Microphonics

An alternative source of microphonic noise is said to be ice inside the Dewar. Two mechanisms have been suggested  

These are listed in the fault-finding guide (Table 12.1)  

Such little information as is available on troubleshooting in high-resolution gamma spectrometry systems is scattered through the manuals of equipment suppliers 1 would particularly commend those of Canberra, PGT and ORTEC. 

Marine environmental measurements are a particular field where considerable efforts have been put into developing low activity measurements to such an extent that the gamma spectrometers are retreating underground, where cosmic ray backgrounds are much lower (Section 13.4.7). One particular reason for such a desire for ultimate MDA is that other techniques, such as ICPMS and AMS, are able to use smaller samples to achieve their ends, and radiometric techniques are beginning to be asked to make do with similar sized samples. Smaller samples facilitate collection, storage and preparation of samples and minimize expensive time on station for the survey ship. 

In Section 5.6, I discussed the principles underlying the concepts of Critical Limit, Limit of Detection and its activity equivalent, the dubious Minimum Detectable Activity (MDA), which you will recall is not the minimum activity detectable. In that section, I pointed out that the common use of MDA as an indication of the upper limit of activity within a measured sample is not justifiable. However, it is the correct parameter to consider when discussing what if in the context of evaluation of 

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Another important application area is the non-destructive defectoscopy of electronic components. Fig.2a shows an X-ray shadow image of a SMC LED. The 3-dimensional displacement of internal parts can only be visualized non-destructively in the tomographic reconstmction. Reconstructed cross sections through this LED are shown in Fig.2b. In the same way most electronic components in plastic and thin metal cases can be visualized. Even small electronic assemblies like hybrid ICs, magnetic heads, microphones, ABS-sensors can be tested by microtomograpical methods. 

The maximum distance between transmitter and receiver is 350 mm. The transmitter on the probe can be rotated making it possible to perform a sequence of scans on a perticular weld and on neighbouring welds without moving the microphone collar. 

The Calibration of the positioning system is carried out using a bar with a given distance which is placed between the referenspoint on the microphone collar and the probe. The distance is then entered into the acquisition software together with informations of the air temperature close to the tested object, pipe dimension, type of UT-probe (probe height) and scanning direction. 

Another feature is a repositioning program (Repos.exe) which is used for the purpose of correcting positioning data collected when scanning on a pipe elbow or when scanning is performed with the microphone collar mounted on the elbow itself... 

Ia early telephoaes, souad (voice) waves caused a carboa microphone s resistance to vary, thus varyiag the current flowing ia a series external circuit. This d-c curreat could thea be used to regeaerate voice waves ia a receiver. Two wires were required to carry a single coaversatioa. With time, telecommunications traffic was eacoded oa a-c carriers, at first usiag ampHtude or frequeacy modulatioa, and more recently pulse code modulation. 

Photoacoustic spectrometry (pas) differs from the other methods in that the detector is a microphone. This makes pas wavelength independent. 

Impedance Tube Test Methods. There are two impedance tube test methods ASTM C384-90a (3) and ASTM E1050-90 (4). Test method C384-90a makes use of a tube with a test specimen at one end, a loudspeaker at the other, and a probe microphone that can be moved inside the tube. Sound emitted from the loudspeaker propagates down the tube and is reflected back by the specimen. A standing wave pattern develops inside the tube. 

ASTM E1050-90 also makes use of a tube with a test specimen at one end and a loudspeaker at the other end, but iastead of a single movable microphone there are two microphones at fixed locations ia the tube. The signals from these microphones are processed by a digital frequency analysis system which calculates the standing wave pattern and the normal iacidence sound-absorption coefficients. 

Method for Impedance and Absorption of Acoustical Materials Usinga Tube, Two Microphones, andaDigitalErequeny Analysis System, ASTME1050-90,... 

A closely related technique useful for localized gas concentrations and leaks is photoacoustic detection and ranging (padar) (90). A laser pulse tuned to an absorption line generates an acoustic signal that is detected by a paraboHc microphone. A range resolution of 1 cm out to 100 m is feasible. 

Hot wire microphone anemometer An instrument for the measurement of fluid flow. 

Another important hut little-known piezoelectric effect is found in some electronic systems. Speaking produces pressure variations that propagate through the air. Forces are produced on anything in contact with this vibrating air so that when contact is with a piezoelectric crystal, tiny voltage variations are produced. A ci ystal microphone is designed to make use of this piezoelectric effect. 

The simplest sound level meter consists of a microphone, an amplifier and a meter of some type. Sound level meters are graded according to British and international standards, and the most common type used for accurate measurement purposes it known as the Precision Grade or Type 1 meter. In practice, a basic sound level meter will incorporate weighting networks with either in-built octave filters or provision for connecting an external filter set (Figure 42.6). 

And that makes Mr. Stuart, who is a gin drinker, who is English, whose parents are pub owners and who said he grew up in the business of drinking, as sad as a ballad sung tightly into a rasping microphone. 

The major piezoelectric applications are sensors (pickups, keyboards, microphones, etc.), electromechanical transducers (actuators, vibrators, etc ), signal devices, and surface acoustic wave devices (resonators, traps, filters, etc ). Typical materials are ZnO, AIN, PbTiOg, LiTaOg, and Pb(Zr.Ti)03 (PZT). 

LDV, PM, and microphone output voltages are simultaneously recorded at a sampling frequency/a = 16,384 Hz during a period of 2 s. It is thus possible to record at least 100 periods for the lowest driving frequency/g = 50 Hz... 

A delay equal to the acoustic time lag r=r/co is also included to account for wave propagation over the distance r between the flame and the microphone M. The resulting signal is then scaled to get a signal dCHVdf(f) with the same amplitude as the pressure fluctuation p (f+x). Owing to the combustion... 

V"-shaped flames submitted to convective waves (Figure 5.2.12—CF, MF, and VF). It should be noted that these results obtained for a microphone located in the near-field of the flame still obey the classical relation derived for far-field conditions. 

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