) : Graham, W ; Greenfield, G


Full Text : https://apps.dtic.mil/dtic/tr/fulltext/u2/009108.pdf


Report Date : Oct 1952


Pagination or Media Count : 107


Abstract : The comb filter designed as an aid in the aural detection of noise tones has pass bands 31 cps apart, extending from 200 to 3500 cps. The width of the bands are adjustable down to 2 cps. The spectrum is aurally monitored for the presence of a tone while the signal is heterodyned with an oscillator whose frequency is varied linearly with time. The comb filter is synthesized using an acoustic delay medium with external electrical feedback. Laboratory listening tests of a 1-kc tone in white noise show an improvement of 7.5 db in signal threshold with the comb filter as compared with naked-ear detection. The system designed to detect the modulation of propeller cavitation noise consists in square-law rectification followed by detection of a resulting tone at the modulation frequency. For a signal 6000 cps wide and 10 seconds long, a 5-db reduction in threshold over naked-ear detection may be attained. Longer observation of the signal will effect additional improvement. The increase in signal threshold due to uncertainty of the signal frequency is evaluated for an ideal detection system. The 50% threshold of detection for a system requiring 30 filters is 3.75 db higher than that for a single-filter system and known signal frequency. With aural detection and a known signal frequency, the 50% threshold was 2.75 db higher. A signal threshold 12 db lower than the aural threshold may be realized with a modified ideal system in detecting a tone in a band of white noise 1200 cps wide with 5 sec of observation time. The use of frequency multiplication followed by analysis with a single scanning filter is discussed.


Descriptors :   *SONAR SOUND ANALYZERS , COMB FILTERS , UNCERTAINTY , LABORATORY TESTS , THRESHOLD EFFECTS , OBSERVATION , WHITE NOISE , ACOUSTIC DETECTION , FEEDBACK , ACOUSTIC FILTERS , BANDWIDTH , MODULATION , HEARING , MULTIPLICATION , AUDIO TONES , SCANNING SONAR , ACOUSTIC DELAY LINES , CAVITATION NOISE , PROPELLER NOISE , FREQUENCY , OSCILLATORS


Subject Categories : Acoustic Detection and Detectors


Distribution Statement : APPROVED FOR PUBLIC RELEASE