When random signal data is analyzed, resolution varies depending on the frequency range, and therefore changing the frequency range changes the Y axis value.

Display with the power spectrum for 1Hz (i.e. power spectrum density) reduces the difference in resolution.

Frequency resolution is 1/800 of the frequency range at a data length of 2048.
At 10kHz and 1kHz it is 12.5Hz and 1.25Hz respectively.
Strictly speaking, it differs slightly. For example, when viewing a 500Hz spectrum, the rms values for the 12.5Hz and 1.25Hz bandwidths are displayed, and the 10kHz range therefore becomes the larger value.
Display with the PSD is display of 1/12.5 and 1/1.25 of the spectrum value, and the difference in size of the spectrum due to the frequency range is therefore reduced.

In order to average and normalize energy, the value obtained by dividing the power spectrum by T (i.e. the power spectrum for each 1sec unit time) is taken into account, and the power spectrum density is defined as the power spectrum per 1Hz.
With random signals, differences in the power spectrum value occur even with the same signal due to FFT resolution (equivalent to bandwidth). Power spectrum resolution (bandwidth) is normalized to 1Hz and displayed, to remove as much as possible the differences due to frequency range.

Energy spectrum density is defined as the power spectrum density multiplied by the data length T.

With random signals, averaging involves the

use of power spectrum density, however with single events (e.g. shockwaves), the energy has a time limit, and the energy spectrum density for which the duration is taken into account is therefore used.

These relationships are as follows. Assuming data length (i.e. time) as T, and frequency resolution as Δf;

* Power spectrum: The spectrum found with a 2048-point FFT
(displays the rms amplitude for each frequency component)

* Power spectrum density: The Power spectrum / Δf
(displays power spectrum per unit frequency)

* Energy spectrum density: Power spectrum density x T
(displays energy spectrum per unit frequency)

T = 2048 / frequency range / 2.56
Δf = Frequency range / 800

If the LIN display value is assumed to be A,

PSD(LIN)=√(A² / Wf / Δf)

Wf: Window adjustment, Hanning = 3/2
Δf: Frequency resolution

This is to ensure that an expression (analysis) independent of the analyzed frequency range is used.

Reference:
Normalized to unit input energy with the frequency response function (transfer function) for expression

Available with the frequency response function. The following functions are available as means of reducing the work in reading data for computation.

(1) List display of 10 points before/after the search.
(2) Search for points of a maximum value -3dB.