Axes of a plotted filter

[Iain W]

When we plot a filter such as the spectral envelope or the filter for comb filtering, I’m assuming the x-axis is frequency. What would the y-axis be? Would it be magnitude?

[Simon]

Key points

You are correct that the x-axis (horizontal) will be frequency, and will be labelled in units of Hertz (Hz).

The vertical axis is magnitude, which is most commonly plotted on a logarithmic scale and is therefore labelled in decibels (dB).

Additional detail

Magnitude is a ratio (in this case, of filter output to its input), and therefore has no units: formally, we say it is dimensionless. So dB are not actually a unit, but a scale.

[Author]

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