Absolute Noise Levels Research Articles

Backscattered microstructural noise in ultrasonic toneburst inspections

A model is presented which relates the absolute backscattered noise level observed in an ultrasonic immersion inspection to details of the measurement system and properties of the metal specimen under study. The model assumes that the backscattered noise signal observed for a given transducer position is an incoherent superposition of echoes from many grains. The model applies to normal-incidence, pulse-echo inspections of weakly-scattering materials using toneburst pulses from either a planar or focused transducer. The model can be used in two distinct ways. Measured noise echoes can be analyzed to deduce a “Figure-of-Merit” (FOM) which is a property of the specimen alone, and which parameterizes the contribution of the microstructure to the observed noise. If the FOM is known, the model can be used to predict the absolute noise levels that would be observed under various inspection scenarios. Tests of the model are reported, using both synthetic noise echoes, and measured noise echoes from metal specimens having simple and complicated microstructures.

Correlation of deep ocean noise (0.4–30 Hz) with wind, and the Holu Spectrum—A worldwide constant

One year of ambient ocean noise data, 0.4 to 30 Hz, from the Wake Island hydrophone array in the northwestern Pacific are compared to surface wind speeds, 0–14 m/s (0–28 kn). Between 0.4 and 6 Hz, noise levels increase with wind speed at rates of up to 2 dB per m/s until a saturation is reached having a slope of about −23 dB/octave and a level of 75 dB relative to 1 μPa/√Hz at 4 Hz. This noise saturation, called the ‘‘Holu Spectrum,’’ likely corresponds to saturation of short-wavelength ocean wind waves. It is probably a worldwide constant. Between 4 and 30 Hz, noise also increases with wind speed at rates of up to 2 dB per m/s, but no saturation level is observed and the slope increases to about 4 dB/octave. This may be acoustic noise from whitecaps. On a hydrophone less than 3 km from Wake, noise between 0.5 and 10 Hz increases with wind speed at a rate up to 2 dB per m/s, but absolute noise levels are significantly higher than levels on the other hydrophones more distant from Wake, and no saturation is apparent. Surf breaking against the shore of the island is the probable source of this noise.

Flexural plate wave devices for chemical analysis

Abstract : The mass-sensitivities, vapor sensitivities, and vapor detection limits of flexural plate wave (FPW) and surface acoustic wave (SAW) vapor sensors are compared both theoretically and experimentally. FPW devices offer high mass sensitivity at much lower operating frequency. Mass sensitivity increases as membranes thickness decreases; frequency decreases at the same time. This scaling law is in contrast to SAW devices where mass sensitivity increases as frequency increases. FPW devices with sorbent polymer films respond to vapors in a manner similar to SAW devices coated with the same polymer. The FPW vapor sensor, however, offers lower absolute noise levels and hence lower vapor detection limits. It is also demonstrated experimentally that FPW devices can monitor changes in polymer films as the polymer undergoes the glass transition. Plots of frequency vs. temperature show a discontinuity in slope at the static glass transition.

Variations in broadband seismic noise at IRIS/IDA stations in the USSR with implications for event detection

AbstractAmbient noise conditions at four IRIS/IDA sites in the USSR are characterized from 0.01 to 100 Hz as part of a study to ascertain the utility of broadband three-component seismic stations in monitoring regional Eurasian seismicity. Estimates of the power spectral density of noise levels were computed for a 5-day period in two seasons (winter and summer), at two times of the day. Of these periods, lower noise conditions were found at night in the summer. In general, at 1 Hz and above, noise levels and their variations correlate predictably with the soundness of vault construction and the proximity of the station to civilization. Absolute noise levels at the IRIS/IDA/USSR sites range from a high of about −120 dB to a low of −155 dB relative to (1 m/s2)2/Hz, between 1 and 5 Hz. A time-of-day variation in noise was observed at all sites, with noise levels during the work day ranging from 7 to 14 dB higher than night levels, depending on the site. This effect was observed only for frequencies above about 1 Hz. Observed seasonal variations (winter versus summer) are highly station dependent, although the seasonal effect is restricted to frequencies below 1 Hz and is in general centered on the microseism peak (0.1 to 0.2 Hz). Below 0.1 Hz, noise levels are influenced by the thermal and barometric isolation of the site. Low-frequency levels were not studied below 0.01 Hz. Minimum detectable magnitudes are estimated for the IRIS/IDA stations using the observed noise levels over 1 Hz. In general, a magnitude 3 event should be detectable at 1,000 km by all stations under night noise conditions if the dominant signal frequency is 1 Hz; the magnitude estimates increase with increasing frequency. These detectability estimates assume a conservative signal-to-noise ratio of 6.High-frequency data recorded by independent equipment co-located with the IRIS/IDA system during a 2-week experiment allow estimation of noise levels at the sites up to 100 Hz. Borehole versus surface noise levels recorded during the high-frequency experiment showed significant noise reduction (20 dB) can be achieved by borehole deployment at sites with exposed surface vaults. With well-isolated surface vaults, borehole noise reduction is about a factor of 2. Absolute noise levels between 1 to 10 Hz observed at IRIS/IDA/USSR sites are systematically higher than average NORESS noise by about 7 dB to 25 dB, depending on the station.

High‐frequency seismic observations in eastern Kazakhstan, USSR, with emphasis on chemical explosion experiments

Two temporary three‐station seismic networks, deploying surface and 100‐m borehole high‐frequency seismometers, of the order of 200 km from the Kazakh test site in the USSR and the Nevada test site in the United States are discussed, with emphasis on chemical explosion experiments. Seismograms attained from the detonation of three buried explosions (10 t, 20 t, 10 t) in eastern Kazakhstan at distances between 156 and 637 km are examined in the frequency band of 1–80 Hz. Observed signal‐to‐noise (S/N) ratios were high, reaching a maximum of 400 for Pg waves and 200 for Lg waves. Good signal‐to‐noise levels persisted to high frequencies; S/N = 2 at about 50 Hz for Lg waves about 250 km from the source, and at about 14 Hz at 680 km distance. For Pg waves, S/N = 2 at about 50 Hz 270 km from the source. Shapes of displacement amplitude spectra were similar, characterized by a broad maximum in signal‐to‐noise levels between 4–8 Hz, and a decay at higher frequencies (e.g. above 10 Hz) of about f−3.5 − f−4.1 for Lg waves, and f−3.1 − f−4.5 for Pg, unconnected for distance. Magnitudes estimated from Lg time domain amplitudes for the 10 t explosion are between 2.8 and 3.3, depending on the magnitude relation used. Spectral characteristics are used to put some constraint on Lg Q. Pg Q is poorly constrained by the data. A similar experiment in southern Nevada showed much lower Pg and Lg signal‐to‐noise levels above 1 Hz, although Kazakh and Nevada absolute noise levels are comparable.

Quantum-limited operation of balanced mixer homodyne and heterodyne receivers

The quantum-limited operation of balanced mixer optical homodyne and heterodyne receiver using a 50-50 percent beam splitter and two identical photodetectors is experimentally confirmed. The absolute quantum noise levels are calibrated with an incoherent GaAs light-emitting diode output. The photoelectron statistics, obtained through the analog photon counting experiment, are reduced to the quantum limit. The photocurrent fluctuation spectral density measured with a spectrum analyzer also agrees with the quantum noise spectral density. Furthermore, the deviations from the absolute quantum noise level are within ±0.02 dB. Finally, complete suppression of local oscillator excess noise is demonstrated in the 0-1.5 GHz frequency region.

A comparison between a downhole seismometer and a seismometer on the ocean floor

Abstract We have compared the signal and noise characteristics recorded by a downhole seismometer and a nearby (within 200 m) ocean bottom seismometer in the deep ocean (5.5 km). Absolute noise levels and signal-to-noise ratios from the two instruments are remarkably similar. We attribute this unexpected similarity to the shallow emplacement depth of the downhole instrument (194 m subbottom) and the soft sediment into which it was clamped.

Noise performance of the new Norton op amps

The noise performance of the new "Norton" single-ended operational amplifier is described in detail. The important noise sources are introduced theoretically, and it is shown that the absolute noise levels can be accurately predicted and simply modeled under any bias conditions. The results are verified experimentally, and an example of a bandpass active filter is used to illustrate the accuracy of the model.

Vertical Directionality of Ambient Noise in the Deep Ocean at a Site near Bermuda

The vertical directionality of ambient ocean noise at 2400 f (fathoms) off Bermuda has been determined experimentally, using a novel data-reduction technique. In agreement with qualitative models, the results indicate that low-frequency noise arrives primarily from the horizontal while noise of higher frequency is incident from overhead. The measured dependence of observed levels on Beaufort-scale wind force is also consistent with the presumed sources of the noise. Absolute noise levels are several decibels lower than those measured near the surface.