Audio frequency current

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. 

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). 

An important application of ferrites is for shielding sensitive equipment (e.g. data-processing, telecommunications and audio-visual equipment) from electromagnetic interference (EMI). Both NiZn- and MZn-based ferrites components are capable of suppressing interference up to the GHz frequency range by virtue of the high impedance they present to high frequency currents. The ferrite parts are made in a variety of shapes to enclose the leads to be shielded, as shown in Fig. 9.17. 

Figure 2. Fhotograph of an audio frequency glow discharge in the magnetic field. The pressure is 50 mtorr, current is 50 mA., Nj gas is employed.

Low-Frequency Limitations. The use of input and output transformers results in cell current and voltage, and thus detector signals that decrease with decreasing frequency. This effect becomes apparent only at low audio frequencies and imposes a practical lower limit of the order of 100-200 Hz with commercial bridges. 

The cold plasma is most often generated in laboratories and industry by an electric glow discharge xmder low pressure using various frequencies of the applied electric field audio frequencies (AF, mainly in the range of 10-50 kHz), radio frequencies (RF, mainly 13.56 MHz), and microwave frequencies (MW, mainly 2.45 GHz). Sometimes, a direct current (DC) discharge is also used. An example of typical parallel plate plasma reactor, one of those being used in our laboratoiy for deposition of thin films, is sketched in Fig. 2. 

The signal coming from the audio input is an alternating, back-and-forth current. An audio sound with a frequency of 1,000 cycles per second, for example, reverses its electric current direction every one-thousandth of a second. When the current is reversed, the North and South poles of the recording head electromagnet are interchanged. Consequently, the nearby magnetic particles embedded in the tape will become reoriented in the opposite direction. 

If the signal is an alternating current sine wave, being at either of the nonlinear ends of the characteristic curve will cause "distortion" of the waveform shape. This causes the formation of "harmonics" of the original frequency, so the linearizing effect of a bias is quite important in such low-distortion applications as audio amplifiers. 

---