Subjects

-N 0-9

534.8 Acoustics

	Measurement of radio noise level in Delhi area part II Author : Chatterjee, R.N. ;Murty, Y.S.N. Source : Defence Science Journal ; Vol:12(1) ; 1962 ; pp 48-54 Subject : 534.8 Acoustics Keywords : Noise level ;Radio noise ;Ionosphere Abstract : Measurement of Radio Noise Level on 150Kc/s were carried out by a direct method at Delhi. The nature of diurnal variation of Mean Peak Noise level on this frequency was studied. The results were compared with those obtained for the same frequency in the American Sub-Arctic region, during 1947. The similarity in the nature of diurnal variation for these two places was observed. For both the places the peak noise level was found to be minimum in the early hours of the morning till sunrise, gradually increasing afterwards. From this observation, it was suggested that the ionosphere contributes most to the Radio Noise level around 150Kc/s. The observed value of peak noise level at Delhi was found to vary between 10uV/Metre/Kc Bandwidth and 0.35uV/Metre/Kc Bandwidth.
	Some preliminary investigations on the propagation of V.H.F. radio-waves at 126Mc/s in Mountainous terrain Author : Rao, B.L.N. ;Roy, B.K. ;Mathur, H.B. ;Gupta, U.C. ;Jai Prakash ;Verma, P.P. Source : Defence Science Journal ; Vol:13(2) ; 1963 ; pp 163-178 Subject : 534.8 Acoustics;53 Applied Physics Keywords : Radio waves ;Vegetation Abstract : "This Paper deals with the propagation of VHF radio waves at 126Mc/s within a distance of 2km, on six different paths. Practically observed field strengths have been considered and compared with those theoretically calculated. The observations indicate a difference of about 10-12db on average from the predicated figures for vertically polarized waves which is expected to arise due to absorption by vegetation and the effect of hillocks close by. The differences tend to zero in simpler cases of diffraction not complicated by vegetation etc. An extreme difference of even 25db was observed to be accounted by absorption due to surroundings of vegetation, and buildings."
	Noise encountered in radar methods of reduction Author : Murty, Y.S.N.;Murthy, A.N. Source : Defence Science Journal ; Vol:11(4) ; 1961 ; pp 36-40 Subject : 534.8 Acoustics Keywords : Radar;Noise Abstract : The different types of noise encountered in a radar such as clutter, receiver noise, cosmic noise are described. The various devices which are employed for minimizing the effect of these noises such as logarithmic amplifiers, FTC, parametric amplifiers and masers, are discussed. Mention also is made of methods for reduction of noise based on information theory. It is concluded that by a judicious application of some of these methods, it may be possible to increase the effective range of a radar.
	Underwater Applications of Acoustical Holography Author : Mehta, P. C. Source : Defence Science Journal ; Vol:34(1) ; 1984 ; pp 97-110 Subject : 534.8 Acoustics;629.5 Marine Sciences Keywords : Acoustical holography;Underwater acoustics;Sonar images;Image processing;Optical visibility;Holograms;Sonars Abstract : The paper describes the basic technique of acoustical holography. Requirements for recording the acoustical hologram are discussed with its ability for underwater imaging in view. Some practical systems for short-range and medium-range imaging are described. The advantages of acoustical holography over optical imaging, acoustical imaging and sonars are outlined.
	Interfering effect of noise on signaling systems Author : Chandrasekhar Aiya, S.V. Source : Defence Science Journal ; Vol:8(3) ; 1958 ; pp 213-216 Subject : 534.8 Acoustics Keywords : Noise ;Signaling system Abstract : Noise is statistical phenomenon. It has a complex wave form. This wave from may undergo modifications in the process of transmission of noise through a signaling system. The interfering effect of noise is always with specific reference to the type of signal that the system handles. Therefore, engineering evaluations of noise interference are more complex than the actual physical problem of noise.
	On noise figures in transistor audio amplifiers Author : Rao, E. Bhagiratha Source : Defence Science Journal ; Vol:12(3) ; 1962 ; pp 1-10 Subject : 534.8 Acoustics Keywords : Frequency ;Collector voltage ;Collector current ;Emitter current ;Input impedance Abstract : The noise figures of two general purpose transistors of Philips and Hitachi (Japanese) make have been determined under identical experimental conditions with a view to see whether they differ in their noise properties. The variation of noise figure of Philips as well as Hitachi transistors with (i) frequency (ii) collector voltage (iii) collector current (iv) emitter current (v) input impedance have been investigated. It was concluded that Philips and Hitachi transistors do not differ much in their noise properties, when operated under identical conditions. For both the transistors the noise figure increase with collector voltage, collector current and emitter current. Increasing input impedance of either of the transistors increases the noise figure when the input impedance exceeds two thousand ohms. For both transistors the noise figure decrease with frequency, then remains constant over a small frequency band and finally increases slowly with frequency.
	Mathematical Modelling of Underwater Reverberation Author : Madhavan, C. Source : Defence Science Journal ; Vol:33(4) ; 1983 ; pp 323-330 Subject : 534.8 Acoustics Keywords : Reverberation ;Underwater reverberation Abstract : A comprehensive treatment of underwater reverberation mentioning the case for reverberation has been discussed. The different approaches adopted by earlier workers for evolving a mathematical model of reverberation is also discussed. The implementation and validation of a model for volume reverberation on a general purpose computer produced by a constant carrier transmit pulse of arbitrary shape and duration, for use by advanced signal processors.
	Effect of noise on reaction time for auditory signals Author : Rao, M.S.Prakash ;Nayak, N.A. Source : Defence Science Journal ; Vol:8(4) ; 1958 ; pp 352-356 Subject : 534.8 Acoustics Keywords : Auditory signals ;Noise Abstract : The effect of noise on the time taken to react to pure tone signals was studied. The reaction time was found to increase with rise in the level of noise. Individual differences become more prominent at the higher noise levels. Greater fluctuation in the same individual was also observed with increase in the noise level.
	Selection of optimum frequency for sonar echoes from underwater targets Author : Iyer, H.M. ;Nanda, J.N. Source : Defence Science Journal ; Vol:8(1) ; 1958 ; pp 1-4 Subject : 623.8 Naval Science;534.8 Acoustics Keywords : Solar ;Water Abstract : Method is given for selecting optimum frequency for detection of echoes from targets using underwater sound. Attenuation of sound in water increases with frequency while diffraction spreading of the echo decreases. The frequency has also to be chosen so that the estimated strength of the echo is above the observed reverberation level.
	Acoustic Emission and Signal Analysis Author : Rao, A.K. Source : Defence Science Journal ; Vol:40(1) ; 1990 ; pp 55-70 Subject : 534.8 Acoustics Keywords : Nondestructive evaluation;AE signal ;Acoustics Abstract : Acoustic emission (AE) is one of the most recent entries into the field of nondestructive evaluation. Due to the uniqueness of the basic principle and the potential for tackling a wide range of applications, the technique has gone through rapid strides in a very short time. Thus, today, two decades after the first application of the technique, AE is used in various industries, such as petro-chemical, refinery, nuclear, transportation and aerospace. While, some of the applications can be dealt with by current state-of-the-art, through simple methods of measurement and analysis, the entire potential of the technique still remains to be exploited as extraction of complete information contained in the signal is not possible with the adaption of only simple data analysis procedures. Currently several scientists in many countries are involved in evolving and implementing advanced concepts for AE signal analysis. These along with the approach adopted by us are discussed in this paper.
	A Solution to the Inverse Problem in Ocean Acoustics Author : Murty, T.V. Ramana;Somayajulu, Y.K.;Mahadevan, R.;Murty, C.S.;Sastry, J.S. Source : Defence Science Journal ; Vol:42(2) ; 1992 ; pp 89-101 Subject : 534.8 Acoustics;629.5 Marine Sciences Keywords : Marine acoustic modelling;Geophysical inverse techniques Abstract : The methodology and software developed to reconstruct a vertical sound speed profile as a part of studies on the marine acoustic modelling, using the ray path lengths and the travel time perturbations in tomographic layers are outlined. For a stratified ocean, considering the range independent nature of the medium, geophysical inverse techniques are employed to reconstruct the sound speed profile. The reconstructed profile for a six layer ocean, with five energetic modes, is in good agreement with that of the assumed profile thereby indicating the usefulness of the model. The effect of noise caused by the excursions of the source and receiver moorings; When expressed in terms of travel-time differences, results in the sound speed changes up to 0.1 per cent.
	The electro acoustic efficiency of a magnetostrictive transducer Author : Srinivasan, D. ;Srivastava, S.S. Source : Defence Science Journal ; Vol:9(1) ; 1959 ; pp 1-5 Subject : 534.8 Acoustics Keywords : Acoustic ;Radiometer ;Electro-acoustic ;Magnetrostrictive Abstract : A vane type radiometer has been constructed in the laboratory for measuring the radiation pressure due to acoustic waves from a magnetostrictive transducers. A method based on the use of this radiometer is described for calibrating underwater applications.
	Underwater Sound Scattering Model of the Topographic Features of the Sea Floor Author : Murty, G.R.K. ;Murty, K.R.G.K.;Srinivasan, D. Source : Defence Science Journal ; Vol:32(1) ; 1982 ; pp 1-8 Subject : 551.46 Oceanography;534.8 Acoustics Keywords : Topographic ;Sea floor;Acoustic Abstract : A description of the sea floor with respect to underwater acoustic transmission giving rise to reflection and scattering is studied. The plane wave reflection coefficients and scattering coefficients are estimated from the topographic features of the floor and the physical properties of the sediments. A bottom loss model for an area off west coast of India is presented.
	Surface Acoustic Wave Devices in Electronic War Fare Author : Joshi, N.K.;Singh, A. Source : Defence Science Journal ; Vol:27(4) ; 1977 ; pp 157-162 Subject : 534.8 Acoustics Keywords : Frequency dependent time delay;Band pass filtering;Coded waveforms Abstract : In this paper the applications of acoustic surface wave device technology to electronic warfare systems have been reviewed. A variety of signal processing tasks such as simple time delay, frequency dependent time delay, programmable and discretely variable time delay, band pass filtering and generation and recognition of various coded waveforms can be performed by surface wave devices to improve the performance of EW system.
	Analysis of Acoustic Emission Signals using Wavelet Transformation Technique Author : Rao, S.V. Subba;Subramanyam, B. Source : Defence Science Journal ; Vol:58(4) ; 2008 ; pp 559-564 Subject : 629.7 Aeronautics;534.8 Acoustics Keywords : Acoustic emission;Wavelets;Wavelet transform;Coherence estimation function Abstract : Acoustic emission (AE) monitoring is carried out during proof pressure testing of pressure vessels to find the occurrence of any crack growth-related phenomenon. While carrying out AE monitoring, it is often found that the background noise is very high. Along with the noise, the signal includes various phenomena related to crack growth, rubbing of fasteners, leaks, etc. Due to the presence of noise, it becomes difficult to identify signature of the original signals related to the above phenomenon. Through various filtering/ thresholding techniques, it was found that the original signals were getting filtered out along with noise. Wavelet transformation technique is found to be more appropriate to analyse the AE signals under such situations. Wavelet transformation technique is used to de-noise the AE data. The de-noised signal is classified to identify a signature based on the type of phenomena.