Scratchy noise with studio monitors

Question

I have a scratching noise coming out from my new active nearfield studio monitors. I tried everything I could think of, and I'm looking for new ideas.

Here is a recording that I made of the sound. I used a simple USB microphone put in front of the speakers, so it fails to render the noise as loud as it can be when I sit in front of my monitors, therefore here is an amplified version (+ 20dB using Audacity).

The noise happens all the time, whether I play a sound or not. This noise is not constant, but very regular, and seems to be more present with the "CPU activity" of my computer (for example when I start a software, the noise is more present). The volume of the noise increases with the volume of the monitors, but not with the output volume of the audio interface.

My hardware :

• audio interface : Focusrite Scarlett 2i2 3rd Gen, connected to my computer through USB-C.
• Monitors : KRK Rokit RP5 G4, linked to the audio interface through standard 6.5mm new cables, 1m long.
• "standard" desktop computer (assembled by IT professionals) without any specific noise coming from it (ventilators are low).

When I use my laptop computer instead of my desktop (no other change in the equipment), the noise is completely gone, and the sound is perfect. This seems to indicate that the problem resides somewhere in my computer. I tried to use all the USB ports I have (front and back), tried other USB cables, all of this with no luck. When I use headphones on my audio interface, I don't have any scratch noise (even without noise reduction).

I thought that maybe there was some kind of electrical problem somewhere in my house setup, so I tried everything I could imagine, from using an other power plug, to plug my computer/my monitors/any combination in between on a uninterruptible power supply (plugged, or on battery). All of this made absolutely no change, the scratching noise is still the same.

My PC runs Linux and Windows, the scratching noise is the same on both OS, so it's probably not a software configuration issue.

Any suggestion ? :)

Answer 1

Trying with your laptop was a great idea and tells us what the problem likely is. There’s probably some kind of grounding issue with your desktop computer and that’s causing noise in the ground connection of the audio path between the interface and the monitors.

Some other things you might try to confirm: try with your laptop again and try with the laptop on battery, try with the laptop plugged in with its charger, and try with the laptop charger plugged into the exact same outlet that the desktop is plugged into.

If the problem never happens on the laptop in any of those tests, then it might an internal problem on the desktop. If the problem does occur on the laptop when the laptop is plugged in anywhere, then it could be that the grounding issue is actual with a power strip or the outlets that the monitors are plugged into.

Another possibility on the desktop is that the USB ports are cheap. You could try inserting a powered USB hub in between the desktop and interface to see if that helps.

If the desktop is a home build (not a name brand like Dell or HP or Alienware or whatever) then you’ve encountered one of the downsides of home builds. If it is a name brand you might reach out to the manufacturer for support, but they are likely to say something like “this system is not intended for professional creative applications. Our web site has a page for our professional workstation products blah blah”.

Answer 2

After trying everything suggested, I ended up buying an USB isolator, and this worked perfectly, all the noise is now gone.

Not posting for advertising, because I think there are a lot other devices that can do the trick, but this is the one I got.