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If you plotted an audio equalizer on a standard linear ruler from 20 Hz to 20,000 Hz, the entire bass range would be crushed into a microscopic 1-millimeter sliver on the left edge.
To give equal visual weight to every musical octave, audio engineers and exam boards use a logarithmic frequency axis paired with a linear decibel grid.
Why does human hearing require this special mathematical grid in the first place?

The Logarithmic Frequency Axis (X-Axis)

The human ear detects sounds from 20 Hz to 20 kHz across ten octaves, where each octave represents a doubling of frequency (20 Hz→40 Hz→80 Hz20\text{ Hz} \to 40\text{ Hz} \to 80\text{ Hz}).
In 1860, psychologist Gustav Fechner formulated the Weber-Fechner Law, proving that human pitch and loudness perception scales proportionally with ratios rather than arithmetic addition.
How do we divide this axis into clear, standardized sections on an exam paper?

Spacing Decades and Intermediate Gridlines

A decade is a ten-fold multiplication of frequency (10 Hz→100 Hz→1 kHz→10 kHz10\text{ Hz} \to 100\text{ Hz} \to 1\text{ kHz} \to 10\text{ kHz}). Each decade takes up an equal width on the page.
The geometric distance of any frequency ff within a decade spanning fstartf_{\text{start}} to fend=10⋅fstartf_{\text{end}} = 10 \cdot f_{\text{start}} is proportional to log⁡10(ffstart)\log_{10}\left(\frac{f}{f_{\text{start}}}\right).
For example, the distance from 100 Hz100\text{ Hz} to 200 Hz200\text{ Hz} covers log⁡10(2)≈30.1%\log_{10}(2) \approx 30.1\% of the decade's width, while the step from 800 Hz800\text{ Hz} to 900 Hz900\text{ Hz} occupies only log⁡10(9/8)≈5.1%\log_{10}(9/8) \approx 5.1\%.
This causes intermediate division lines to compress toward the upper end of each decade. For example, moving from 100 Hz100\text{ Hz} to 200 Hz200\text{ Hz} occupies log⁡10(2)≈30.1%\log_{10}(2) \approx 30.1\% of the decade width, while 800 Hz800\text{ Hz} to 900 Hz900\text{ Hz} takes up only log⁡10(9/8)≈5.1%\log_{10}(9/8) \approx 5.1\%.