Trivia. Is the output of your HU linear...? RCA Output Voltage Curve

[Wicks]

So I was testing my HU the other day and based on the results of what I was seeing, I figured this would make a good trivia post.

Conventional thinking would assume that as you turn your volume knob up on your HU (headunit) that your output voltage increases linearly with the volume knob......right?

The sound seems to increase linearly as you crank it up doesn't it?

I inserted my copy of the DD-1 Test Tone Disc into my Kenwood and looped Track1 (40Hz, 0dB).

Then I connected one of the front RCA outputs to a regular Fluke 179 true-RMS DMM and measured the output voltage with each increasing click of the volume knob.

My HU volume goes from 0-35 with a clean unclipped signal at max volume.

Here's what I found:

Focus on the green line, that's the RCA output voltage (Y-axis scale on the right). The X-axis at the bottom is the volume knob position.

Notice how the output goes up exponentially and that I don't get my "5V rated output" until the last click on the volume knob (35).

The red line is the dB change in voltage with each click of the volume, referencing the previous knob position.

The blue line is the dB change in the voltage with reference to volume position Zero.

Y-axis scale on the left.

So who thinks they know why the output voltage goes up exponentially?

Hint, the answer is in the chart if you know what you're looking for...

[skittlesRgood]

easy. its the 2x power = 3db gain formula. if each click of the volume increased voltage the same amount then it would sound less effective at higher volumes.

That's really cool to look at though. I never would have thought about it. experiments like these are always fun.

[Noel989]

[Wicks]

skittles is on the right track.

Sure, mathematically twice the power equals an increase of 3dB, but I was looking for a little bit more detail on why that has anything to do with your hearing...?

[Wicks]

Any other thoughts?

[Jessica]

Human hearing is not linear? Idunno?

[SnowDrifter]

Makes sense.

Our hearing is logarithmic. And decibels, which is based on human hearing, is logarithmic too.

[Wicks]

Makes sense.

Our hearing is logarithmic. And decibels, which is based on human hearing, is logarithmic too.

That's what I wanted to hear!

Notice the blue line in the chart above.

That's the logarithmic plot of the output.

Notice how that line is linear with the volume knob and is how you'd hear the output.