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23<br />

Decibels, Filters,<br />

and Bode Plots<br />

23.1 LOGARITHMS<br />

The use of logarithms in industry is so extensive that a clear understanding<br />

of their purpose and use is an absolute necessity. At first exposure,<br />

logarithms often appear vague and mysterious due to the mathematical<br />

operations required to find the logarithm and antilogarithm<br />

using the longhand table approach that is typically taught in mathematics<br />

courses. However, almost all of today’s scientific calculators have<br />

the common and natural log functions, eliminating the complexity of<br />

applying logarithms and allowing us to concentrate on the positive<br />

characteristics of the function.<br />

Basic Relationships<br />

Let us first examine the relationship between the variables of the logarithmic<br />

function. The mathematical expression<br />

N � (b) x<br />

states that the number N is equal to the base b taken to the power x. A<br />

few examples:<br />

100 � (10) 2<br />

27 � (3) 3<br />

54.6 � (e) 4<br />

where e � 2.7183<br />

If the question were to find the power x to satisfy the equation<br />

1200 � (10) x<br />

the value of x could be determined using logarithms in the following<br />

manner:<br />

x � log10 1200 � 3.079<br />

revealing that<br />

10 3.079 � 1200<br />

dB

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