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Say I have some audio device with the following absolute frequency response:

enter image description here

I want to shift the curve up, so that the 1kHz point is the 0dB point, and everything else is relative to this point. Am I correct, that I cannot simply add 0.35dB to the entire curve, but need to compute back to Vout from:

dB = 20log(Vout/Vin)

add whatever voltage is necessary to the initial Vout to make the 1kHz point 0dB (we'll call this added voltage "curve offset voltage"), then offset all of the Vout values across the measurement range with that same curve offset voltage to obtain the frequency response relative to 1kHz?

Initially I just added +0.35dB to the entire curve but after thinking about it, I don't think that makes sense, since it would be changing the output magnitudes at each frequency exponentially.

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    \$\begingroup\$ Just adding to it is perfectly correct. You cannot simply add something to each voltage level. You must multiply them by some factor. Adding logarithms is the same as multiplying. That's how sliderules work. And analog multipliers. \$\endgroup\$ – JRE Nov 7 '18 at 7:07
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... add whatever Vout is necessary to make the 1kHz point 0dB, then offset all of the Vout values across the measurement range with that same "offset" voltage to obtain the frequency response relative to 1kHz?

You will make the +0.35 dB adjustment by amplifying the signal by +0.35 dB and not by adding an "offset" voltage. This will raise the whole curve by +0.35 dB.

Initially I just added +0.35dB to the entire curve but after thinking about it, I don't think that makes sense, since it would be changing the output magnitudes at each frequency exponentially.

Not exponentially. "Ratiometrically" would be a better description.

That's what most amplifiers or attenuators are designed to do. Think of a potentiometer in a volume control application, for example. It attenuates by a fixed ratio across the full spectrum (ignoring any parasitic capacitance).

One of the handy features of using decibels is that we can just add them rather than multiplying ratios and, for audio work, it gives a linear value which is a good approximation to the ears' perceived volume. For example, if we had a -3 dB attenuator followed by a +5 dB amplifier and a -9 dB attenuator we can get the full circuit gain as -3 + 5 -9 = -7 dB.

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