Low-frequency noise

A standard probe (type MWB or SWB) is fixed to the probe holder and is mechanically connected to a further piezoelectric receiver. A noise generator, which is coupled to any point of the test object, provides a low frequency noise signal which is picked up by the piezoelectric receiver. The intensity of the signal allows the evaluation of the coupling quality. 

Steam injection introduces an additional source of noise. An effective flare tip is one which achieves a good balance of smoke and luminosity reduction without exceeding acceptable noise levels. Low-frequency noise is encountered at relatively high steam to hydrocarbon ratios. 

Landsrrom, U., Kjellherg, A., and Soderberg, L. (1991). Spectral character, exposure lei fcls and adverse eft ects of ventilation noise in offices. /. Low frequency Noise and Vibration 10(,i i. 

Paakkonen, R. (1988). Low frequency noise and complaints about indoor climate. Procetdmgs of Nordic Acoustical Meeting, 15-17 June, Tampere. 

Landstrom, Eh, Holmberg, K., Kjellberg, A.,. Soderberg, L., and Tesarz, M. (1996). Exposure time and irs influence on noise annoyance at work. /. Low Frequency Noise and Vibration 14(41. 

I.andstrdm, L). (1995). Exposure parameters Involved in low frequency noise annoyance. Proceeding .Assessing, ind Controlling Community Noise with I.ow Frequenev Com[ioncms. Copenhagen, December. 

Low-frequency noise (in the range 3-50 Hz) may have other injurious effects on the body. Research has also indicated that a type of fatigue caused by low-frequency noise has a similar effect to that caused by alcohol. Infrasound (low-frequency sound) also has a synergistic effect with alcohol. Low-frequency noise is particularly important in the case of workers operating machinery (e.g. vehicles, cranes, etc.). It must also be remembered that very high power levels may be generated at low frequency and may not be readily detected by the ear. Attenuation of low-frequency noise is very difficult (see Section 42.7). 

Reciprocating compressors These cause very high noise levels at low frequency (typically, below 420 Hz). The low-frequency noises are very difficult to attenuate, and the most popular solution is to use rotary (vane type) compressors. These are inherently quieter and have the further advantage that the noise they generate is at high frequency (typically, above 1 kHz) and is therefore easy to attenuate. 

Another unwanted effect of smoothing is the alteration of the frequency characteristics of the noise. This calls for caution. Because low frequencies present in the noise are not removed, the improvement of the signal-to-noise ratio may be limited. This is illustrated in Fig. 40.23 where one can see that after smoothing with a 25 point window low-frequency noise is left. In Section 40.5.3 filtering... 

However the carbon specific heat was not as low as the crystalline materials employed later. Also important, the resistor material exhibits an excess low-frequency noise. Nowadays, two types of resistance sensors are used to realize LTD NTD Ge sensors and TES. 

The next step was the introduction of ion implantation to dope Si for thermometers. Downey et al. [66] used micromachining to realize a Si bolometer with an implanted thermometer. This bolometer had very little low-frequency noise. The use of thermometers doped by neutron transmutation instead of melt doping is described by Lange et al. [67], The evolution of bolometers sees the replacement of the nylon wires to make the conductance to the bath, used by Lange et al. with a micromachined silicon nitride membrane with a definite reduction in the heat capacity associated to the conductance G [68],... 

The large fans associated with hoods may cause severe vibration problems unless they are appropriately mounted at some considerable distance. The mounting of blower motors behind the building can reduce unwanted vibrations in the laboratories, but care must then be taken to avoid irritating low-frequency noise from the lengthy duct work. Room air should be delivered through a perforated dropped ceiling, as it is thus more evenly delivered at lowered velocities to reduce turbulent flows. 

If we utilise the above equations to describe the low frequency noise signals observed with electrochemical systems, it is apparent that the potential noise signal will provide information pertaining to the value of the Stern Geary constant since ... 

Xu G, Liu F, Han S et al (2008) Low-frequency noise in top-gated ambipolar carbon nanotube field effect transistors. Appl Phys Lett 92 223114... 

The high-frequency noise can be diminished either by slowing the time constant of the amplification circuitry or by averaging the signals on the computer. The low-frequency noise is caused by thermal fluctuation in the instrument. If one observes the CD spectrum with extremely small CD magnitudes, a two or three hour warmup... 

Summary. Two principal methods for removal of low frequency noise transients are currently available. The model-based separation approach has shown more flexibility and generality, but is computationally rather intensive. It is felt that future work in the area should consider the problem from a realistic physical modelling perspective, which takes into account linear and non-linear characteristics of gramophone and film sound playback systems, in order to detect and correct these artifacts more effectively. Such an approach could involve both experimental work with playback systems and sophisticated non-linear modelling techniques. Statistical approaches related to those outlined in the click removal work (section 4.3.4) may be applicable to this latter task. 

Signal waveform degraded by low frequency noise transient 157... 

A further problem which is common to several recording media including gramophone discs and optical film sound tracks is that of low frequency noise pulses. This form of degradation is typically associated with large scratches or even breakages in the... 

FIGURE 2.10 Low-frequency noise superimposed on the 480-V bus after switching off the capacitor bank. 

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