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Sound in Electrical Form

Although the sound that one hears is due to compression and rarefaction of the air, it is often necessary to convert sound into an electrical form in order to perform operations on it such as amplification, recording and mixing. It is the job of the microphone to convert sound from an acoustical form into an electrical form. The process of conversion will not be described here, but the result is important because if it can be assumed for a moment that the microphone is perfect then the resulting electrical waveform will be exactly the same shape as the acoustical waveform which caused it.


The equivalent of the amplitude of the acoustical signal in electrical terms is the voltage of the electrical signal. If the voltage at the output of a microphone were to be measured whilst the microphone was picking up an acoustical sine wave, one would measure a voltage which changed sinusoidal as well. Figure below shows this situation, and it may be seen that an acoustical compression of the air corresponds to a positive-going voltage, whilst an acoustical rarefaction of the air corresponds to a negative-going voltage. (This is the norm, although some sound reproduction systems introduce an absolute phase reversal in the relationship between acoustical phase and electrical phase, such that an acoustical compression becomes equivalent to a negative voltage. Some people claim to be able to hear the difference.)



 A microphone converts  variations in acoustical  sound pressure into  variations in electrical  voltage. Normally, a  compression of the air  results in a positive voltage  and a rarefaction results in  a negative voltage.

The other important quantity in electrical terms is the current flowing down the wire from the microphone. Current is the electrical equivalent of the air particle motion discussed in ‘ How sound travels in air ’ , above. Just as the acoustical sound wave was carried in the motion of the air particles, so the electrical sound wave is carried in the motion of tiny charge carriers which reside in the metal of a wire (these are called electrons). When the voltage is positive the current moves in one direction, and when it is negative the current moves in the other direction. Since the voltage generated by a microphone is repeatedly alternating between positive and negative, in sympathy with the sound wave’s compression and rarefaction cycles, the current similarly changes direction each half cycle. Just as the air particles in ‘ Characteristics of a sound wave ’ , above, did not actually go anywhere in the long term, so the electrons carrying the current do not go anywhere either –they simply oscillate about a fixed point. This is known as alternating current or AC.


Ohm ’s law states that there is a fixed and simple relationship between the current flowing through a device ( I ), the voltage across it ( V ), and its resistance ( R ),


V=I/R


or, I=V/R


or, R=V/I


Thus if the resistance of a device is known, and the voltage dropped across it can be measured, then the current flow may be calculated.


There is also a relationship between the parameters above and the power in watts (W) dissipated in a device:




In AC systems, resistance is replaced by impedance, a complex term which contains both resistance and reactance components. The reactance part varies with the frequency of the signal; thus the impedance of an electrical device also varies with the frequency of a signal. Capacitors (basically two conductive plates separated by an insulator) are electrical devices which present a high impedance to low-frequency signals and a low impedance to high-frequency signals. They will not pass direct current. Inductors (basically coils of wire) are electrical devices which present a high impedance to high-frequency signals and a low impedance to low-frequency signals. Capacitance is measured in farads, inductance in henrys.


a) An oscilloscope displays the waveform of an electric signal by means of a moving spot which is deflected up by a positive signal and down by a negative signal. (b) A spectrum analyzer displays the frequency spectrum of an electrical waveform in the form of lines representing the amplitudes of different spectral components of the signal.

An oscilloscope is used for displaying the waveform of a sound, and a spectrum analyzer is used for showing which frequencies are contained in the signal and their amplitudes. Examples of such devices are pictured in Figure above (a) . Both devices accept sound signals in electrical form and display their analyses of the sound on a screen. The oscilloscope displays a moving spot which scans horizontally at one of a number of fixed speeds from left to right and whose vertical deflection is controlled by the voltage of the sound signal (up for positive, down for negative). In this way it plots the waveform of the sound as it varies with time. Many oscilloscopes have two inputs and can plot two waveforms at the same time, and this can be useful for comparing the relative phases of two signals (see ‘ Phase ’ , above). The spectrum analyzer works in different ways depending on the method of spectrum analysis. A real-time analyzer displays a constantly updating line spectrum and shows the frequency components of the input signal on the horizontal scale together with their amplitudes on the vertical scale.


Bibliography:


  1. Sound and Recording, Sixth Edition, Francis Rumsey and Tim McCormick.

  2. Designing Sound, MIT

  3. Sound Design, Maurizio Giri.

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