Audio frequencies

In earlier years, to reach a remote area, where. separate telephone lines had not been laid it was normal practice to rttn them through the same poles as the HT power distribution lines (generally 11-33 kV). This was particularly true of internal communications of the electricity companies for ease of operation and to save costs and time. This commitnication was known as the magneto-telephone system. But the proximity of telephone lines to power lines adversely affected the performance of the telephone lines due to generation of overvoltages (Chapter 20) and eleetrical interferences (conductive and inductive interferences, discussed later) on the telephone lines by the power lines.. Some of these interferenees, particularly system harmonics, had the same frequency as the audio frequency of the telephone lines and alTected their audio quality. 

The running of telephone lines through power lines is long discontinued. They are now run on separate structures within a city and nearby areas at audio frequency (— 0.3-3.4 kHz), and maintain enough distance from HT power distribution lines. They are therefore almost unaffected from such disturbances. Nevertheless, interferences must be kept in mind when installing these lines so that they are out of the inductive interference zone of the power lines. The latest method in the field of communications to avoid disturbances is to use underground optical fibre cables, where possible, as discussed later. Optical fibre cables are totally immune to such disturbances. 

Other than the system harmonics, electrical interferences are also caused by line disturbances, which may be caused by lightning, switching, sparking or a fault. As discussed in Chapter 17, line disturbances occur at very high frequencies but some may coincide with the audio frequency of telephone lines, and cause disturbance in the audio quality of the telephone system. All these disturbances are referred to as inductive interferences. 

These are meant to be used with a capacitor to tune a filter circuit, with resonances in the audio frequency range for reducing and filtering the harmonics or communication frequencies. They provide a near short-circuit for the required harmonics to filter them out of circuit. They may be single-phase or three-phase and connected in series or parallel of the capacitor circuit and may have a fixed or variable reactance, rated continuously with saturated magnetic characteristics. They may incur heavy losses. 

Resistances in and of electrolytes are exclusively measured with low or audio frequency ac so as not to falsify the results with polarization effects. Measurement is mainly by four electrodes, which eliminates voltages due to the grounding field resistances of the measuring electrodes. 

Hbrer, m. hearer (telephone) receiver. Hbrfrequenz, /. audio frequency. 

Bhargava109 has reported that no ac polarographic wave is obtained for the first step of the reduction, and for the second step of reduction, the ac wave is conspicuous only at low audio frequencies, indicating that both steps are very slow and irreversible. The diffusion coefficient of isatin in a buffer of pH 1.1 is reported to be 1.85 x 10-6cm2/s and the value of n for the second step is 2. 

Newman and Lerner (N2) have used an arrangement where the signal picked up by a microphone attached to the flat surface below the orifice plate is amplified and fed to a loud speaker. The amplified bubble signal is then fed to one pair of fixed contacts of a double-pole double-throw switch of which the other pair of fixed contacts is connected to an audio-frequency generator. The movable contacts of the switch are connected to the vertical and ground terminals of an oscilloscope. This arrangement permits the observation of either the bubble signal or the sine wave as a function of the internal linear time-base of the oscilloscope. 

Energy losses in soft magnetic materials arise due to both hysteresis and eddy currents, as described in the previous section. Eddy current losses can be reduced by increasing the electrical resistivity of the magnetic material. This is one reason why solid-solution iron-silicon alloys ( 4% Si) are used at power frequencies of around 60 Hz and why iron-nickel alloys are used at audio frequencies. Some magnetically soft ferrites (see Section 6.2.2.1) are very nearly electrical insulators and are thus immune to eddy current losses. Some common soft magnetic materials and their properties are listed in Table 6.19. Soft magnetic alloys are described further in Section 6.2.1.6. 

Apparatus. All electrical resistances were measured with an electrolytic conductivity bridge (Leeds and Northrup model 4666) which was constructed according to specifications set forth by Jones (28) and described by Dike (29). The audio-frequency source was a General Radio Co. type 1311-A audio oscillator used with the frequency regulated at 1000 Hz and the output at about 5 V. The detector circuit consisted of a high-gain low-noise tuned amplifier and null detector (General Radio Co. type 1232-A) and an oscilloscope (Heathkit model O-ll) ... 

The crystals are placed between the plates of a condenser which forms part of an oscillating circuit. An audio-frequency amplifier, with headphones or speaker, is connected to the oscillator. When the frequency of the oscillator is changed continuously by means of a variable condenser in the circuit, clicks (or, for a large number of small crystals, rustling noises) are heard. The reason is that whenever the frequency of the oscillator happens to coincide with a natural frequency of one of the crystals, there is a sudden change of current through the condenser and consequently an impulse which is amplified by the audio-frequency amplifier. For a suitable circuit see Wooster (1957). 

DTGS device will be slow, but can extend into the kHz range, which fits well with the amplitude modulation audio frequencies typical of an FT instrument. 

This work led to the important conclusion that the capacitance of all biological membranes, including cellular membranes and those of subcellular organelles, such as mitochondria, is of the order of 1 yF/cm. This value is apparently independent of frequency in the total RF range at low audio frequencies, capacitance values increase with decreasing frequencies due to additional relaxation mechanisms in or near the membranes Q, 26). These mechanisms will not be discussed here and have been summarized elsewhere (1, 26). 

For polyelectrolyte NaPSS samples, significant nonlinear effects and significant dispersion effects in audio frequency range were recorded quantitatively. Figures 5 and 6 summarize these results. Qualitative behaviors of these phenomena are also summarized at the end of the Results section. These phenomena were not observed significantly in saline solutions as shown in Figure 8. 

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