Why does my car radio volume go up and down?

Why does the volume on my car radio go up and down? This is especially noticeable when you are going over a bump in the road.

How car radio works

During normal operation, as long as your vehicle’s ignition is turned on, power will be sent to the amplifier at all times. In this case, all it takes for the amp to send a high-power signal out of its speaker outputs is an input from your radio. When your volume knob is set to off or “0”, no signals are being sent to your speakers. However, if there exists any level of signal (even just 2 volts), that small current flow will turn on the amplifier. It doesn’t take much;’s why factory radio output levels are kept at nearly the lowest possible level.

When you turn your radio up, it sends more power to the amplifier. If this small amount of power is allowed to flow through to the speaker output stages, they will amplify this weak signal and send it on to the speakers which will play louder. Since no other part in the system has changed, there should be no change in what caused that voltage in the first place; you merely increased its amplitude. The speaker was already moving before you increased your radio volume; all you did was boost its “movement” (the amount of sound produced).

At least…that’s how it should work. Depending on several factors, this small amount of input power can cause some rather large problems if not dealt with properly.

Why does car radio volume go up and down?

When you are driving down the road, your vehicle goes over bumps and dips which cause the suspension to compress and expand. In doing this, it changes the length of your speaker cables very slightly at different points in time. This compression and expansion is enough to allow a small amount of current from your radio signal to bleed into other parts of the system where it should not be. If there was no amplifier before, this extra power could go completely unnoticed….but now that you have an amp involved, these minute fluctuations can make a much larger difference in what gets amplified for playback.

A volume fluctuation at 9 volts caused by a 1-millisecond change in cable length is equivalent to a whopping 1,000 volts coming out of your speaker terminals. That is well above the 100-volt legal limit so it can cause some serious damage!

So what causes this fluctuation in your speaker cables?

It could be caused by various factors but most likely you are experiencing it right at that time when the front end of your car is compressing and/or the rear end of your car is decompressing. This translates into miniscule movement between your chassis and body where there are no isolators or anti-squeak devices installed in nearly every vehicle ever built!

There are about 10 million different solutions to fix this problem. The most common solution I have seen is placing electrical tape over the cable connections to stop the problem (although that only solves the short-term solution; it is not a real fix).

Another popular cure is to purchase some isolators and install them between your car body and chassis. That should solve your cable movement variation problems…right? Well, no actually. Isolators do reduce the transfer of energy (decreasing speaker cable movement) but they don’t eliminate it completely. Plus, what about all those other points where your speaker cables touch something? Each point can cause a slight decrease in voltage and each decrease adds up until there’s nothing left to amplify! Since isolators were never really designed for this purpose, they do not properly address these issues. So I would say that isolators are good tools for keeping your speaker cables in their original positions….but they’re not very effective when it comes to keeping them from moving.

So how do you solve this problem?

Here’s the easy answer: if you want your music loud and crystal clear with no unwanted distortion, don’t use factory head units! With today’s digital technology, there are simply too many compromises in terms of cost versus quality. They simply haven’t gotten the message yet that people want clean sound without all the extra noise. There are exceptions but I’m truly disappointed in most factory car stereos out these days.

If you really want good sound from a head unit, get something aftermarket which has been specifically designed for car audio applications. You will notice a tremendous difference in sound quality by simply changing head units alone.