This is a question of curiosity. I have a '98 Acura Integra, a cheap car stereo with a 3.5mm aux in, a cheap inverter and a smart phone. If I have the smart phone audio hooked up to the car stereo and the inverter charging the phone I get a high pitched tone through the speakers at around 4k RPM that increases in pitch with the RPM of the engine.

I know the solution is to not use the inverter, there is no reason to be doing that, but I'm curious why the pitch of the noise fluctuates with the engine RPM. Shouldn't the output of that DC outlet be mostly consistent?

6 Answers 6


Most of the time a ground loop is the cause of this problem. This problem is common especially if your stereos speakers are amplified, and can be fixed my making sure that your stereo unit and/or amplifier have a good ground connection. This can be done by either finding a more direct ground connection to the cars frame, sanding the area around your ground connection to remove any rust or paint that could be adding grounding resistance, or even running a ground wire from your stereo or amp directly to your battery.

In fact, it is most likely that you are constantly experiencing the whining sound. With the engine idling if you put your ear up to the speaker you will probably hear the sound, it just doesn't become noticeable until higher revs.

Note: Making sure any wires you add (+ or -) are properly insulated always helps too!

See this link for a full description.


  • 2
    Good point - I hadn't even thought of ground loops in this context. +1
    – Rory Alsop
    Commented May 5, 2012 at 12:03

For anyone who has similar issues

I had a similar issue however I don't think it was exactly same. I tried the steps in the accepted answer with no success.

Research on my own

I tried different phones and noticed that some would have the static at low volumes and other would not. I looked online and sometimes the phone can cause this to happen.

How I solved the issue

When talking to the guys at an auto/stereo they mentioned that a there is a product called a Ground Loop Isolator (3.5mm) that could reduce the noise. I found and ordered one up, so far it has considerably reduced the static noise (when using the phone at low volumes).

I hope this helps others like it helped myself.

Note: I paid ~30$ CAD (25 USD) for the isolator but later found them online for less than half the price. I bought the one online and it worked just as well as the expensive one.

Update: On second read-through I see that I missed the attached document on the answer and it does mention Isolators, my apologies.


When converting the 12V DC from your battery to whatever voltage the inverter puts out, there are conversion steps internally to AC and back through switching the voltage.

The switching circuit frequency is probably sensitive to the voltage, and as this varies (can be from 11 to even 15v in some cars) that could change the speed of the switching stage.


This question is already answered, but it is worth noting that a distinct stereo whine that changes pitch with engine speed can be indicative of an issue with the alternator and/or voltage regulator. The noise sounds like supercharger whine in such cases.


I'd check to make sure no power wire is near the spark plug wires. That's caused similar sound for me in my '88 civic wagon with aftermarket stereo. RPM dependant, almost like a turbo whine hahah. 3yrs old but maybe this could help someone else?


Sounds a bit like alternator whine. A noise suppression filter capacitor may be bad. A unfiltered alternator would send pulsating DC ripple into the radio creating a whine that varies in frequency. It is usually located on the back of the alternator(or at least it used to be).

Alternators generate AC (Alternating Current) which is then converted into pulsating DC(Direct Current). This then has to be filtered in order to become pure DC. This capacitor shorts out any residual AC and passes pure DC.

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