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Q. How can I track down unwanted HF noises in my studio?

By Hugh Robjohns

Recently I started hearing a noise in my studio around 10-15 kHz. If I apply a lot of HF boost using an EQ the analyser display reveals a 20kHz tone. How can I find the cause and remove this unwanted noise? I'm using a UPS [uninterruptible power supply), a TV, a PC, and a [Steinberg] UR242 USB audio interface.

SOS Forum post

SOS Technical Editor Hugh Robjohns replies: I'm not clear whether your sound source is from the output of a microphone recording, or something generated inside the computer, or is coming in via the mains power supply. I note you're using a UPS and I have come across situations where an online UPS deposited high-frequency whistles on the supply from its battery inverter. Powering your equipment temporarily from the raw mains (while removing or disabling the UPS so it can't contaminate the local area) would soon reveal if this is the culprit.

Annoying high-frequency sounds can be difficult to locate by ear, but with a bat scanner you may be able to find and solve the problem swiftly.Annoying high-frequency sounds can be difficult to locate by ear, but with a bat scanner you may be able to find and solve the problem swiftly.However, assuming your unwanted noise is being picked up acoustically, I would say that it's not that unusual to have high‑pitched whistles in studios and control rooms these days. Potential sources include the power supplies and high-frequency inverters often used in the fluorescent backlighting of computer screens and TVs, as well as in online UPS systems, and the inverters employed in line-lump and internal switch-mode power supplies. These devices tend to be designed to oscillate at low ultrasonic frequencies (typically 15kHz up to 40kHz or so) and consequently can be almost impossible to track down with your own ears. Inevitably, though, some mics — especially small diaphragm mics — will often pick them up acoustically!

I have had some success locating elusive HF whistles like these by using a handheld 'bat detector'. In my case a Ciel CDB101 (which I don't think is available any more), although there are many other similar affordable devices on the market. These bat detectors use heterodyne technology to pitch-shift ultrasonic sounds down into a more easily audible range (bats use ultrasonic clicks and squeaks to echo-locate, of course). So by turning the sensitivity down on a suitable bat-detector and moving it slowly around in your control room it is often possible to identify numerous ultrasonic whines and whistles coming from all manner of equipment! You may need to experiment with the frequency control to identify and differentiate between the different ultrasonic noise-making devices.

Having tracked down any culprits your options are usually pretty limited when it comes to dealing with the noises: replacement of the offending equipment is often not practical. Sometimes re-orientating the device helps as the ultrasonic noise emissions are often quite directional, especially with line-lump PSUs. Alternatively, you might be able to fabricate simple acoustic covers for the offending items (being aware of any potential heat/fire risks, of course). Given the ultrasonic nature of these sounds, it doesn't take much in the way of a foam-lined baffle to reduce the radiation of unwanted noise dramatically.

Published March 2019