All manner of experiments have been conducted like the one on "the hypersonic effect, a term coined to describe a phenomenon reported in a controversial scientific study by Tsutomu Oohashi et al which claims that, although humans cannot consciously hear ultrasound (sounds at frequencies above approximately 20kHz), the presence or absence of those frequencies has a measurable effect on their physiological and psychological reactions." – Wikipedia. Others claim no effect when tests are done with people listening to recordings with and without ultrasonic content. Just as with infrasound, some morons created ultrasonic weapons, thus proving that humans do react to beyond-tweeter sounds. Of course recorded music contains at best a limited amount of ultrasonics, format permitting, whilst acoustic live performances definitely sport ultrasonic content in abundance.

Meanwhile the electronics we use to recreate the original musical experience from our carriers often include parts that oscillate at high frequencies. We know that PCs and Macs, depending on the model, are full of ultrasonically active parts. Clocks oscillate at MHz frequencies, memory chips at multiples of that, CPUs in the GHz range. All of it makes for a noisy environment. DACs used to convert a digital input signal run on high-frequency clocks and handle data streams up to 22.6MHz for DSD512. These high frequencies have an effect on the listener. What's more, some of them seep into the audio signal where they can intermodulate with and affect the audible range. Other ultrasonics generated by our gear simply radiate into our listening space where they not only affect nearby gear but again the human listener who, as mentioned repeatedly, is susceptible to HF. In some quarters, this pollution is known as electrosmog. It includes the Bluetooth, WiFi, cellular and radar bands. The above spectrum allocation map breaks it down.

It's no secret that most manufacturers overlook the effects of their high-frequency spillage. That's become the raison d'être for companies like Shakti, Verictum, LessLoss and others who successfully combat EMI/RFI. In a perfect world, manufacturers would clad their gear in copper or solid wood to contain electromagnetic noise, use power supplies far cleaner than CE marking specs require, internally partition their gear for improved shielding. Alas, we don't live in a perfect world. We're at the mercy of the audio gods. Fortunately, we still can set up our own protection scheme by turning to suppliers of noise-reduction solutions. These come in many ways. We have a few highly efficient examples on hand and apply them whenever trying to extract the best possible sound from a setup. Call them tweaks if you like but we only apply those which really make a difference.

For room acoustics, we can't do without the Acoustic System resonators. For noisy mains, PS Audio's good old Premier delivers reconditioned power and acts as galvanic separator, great for analog sources. IsoTek's Evo3 Synchro keeps nasty DC atop the AC from power supplies and Shakti stones absorb RFI/EMI from gear operating at high frequencies. Is that it? No, there's also intra-gear pollution. When there is close physical proximity and a shared power distribution block, there is crosstalk of noise. Finally there's noise 'on the air' from WiFi, 3G and 4G cellular networks which tries, often successfully, to infiltrate our hifi equipment. Countermeasures for this type noise are the tuning sticks and Triple AC Enhancer from Dutch firm Akiko Audio. We reviewed them here and here and still enjoy their benefits. Sometimes their effects are highly appreciated and very substantial.