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2008 Chevrolet 1500 - Audio question

why do my speakers from my truck get lower and higher ( volume wise ) at random times?

Are they set to “Speed-responsive volume control?”

John, hi. You may not be aware of this, but Ford actually introduced speed sensitive radio volume on the '55 T-Bird. This of course was in the days of vacuum tubes and mechanical volume controls. I’d love to see how they did it, but I’ve been too lazy to find a schematic.

Wow. I had no idea. I thought it was a relatively new thing. Cool!

Designing a circuit to do speed-controlled-volume wouldn’t be that complicated using vacuum tubes. The main problem for the design engineer would be how to make it all fit inside the space allotted for the radio. The thing the designer would have going for them, vacuum tubes are more versatile in the number of inputs and outputs they allow, compared to to semi-conductor transistors. I expect that’s what made that function feasible in a 55 T-Bird. Years ago I had a 45 rpm record player in my car, which used vacuum tubes.

What would they have used as a speed input to the radio?

I’d have guessed the radio designers would use a microphone to measure the amount of audio ambient noise inside the passenger compartment. That would increase with vehicle speed but also if you had the window down, etc. But it could be done using a signal from the speedometer too.

It is because they adjust for the exterior noise in the cabin . Have a 2008 Envoy that does this . If it bothers you I believe you can shut it off too .