165,817 research outputs found
Scan and paint: theory and practice of a sound field visualization method
Sound visualization techniques have played a key role in the development of acoustics throughout history. The development of measurement apparatus and techniques for displaying sound and vibration phenomena has provided excellent tools for building understanding about specific problems. Traditional methods, such as step-by-step measurements or simultaneous multichannel systems, have a strong tradeoff between time requirements, flexibility, and cost. However, if the sound field can be assumed time stationary, scanning methods allow us to assess variations across space with a single transducer, as long as the position of the sensor is known. The proposed technique, Scan and Paint, is based on the acquisition of sound pressure and particle velocity by manually moving a P-U probe (pressure-particle velocity sensors) across a sound field whilst filming the event with a camera. The sensor position is extracted by applying automatic color tracking to each frame of the recorded video. It is then possible to visualize sound variations across the space in terms of sound pressure, particle velocity, or acoustic intensity. In this paper, not only the theoretical foundations of the method, but also its practical applications are explored such as scanning transfer path analysis, source radiation characterization, operational deflection shapes, virtual phased arrays, material characterization, and acoustic intensity vector field mapping
Investigation into the selection of viewing configurations for three-component planar Doppler velocimetry measurements.
A method for the calculation of three orthogonal velocity components in planar
Doppler velocimetry (PDV) using four or more measured velocity components (to
the three typically used) is presented. The advantages and disadvantages are
assessed by use of a Monte Carlo simulation and experimental measurements of the
velocity field of a rotating disk. The addition of a fourth velocity component
has been shown to lead to reductions in the final errors of up to 25%. The
selection of viewing configurations for experiments is discussed by simulation
of the level of errors in measured velocity components and investigation of the
final level of errors in the orthogonal velocity components. Experimental
measurements of the velocity field of a rotating disk are presented,
demonstrating the effect of the viewing configuration on the final level of
error
Quantum noise in the position measurement of a cavity mirror undergoing Brownian motion
We perform a quantum theoretical calculation of the noise power spectrum for
a phase measurement of the light output from a coherently driven optical cavity
with a freely moving rear mirror. We examine how the noise resulting from the
quantum back action appears among the various contributions from other noise
sources. We do not assume an ideal (homodyne) phase measurement, but rather
consider phase modulation detection, which we show has a different shot noise
level. We also take into account the effects of thermal damping of the mirror,
losses within the cavity, and classical laser noise. We relate our theoretical
results to experimental parameters, so as to make direct comparisons with
current experiments simple. We also show that in this situation, the standard
Brownian motion master equation is inadequate for describing the thermal
damping of the mirror, as it produces a spurious term in the steady-state phase
fluctuation spectrum. The corrected Brownian motion master equation [L. Diosi,
Europhys. Lett. {\bf 22}, 1 (1993)] rectifies this inadequacy.Comment: 12 pages revtex, 2 figure
Quantum copying can increase the practically available information
While it is known that copying a quantum system does not increase the amount
of information obtainable about the originals, it may increase the amount
available in practice, when one is restricted to imperfect measurements. We
present a detection scheme which using imperfect detectors, and possibly noisy
quantum copying machines (that entangle the copies), allows one to extract more
information from an incoming signal, than with the imperfect detectors alone.
The case of single-photon detection with noisy, inefficient detectors and
copiers (single controlled-NOT gates in this case) is investigated in detail.
The improvement in distinguishability between a photon and vacuum is found to
occur for a wide range of parameters, and to be quite robust to random noise.
The properties that a quantum copying device must have to be useful in this
scheme are investigated.Comment: 10 pages, 6 figures, accepted PR
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