Radiation impedance

So far, we have developed a transfer function V(z) which is defined as the ratio of the volume velocity of the lips over the voliune velocity at the glottis. In practice however, when we measure sound, we normally in fact measure pressure signals, as this is what most microphones are designed to respond to. Most microphones exist in the far-field, that is at some distance fi om the lips, and hence the signal is influenced by a radiation impedance from the hps. This can be modelled by another transfer function, R(z), which, from Equation 11.4 is defined as... 

In more advanced discussions, there are three kinds of impedances specific acoustic impedance (pressure/particle speed), acoustic impedance (pressure/volume speed) and radiation impedance (force/speed). See Ref. 6 for the details. 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

Liquid Level. The most widely used devices for measuring Hquid levels involve detecting the buoyant force on an object or the pressure differential created by the height of Hquid between two taps on the vessel. Consequently, care is required in locating the tap. Other less widely used techniques utilize concepts such as the attenuation of radiation changes in electrical properties, eg, capacitance and impedance and ultrasonic wave attenuation. 

The maximum velocity at the axis is twice the average, whereas the velocity at the wall is zero. The effect of the burner wall is to cool the flame locally and decrease the burning velocity of the mixture. This results in flame stabilization. However, if the heat-transfer processes (conduction, convection, and radiation) involved in cooling the flame are somehow impeded, the rate of heat loss is decreased and the local reduction in burning velocity may no longer take place. This could result in upstream propagation of the flame. 

As with alternating electrical currents, phase-sensitive measurements are also possible with microwave radiation. The easiest method consists of measuring phase-shifted microwave signals via a lock-in technique by modulating the electrode potential. Such a technique, which measures the phase shift between the potential and the microwave signal, will give specific (e.g., kinetic) information on the system (see later discussion). However, it should not be taken as the equivalent of impedance measurements with microwaves. As in electrochemical impedance measurements,... 

There are several consequences of these anomalies in pressnre gradients for the delivery and distribution of drugs and macromolecules within the tnmonr interstitinm. First, high interstitial pressures mean that the central regions of the tnmonr, already poorly perfused, demonstrate low or non-existent convective flow into the interstitinm. Fnrthermore, interstitial convective flow will tend to radiate outward from the centre, towards the periphery and regions of lower interstitial pressure. Therefore, only small amonnts of drngs or macromolecules will reach cells in the centre of the tumour. At the tumour periphery, where convective transfer across the blood vessel wall might take place, further movement towards the centre of the tumour will be impeded by bulk flow in the opposite direction. 

The magnitude of AV is measured most conveniently by placing an air electrode (a radiation emitter Po210 [alpha-emitter]), near the surface (ca. millimeter in air), connected to an impedance electrometer. This is required since resistance in air is very high, but it is appreciably reduced by the radiation electrode. However, proper isolation is essential for the data to be reliable. 

Rayleigh angle as usual. The value of (7.45) has the form of a ratio of impedances multiplied by a ratio of velocities, and it provides a means of relating Rayleigh wave attenuation (due to radiation into the fluid) to the density of the specimen. With the approximation of (7.45), the term in the large curly brackets in (7.42) becomes... 

A more satisfactory procedure involves a radioactive probe held a little above the water surface. The radioactive radiations ionise the air between the probe and the surface and ensure that they are at the same potential. The probe is connected to an electrometer having a very high input impedance which reads the surface potential. This procedure was pioneered by Guyot (70J and Frumkin [71J in the 1920s but has become much more convenient to use with the introduction of artificial radioactive isotopes and modern electronics. Americium 241 is particularly useful as the low energies of the a and y radiations produced only ionise the air in the immediate vicinity of the probe. 

For efficient transfer of power from the generator to the medium, usually water, the two must be acoustically matched. The discontinuity can be smoothed by fixing a 2/4 thick layer of material having an acoustic impedance intermediate between that of the radiating surface material and water, and polymers having impedances of about 3.5 Mrayl are readily available. The velocity of sound in them is approximately 2500 ms-1 so that the thickness required at 50 kHz is about 12 mm. In practice the transducer is often bonded to an ultrasonic cleaning tank and then the tank and water become a complicating part of the transducer. 

But experiments to resolve the fine structure of the Balmer lines were difficult as you all know, resolution was impeded by the Doppler broadening of components. So ionized helium comes into the picture, because, as Sommerfeld s formula predicted, fine structure intervals are a function of (aZ)2, so in helium they are of order four times as wide as in hydrogen and one has more chance of resolving the Doppler-broadened lines. So PASCHEN [40], in 1916. undertook an extensive study of the He+ lines and in particular, 4686 A (n = 4->3). Fine structure, indeed, was found and matched against Sommerfeld s formula. The measurements were used to determine a value of a. But the structure did not really match the theory in that the quantum numbers bore no imprint of electron spin, so even the orbital properties - which dominated the intensity rules based on a correspondence with classical radiation theory - were wrongly associated with components, and the value of a derived from this first study was later abandoned. 

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