Weber-Fechner Law

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The Weber - Fechner law describes the human perception of various physical stimuli. Ernst Heinrich Weber (1795-1878), was one of the first people to quantitatively study the human response to a physical stimulus. Weber's findings were later popularized by Gustav Theodor Fechner (1801-1887), hence the name.

In one of his classic experiments, Weber gradually increased the weight that a blindfolded man was holding and asked him to respond when he first felt the increase. Weber found that the response was proportional to a relative increase in the weight. That is to say, if the weight is 1 kg, an increase of a few grams will not be noticed. Rather, when the mass in increased by a certain factor, an increase in weight is perceived. If the mass is doubled, the threshold is also doubled. This kind of relationship can be described by a differential equation as,

where dp is the differential change in perception, dS is the differential increase in the stimulus and S is the stimulus at the instant. A constant factor k is to be determined experimentally.

Integrating the above equation

with C is the constant of integration, ln is the natural logarithm.

To determine C, put p = 0, i.e. no perception; then

where is that threshold of stimulus below which it is not perceived at all.

Therefore, our equation becomes

The relationship between stimulus and perception is logarithmic. This logarithmic relationship means that if the perception is altered in an arithmetic progression (i.e. add constant amounts) the corresponding stimulus varies as a geometric progression (i.e. multiply by a fixed factor).

The point is that this logarithmic relationship is valid, not just for the sensation of weight, but for other stimuli as well.

Take the case of vision. The eye senses brightness logarithmically. Hence stellar magnitude is measured on a logarithmic scale. This magnitude scale was invented by the ancient Greek astronomer Hipparchus in about 150 B.C. He ranked the stars he could see in terms of their brightness, with 1 representing the brightest down to 6 representing the faintest, though now the scale has been extended beyond these limits. An increase in 5 magnitudes corresponds to a decrease in brightness by a factor 100.

Still another logarithmic scale is the decibel scale of sound intensity (see the page for details). And yet another is pitch. In the case of perception of pitch, humans hear pitch in a logarithmic or "geometric" ratio-based fashion. For instance, the "pitch distance" between 100 Hz and 150 Hz sounds the same as 1000 Hz and 1500 Hz. The frequency of corresponding notes of adjacent octaves differ by a factor of 2. For notes spaced equally apart to the human ear, the frequencies are related by a multiplicative factor. For instance, for a 12-tone scale in equal temperament, this factor is (twelfth root of 2). So the frequency of the A# note is the frequency of the A times the 12th root of 2, for any octave.

Weber-Fechner Law

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